Sulfur processing in the marine atmospheric boundary layer: A review and critical assessment of modeling uncertainties

Atmospheric Environment 43 (2009) 2841–2854

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Review

Sulfur processing in the marine atmospheric boundary layer: A review and critical assessment of modeling uncertainties Ian Faloona* Department of Land, Air and Water Resources, University of California Davis, One Shields Avenue, Davis, CA 95616, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 December 2008 Received in revised form 17 February 2009 Accepted 18 February 2009

Sulfur is an extremely motile and vital element in the Earth’s biogeochemical environment, one whose active redox chemistry maintains small reservoirs in the atmosphere and biosphere yet large fluxes through both. Essential for life, intimately linked to the climate state, and an important component of air quality, sulfur and its transport and processing in the atmosphere have been the subject of active research for several decades. This review article describes the current state of our understanding of the atmospheric sulfur cycle, focusing on the marine atmospheric boundary layer, with the aim of identifying the largest roots of uncertainty that most inhibit accurate simulation of sulfur cycling in the atmosphere. An overview of the emissions by phytoplankton and shipping, dispersion and entrainment in the marine boundary layer, and chemical processing by aerosols, clouds, and dry deposition is presented. Analysis of 20 contemporary modeling studies suggests that the greatest ambiguity in global sulfur cycling derives from (in descending order) wet deposition of aerosol sulfate, dry deposition of sulfur dioxide to the Earth’s surface, and the heterogeneous oxidation of SO2 in aerosols and clouds. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Sulfur cycle Atmospheric chemistry Marine boundary layer Heterogeneous oxidation

1. Introduction The element sulfur is an essential and highly mobile component of the Earth’s biogeochemical system, and has experienced significant perturbations from human activity over the last century (Rodhe, 1999). A chemically supple element, sulfur is important in determining proton activity in solution, fueling respiration processes in anaerobic media, and inducing gas to particle conversion in the atmosphere. Not surprisingly sulfur is central to many environmental issues including acid rain and climate change. Sulfur compounds are central to the aerosol budget in the marine atmosphere because of the vast source pool in the ocean, and because their oxidation products (predominantly sulfate, SO2 4 ) affect aerosol pH and hygroscopicity. This connection is why sulfur is central to the principle feedback hypothesis of biological climate self-regulation – the so-called ‘‘CLAW’’ hypothesis (Charlson et al., 1987). The basic tenet is that increased ocean productivity engendered under a warming climate might increase the net emission of dimethyl sulfide (DMS) from the ocean surface to the atmosphere. The resultant oxidation of this reduced biogenic gas to sulfur dioxide, SO2, and then subsequently to aerosol sulfate, could

* Tel.: þ1 530 752 2044; fax: þ1 530 752 1522. E-mail address: [email protected] 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.02.043

possibly increase the Earth’s albedo by both direct backscattering of solar radiation (Shaw, 1983) and indirectly by increasing cloud condensation nuclei (CCN). The enhancement of CCN will in turn increase cloud droplet number concentrations when cloud formation occurs thereby enhancing the reflectivity of marine clouds (Twomey, 1977). The former effect of backscattering shortwave radiation is known as the ‘‘direct effect’’, and the latter is the ‘‘1st indirect effect’’ or the more straightforward ‘‘cloud albedo effect.’’ Aside from increasing cloud reflectivity, increments of cloud droplet numbers can increase cloud lifetimes (Albrecht, 1989), alter the liquid water content of clouds, suppress drizzle, and augment cloud heights (Pincus and Baker, 1994), all of which can potentially alter the climatology of low altitude, reflective clouds. These kinds of influence are much more difficult to quantify and isolate and are intimately tied to climate feedbacks on the hydrologic cycle. Collectively they are referred to as the ‘‘2nd indirect effect’’ or ‘‘cloud lifetime effects.’’ While the CLAW hypothesis was initially framed as the cloud albedo effect, it is all of these complex connections to the climate system via aerosols and clouds, whether from anthropogenic or natural sources, that currently drives most of the research in atmospheric sulfur. In the past, human sulfur mobilization was considered important in the context of direct effects to public health and the acidification of precipitation which was found to adversely impact

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lacustrine and terrestrial ecosystems with insufficient buffering capacity. These concerns helped establish SO2 as one of the primary ‘criteria pollutants’ that was regulated in the Clean Air Act and its amendments. The primary anthropogenic sources are from fuel and coal combustion, and the smelting of metallic ores, especially in industrial copper production. In part due to successful abatement of the acid rain problem in the American Northeast (Saltman, 2005) and Northern Europe, and because of the rise of climate change as the preeminent environmental concern, focus on the sulfur cycle has shifted towards its influence on the Earth’s radiative balance via sulfate aerosols. Sulfur is mostly emitted into the atmosphere in a reduced form such as DMS and H2S (biogenic) or in a partially oxidized state such as SO2 (anthropogenic and volcanic), and is efficiently oxidized to SO2 4 in a matter of days in either the gas phase or in cloud or aerosol solutions. Most of the environmental impacts are concerned with this final product of atmospheric oxidation: sulfate. Because the vapor pressure of H2SO4 is relatively low, most atmospheric sulfate ultimately resides in aerosol phases – partially crystalline and/or concentrated droplets. Forecasts of the ultimate impacts of sulfur emissions therefore rely on accurate quantification of the multiphase, branched processes that oxidize sulfur in the atmosphere. These processes are outlined in the cartoon of Fig. 1, but their relative rates are strongly dependent on the meteorological setting and many aspects of this overall process are still highly uncertain. Fig. 1 attempts to crudely encapsulate the best estimates of the branching ratio of the main processes along with a rough estimate of the expected error. Because of the relatively high solubility of SO2, its oxidation is complex, as it is known to stick to surfaces (dry deposition), react with OH directly in the gas phase, and be taken up and processed (aqueous phase oxidation) or delivered back to the surface in dilute solutions of rainwater (wet deposition). Identification of the best physical description of these processes and the primary sources of uncertainties in their parameterization are the main objectives of this scientific review.

2. Overview of global sulfur cycle 2.1. Radiative forcing of sulfate and other environmental effects A recent review by Haywood and Boucher (2000) estimated the direct radiative forcing (RF) of sulfate aerosols to range between 0.26 and 0.82 W m2, and the IPCC Fourth Assessment Report (Forster et al., 2007) has come down on a value of 0.40  0.20 W m2 (the negative sign indicates a net cooling of the Earth’s climate). Approximately one-half to two-thirds of that effect is attributable to anthropogenic sulfur. The best estimate of climate forcing from the cloud albedo effect is reported to be 0.7 W m2, with a 90% confidence interval from 0.3 to 1.8 W m2 (Solomon et al., 2007). The level of scientific understanding is a way that the IPCC attempts to quantify uncertainties in its conclusions, and assigns a level of medium-to-low for the direct effect, and low for the cloud albedo effect. These correspond to roughly a 2–4 chance in 10 of being correct. For reference, the RF from CO2 is estimated with a high level of certainty (chance of 8 in 10) at 1.7 W m2. The aerosol climate forcing remains a substantially large uncertainty in our understanding of the Earth system; however, it is known with a fair degree of confidence that the presence of sulfate aerosols is responsible for globally averaged temperatures being lower than expected from greenhouse gas (GHG) concentrations alone (Denman et al., 2007). Unlike long-lived, and thus well-mixed, GHGs though, aerosols are distinctly regional in nature because they are effectively scrubbed from the atmosphere on synoptic time scales. It turns out that such rapid and localized removal processes are also responsible for the high degree of uncertainty in modeling the sulfur cycle. Many epidemiological studies have shown premature death and respiratory illness are statistically associated with exposure to particulate matter < 2.5 mm (PM2.5) in diameter. Although sulfate and associated aerosol acidity have been implicated as harmful agents in many such studies, untangling the interrelated causality web can be extremely difficult and contradictory conclusions can

Fig. 1. Schematic representation of important processes influencing the sulfur cycle in the marine atmospheric boundary layer. Chemical conversion fractions are means and standard deviations (of the mean) of the sulfur models reviewed f(RH) signifies a functional dependence on relative humidity.

I. Faloona / Atmospheric Environment 43 (2009) 2841–2854

2.2. Recent model intercomparisons Because of the current interest in the climatic effects of aerosols, of which sulfate is an important component (0.4  0.2 W m2 out of a total direct aerosol RF estimated at 0.5 W m2; Solomon et al., 2007), there have been many large-scale model intercomparison projects initiated in the past decade. Liu et al. (2007) studied the variability in a state-of-the-art modular 3D chemistry and transport model under the NASA enterprise known as the Global Modeling Initiative (GMI). NASA GMI is an ongoing project (http://gmi.gsfc. nasa.gov/gmi.html) that is serving as a testbed for model improvement. The Liu et al. (2007) study used three distinct meteorological data sets to drive the same aerosol and chemistry model in order to determine the variability derived from the 3D wind field alone. They found substantial differences in the modeled surface sulfate concentrations upwards of a factor of 3, which they attributed to differing strengths of convection/precipitation in the data. This variability was found to be larger than that observed in a multiyear run with a single continuous meteorological data set. Earlier model intercomparisons, like the internationally coordinated study of large-scale sulfate aerosol models (COSAM) (Barrie et al., 2001), focused specifically on sulfur chemistry because sulfates were the only major aerosol type for which there was a sufficient number of models and global observations to warrant an international comparison. Just a few years beforehand, in a review of the World Climate Research Program workshop of 1995 in Cambridge, Rasch et al. (2000b) concluded, ‘‘the ability to model the transport, scavenging and transformation of SO2 and the production, transport and scavenging of SO2 4 is still in its infancy.’’ The overview of the 11 models that participated in COSAM found that while surface level sulfates compared fairly well with observations, SO2 mixing ratios were overpredicted by a factor of 2 or more (Barrie et al., 2001). The study further suggested that a major source of variability arises from infidelity of the vertical transport from the planetary boundary layer (PBL) to the free troposphere (FT), especially in strong source regions. Further details of this study with respect to specific aspects of vertical distributions and regional comparisons can be found in Lohmann et al. (2001) and Roelofs et al. (2001), respectively. More recently the Aerosol Comparisons between Observations and Models project (AEROCOM) has begun an international effort to improve the understanding of global aerosols and their impacts on the Earth’s climate. Motivated by the ever increasing availability of satellite and ground network aerosol observations, AEROCOM provides a standardized platform for detailed evaluation of global multi-component aerosol models. Details of the project and links to emission maps, protocols, and interactive results can be found at http://dataipsl.ipsl.jussieu.fr/AEROCOM/. Textor et al. (2006) compare the results of 16 global aerosol models using the idea of model ‘‘diversity.’’ They define the diversity of various aspects of the aerosol budgets as the standard deviation of the properties

(emission rates, atmospheric burdens, deposition rates, etc.) among the various models divided by the ensemble’s mean. They opt to use the term diversity rather than uncertainty because the latter implies some level of knowledge of the true value, whereas the spread in model inputs represents merely the variability of different estimates regardless of their actual accuracy. In this work I continue this convention of inspecting model diversity but report them as absolute sulfur fluxes (Tg Sa1) instead of relative values (%) as used in Textor et al. (2006). The diversities are, in a sense, a measure of the precision, rather than the accuracy, of modeling efforts in the field. The analysis of Textor et al. (2006) illustrates the diversities of the principal terms of the sulfate budget based on 10 global aerosol models (see their Fig. 2). The emissions of anthropogenic SO2 and biogenic DMS are an average of 70% and 25%, respectively, of the sulfate precursor gases, but because the DMS emissions are based on various surface ocean distributions, and surface wind speeds, and air–sea transfer parameterizations, the diversity of the biogenic source is much larger (45%) than that of the anthropogenic SO2 source (8%). However, the overall precursor emission diversity is seen to be only 10%, indicating that models with higher emissions of one gas tend to be internally offset by lower emissions of the other. A similar compensation effect is seen in the diversities of the homogeneous, heterogeneous, and total chemical oxidation pathways for producing sulfate aerosols from SO2. This most likely emerges as a result of tuning processes in a model’s development and the ongoing comparison with a limited observational database. In the end, the study by Textor et al. (2006) points out that the greatest relative variability in sulfate aerosol source strengths seems to reside in the depositional (mostly dry) loss of SO2 before being fully oxidized to sulfate. In their analysis of the sulfate losses, however, it is clear that the wet deposition of aerosol sulfate has the greatest diversity in terms of an absolute sulfur flux. Table 1 is an amalgam of the principal components of the global sulfur budget based on a synthesis of 20 atmospheric chemical modeling studies. The table provides a general sense of the spread in the various terms that control the levels and distributions of sulfur species in the atmosphere. Assuming that the models’ sulfur terms are uncorrelated and normally distributed, assumptions of admittedly dubious validity, the standard deviations of each term

105

Plume Width (m)

be made from the existing data (Schlesinger, 2007). Nevertheless, inasmuch as sulfate can compound PM2.5 problems it can become a health hazard in some environments. Another, most likely tertiary, environmental connection of anthropogenic sulfur was recently put forward by Doney et al. (2007). This study calculated anthropogenic SOx (total oxides of sulfur) and NOx inputs to the ocean, where approximately one-third of these pollutants is deposited, and estimated their contribution to the observed acidification of the world’s oceans. While the calculations link these pollutants to the level of only a few percent globally, they point out that the effect can be particularly important in coastal areas of western ocean boundaries such as offshore of the eastern seaboard of the U.S.

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104

Gifford 1995 Heffter 1965 Durkee 2000 (by eye) von Glasow 2004 EPA’s OCD (σv/u = 0.1)

103

102 102

103

104

105

Time Since Emission (s) Fig. 2. Comparison of several horizontal plume dispersion parameterizations used in marine environments. The EPA’s Offshore and Coastal Dispersion (OCD) model is shown for a nominal value of the relative horizontal wind deviation (sv/u) of 0.1.

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Table 1 Global budgets of the main sulfur species (Tg S y1) and their lifetimes (d). Models’ Range of other median modelsa Min DMS Sources Global lifetime

19.4 1.95

Estimated Approximate std. deviationb diversityc

Max

10.7

27.9

4.4

0.9

5

1.0

8.7 –

SO2 Sources Anthropogenic Ship Biomass burn Volcanic DMS oxidation

95.7 67.2 – 2.3 7.8 18.5

79.0 52.9 2.5 2.1 3.4 10.0

121.6 92.0 4.7 3.0 14.6 27.5

10.9 10.0 0.6

9.6 5.4 0.3

2.9 4.5

0.6 8.3

Sinks Dry deposition Wet deposition Oxidation (/SO4)

96.4 34.6 7.3 51.6

80.1 16.0 0.2 26.2

122.8 55.0 19.9 74.0

– 9.9 5.0 –

– 11.1 2.3 –

1.8

0.6

2.8

0.6

NSS-sulfate Sources 53.3 2.0 Direct emissionsd Heterogeneous ox. 42.0 Homogeneous ox. 11.0

27.6 1.1 15.2 6.1

78.4 3.5 55.5 17.3

13.0 0.6 10.3 2.9

11.7 1.4 9.2 3.9

Sinks Dry deposition Wet deposition

57.6 6.4 44.6

49.1 3.2 24.7

65.7 17.0 74.1

– 3.5 12.6

– 3.5 9.8

4.6

3.4

7.2

1.0

0.8

Global lifetime

Global lifetime

–

a

Models included: Langner and Rodhe (1991), Pham et al. (1995), Chin et al. (1996, 2000), Feichter et al. (1996), Graf et al. (1997), Lohmann and Feichter (1997), Chuang et al. (1997), Roelofs et al. (1998), Koch et al. (1999), Lohmann et al. (1999), Restad et al. (1998), Rasch et al. (2000a), Liu et al. (2007), Chuang et al. (2002), Rotstayn and Lohmann (2002), Easter et al. (2004), Koch et al. (2006), Kloster et al. (2006), and Verma et al. (2007). b Standard deviation is estimated assuming the range spans a w95% confidence interval. c Diversity is from relative values reported by Textor et al. (2006) times mean budget terms (not shown). d Direct sulfate emissions are typically estimated as 2–5% of anthropogenic SO2 emissions.

are estimated based on the observed range spanning the 95% confidence interval. This is presented to suggest a reasonable approximation of the diversity, or spread of estimates, in modern global sulfur models along the lines of the Textor et al. (2006) work. The final column of Table 1 is an attempt to derive an absolute diversity value by using the relative values of Textor et al. (2006) multiplied by the mean budget terms. The mean values are not reported in the table but, aside from the deposition terms, differed from the median values by less than 10%. A comparison of the absolute magnitudes of the deviations in sulfur budget terms allows for a somewhat objective prioritization of the uncertainties in the field of global sulfur modeling. Estimating the spread of the model terms by the average of the last two columns of Table 1 indicates that the largest individual source of variability in the sulfur cycle is the wet deposition of sulfate aerosols, followed by the dry deposition and heterogeneous oxidation of SO2. The large discrepancy between the DMS spread of the sulfur models included in this analysis and the diversity estimated using the Textor et al. (2006) analysis lies in the various ways in which the oceanic source is estimated. In many chemical modeling studies the source strength is optimized to observations of DMS in the atmosphere balanced with the main photochemical sink, believed to be reaction with OH. The other method of estimating this source

strength is by using a seawater DMS climatology and then parameterizing the air–sea exchange with the model meteorology. This latter bottom-up method, more prevalent in the Textor et al. (2006) models, leads to much greater variations in DMS emission estimates. Table 1 further shows that although anthropogenic sources overshadow DMS sources globally by a factor of 3–4, the absolute diversity of DMS emissions (4.5–8.5 Tg Sa1) is on par with that of anthropogenic SO2 emission estimates (5.5–10 Tg Sa1). The comparable uncertainty reinforces the importance of continuing DMS flux observational studies. 2.3. Sources of sulfur to the atmosphere Human activities have influenced the sulfur cycle primarily through massive emissions of SO2 directly from fuel combustion, ore processing (e.g., in copper smelting), and biomass burning. The natural sources of sulfur include emissions of the gas DMS, an ecosystem product of assimilatory reduction of the ocean’s abundant sulfate reservoir by phytoplankton, and from smoldering and eruptive volcanoes, and a tiny fraction from the continental biosphere. The anthropogenic emissions are believed to exceed natural sources by a factor of 2–3 (Rodhe,1999), and in heavily industrialized regions the ratio can be well over 10 times, although global industrial emissions have been on the decline in the last two decades. Table 1 presents a representative breakdown of this budget, and a range documented in the literature from the past 15 years of atmospheric sulfur modeling. The literature estimates of anthropogenic emissions range from 53 to 92 Tg Sa1 worldwide, and a recent review of marine sulfur isotope cycling (Bottrell and Newton, 2006) puts the human impacts in geologic perspective. The best estimates of contemporary riverine fluxes of sulfate runoff from the continents to the oceans are w100 Tg Sa1, and that of the net pyrite burial (the ultimate geologic sink) is w60 Tg Sa1. Seen alongside these other geophysical fluxes it becomes apparent that the human perturbations to this elemental cycle are significant even from a geologic perspective. 2.3.1. Anthropogenic emissions Anthropogenic sulfur emissions are believed to have been decreasing since the late 1980s, due to improved control engineering and cleaner fuels driven in large part by the environmental concerns about acid rain. The estimated global reductions range from 13% to 24% (Stern, 2006; Streets et al., 2006, respectively), between 1988 and 2000; however, the decrease has occurred preferentially in North America and Europe, while Asian emissions appear to still be on the rise. Emissions of U.S. sulfur have dropped by one-third (12–8 Tg S y1) in two decades since 1980 (EPA, 2003). Emissions from 25 European nations (Vestreng et al., 2004) were reduced from 18 to 4 Tg S y1 in that same period; however, Asian emissions are at an all time high estimated by to be 17.2 Tg S y1, with China contributing 10.2 Tg S y1 of that total (Streets et al., 2003). Even in the case of documented sulfur (SO2) emission reductions, the efficiency with which the reduction translates into a decline in atmospheric sulfate burden varies regionally. Manktelow et al. (2007) use an British 3D chemical transport model (TOMCAT) with a sulfur/ aerosol module (UKCA) to estimate that whereas every 1% decrease in SO2 release in Europe and the USA leads to column sulfate reduction by 0.65%, every 1% increase in Asia results in a 0.88% rise in sulfate. Their modeling work attributes this difference to the availability of in-cloud oxidant, and further suggests that the Asian increases, occurring at generally lower latitudes and thus in the presence of more abundant oxidative capacity, leads to a diminished efficiency of the global sulfate abatement. The diminution of anthropogenic sulfur emissions has been suggested to have played a part in the transition from global

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dimming to ‘brightening’ (reduction of dimming), allowing the warming due to a buildup of greenhouse gases in our atmosphere to be experienced more completely during the last decade. Many observational records revealed a gradual fall of surface insolation of about 5% from 1960 to 1990 (global dimming), giving way to a reversal since then (Wild et al., 2005). Modeling work by Streets et al. (2006) showed that these trends match the global emissions of sulfate and black carbon, which peaked near 1989. Although all of these trends imply that the human impact on the global sulfur cycle is substantial and quantifiable, many large uncertainties hamper our predictive power and complete understanding. What can be comprehended from the current body of work is that while emissions from the continental U.S. & Europe are on the fall, those from Asia, other rapidly developing countries, and the marginally regulated shipping industry will continue to rise, at least in the near term. Consequently, an increase in the relative importance of Asian and ship traffic emissions is expected. Because the lifetime of sulfur in any form in the atmosphere does not exceed 5–7 days, for western North America atmospheric processing over the ocean is of the utmost importance in understanding the impact of changing sulfur emissions on air quality and regional climate there. 2.3.2. Ship emissions Traditionally, the cheapest fuel for commercial shipping fleets has been residual fuel oil that can contain as much as 5% S but averages about 2.8% (Streets et al., 2000). The latest inventories of emissions indicate that somewhere between 4 and 17% of the NOx and 2.5–7.7% of the SO2 emitted worldwide from anthropogenic sources come from maritime shipping activity (Corbett et al., 1999; Olivier and Berdowski, 2001; Corbett and Koehler, 2003; Endresen et al., 2003; Liu et al., 2007, and references therein). This is large compared to the approximately 2% of CO2 released from ship combustion, making shipping among the world’s greatest polluting combustion sources per ton of fuel consumed (Corbett and Fischbeck, 1997). Ship emissions are thus 10–20% as large as the global estimates of marine biogenic sulfur emissions, mostly in the form of DMS. Because the former are delivered predominantly to the Northern Hemisphere MBL (85% according to Corbett et al. (1999)), while two-thirds of the latter is emitted by the southern oceans (Kloster et al., 2006), this proportion is probably greater in many regions of the North Pacific and Atlantic. In fact, zonal average S emissions reported in Corbett et al. (1999) show comparable magnitudes to DMS emissions between 35 and 55 N (Chin et al., 1996). While NOx emitted per ton of fuel depends on engine load, combustion system, and other operational factors, the SO2 emissions scale solely with the fuel-bound sulfur. A straightforward empirical relationship of 10 kg ton1 of fuel burned (per %S in fuel) can be used to estimate emissions. This means that the average residual fuel oil consumption delivers 28 kg ton1 fuel to the atmosphere. Capaldo et al. (1999) use a global chemical transport model with an updated ship emissions inventory to estimate that somewhere between 5 and 30% of coastal atmospheric SO2 may come from maritime shipping activity. They further predict that this leads to an added 5–20% of the atmospheric NSS-SO4 burden along western Europe, Africa, and North America as well as Japan, Scandinavia, and Indonesia. Of course the model used to generate these estimates is plagued by the same uncertainties discussed below regarding the SO2/SO4 conversion efficiencies. 2.3.3. Biogenic emissions The main reduced sulfur species of biological origin are carbon disulfide (CS2), hydrogen sulfide (H2S), carbonyl sulfide (OCS), and dimethyl sulfide (CH3SCH3 or DMS). An extensive review of the fluxes

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of these gases to the atmosphere prepared by Watts (2000) shows that DMS represents the largest biogenic S flux by far, although he points out that estimates of this predominantly marine source can range from 15 to 109 Tg Sa1 (Andreae, 1990; Erickson et al., 1990; Kelly and Smith, 1990; Bates et al., 1992; Schlesinger, 1997). The breadth of these estimates arises from uncertainties in the bottom-up approach to DMS air–sea exchange, as mentioned previously. Oceanside estimates of DMS emission to the atmosphere are based on spotty observations of dissolved DMS in the ocean and highly contested exchange rates parameterized on the mean wind speed near the ocean surface. When these attempts are globally averaged over different ecosystems, seasons, and reanalyzed meteorological wind fields, the results span a factor of seven. It is generally considered that this span has narrowed over the past two decades; however, Kettle and Andreae (2000) showed that global estimates can vary from 15 to 33 Tg Sa1 based only on the choice of air–sea exchange parameterization. The other method of estimating the oceanic DMS flux invoke balancing a budget where the atmospheric lifetime is believed to be constrained to reactivity with OH (and NO3 in polluted regions at night), and observations of atmospheric concentrations. Because the former method is considered to embody more uncertain processes, the latter tends to be preferred and a plurality of sulfur studies assume DMS emissions to amount to w20–25 Tg Sa1. A global comprehensive database of seawater DMS concentrations is supplied to the research community by the atmospheric chemistry program of NOAA’s Pacific Environmental Laboratory (http://saga.pmel.noaa. gov/dms/). Watts (2000) estimates the other reduced sulfur sources from H2S, OCS, and CS2 to be 7.7  1.3, 1.3  0.3, and 0.7  0.2 Tg Sa1, respectively. Note that these estimates include contributions from anthropogenic activity. Nevertheless, their contributions to the global sulfur cycle are usually considered insignificant, although Watts (2000) points out that the constraints on the H2S budget are particularly unsubstantiated. 3. Physico-chemical aspects of sulfur modeling 3.1. Boundary layer entrainment When a turbulent fluid lies adjacent to a laminar fluid, like the marine boundary layer below a capping thermal inversion and the mildly stable free troposphere above, an irreversible mixing continuously occurs across the interface called entrainment. The ingestion of the more stable air from above down into the MBL consumes (turbulent kinetic) energy in order to overcome the potential energy of the stratification. This process is ubiquitous over the oceans and the mass entrained is balanced in the long-term by a convective mass flux in regions of vertical instability. These exchange processes are absolutely essential to most considerations of trace gas or aerosol budgets in the MBL. Most commonly the exchange flux is parameterized as an entrainment velocity multiplied by a difference in species concentration across the interface. Because many estimates of the exchange velocity fall around 5 mm s1, an approximate time scale for this process in an MBL of w1000 m height is 50–60 h (Faloona et al., 2005). Therefore any other process that occurs on similar or longer time scales will be strongly influenced by this micrometeorological action. Results of the model compilation in Table 1 show that the median lifetimes of all the sulfur species are 2–5 days making them all potentially susceptible to the influence of entrainment, given that there exist appreciable gradients between the MBL and the overlying lower layers of the free troposphere. The influence of turbulent exchange between the marine boundary layer and the free troposphere has long been discussed vis-a`-vis aerosols and sulfate in the MBL. Raes (1995) points out that the multi-step process of oxidizing DMS to create large enough

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aerosols that can act as decent cloud condensation nuclei (CCN) is greater than or equal to about 3 days, therefore entrainment from the FT into the MBL should have a strong influence, at least on the aerosol number concentration. He noted that the mass of NSSsulfate is mostly controlled by emissions of DMS from the ocean surface (in remote regions). A study by Bates et al. (1998) in the Southern Ocean arrived at conclusions confirming the importance of entrainment: ‘‘The dominant process affecting the three smaller modes (<300 nm) was the dynamical mixing between the FT and the MBL.’’ In this case the free troposphere was believed to provide ultrafine aerosols and dilute larger aerosols, the exact outcome depending on the vertical gradients of each species. There is not an overabundance of observations of entrainment velocities, especially over the ocean. Kritz (1983) used radon decay products and aerosol lifetimes around Hawaii to estimate exchange velocities from 3 to 4 mm s1. Ayers and Galbally (1995) released a long-term study of chemical budgets at Cape Grim that reported an average entrainment velocity of 4 (2) mm s1. An experiment on Christmas Island used ozonesondes and budgets to derive an entrainment velocity of 6 (2) mm s1 over the Equatorial Pacific (Clarke et al., 1996). Airborne studies presented in Kawa and Pearson (1989) and Deroode and Duynkerke (1997) indicate wide ranges of entrainment velocities, from 2 to 5 mm s1 in daytime stratocumulus offshore of California to 9–12 mm s1 over the Northeast Atlantic. These studies, however, suffered from very large errors (estimated uncertainties of w10 mm s1). More recently, much more direct and accurate determinations were made off the coast of Southern California by airborne measurements of DMS fluxes and gradients across the stratocumulus (SC) topped MBL (Faloona et al., 2005) yielding an average value of 6 (1) mm s1, but these measurements were made under mostly nighttime conditions when radiative cooling of the stratus deck top generates a much more turbulent MBL than in the daytime when solar heating of the remaining cloud deck acts to stabilize the boundary layer. The authors further propose a parameterization of the entrainment velocity based on the convective Richardson number; however, this requires a priori knowledge of the inversion strength and a convective velocity scale. 3.2. Ship plume dispersion Modeling individual ship exhaust plume dispersion ranges from detailed large-eddy simulation of the effluent (Liu et al., 2000) to the much more standard air quality Gaussian plume model (Song et al., 2003a; von Glasow et al., 2003). The detailed LES study is instructive for the very near field of the plume, but the spatial extent of such a detailed simulation is typically limited to no more than 10 km (Sullivan et al., 1996) which can only encompass the first 20–30 min of ship plume evolution. Thus to investigate the regional (mesoscale) chemical effects of ship effluents, one must resort to average dispersion models. The studies of Song et al. (2003b) and von Glasow et al. (2003) were undertaken to investigate the findings of Kasibhatla et al. (2000) and Davis et al. (2001) that indicated observations of NOx in the MBL were nearly an order of magnitude less than those predicted from chemical transport models incorporating realistic maritime traffic emissions. The cause of this discrepancy is the non-linear photochemistry that occurs at the elevated concentrations within individual ship exhaust plumes, rendering elevated OH and shortening the lifetime of NOx by factors >3 within the plume relative to background conditions (Song et al., 2003b; von Glasow et al., 2003; Chen et al., 2005). This non-linear effect of the photochemistry in highly concentrated plume exhaust is nearly negligible for the anthropogenic SOx because gas phase OH oxidation contributes but a small fraction to the chemical lifetime of SO2. The modeling studies of Song et al. (2003a) and von Glasow et al. (2003) both confirm this fact for ship sulfur emissions.

There is, however, a subtle point lurking in the study by von Glasow et al. (2003) regarding the ship impacts on the regional sulfur budget. In order to attempt to explain the discrepancy between model predictions of MBL NOx and the much lower observations described in Kasibhatla et al. (2000) and Davis et al. (2001), they compare a continuously distributed ship emission inventory against discrete ship plumes, both containing the same overall emission rates. Their model of the sum of individual ship plumes predicts OH concentrations of nearly a factor of two less (and reduced NOx by about a factor of 5). Again, this is a consequence of the non-linear NOx loss that occurs in the elevated plume concentrations as opposed to evenly distributed emissions that would uniformly increase the NOx, in turn systematically elevating the OH (in the generally low background NOx conditions of the MBL). In the latter case, the oxidation rate of background DMS will increase in step yielding approximately twice as much biogenically derived SO2. Thus while the ship SO2 contribution will not differ in either scenario, the absolute SO2 levels derived from DMS oxidation, and therefore the ratio of anthropogenic to biogenic S, will. This fact implies that in order to get the relative contributions to the MBL sulfur cycle correct, one must somehow incorporate this effect of discrete ship plume chemistry into any modeling effort. The work by von Glasow et al. (2003) highlights the importance of plume dilution in modeling results of the chemical evolution of ship exhaust in the MBL. The authors estimate the dilution lifetime of a ship plume (defined as when its contrast to the background has fallen to 5% its initial value) to be w2 days, based on their lateral mixing parameterization. It should be pointed out, however, that the plume dilution parameterization developed by von Glasow et al. (2003) is based on just a few ship track cases from Durkee et al. (2000) spanning a limited plume evolution time and does not agree very well with other common dispersion models. Fig. 2 shows the von Glasow et al. (2003) relationship compared with other more common parameterizations such as that from the EPA’s Offshore and Coastal Dispersion (OCD) model, and the power laws of Heffter (1965) and the more recent overview by Gifford (1995). The scatter in the data used by each of these is fairly large, but it is apparent that the von Glasow et al. (2003) is too large in the near field and probably too small in the far field. The method of using satellite images of ship tracks in marine stratocumulus (Durkee et al., 2000) is complicated because of the requisite coupling between the ship effluent and the ambient cloud field. It is recommended that some average power law of the OCD, Gifford (1995), and Heffter (1965) be used in future work. 3.3. Atmospheric chemical processing 3.3.1. Homogeneous photochemistry The main biogenic sulfur species, DMS, is a highly volatile compound and therefore is preponderantly oxidized in the gas phase. The oxidation products are multitudinous and include SO2, dimethylsulfoxide (CH3SOCH3, DMSO), dimethylsulfone (CH3S(O2)CH3, DMSO2), sulfuric acid (H2SO4), methane sulfinic acid (CH3S(O)OH, MSIA), and methane sulfonic acid (CH3S(O2)OH, MSA). It is believed that OH is the most important oxidizing species in the remote MBL, and in more polluted regions at night NO3 may play a role. Both react by a hydrogen abstraction mechanism, but OH oxidation also proceeds via an addition channel. Because the addition path involves a thermally unstable reaction intermediate its overall rate is characterized by a negative temperature dependence. At w8  C the reaction rate coefficients are equal, but at 20  C the abstraction channel outpaces the addition by a factor of two. The details of the multiple DMS oxidation pathways are still somewhat uncertain, and including all known reactions is not practical for most modeling efforts. Karl et al. (2007) carried out

I. Faloona / Atmospheric Environment 43 (2009) 2841–2854

a model intercomparison of 7 different DMS oxidation schemes with three different long-term monitoring sites. Fortunately, their results indicated that the mechanism developed by Chin et al. (1996), for use in global chemical transport models, performed extremely well with only 6 reactions in comparison to many of the more complex models. It is recommended that this reduced mechanism be used in sulfur modeling, but with the DMS þ OH reaction rates updated to those from Atkinson et al. (2004), which deviate particularly at lower temperatures. From the comparison of 20 different process and modeling studies represented in Table 2, it is evident that there is still considerable uncertainty in the yield of SO2 from DMS oxidation. The mean of all these values is 71%, which may be used as a best guess pending improved understanding of the DMS oxidation mechanism. Nevertheless, the apparently successful Chin et al. (1996) mechanism uses 75% yield only for the addition channel and 100% SO2 yield for all other channels, resulting in a global average yield closer to 95%. One aspect of DMS oxidation that is gradually emerging is that the standard OH and NO3 mechanisms as described above do not fully account for diurnal decay rates typically observed in the MBL. Barnes et al. (1989) discovered that BrO also adds oxygen to DMS, and Toumi (1994) calculated that this could be responsible for a significant rate of oxidation in the atmosphere at concentrations of merely 1 pptv. As early as 1983 it was noted in the literature (Nguyen et al., 1983) that the production of SO2 from DMS appeared faster than could be explained by the known oxidation chemistry. Later Cooper and Saltzman (1991) showed more directly that their observations of maximum to minimum diurnal DMS concentrations from multiple sites exceeded, by about a factor of two, their expectations based on a simple photochemical model. This feature of the DMS/SO2 system has since been observed on many other occasions (Chin et al., 1996; Yvon et al., 1996; James et al., 2000; Sciare et al., 2000; De Bruyn et al., 2006). Evidence of an overlooked oxidant in the MBL was presented by Wingenter et al. (2005), independent of the sulfur cycle, in a study of the ethane budget

Table 2 Sulfur cycle branching ratios (%) reported in the literature. Authors (pub year) Langner and Rodhe (1991) Hertel et al. (1994) Pham et al. (1995) Bandy et al. (1996) Chin et al. (1996) Feichter et al. (1996) Yvon et al. (1996) De Bruyn et al. (1998) Roelofs et al. (1998) Suhre et al. (1998) Davis et al. (1999) Koch et al. (1999) Mari et al. (1999) Chen et al. (2000) Chin et al. (2000) Rasch et al. (2000a,b) Sciare et al. (2000) Shon et al. (2001) De Bruyn et al. (2002) Boucher et al. (2003) Berglen et al. (2004) Kloster et al. (2006) Koch et al. (2006) Textor et al. (2006) Verma et al. (2007) Liu et al. (2007) Number of references Mean value Standard deviation

DMS / SO2 SO2 / SO4 Gas phase Heterogeneous 40 89 62 95 41 40 80 72

53

16

84

50

10

90

51 51 42 83

15 33 12

85 67 88

61 56

25

75

68 65 90 68 39 70 31 85 91 84 92

52 83 31

90 98 20 71 22

51 66

15 34 42 27 29 25

85 66 58 73 71 75

14 57 14

12 24 10

12 76 10

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over the Equatorial Pacific. Their results indicated the presence of a daytime ethane oxidant equivalent to 6–8  104 molecules cm3 Cl or 1.3 ppt BrO. Because of the complexity of the multiphase halogen chemistry in the troposphere, and a dearth of observations, substantiation of these levels of reactive halogens has been long in coming. Observations of an abundance of reactive Cl in the MBL by Pszenny et al. (1993), followed by a theoretical mechanism of autocatalytic reactive Br release from sea-salt aerosols put forward by Vogt et al. (1996), gave great credence to the idea that halogen chemistry was playing an important role in the MBL. More recently, differential optical absorption spectroscopy results reported by Saiz-Lopez et al. (2006) along the coast of Ireland indicated nearsurface levels of BrO typically ranged from 2 to 6 pptv during the day, and observations from the Cape Verde Islands indicate that midday concentrations of BrO are between 2 and 3 pptv even in the open ocean (Read et al., 2008). Moreover, a recent modeling effort by Yang et al. (2005) has produced similar concentrations globally. Thus, it now appears that halogen chemistry is an integral part of the natural rate of DMS oxidation rate, but modeling this chemistry is severely hindered by a paucity of observations of the heterogeneous precursors. The gas phase (homogeneous) oxidation of SO2 is believed to be dominated by reaction with the hydroxyl radical via a threebody association mechanism following a modified Lindemann– Hinshelwood rate expression (Atkinson et al., 2004) (this latest assessment is 9% slower at 20  C than the previous rate recommended by JPL; Demore et al., 1997). The resultant HOSO2 adduct quickly reacts with oxygen in the atmosphere to form SO3, and this species rapidly reacts with water vapor: SO2 DOH/HOSO2 ; DO2 /HO2 DSO3 ; DH2 O/H2 SO4 ðgÞ

(G1)

The resultant sulfuric acid vapor is thought to be extremely important in nucleating new particles, but can only do so when concentrations swell in the absence of preexistent aerosol surface area because of its very low volatility. Most studies have found that new particle production from H2SO4 is rare in the MBL (Raes, 1995; Clarke et al., 1996; Pirjola et al., 2000) and that it more often occurs higher up in the free troposphere, particularly near convective outflow regions where aerosol surface area has been effectively scrubbed (Twohy et al., 2002). Nevertheless, the homogeneous oxidation of SO2 by OH in the MBL (lifetime of w1 week) is a steady source of sulfuric acid vapor that winds up ultimately on the aerosol surfaces as non-sea-salt sulfate or in the ocean. 3.3.2. Heterogeneous processing Marine aerosols and sulfur processing are intricately coupled both by direct heterogeneous reaction and through cloud processing. Because the dissolution of SO2 involves a weak acid dissociation, analogous to CO2, both the uptake and the reactivity of sulfur in solution is strongly pH dependent. The process involves hydrated 2 SO2, and both the bisulfite (HSO 3 ) and the sulfite (SO3 ) ions: SO2 ðgÞDH2 O5SO2 $H2 OðaqÞ

(A1)

D SO2 $H2 OðaqÞ5HSOL 3 DH

(A2)

2L D HSOL 3 5SO3 DH

(A3)

Collectively these aqueous sulfur species are known as S(IV), due to their oxidation state of 4 (the same as SO2). It is clear from LeChatelier’s principle that the lower the pH of the solution the less effectively soluble SO2 becomes, decreasing the overall availability of S(IV) in the aqueous phase, but also shifting which of the forms predominates. Once dissolved in atmospheric waters, however,

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S(IV) is oxidized to sulfate (SO2 4 , S(VI)) by ozone and hydrogen peroxide primarily. Both of these oxidants are partitioned into solution from the background atmosphere, although in solutions rich in organics and/or metals, H2O2 can be further supplied by aqueous photochemistry (Anastasio et al., 1997). Ozone reacts with all the S species: 2L 2L SO2 $H2 OðaqÞ; HSOL 3 ; SO3 DO3 /SO4 D.

(A4)

but in an increasingly rapid fashion with the deprotonated forms, so the overall reaction rate increases strongly at higher pH along with the S(IV) availability. The reaction with hydrogen peroxide, on the other hand, is strongly acid catalyzed and proceeds much faster at low pH: SðIVÞDH2 O2 DHD //SO2L 4 D.

(A5)

It turns out that the reaction rate increase at lower pH is nearly offset by the diminished availability of S(IV) and so the overall oxidation by H2O2 is nearly pH independent under most atmospherically relevant conditions. Another S(IV) oxidation mechanism has been proposed in the last decade involving the autocatalytic release of reactive bromine species from acidified aerosols. The net multiphase reaction, first fully described by Vogt et al. (1996), is 2HO2 ðgÞD2O3 ðgÞDHD ðaqÞDBrL ðaqÞDhv/HOBrD4O2 DH2 O (A6) such that gas phase oxidants, HO2 and O3, extract reactive Br from the natural salts in marine aerosols through autocatalytic production of hypobromous acid (HOBr). The last reactant in (A6) represents the requirement of actinic radiation because the molecular bromine liberated initially is unreactive prior to being photolyzed. The resultant HOBr is efficiently scavenged by the aerosol and can contribute to S(IV) oxidation: L L SO2L 3 DHOBr/HSO4 DBr

(A7)

Vogt et al. (1996) suggested that this reaction and its chlorine analog involving hypochlorous acid (HOCl) probably occur at a similar rate with the bisulfite form as well, so that they are effective throughout a wide range of pH. Their detailed chemical model of the multiphase chemistry indicated that over half of the S(IV) oxidation could be due to this additional oxidant in the remote MBL ([O3] ¼ 40 ppbv, [H2O2] w 1 ppbv). However, their model’s simplifications neglect many aerosol acidification processes particularly active in the submicron aerosols, and the contribution of these halogen sulfur oxidation pathways is strongly dependent on highly uncertain factors: the reactant concentrations due to complex heterogeneous chemistry, the actual reaction rates which are not well established, and the pH of the aerosols which is not easily observed (Keene et al., 1998; Jacob, 2000). Furthermore, modeling experiments performed by von Glasow and Crutzen (2004) indicate possible contributions by HOBr/HOCl of about 10– 75% in the overall NSS-SO4 production rate. Nevertheless, pending more detailed investigations of reactive halogen species in the MBL, these oxidation pathways are not easily incorporated into sulfur cycling models and not considered further here. Other trace species in aerosols and cloud water are believed to play small parts in S(IV) oxidation, such as HOx radicals and dissolved O2 catalyzed by Fe, Mn, Cu(I) and Cl (Zhang and Millero, 1991; Erel et al., 1993; Berglund and Elding, 1995; Finlayson-Pitts and Pitts, 2000) but these are thought to be unimportant in the remote MBL where most of the requisite precursors are scarce and

the pH is never extremely high (Hoppel and Caffrey, 2005). For these reasons I will spend the rest of this work focusing on the two S(IV) heterogeneous oxidation agents considered most commonly in sulfur processing: O3 and H2O2. Aside from the availability of reactive surface area (aerosols and clouds) in the MBL, the next most important factor in determining the rate of sulfate production is the pH of the atmospheric solutions present. The pH of condensed phase atmospheric solutions is a complex function of their composition, and has eluded accurate observation thus far. In the remote MBL sea-salt aerosols, generated by wind stress on the ocean surface, tend to dominate the aerosol mass, while non-sea-salt sulfate is prevalent in the smaller aerosols that more strongly govern the overall numbers. Fresh sea-salt aerosols generated at the surface rapidly dehydrate and come into equilibrium with the ambient relative humidity (RH), which normally increases with height in the MBL due to the natural temperature lapse rate in the typical conditions of a well-mixed boundary layer. Aerosols so formed initially contain natural sea salts in approximate proportion to their abundance in seawater 2 2þ 2þ þ  ([Naþ], [Cl], > [SO2 4 ] > [Mg ] > [Ca ] > [K ] > [CO3 ] > [Br ]), but when equilibrated with the ambient RH they shrink in size, develop greater ionic strength, and tend to increase in pH. Suspended in the atmosphere they are then subjected to titration by both basic (NH3) and acidic gases (HCl, HNO3, H2SO4, MSA, HCOOH, etc.), as well as internal reaction and volitilaztion of products. Detailed accounting of this aqueous menagerie is a field of active research but tends to be underconstrained by observations of the various components and their gas phase mediators. Most germane to the present question of S(IV) oxidation is the resultant pH of the system because of its direct influence on reactions (A1)–(A5). Aerosol phase pH is usually estimated by indirect means such as inference from gas–aerosol phase partitioning of compounds with pH dependent solubilities (e.g., HCl, HNO3, HCOOH or NH3) (Keene and Savoie, 1998; Fridlind and Jacobson, 2000; Keene et al., 2004), or ion balance of aerosol collection analysis (Kerminen et al., 2001), or direct measurements of aerosol aqueous extracts (Winkler, 1980; Keene et al., 2002, 2004). The relative paucity of pH measurements gives rise to considerable confusion about the exact reactivity of marine aerosols with respect to S oxidation, but some trends appear universal. In general, the pH is observed or calculated to be substantially lower in submicron aerosols, and pH is found to effectively equilibrate within each mode due to the acidic redistribution of HCl vapor (Keene and Savoie, 1998; Fridlind and Jacobson, 2000). Table 3 is a compilation of aerosol and cloud pH values either experimentally derived, inferred, or modeled by various researchers over the past few decades. While there appears to be considerable scatter in these estimates, the general picture emerges of an acidified submicron population (roughly speaking, the accumulation mode) distinct from a neutral to slightly acidic supermicron fraction (coarse mode). Another variable controlling the pH of the larger aerosols in particular is the ambient RH, as many studies have confirmed that for any particular environment higher relative humidity tends to reduce the aerosol pH (Chameides and Stelson, 1992; Keene and Savoie, 1998; Fridlind and Jacobson, 2000). von Glasow and Sander (2001) explained this observation by the Henry’s Law equilibration of gaseous HCl (not significantly altered by changes in the low liquid water contents of aerosols) in conjunction with a dilution of the seasalt chloride ions. The result is the counterintuitive rise in hydronium ion concentration with increasing RH ([Hþ]  [Cl] ¼ H0 HCl [HCl (g)] z constant). H0 HCl in this case is the Henry’s Law coefficient for hydrogen chloride, and thus as aerosols grow, [Cl] falls from dilution and [Hþ] must rise in compensation. Thus some of the variability in the reported pH values of Table 3 is most likely due to variability in the ambient RH. von Glasow and Sander (2001) calculate a change of w2 pH units over 500 m in a cloud-free MBL.

I. Faloona / Atmospheric Environment 43 (2009) 2841–2854 Table 3 Estimates of pH in marine atmospheric multiphase systems. Accumulation mode

Coarse mode

1–3

6–9 3–5 5.5 2.5–3.5 6–9 5–8 2–5 3.5–5.5 3.8–4.6 2–5 4.5–5.4 7.5–8.5

1–0 0–2 1.5–2 1–4 2.6–5.3

1.4

5.3

Sc cloud droplets

aerosols is calculated using the method of Schwartz (1986) (e.g., Jacob, 2000):

Source Winkler (1980) Chameides and Stelson (1992) Vogt et al. (1996) Keene and Savoie (1998) Katoshevski et al. (1999) Erickson et al. (1999) Fridlind and Jacobson (2000) von Glasow and Sander (2001) Keene et al. (2002) Keene et al. (2004) Pszenny et al. (2004) Hoppel and Caffrey (2005)

4.0–4.5 3.7–5 4.2–4.4 3.8–5.2 3.3–4.8

Hegg et al. (1984) Lenschow et al. (1988) Vong et al. (1997) Watanabe et al. (2001) Straub et al. (2007)

4.3

Mean value

Most models expect that large sea-salt aerosols are rapidly titrated by acidic gases readily available in the marine environment, viz: HCl, H2SO4, HCOOH, and in more anthropogenically influenced regions, HNO3. The equilibration is believed to occur within minutes to at most an hour or two (Chameides and Stelson, 1992; Erickson et al., 1999). In a review and model evaluation of aerosol pH in the MBL, Keene et al. (1998) summarized the sulfate production mechanisms described above. In their model of aerosol processing at a pH of 8, the ozone oxidation path dominates and leads to greater overall heterogeneous sulfate production than possible at lower pH. At a pH of 5.5 the dominant S oxidation occurs via the HOBr/HOCl reactions, while at pH 3 the hydrogen peroxide is most important. Thus the most important difference in the various model and observational studies of aerosol pH lies in the exact extent to which the coarse mode maintains an elevated pH due to their origins as seawater (pH of about 8.1). Debate on this subject is still quite vigorous. Recently Laskin et al. (2003) proposed that NaOH production on deliquesced sea-salt aerosols might increase the pH of the substrate enough to substantially increase the heterogeneous SO2 / SO2 4 conversion rates. Also, Sievering et al. (2004) have reported Ca excesses in coarse mode aerosols indicative of alkalinities in excess of bulk seawater (up to 40 times), thus rendering aqueous phase ozone oxidation extremely efficient. They claim that biogenic sources of Ca enhance NSS-sulfate production in large aerosols that are rapidly deposited back to the ocean, effectively short circuiting the oceanic source of biogenic sulfur. Using oxygen isotopic analysis of marine aerosols, Alexander et al. (2005) proposed that sea-salt alkalinity is rapidly titrated away by uptake of acidic gases suppressing the O3 oxidation pathway. However, this study was made during the Indian Ocean Experiment (INDOEX) where there was sufficient anthropogenic HNO3 to titrate the aerosol. Elsewhere, Patris et al. (2007) using a similar technique on aerosols collected at Trinidad Head in Northern California (an environment with presumably less anthropogenic influence), argue that the oxygen isotope anomalies of the NSS-SO4 are consistent with ozone-driven oxidation in the coarse mode, indicating that the sea-salt alkalinity is sufficient to buffer rapid acid titration and keep the ozone oxidation channel active. In order to better grasp the aerosol size and pH dependence of heterogeneous sulfur oxidation, I have assembled a simple masstransfer and S oxidation model of the problem applied to a representative MBL aerosol size spectrum. The mass-transfer rate to the

2849

kmt ¼

Z

w50 mm  w0

rp 4 þ Dg cg

1

  4prp2 N rp drp

where the surface area of each aerosol size radius, rp, is integrated with its number density, N(rp), the diffusion rate of the gas to each particle, Dg, and the transfer rate across the aerosol surface comprised of the mean molecular speed of the gas, c, and the surface reaction probability, g. The chemistry mechanism is the standard one used by Mari et al. (1999), Song et al. (2003a), and others which includes only O3 and H2O2 oxidation. A trimodal (Aitken, accumulation, and sea salt) log-normal aerosol distribution, shown in Fig. 3, is assumed with number concentrations, geometric mean diameters and standard deviations amalgamated from various reports in the literature (Fitzgerald, 1991; Frick and Hoppel, 1993; O’dowd et al., 1997; Heintzenberg et al., 2000). The overall number concentration of this distribution is 320 cm3, with a total surface area concentration of w150 mm2 cm3. Atmospheric hydrogen peroxide and ozone are set at levels typical of the remote MBL of 1 and 25 ppbv, respectively. Table 3 lists various model and observational estimates of aerosol pH for MBL aerosols categorized by size. To the best of our knowledge it appears that the smaller submicron aerosols maintain a very low pH between 0 and 4, while the supermicron aerosols tend to develop higher pH with estimates varying between 2 and 9. The aerosol pH in the present model was assumed to run from 1 in the Aitken and accumulation modes until rp w 0.5 mm, then to rise rapidly to pH ¼ 7 from w2.0 mm and larger. Fig. 4 illustrates the size distribution of heterogeneous S(IV) oxidation reactivity under these marine conditions. The dashed line shows the mass-transfer rate of SO2 to the aerosols peaking in the accumulation mode where the maximum surface area contribution resides. The dash-dot and solid-dotted lines show the components for the oxidation of H2O2 and O3, respectively, reacting simultaneously with the S(IV) once inside the aerosol phase. The thick solid line traces the overall first order reaction rate for all these processes taken together, showing

Fig. 3. Aerosol (thick solid) and cloud droplet (open circle-line) size distributions used in this study as representative of marine boundary layer conditions. The aerosol size spectrum is an amalgam of several published data sets, and the cloud spectrum comes from a synthesis of observations by Miles et al. (2000). The aerosol surface area distribution (diamond-line) is displayed (in different units) to compare the relative importance of the different size modes to heterogeneous chemistry.

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Fig. 4. Logarithmic size distribution of the three main processes in hetereogeneous S oxidation: mass-transfer to the aerosol surface (dotted line), reaction with hydrogen peroxide (dashed-dot line), and reaction with ozone (dot-solid line). The overall reaction rate distribution is also shown (thick solid line). The parent aerosol distribution is shown in Fig. 3 assuming pH ¼ 1 for the submicron aerosols, and pH ¼ 7 for the supermicron mode. The integrated reaction rate of S(IV) oxidation for this case is 0.074 h1.

that the total oxidation rate (0.074 h1) is strongly dominated by the coarse (sea salt) mode aerosols of near-neutral pH. Running the same model with the pH of the supermicron aerosols reaching only 6 (not shown), yields an overall rate mostly limited by the H2O2 oxidation, and lower than that shown in Fig. 4 by nearly two orders of magnitude (0.001 h1). This subtle change in supermicron aerosol pH then shifts the resultant heterogeneous chemical lifetime of SO2 in the marine boundary layer from half a day to over a month, meaning that in order for S(IV) oxidation on marine aerosols to be significant, the pH of sea-salt aerosols must remain well above 6. Because the pH dependence of the partioning of SO2 to the aerosol phase and the H2O2 oxidation nearly balance as discussed earlier, the total heterogeneous reaction rates are very insensitive to the pH of the aerosols below pH w6, but are 5–10 times slower than homogeneous reaction with OH (a diurnal mean of 2  106 molecules cm3 is used here). The oxidation of S(IV) by shallow marine cloud droplets was also investigated using the same methods of mass-transfer and reactivity with dissolved H2O2 and O3. Fig. 3 shows the log-normal droplet size distribution, gleaned from an overview by Miles et al. (2000) of low-level stratiform clouds, that was further incorporated into the model. The assembled measurements of cloud water pH from marine stratocumulus environments assembled in Table 3 show surprisingly little spread, and the variations in cloud observed by Straub et al. (2007) indicate that differences may be mostly controlled by liquid water content. In the case of cloud water the pH does increase with LWC as expected by direct dilution of hydronium ions. Cloud droplets are not subject to the previously discussed pH reduction with increased relative humidity, and consequent size swelling, as aerosols are. The 3–4 orders of magnitude larger LWC in clouds substantially draw down HCl, and any other acid vapors present, preventing their redistribution throughout the aerosol modes. In fact, the uniformly low (3–5) cloud pH observations compiled in Table 3 are most likely due not only to the dissolution of acidic CCN, but also to the effective scavenging of acidic gases such as HCl, H2SO4, and HNO3 by cloud water. In order to reveal the pH dependence of the overall oxidation rate of SO2 in generally clean marine environments, Fig. 5 shows

Fig. 5. pH dependence of the various S oxidation pathways in the marine boundary layer. Homogeneous oxidation is calculated with an average [OH] ¼ 2  106 molecules cm3, and the hetereogeneous rates are for gas phase [O3] ¼ 25 ppbv, [H2O2] ¼ 1 ppbv, total aerosol volume concentration of 6.8  1011, and a mean stratocumulus liquid water content of 0.16 g m3 filling the top third of the MBL.

the integrated rates of various components as a function of pH for the aerosol and cloud conditions described herein. The thick solid line is the in cloud oxidation rate which is nearly three orders of magnitude larger than the overall aerosol oxidation rate (solid line with dots) due to the increased reaction volume available incloud droplets. It is clear that the presence of clouds is a strong determinant of the overall oxidation of SO2 in the MBL, and not until pHs in excess of 6 or less than 1 does the acidity of the particle or droplet influence the oxidation. Furthermore, above a pH of 7–7.5 the reactivity is limited by mass-transfer of SO2 to the condensed phase, rendering further alkalinity ineffective except insofar as it may prolong acidification. Fig. 5 also shows that at aerosol pHs between 2 and 6, the homogeneous oxidation of SO2 by OH outpaces heterogeneous processes in aerosols by about a factor of five. 3.3.3. Dry deposition Table 1 indicates that the dry deposition of SO2 to the Earth’s surface makes up about one-quarter to one-third of that species’ total budget. The model diversity of this process is between 10 and 11 Tg Sa1, approximately 10% of the budget, making dry deposition of SO2 one of the three main sources of scatter in models of the global sulfur cycle. Until very recently, the fluxes of SO2 over water have not been directly observable, and the estimation of this process rate is mostly accomplished with standard micrometeorological methods (Wesely, 1989; Ganzeveld and Lelieveld, 1995). Because of the large partitioning of dissolved SO2 into bisulfite (A2) in the high pH of seawater (the effective Henry’s Law coefficient is about 3  107 M atm1) its solubility presents a negligible resistance to uptake. Its dry deposition is therefore limited primarily by the turbulent transfer of the atmospheric flow in the surface layer. Virtually all global chemical models tend to use the methods of Wesely (1989), and report deposition velocities varying between 0.3 and 2.0 cm s1 over the oceans depending on surface wind speed and stability. While some models have reported using standard average rates over the ocean of 0.8 or 0.9 (Feichter et al., 1996; Karl et al., 2007), more specific estimates based on chemical budgets or airborne eddy covariance indicate more realistic values of 0.2–0.5 cm s1 (Davis et al., 1999; Thornton et al., 2002).

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Although the net loss of sulfate to dry deposition is believed to be only w15% of that due to rain out, it also exhibits broad variability among models, and for ship effluent in arid regions this could play a much larger role. Ganzeveld et al. (1998) modeled the deposition of sulfate aerosols over the sea using a parameterization by Hummelshøj (1992), and showed that because of the strong size dependence of particle dry deposition velocity, it is necessary to explicitly calculate the deposition of the entire sulfate aerosol spectrum rather than simply using a mean radius. More specifically, the rise in deposition velocity in the coarse mode due to the influence of gravitationally settling yields overall sulfate aerosol concentrations over the ocean to be nearly a factor of two smaller than when using a single deposition velocity for the mass mean of the particle size distribution.

4. Conclusions The atmospheric processing of volatile sulfur compounds has long been understood to have an important bearing on the acidity of aerosols, clouds, and precipitation. The influence of sulfur on the pH of the environment results in strong modification of the chemical reactivity of multiphase atmospheric systems, and potential damage to lacustrine and terrestrial ecosystems that are subjected to its precipitation. In the past two decades, however, environmental focus has shifted towards the impact that non-seasalt derived sulfate in aerosols has on radiative transfer and thus on the global energy budget of the climate system. The thermodynamic endpoint of sulfur in the Earth’s atmosphere is sulfate which contributes significantly to the overall aerosol burden. Sulfate aerosols affect the climate by increasing the atmosphere’s shortwave reflectivity (RF < 0), and by influencing the radiative and microphysical properties of clouds that condense onto them, but the exact magnitude of these effects remains poorly constrained. The variability of radiative forcing by sulfate aerosols, even for a given atmospheric burden, is large and strongly dependent on humidity fields and cloud schemes (Haywood and Boucher, 2000), with different modeled RH fields generating differences up to 60% (Myhre et al., 2004) in RF. The forcing is a strong function of RH but relatively insensitive to composition (Nemesure et al., 1995). Furthermore, SO2 oxidation rates are most strongly dependent on the liquid water content of both the aerosols and the clouds (if any) in the MBL. Therefore, successful computer predictions of sulfur oxidation rates and radiative impacts will require greater fidelity in model RH fields, moist convection, and precipitation processes. A survey of global sulfur modeling over the past two decades indicates that the broadest scatter of component sulfur fluxes exists, in wet deposition of sulfate aerosols, followed by dry deposition and heterogeneous oxidation of SO2, respectively. The heterogeneous oxidation of SO2 in the MBL depends strongly on the presence of clouds, where liquid water is abundant enough to promote fast reaction with O3 and H2O2 at typical mid-to-low pH (S(IV) lifetime of w1 h). In clear skies the uncertainty in heterogeneous reaction rates is centered around our inability to ascertain the pH of supermicron aerosols, because ozone oxidation exhibits a rise of nearly 3 orders of magnitude per pH unit above 6. The great majority of the results described herein has been simulated with large chemical transport models, assimilated meteorology, simplified aerosol dynamics, etc. The dramatic imbalance of model over observational data clearly bespeaks a need for further observational studies of sulfur and its oxidation in the marine atmosphere. Based on the variability in modeling of the sulfur cycle it is proposed that focused observational research efforts be made of precipitation processing of sulfate aerosols, and turbulent surface fluxes and heterogeneous uptake of SO2.

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Acknowledgements This research was sponsored by a grant from the California Air Resources Board (grant #18530), and made vital by the author’s involvement with the Pacific Atmospheric Sulfur Experiment sponsored by National Science Foundation. The work was also fueled by fruitful discussions with Barry Huebert, Tony Clarke, and Roland von Glasow. Suggestions from two anonymous reviewers further improved this manuscript. References Albrecht, B., 1989. Aerosols, cloud microphysics, and fractional cloudiness. Science 245, 1227–1230. Alexander, B., Park, R.J., Jacob, D.J., Li, Q.B., Yantosca, R.M., Savarino, J., Lee, C.C.W., Thiemens, M.H., 2005. Sulfate formation in sea-salt aerosols: constraints from oxygen isotopes. Journal of Geophysical Research – Atmospheres 110 (D10), D10307. Anastasio, C., Faust, B.C., Rao, C.J., 1997. Aromatic carbonyl compounds as aqueousphase photochemical sources of hydrogen peroxide in acidic sulfate aerosols, fogs, and clouds. 1. Non-phenolic methoxybenzaldehydes and methoxyacetophenones with reductants (phenols). Environmental Science & Technology 31 (1), 218–232. Andreae, M.O., 1990. Ocean–atmosphere interactions in the global biogeochemical sulphur cycle. Marine Chemistry 30, 1–29. Atkinson, R., Baulch, D.L., Cox, R.A., Crowley, J.N., Hampson, R.F., Hynes, R.G., Jenkin, M.E., Rossi, M.J., Troe, J., 2004. Evaluated kinetic and photochemical data for atmospheric chemistry: volume I – gas phase reactions of Ox, HOx, NOx and SOx species. Atmospheric Chemistry and Physics 4, 1461–1738. Ayers, G.P., Galbally, I.E., 1995. A preliminary estimation of boundary-layer-free troposphere entrainment velocity at Cape Grim. In: Dick, A.C., Gras, J.L. (Eds.), Baseline 92. Australian Bureau of Meteorology, pp. 10–15. Bandy, A., Thornton, D.C., Blomquist, B.W., Chen, S., Wade, T.P., Ianni, J.C., Mitchell, G.M., Nadler, W., 1996. Chemistry of dimethyl sulfide in the equatorial Pacific atmosphere. Geophysical Research Letters 23 (7), 741–744. Barnes, I., Becker, K.H., Martin, D., Carlier, P., Mouvier, G., Jourdain, J.L., Laverdet, G., Lebras, G., 1989. Impact of halogen oxides on dimethyl sulfide oxidation in the marine atmosphere. ACS Symposium Series 393, 464–475. Barrie, L.A., Yi, Y., Leaitch, W.R., Lohmann, U., Kasibhatla, P., Roelofs, G.J., Wilson, J., Mcgovern, F., Benkovitz, C., Melieres, M.A., Law, K., Prospero, J., Kritz, M., Bergmann, D., Bridgeman, C., et al., 2001. A comparison of large-scale atmospheric sulphate aerosol models (COSAM): overview and highlights. Tellus Series B – Chemical and Physical Meteorology 53 (5), 615–645. Bates, T.S., Kapustin, V.N., Quinn, P.K., Covert, D.S., Coffman, D.J., Mari, C., Durkee, P.A., De Bruyn, W.J., Saltzman, E.S., 1998. Processes controlling the distribution of aerosol particles in the lower marine boundary layer during the first aerosol characterization experiment (ACE 1). Journal of Geophysical Research – Atmospheres 103, 16369–16383. Bates, T.S., LambGeunther, B.K., Dignon, A., Stoiber, R.E., 1992. Sulfur emissions to the atmosphere from natural sources. Journal of Atmospheric Chemistry 14, 315–317. Berglen, T.F., Berntsen, T.K., Isaksen, I.S.A., Sundet, J.K., 2004. A global model of the coupled sulfur/oxidant chemistry in the troposphere: the sulfur cycle. Journal of Geophysical Research – Atmospheres 109 (D19), D19310. Berglund, J., Elding, L.I., 1995. Manganese-catalyzed autoxidation of dissolved sulfur-dioxide in the atmospheric aqueous-phase. Atmospheric Environment 29 (12), 1379–1391. Bottrell, S.H., Newton, R.J., 2006. Reconstruction of changes in global sulfur cycling from marine sulfate isotopes. Earth-Science Reviews 75 (1–4), 59–83. doi:10.1016/j.earscirev.2005.10.004. Boucher, O., Moulin, C., Belviso, S., Aumont, O., Bopp, L., Cosme, E., Von Kuhlmann, R., Lawrence, M.G., Pham, M., Reddy, M.S., Sciare, J., Venkataraman, C., 2003. DMS atmospheric concentrations and sulphate aerosol indirect radiative forcing: a sensitivity study to the DMS source representation and oxidation. Atmospheric Chemistry and Physics 3, 49–65. Capaldo, K., Corbett, J.J., Kasibhatla, P., Fischbeck, P., Pandis, S.N., 1999. Effects of ship emissions on sulphur cycling and radiative climate forcing over the ocean. Nature 400, 743–746. Chameides, W.L., Stelson, A.W., 1992. Aqueous-phase chemical processes in deliquescent sea-salt aerosols – a mechanism that couples the atmospheric cycles of s and sea salt. Journal of Geophysical Research – Atmospheres 97 (D18), 20565–20580. Article. Charlson, R.J., Lovelock, J.E., Andreae, M.O., Warren, S.G., 1987. Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326, 655–661. Chen, G., Davis, D.D., Kasibhatla, P., Bandy, A.R., Thornton, D.C., Huebert, B.J., Clarke, A.D., Blomquist, B.W., 2000. A study of DMS oxidation in the tropics: comparison of Christmas Island field observations of DMS, SO2, and DMSO with model simulations. Journal of Atmospheric Chemistry 37 (2), 137–160. Chen, G., Huey, L.G., Trainer, M., Nicks, D., Corbett, J., Ryerson, T., Parrish, D., Neuman, J.A., Nowak, J., Tanner, D., Holloway, J., Brock, C., Crawford, J., Olson, J.R., Sullivan, A., et al., 2005. An investigation of the chemistry of ship emission plumes during ITCT 2002. Journal of Geophysical Research – Atmospheres 110 (D10), D10s90.

2852

I. Faloona / Atmospheric Environment 43 (2009) 2841–2854

Chin, M., Rood, R.B., Lin, S.J., Muller, J.F., Thompson, A.M., 2000. Atmospheric sulfur cycle simulated in the global model GOCART: model description and global properties. Journal of Geophysical Research – Atmospheres 105 (D20), 24671– 24687. Chin, M.A., Jacob, D.J., Gardner, G.M., Foremanfowler, M.S., Spiro, P.A., Savoie, D.L., 1996. A global three-dimensional model of tropospheric sulfate. Journal of Geophysical Research – Atmospheres 101 (D13), 18667–18690. Chuang, C.C., Penner, J.E., Prospero, J.M., Grant, K.E., Rau, G.H., Kawamoto, K., 2002. Cloud susceptibility and the first aerosol indirect forcing: sensitivity to black carbon and aerosol concentrations. Journal of Geophysical Research – Atmospheres 107 (D21), 4564. Chuang, C.C., Penner, J.E., Taylor, K.E., Grossman, A.S., Walton, J.J., 1997. An assessment of the radiative effects of anthropogenic sulfate. Journal of Geophysical Research – Atmospheres 102 (D3), 3761–3778. Clarke, A.D., Li, Z., Litchy, M., 1996. Aerosol dynamics in the equatorial Pacific marine boundary layer: microphysics, diurnal cycles and entrainment. Geophysical Research Letters 23 (7), 733–736. Cooper, D.J., Saltzman, E.S., 1991. Measurements of atmospheric dimethyl sulfide and carbon-disulfide in the western Atlantic boundary-layer. Journal of Atmospheric Chemistry 12 (2), 153–168. Corbett, J.J., Fischbeck, P., 1997. Emissions from ships. Science 278 (5339), 823–824. Corbett, J.J., Fischbeck, P.S., Pandis, S.N., 1999. Global nitrogen and sulfur inventories for oceangoing ships. Journal of Geophysical Research – Atmospheres 104 (D3), 3457–3470. Corbett, J.J., Koehler, H.W., 2003. Updated emissions from ocean shipping. Journal of Geophysical Research – Atmospheres 108 (D20), 4650. Davis, D., Chen, G., Bandy, A., Thornton, D., Eisele, F., Mauldin, L., Tanner, D., Lenschow, D., Fuelberg, H., Huebert, B., Heath, J., Clarke, A., Blake, D., 1999. Dimethyl sulfide oxidation in the equatorial Pacific: comparison of model simulations with field observations for DMS, SO2, H2SO4(g), MSA(g), MS, and NSS. Journal of Geophysical Research – Atmospheres 104 (D5), 5765–5784. Davis, D.D., Grodzinsky, G., Kasibhatla, P., Crawford, J., Chen, G., Liu, S., Bandy, A., Thornton, D., Guan, H., Sandholm, S., 2001. Impact of ship emissions on marine boundary layer NOx and SO2 distributions over the pacific basin. Geophysical Research Letters 28 (2), 235–238. De Bruyn, W.J., Bates, T.S., Cainey, J.M., Saltzman, E.S., 1998. Shipboard measurements of dimethyl sulfide and SO2 southwest of Tasmania during the first Aerosol Characterization Experiment (ACE 1). Journal of Geophysical ResearchAtmospheres 103 (D13), 16703–16711. De Bruyn, W.J., Dahl, E., Saltzman, E.S., 2006. DMS and SO2 measurements in the tropical marine boundary layer. Journal of Atmospheric Chemistry 53 (2),145–154. doi:10.1007/s10874-005-9000-z. De Bruyn, W.J., Harvey, M., Cainey, J.M., Saltzman, E.S., 2002. DMS and SO2 at Baring Head, New Zealand: implications for the yield of SO2 from DMS. Journal of Atmospheric Chemistry 41 (2), 189–209. Demore, W.B., Golden, D.M., Hampson, R.F., Kurylo, M.J., Howard, C.J., Ravishankara, A.R., Kolb, C.E., Molina, M.J., 1997. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. NASA, Pasadena, CA. Denman, K.L., Brasseur, G., Chidthaisong, A., Ciais, P., Cox, P.M., Dickinson, R.E., Hauglustaine, D., Heinze, C., Holland, E., Jacob, D., Lohmann, U., Ramachandran, S., Da Silva Dias, P.L., Wofsy, S.C., Zhang, X., 2007. Couplings between changes in the climate system and biogeochemistry. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK; New York, NY, USA. De Roode, S.R., Duynkerke, P.G., 1997. Observed Lagrangian transition of stratocumulus into cumulus during ASTEX: mean state and turbulence structure. Journal of the Atmospheric Sciences 54 (17), 2157–2173. Doney, S.C., Mahowald, N., Lima, I., Feely, R.A., Mackenzie, F.T., Lamarque, J.F., Rasch, P.J., 2007. Impact of anthropogenic atmospheric nitrogen and sulfur deposition on ocean acidification and the inorganic carbon system. Proceedings of the National Academy of Sciences of the United States of America 104 (37), 14580–14585. doi:10.1073/pnas.0702218104. Durkee, P.A., Chartier, R.E., Brown, A., Trehubenko, E.J., Rogerson, S.D., Skupniewicz, C., Nielsen, K.E., Platnick, S., King, M.D., 2000. Composite ship track characteristics. Journal of the Atmospheric Sciences 57 (16), 2542–2553. Easter, R.C., Ghan, S.J., Zhang, Y., Saylor, R.D., Chapman, E.G., Laulainen, N.S., AbdulRazzak, H., Leung, L.R., Bian, X.D., Zaveri, R.A., 2004. Mirage: model description and evaluation of aerosols and trace gases. Journal of Geophysical Research – Atmospheres 109 (D20), D20210. Endresen, O., Sorgard, E., Sundet, J.K., Dalsoren, S.B., Isaksen, I.S.A., Berglen, T.F., Gravir, G., 2003. Emission from international sea transportation and environmental impact. Journal of Geophysical Research – Atmospheres 108 (D17), 4560. EPA, U.S., 2003. National Air Quality and Emissions Trends Report, Special Studies Edition 2003. US Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. Erel, Y., Pehkonen, S.O., Hoffmann, M.R., 1993. Redox chemistry of iron in fog and stratus clouds. Journal of Geophysical Research – Atmospheres 98 (D10), 18423– 18434. Erickson, D.J., Ghan, S.J., Penner, J.E., 1990. Global ocean-to-atmosphere dimethyl sulfide flux. Journal of Geophysical Research – Atmospheres 95 (D6), 7543– 7552. Erickson, D.J., Seuzaret, C., Keene, W.C., Gong, S.L., 1999. A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination:

reactive chlorine emissions inventory. Journal of Geophysical Research – Atmospheres 104 (D7), 8347–8372. Faloona, I., Lenschow, D.H., Campos, T., Stevens, B., Van Zanten, M., Blomquist, B., Thornton, D., Bandy, A., Gerber, H., 2005. Observations of entrainment in eastern pacific marine stratocumulus using three conserved scalars. Journal of the Atmospheric Sciences 62 (9), 3268–3285. Article. Feichter, J., Kjellstrom, E., Rodhe, H., Dentener, F., Lelieveld, J., Roelofs, G.J., 1996. Simulation of the tropospheric sulfur cycle in a global climate model. Atmospheric Environment 30 (10–11), 1693–1707. Finlayson-Pitts, B.J., Pitts, J.N., 2000. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications. Academic Press, San Diego, CA, p. 969. Fitzgerald, J.W., 1991. Marine aerosols – a review. Atmospheric Environment Part A – General Topics 25 (3–4), 533–545. Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D.W., Haywood, J., Lean, J., Lowe, D.C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., Van Dorland, R., 2007. Changes in atmospheric constituents and in radiative forcing. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK; New York, NY, USA. Frick, G.M., Hoppel, W.A., 1993. Airship measurements of aerosol-size distributions, cloud droplet spectra, and trace gas concentrations in the marine boundarylayer. Bulletin of the American Meteorological Society 74 (11), 2195–2202. Fridlind, A.M., Jacobson, M.Z., 2000. A study of gas–aerosol equilibrium and aerosol pH in the remote marine boundary layer during the first aerosol characterization experiment (ACE 1). Journal of Geophysical Research – Atmospheres 105 (D13), 17325–17340. Ganzeveld, L., Lelieveld, J., 1995. Dry deposition parameterization in a chemistry general-circulation model and its influence on the distribution of reactive trace gases. Journal of Geophysical Research – Atmospheres 100 (D10), 20999–21012. Ganzeveld, L., Lelieveld, J., Roelofs, G.J., 1998. A dry deposition parameterization for sulfur oxides in a chemistry and general circulation model. Journal of Geophysical Research – Atmospheres 103 (D5), 5679–5694. Gifford, F.A., 1995. Some recent long-range diffusion observations. Journal of Applied Meteorology 34 (7), 1727–1730. Graf, H.F., Feichter, J., Langmann, B., 1997. Volcanic sulfur emissions: estimates of source strength and its contribution to the global sulfate distribution. Journal of Geophysical Research – Atmospheres 102 (D9), 10727–10738. Haywood, J., Boucher, O., 2000. Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: a review. Reviews of Geophysics 38 (4), 513–543. Heffter, J.L., 1965. The variation of horizontal diffusion parameters with time for travel periods of one hour or longer. Journal of Applied Meteorology 4, 153–156. Hegg, D.A., Radke, L.F., Hobbs, P.V., 1984. Measurements of transformations in the physical and chemical-properties of clouds associated with onshore flow in Washington State. Journal of Climate and Applied Meteorology 23 (6), 979–984. Heintzenberg, J., Covert, D.C., Van Dingenen, R., 2000. Size distribution and chemical composition of marine aerosols: a compilation and review. Tellus Series B – Chemical and Physical Meteorology 52 (4), 1104–1122. Hertel, O., Christensen, J., Hov, O., 1994. Modeling of the end-products of the chemical decomposition of DMS in the marine boundary-layer. Atmospheric Environment 28 (15), 2431–2449. Hoppel, W.A., Caffrey, P.F., 2005. Oxidation of S(IV) in sea-salt aerosol at high pH: ozone versus aerobic reaction. Journal of Geophysical Research – Atmospheres 110 (D23), D23202. Hummelshøj, P.N., 1992. Particle dry deposition to a sea surface. In: Schwartz, S.E., Slinn, W.G.N. (Eds.), Precipitation Scavenging and Atmosphere–Surface Exchange, pp. 829–840. Washington, D.C. Jacob, D.J., 2000. Heterogeneous chemistry and tropospheric ozone. Atmospheric Environment 34 (12–14), 2131–2159. James, J.D., Harrison, R.M., Savage, N.H., Allen, A.G., Grenfell, J.L., Allan, B.J., Plane, J.M.C., Hewitt, C.N., Davison, B., Robertson, L., 2000. Quasi-Lagrangian investigation into dimethyl sulfide oxidation in maritime air using a combination of measurements and model. Journal of Geophysical Research – Atmospheres 105 (D21), 26379–26392. Karl, M., Gross, A., Leck, C., Pirjola, L., 2007. Intercomparison of dimethylsulfide oxidation mechanisms for the marine boundary layer: gaseous and particulate sulfur constituents. Journal of Geophysical Research – Atmospheres 112 (D15), D15304. Kasibhatla, P., Levy, H., Moxim, W.J., Pandis, S.N., Corbett, J.J., Peterson, M.C., Honrath, R.E., Frost, G.J., Knapp, K., Parrish, D.D., Ryerson, T.B., 2000. Do emissions from ships have a significant impact on concentrations of nitrogen oxides in the marine boundary layer? Geophysical Research Letters 27 (15), 2229– 2232. Katoshevski, D., Nenes, A., Seinfeld, J.H., 1999. A study of processes that govern the maintenance of aerosols in the marine boundary layer. Journal of Aerosol Science 30 (4), 503–532. Kawa, S.R., Pearson, R., 1989. Ozone budgets from the dynamics and chemistry of marine stratocumulus experiment. Journal of Geophysical Research – Atmospheres 94 (D7), 9809–9817. Keene, W.C., Pszenny, A.A.P., Maben, J.R., Sander, R., 2002. Variation of marine aerosol acidity with particle size. Geophysical Research Letters 29 (7), 1101. doi:10.1029/2001gl013881. Keene, W.C., Pszenny, A.A.P., Maben, J.R., Stevenson, E., Wall, A., 2004. Closure evaluation of size-resolved aerosol pH in the New England coastal atmosphere

I. Faloona / Atmospheric Environment 43 (2009) 2841–2854 during summer. Journal of Geophysical Research – Atmospheres 109 (D23), D23307. Keene, W.C., Sander, R., Pszenny, A.A.P., Vogt, R., Crutzen, P.J., Galloway, J.N., 1998. Aerosol pH in the marine boundary layer: a review and model evaluation. Journal of Aerosol Science 29 (3), 339–356. Keene, W.C., Savoie, D.L., 1998. The pH of deliquesced sea-salt aerosol in polluted marine air. Geophysical Research Letters 25 (12), 2181–2184. Kelly, D.P., Smith, N.A., 1990. Organic sulfur compounds in the environment. Advances in Microbial Ecology 11, 345–385. Kerminen, V.M., Hillamo, R., Teinila, K., Pakkanen, T., Allegrini, I., Sparapani, R., 2001. Ion balances of size-resolved tropospheric aerosol samples: implications for the acidity and atmospheric processing of aerosols. Atmospheric Environment 35 (31), 5255–5265. Kettle, A.J., Andreae, M.O., 2000. Flux of dimethylsulfide from the oceans: a comparison of updated data seas and flux models. Journal of Geophysical Research – Atmospheres 105 (D22), 26793–26808. Article. Kloster, S., Feichter, J., Reimer, E.M., Six, K.D., Stier, P., Wetzel, P., 2006. DMS cycle in the marine ocean–atmosphere system – a global model study. Biogeosciences 3 (1), 29–51. Koch, D., Jacob, D., Tegen, I., Rind, D., Chin, M., 1999. Tropospheric sulfur simulation and sulfate direct radiative forcing in the Goddard Institute for Space Studies general circulation model. Journal of Geophysical Research – Atmospheres 104 (D19), 23799–23822. Koch, D., Schmidt, G.A., Field, C.V., 2006. Sulfur, sea salt, and radionuclide aerosols in GISS ModelE. Journal of Geophysical Research – Atmospheres 111 (D6), D06206. Kritz, M.A., 1983. Use of long lived radon daughters as indicators of exchange between the free troposphere and the marine boundary-layer. Journal of Geophysical Research – Oceans and Atmospheres 88 (NC13), 8569–8573. Langner, J., Rodhe, H., 1991. A global 3-dimensional model of the tropospheric sulfur cycle. Journal of Atmospheric Chemistry 13 (3), 225–263. Laskin, A., Gaspar, D.J., Wang, W.H., Hunt, S.W., Cowin, J.P., Colson, S.D., FinlaysonPitts, B.J., 2003. Reactions at interfaces as a source of sulfate formation in seasalt particles. Science 301 (5631), 340–344. Lenschow, D.H., Paluch, I.R., Bandy, A.R., Pearson, R., Kawa, S.R., Weaver, C.J., Huebert, B.J., Kay, J.G., Thornton, D.C., Driedger, A.R., 1988. Dynamics and chemistry of marine stratocumulus (DYCOMS) experiment. Bulletin of the American Meteorological Society 69 (9), 1058–1067. Liu, Q.F., Kogan, Y.L., Lilly, D.K., Johnson, D.W., Innis, G.E., Durkee, P.A., Nielsen, K.E., 2000. Modeling of ship effluent transport and its sensitivity to boundary layer structure. Journal of the Atmospheric Sciences 57 (16), 2779–2791. Liu, X.H., Penner, J.E., Das, B.Y., Bergmann, D., Rodriguez, J.M., Strahan, S., Wang, M.H., Feng, Y., 2007. Uncertainties in global aerosol simulations: assessment using three meteorological data sets. Journal of Geophysical Research – Atmospheres 112 (D11), D11212. Lohmann, U., Feichter, J., 1997. Impact of sulfate aerosols on albedo and lifetime of clouds: a sensitivity study with the ECHAM4 GCM. Journal of Geophysical Research – Atmospheres 102 (D12), 13685–13700. Lohmann, U., Leaitch, W.R., Barrie, L., Law, K., Yi, Y., Bergmann, D., Bridgeman, C., Chin, M., Christensen, J., Easter, R., Feichter, J., Jeuken, A., Kjellstrom, E., Koch, D., Rasch, P., et al., 2001. Vertical distributions of sulfur species simulated by large scale atmospheric models in COSAM: comparison with observations. Tellus Series B – Chemical and Physical Meteorology 53 (5), 646–672. Lohmann, U., Von Salzen, K., Mcfarlane, N., Leighton, H.G., Feichter, J., 1999. Tropospheric sulfur cycle in the Canadian general circulation model. Journal of Geophysical Research – Atmospheres 104 (D21), 26833–26858. Manktelow, P.T., Mann, G.W., Carslaw, K.S., Spracklen, D.V., Chipperfield, M.P., 2007. Regional and global trends in sulfate aerosol since the 1980s. Geophysical Research Letters 34 (14), L14803. doi:10.1029/2006gl028668. Mari, C., Suhre, K., Rosset, R., Bates, T.S., Huebert, B.J., Bandy, A.R., Thornton, D.C., Businger, S., 1999. One-dimensional modeling of sulfur species during the first aerosol characterization experiment (ACE 1) Lagrangian b. Journal of Geophysical Research – Atmospheres 104 (D17), 21733–21749. Miles, N.L., Verlinde, J., Clothiaux, E.E., 2000. Cloud droplet size distributions in lowlevel stratiform clouds. Journal of the Atmospheric Sciences 57 (2), 295–311. Myhre, G., Stordal, F., Berglen, T.F., Sundet, J.K., Isaksen, I.S.A., 2004. Uncertainties in the radiative forcing due to sulfate aerosols. Journal of the Atmospheric Sciences 61 (5), 485–498. Nemesure, S., Wagener, R., Schwartz, S.E., 1995. Direct shortwave forcing of climate by the anthropogenic sulfate aerosol: sensitivity to particle size, composition, and relative humidity. Journal of Geophysical Research – Atmospheres 100 (D12), 26105–26116. Nguyen, B.C., Bonsang, B., Gaudry, A., 1983. The role of the ocean in the global atmospheric sulfur cycle. Journal of Geophysical Research – Oceans and Atmospheres 88 (NC15), 903–914. O’dowd, C.D., Smith, M.H., Consterdine, I.E., Lowe, J.A., 1997. Marine aerosol, seasalt, and the marine sulphur cycle: a short review. Atmospheric Environment 31 (1), 73–80. Olivier, J.G.J., Berdowski, J.J.M., 2001. Global emissions sources and sinks. In: Berdowski, J., Guicherit, R., Heij, B.J. (Eds.), The Climate System. A.A. Balkema, Brookfield, VT, pp. 33–78. Patris, N., Cliff, S.S., Quinn, P.K., Kasem, M., Thiemens, M.H., 2007. Isotopic analysis of aerosol sulfate and nitrate during ITCT-2k2: determination of different formation pathways as a function of particle size. Journal of Geophysical Research – Atmospheres 112 (D23), D23301.

2853

Pham, M., Muller, J.F., Brasseur, G.P., Granier, C., Megie, G., 1995. A three-dimensional study of the tropospheric sulfur cycle. Journal of Geophysical Research – Atmospheres 100 (D12), 26061–26092. Pincus, R., Baker, M.B., 1994. Effect of precipitation on the albedo susceptibility of clouds in the marine boundary-layer. Nature 372 (6503), 250–252. Pirjola, L., O’dowd, C.D., Brooks, I.M., Kulmala, M., 2000. Can new particle formation occur in the clean marine boundary layer? Journal of Geophysical Research – Atmospheres 105 (D21), 26531–26546. Pszenny, A.A.P., Keene, W.C., Jacob, D.J., Fan, S., Maben, J.R., Zetwo, M.P., Springeryoung, M., Galloway, J.N., 1993. Evidence of inorganic chlorine gases other than hydrogen-chloride in marine surface air. Geophysical Research Letters 20 (8), 699–702. Pszenny, A.A.P., Moldanov, J., Keene, W.C., Sander, R., Maben, J.R., Martinez, M., Crutzen, P.J., Perner, D., Prinn, R.G., 2004. Halogen cycling and aerosol pH in the Hawaiian marine boundary layer. Atmospheric Chemistry and Physics 4, 147–168. Raes, F., 1995. Entrainment of free tropospheric aerosols as a regulating mechanism for cloud condensation nuclei in the remote marine boundary-layer. Journal of Geophysical Research – Atmospheres 100 (D2), 2893–2903. Rasch, P.J., Barth, M.C., Kiehl, J.T., Schwartz, S.E., Benkovitz, C.M., 2000a. A description of the global sulfur cycle and its controlling processes in the national center for atmospheric research community climate model, version 3. Journal of Geophysical Research – Atmospheres 105 (D1), 1367–1385. Rasch, P.J., Feichter, J., Law, K., Mahowald, N., Penner, J., Benkovitz, C., Genthon, C., Giannakopoulos, C., Kasibhatla, P., Koch, D., Levy, H., Maki, T., Prather, M., Roberts, D.L., Roelofs, G.J., et al., 2000b. A comparison of scavenging and deposition processes in global models: results from the WCRP Cambridge Workshop of 1995. Tellus Series B – Chemical and Physical Meteorology 52 (4), 1025–1056. Read, K.A., Mahajan, A.S., Carpenter, L.J., Evans, M.J., Faria, B.V.E., Heard, D.E., Hopkins, J.R., Lee, J.D., Moller, S.J., Lewis, A.C., Mendes, L., Mcquaid, J.B., Oetjen, H., Saiz-Lopez, A., Pilling, M.J., et al., 2008. Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean. Nature 453 (7199), 1232– 1235. doi:10.1038/nature07035. Restad, K., Isaksen, I.S.A., Berntsen, T.K., 1998. Global distribution of sulphate in the troposphere. A three-dimensional model study. Atmospheric Environment 32 (20), 3593–3609. Rodhe, H., 1999. Human impact on the atmospheric sulfur balance. Tellus Series A – Dynamic Meteorology and Oceanography 51 (1), 110–122. Roelofs, G.J., Kasibhatla, P., Barrie, L., Bergmann, D., Bridgeman, C., Chin, M., Christensen, J., Easter, R., Feichter, J., Jeuken, A., Kjellstrom, E., Koch, D., Land, C., Lohmann, U., Rasch, P., 2001. Analysis of regional budgets of sulfur species modeled for the COSAM exercise. Tellus Series B – Chemical and Physical Meteorology 53 (5), 673–694. Roelofs, G.J., Lelieveld, J., Ganzeveld, L., 1998. Simulation of global sulfate distribution and the influence on effective cloud drop radii with a coupled photochemistry sulfur cycle model. Tellus Series B – Chemical and Physical Meteorology 50 (3), 224–242. Rotstayn, L.D., Lohmann, U., 2002. Simulation of the tropospheric sulfur cycle in a global model with a physically based cloud scheme. Journal of Geophysical Research – Atmospheres 107 (D21), 4592. Saiz-Lopez, A., Shillito, J.A., Coe, H., Plane, J.M.C., 2006. Measurements and modelling of I-2, IO, OIO, BrO and NO3 in the mid-latitude marine boundary layer. Atmospheric Chemistry and Physics 6, 1513–1528. Saltman, T., 2005. Executive Summary. National Acid Precipitation Assessment Program Report to Congress: an Integrated Assessment. NOAA, Silver Spring, MD. Schlesinger, R.B., 2007. The health impact of common inorganic components of fine particulate matter (PM2.5) in ambient air: a critical review. Inhalation Toxicology 19 (10), 811–832. doi:10.1080/08958370701402382. Review. Schlesinger, W.H., 1997. Biogeochemistry: an Analysis of Global Change. Academic Press, Amsterdam. Schwartz, S.E., 1986. Mass-transport consideration pertinent to aqueous-phase reactions of gases in liquid-water clouds. In: Jaechske, W. (Ed.), Chemistry of Multiphase Atmospheric Systems. Springer, Heidelberg, pp. 415–471. Sciare, J., Baboukas, E., Kanakidou, M., Krischke, U., Belviso, S., Bardouki, H., Mihalopoulos, N., 2000. Spatial and temporal variability of atmospheric sulfurcontaining gases and particles during the albatross campaign. Journal of Geophysical Research – Atmospheres 105 (D11), 14433–14448. Shaw, G.E., 1983. Bio-controlled thermostasis involving the sulfur cycle. Climatic Change 5 (3), 297–303. Shon, Z.H., Davis, D., Chen, G., Grodzinsky, G., Bandy, A., Thornton, D., Sandholm, S., Bradshaw, J., Stickel, R., Chameides, W., Kok, G., Russell, L., Mauldin, L., Tanner, D., Eisele, F., 2001. Evaluation of the DMS flux and its conversion to SO2 over the Southern Ocean. Atmospheric Environment 35 (1), 159–172. Sievering, H., Cainey, J., Harvey, M., Mcgregor, J., Nichol, S., Quinn, P., 2004. Aerosol non-sea-salt sulfate in the remote marine boundary layer under clear-sky and normal cloudiness conditions: ocean-derived biogenic alkalinity enhances seasalt sulfate production by ozone oxidation. Journal of Geophysical Research – Atmospheres 109 (D19), D19317. doi:10.1029/2003jd004315. Solomon, S., Qin, D., Manning, M., Alley, R.B., Berntsen, T., Bindoff, N.L., Chen, Z., Chidthaisong, A., Gregory, J.M., Hegerl, G.C., Heimann, M., Hewitson, B., Hoskins, B.J., Joos, F., Jouzel, J., et al., 2007. Technical summary. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK; New York, NY, USA.

2854

I. Faloona / Atmospheric Environment 43 (2009) 2841–2854

Song, C.H., Chen, G., Davis, D.D., 2003a. Chemical evolution and dispersion of ship plumes in the remote marine boundary layer: investigation of sulfur chemistry. Atmospheric Environment 37 (19), 2663–2679. doi:10.1016/s13522310(03)00198-5. Song, C.H., Chen, G., Hanna, S.R., Crawford, J., Davis, D.D., 2003b. Dispersion and chemical evolution of ship plumes in the marine boundary layer: investigation of O3/NOy/HOx chemistry. Journal of Geophysical Research – Atmospheres 108 (D4), 4143. doi:10.1029/2002jd002216. Stern, D.I., 2006. Reversal of the trend in global anthropogenic sulfur emissions. Global Environmental Change – Human and Policy Dimensions 16 (2), 207–220. doi:10.1016/j.gloenvcha.2006.01.001. Straub, D.J., Lee, T., Collett, J.L., 2007. Chemical composition of marine stratocumulus clouds over the eastern Pacific Ocean. Journal of Geophysical Research – Atmospheres 112 (D4), D04307. doi:10.1029/2006jd007439. Streets, D.G., Bond, T.C., Carmichael, G.R., Fernandes, S.D., Fu, Q., He, D., Klimont, Z., Nelson, S.M., Tsai, N.Y., Wang, M.Q., Woo, J.H., Yarber, K.F., 2003. An inventory of gaseous and primary aerosol emissions in Asia in the year 2000. Journal of Geophysical Research – Atmospheres 108 (D21), 8809. doi:10.1029/ 2002jd003093. Streets, D.G., Guttikunda, S.K., Carmichael, G.R., 2000. The growing contribution of sulfur emissions from ships in Asian waters, 1988–1995. Atmospheric Environment 34 (26), 4425–4439. Streets, D.G., Wu, Y., Chin, M., 2006. Two-decadal aerosol trends as a likely explanation of the global dimming/brightening transition. Geophysical Research Letters 33 (15), L15806. doi:10.1029/2006gl026471. Suhre, K., Mari, C., Bates, T.S., Johnson, J.E., Rosset, R., Wang, Q., Bandy, A.R., Blake, D.R., Businger, S., Eisele, F.L., Huebert, B.J., Kok, G.L., Mauldin, R.L., Prevot, A.S.H., Schillawski, R.D., et al., 1998. Physico-chemical modeling of the first aerosol characterization experiment (ACE 1) Lagrangian b – 1. A moving column approach. Journal of Geophysical Research – Atmospheres 103 (D13), 16433–16455. Sullivan, P.P., Mcwilliams, J.C., Moeng, C.H., 1996. A grid nesting method for largeeddy simulation of planetary boundary-layer flows. Boundary-Layer Meteorology 80 (1–2), 167–202. Textor, C., Schulz, M., Guibert, S., Kinne, S., Balkanski, Y., Bauer, S., Berntsen, T., Berglen, T., Boucher, O., Chin, M., Dentener, F., Diehl, T., Easter, R., Feichter, H., Fillmore, D., et al., 2006. Analysis and quantification of the diversities of aerosol life cycles within AeroCom. Atmospheric Chemistry and Physics 6, 1777–1813. Thornton, D.C., Bandy, A.R., Tu, F.H., Blomquist, B.W., Mitchell, G.M., Nadler, W., Lenschow, D.H., 2002. Fast airborne sulfur dioxide measurements by atmospheric pressure ionization mass spectrometry (APIMS). Journal of Geophysical Research-Atmospheres 107 (D22), 12, 4632. doi:10.1029/2002JD002289. Toumi, R., 1994. Bro as a sink for dimethylsulfide in the marine atmosphere. Geophysical Research Letters 21 (2), 117–120. Twohy, C.H., Clement, C.F., Gandrud, B.W., Weinheimer, A.J., Campos, T.L., Baumgardner, D., Brune, W.H., Faloona, I., Sachse, G.W., Vay, S.A., Tan, D., 2002. Deep convection as a source of new particles in the midlatitude upper troposphere. Journal of Geophysical Research – Atmospheres 107 (D21), 4560. doi:10.1029/2001jd000323. Twomey, S., 1977. Influence of pollution on shortwave albedo of clouds. Journal of the Atmospheric Sciences 34 (7), 1149–1152.

Verma, S., Boucher, O., Reddy, M.S., Upadhyaya, H.C., Le Van, P., Binkowski, F.S., Sharma, O.P., 2007. Modeling and analysis of aerosol processes in an interactive chemistry general circulation model. Journal of Geophysical Research – Atmospheres 112 (D3), D03207. doi:10.1029/2005jd006077. Vestreng, V., Adams, M., Goodwin, J., 2004. Inventory Review 2004. EMEP/EEA Joint Review Report on Emission Data. Vogt, R., Crutzen, P.J., Sander, R., 1996. A mechanism for halogen release from sea-salt aerosol in the remote marine boundary layer. Nature 383 (6598), 327–330. von Glasow, R., Crutzen, P.J., 2004. Model study of multiphase DMS oxidation with a focus on halogens. Atmospheric Chemistry and Physics 4, 589–608. von Glasow, R., Lawrence, M.G., Sander, R., Crutzen, P.J., 2003. Modeling the chemical effects of ship exhaust in the cloud-free marine boundary layer. Atmospheric Chemistry and Physics 3, 233–250. von Glasow, R., Sander, R., 2001. Variation of sea salt aerosol pH with relative humidity. Geophysical Research Letters 28 (2), 247–250. Vong, R.J., Baker, B.M., Brechtel, F.J., Collier, R.T., Harris, J.M., Kowalski, A.S., Mcdonald, N.C., Mcinnes, L.M., 1997. Ionic and trace element composition of cloud water collected on the Olympic Peninsula of Washington State. Atmospheric Environment 31 (13), 1991–2001. Watanabe, K., Ishizaka, Y., Takenaka, C., 2001. Chemical characteristics of cloud water over the Japan sea and the Northwestern Pacific Ocean near the central part of Japan: airborne measurements. Atmospheric Environment 35 (4), 645–655. Watts, S.F., 2000. The mass budgets of carbonyl sulfide, dimethyl sulfide, carbon disulfide and hydrogen sulfide. Atmospheric Environment 34 (5), 761–779. Wesely, M.L., 1989. Parameterization of surface resistances to gaseous dry deposition in regional–scale numerical-models. Atmospheric Environment 23 (6), 1293–1304. Wild, M., Gilgen, H., Roesch, A., Ohmura, A., Long, C.N., Dutton, E.G., Forgan, B., Kallis, A., Russak, V., Tsvetkov, A., 2005. From dimming to brightening: decadal changes in solar radiation at earth’s surface. Science 308 (5723), 847–850. doi:10.1126/science.1103215. Wingenter, O.W., Sive, B.C., Blake, N.J., Blake, D.R., Rowland, F.S., 2005. Atomic chlorine concentrations derived from ethane and hydroxyl measurements over the equatorial Pacific Ocean: implication for dimethyl sulfide and bromine monoxide. Journal of Geophysical Research – Atmospheres 110 (D20), D20308. doi:10.1029/2005jd005875. Winkler, P., 1980. Observations on acidity in continental and in marine atmospheric aerosols and in precipitation. Journal of Geophysical Research – Oceans and Atmospheres 85 (NC8), 4481–4486. Yang, X., Cox, R.A., Warwick, N.J., Pyle, J.A., Carver, G.D., O’connor, F.M., Savage, N.H., 2005. Tropospheric bromine chemistry and its impacts on ozone: a model study. Journal of Geophysical Research – Atmospheres 110 (D23), D23311. doi:10.1029/2005jd006244. Yvon, S.A., Saltzman, E.S., Cooper, D.J., Bates, T.S., Thompson, A.M., 1996. Atmospheric sulfur cycling in the tropical pacific marine boundary layer (12 S,135 W): a comparison of field data and model results. 1. Dimethylsulfide. Journal of Geophysical Research – Atmospheres 101 (D3), 6899–6909. Zhang, J.Z., Millero, F.J., 1991. The rate of sulfite oxidation in seawater. Geochimica et Cosmochimica Acta 55 (3), 677–685.