Dust characteristics over the North Pacific observed through shipboard measurements during the ACE-Asia experiment

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Atmospheric Environment 41 (2007) 7907–7922 www.elsevier.com/locate/atmosenv

Dust characteristics over the North Pacific observed through shipboard measurements during the ACE-Asia experiment Yuan Gaoa,, James R. Andersonb, Xin Huab a

Department of Earth and Environmental Sciences, Rutgers University, Newark, NJ 07102, USA Department of Mechanical & Aerospace Engineering, Arizona State University, Tempe, AZ 85287, USA

b

Received 22 September 2006; received in revised form 12 March 2007; accepted 7 June 2007

Abstract To examine the diversity of chemical and physical properties of aerosol particles, in particular dust, over the North Pacific, aerosols were collected along 321N latitude between 1401E and 1701W longitude aboard the NOAA R/V Ronald H. Brown during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) in the spring 2001. A total of 11,482 aerosol particles were examined through individual-particle analysis. Results indicate that dust particles over this region were dominated primarily by Si-rich particles, including aluminosilicates that contain Fe. Fe is also present as separate Fe-rich particles. Additional common particle types include Ca- and S-rich particles; many of the later appear to represent soil-derived calcium carbonate and its reaction products whereas the former are predominantly reaction products of sea salt and sulfate. Particles are often aggregates of different types including pollution-derived substances and highly heterogeneous, both internally and externally. Dust particles are non-spherical, having circularities from 1.0 up to 4.5, suggesting the high degree of complexity of particle shape. The majority of dust particles were dominated by particles with median diameters from 0.67 to 1.26 mm. However, dust particles with diameters of 5 mm or even larger do exist associated with those events of dust originated from Asian desert areas. The existence of soot and Fe-rich particles over this region indicates the influence of fossil fuel sources in Asia. Aerosol Fe from both Asian desert and fossil fuel combustion may contribute to the nutrient Fe in the surface waters of the North Pacific basin. Therefore, the transport of Asian dust associated with species of fossil fuel burning in the spring may play an important role in altering the natural composition of aerosols over the North Pacific. r 2007 Elsevier Ltd. All rights reserved. Keywords: Dust; Asian aerosols; Shipboard sampling; Soot; Iron

1. Introduction Dust characterization is relevant for the study of global climate change because dust particles play an important role in a number of atmospheric proCorresponding author. Tel.: +1 973 373 1139; fax: +1 973 353 1965. E-mail address: [email protected] (Y. Gao).

1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.06.060

cesses, especially their direct and indirect effects on the earth’s radiation budget (Dentener et al., 1996; Tegen et al., 1996). The degrees of dust–climate effects are dominated by chemical and physical properties of mineral particles, including their type, size, chemical composition, shape, particle number concentration, etc. (Sokolik and Toon, 1999). Many dust particles are internally mixed (Anderson et al., 1996; Gao and Anderson, 2001), and they probably

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exhibit intermediate properties, which causes difficulties in quantifying dust–climate forcing. Thus, detailed information on the chemical and physical properties of dust particles is critical to climate studies. East Asia, particularly in China, is a major dust source for the North Pacific. Dust storms in China have been documented for 42000 years (Zhang, 1984). Dust derived from deserts of western and northern central China affects large areas of East Asia and North Pacific under westerly winds in the spring (Duce et al., 1980; Uematsu et al., 1985; Prospero et al., 2002; Merrill and Kim, 2004), and its influence even reaches North America (Jaffe et al., 1999; VanCuren and Cahill, 2002; VanCuren, 2003; de Gouw et al., 2004). On the other hand, East Asia is also a major source for anthropogenic aerosols over the Northern Hemisphere. Of particular importance is the perturbation of regional and global climate and biogeochemical cycles by natural and pollution emissions from this region. Sulfate aerosol from coal burning is an important fraction of global anthropogenic sulfate that affects climate (Kato and Akimoto, 1992). Enhanced coal burning and consequent oxidation of nitrogen and sulfur in air leads to severe acid precipitation, especially in southern China (Zhao et al., 1988). Coal combustion also emits a variety of trace elements into the atmosphere, which affects their natural cycles (Gao et al., 1997). One important consequence is the strong interactions between dust and pollutionderived aerosols from Asia (Huebert et al., 2003; Matsumoto et al., 2003; Arimoto et al., 2004; Bates et al., 2004; Clarke et al., 2004; Quinn et al., 2004). Dust particles could provide reaction sites for many heterogeneous reactions involving SO2, NOy, HOx, O3, etc., and serve as conveyors carrying anthropogenic substances far distance from its sources (Zhang et al., 1994; Carmichael et al., 1996; Perry et al., 2004; VanCuren et al., 2005; Sullivan et al., 2007). Such reaction could cause certain modification of the properties of both dust and anthropogenic aerosols. Under such complicated circumstances, dust particles originated from East Asia and transported over the North Pacific are highly mixed internally and externally (Ooki and Uematsu, 2005; Tang et al., 2004). Recent studies found that combustion particles and dust particles are commonly transported to North America from Asia (VanCuren and Cahill, 2002; VanCuren, 2003). Another impact of dust is its role as a source of iron (Fe), a limiting nutrient in certain parts of the

ocean. Input of Asian dust may promote the growth of phytoplankton in certain areas of the North Pacific (Bishop et al., 2002), affecting oceanic carbon cycles. However, dust properties over the North Pacific, in particular the nature of Fecontaining particles, still need to be addressed in more detail. In this paper, we present the results of aerosol measurements conducted in the mid-latitude of the North Pacific in March–April 2001 during a cruise on board the NOAA R/V Ronald H. Brown from Hawaii to Japan. We characterized the chemical composition, degree of aggregation, size, and shape of the particles through individual-particle analysis using an automated scanning electron microscope (SEM) and a field emission scanning electron microscope (FESEM). We also used the cluster and principal component analysis to explore the complexity and heterogeneous nature of dust particles in the region far away from their sources. Our intention is to illustrate the range of diversity of dust particles during the outflow of aerosols from East Asia over the North Pacific and to provide the insight of dust properties for better assessments of dust radiative forcing and biogeochemical impacts on the ocean. 2. Methodology 2.1. Sample collection Aerosol particles were collected aboard the NOAA R/V Ronald H. Brown between Hawaii (201N, 1561W) and Hachijo (331N, 1371E) during March–April 2001 as part of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) (Fig. 1). Samples were selected from 10 locations along 321N between 1401E and 1701W (Table 1) as this latitudinal band is on the path of Asian dust transport that often occurs in the spring. We used Julian Day and latitude/longitude to indicate each sampling location and period. A brief information of air mass back trajectories for three arrival altitudes (500, 2500, and 5500 m) for the ship’s locations were included in Table 1 to indicate the potential sources of samples collected at each location (http://saga.noaa.gov/Field/aceasia/ rhb_data/traject/index). During this period, the ship moving westward on this transect encountered the influence of dust air masses originated from Asia and passing over the polluted regions from time to time, as seen often in the spring (Huebert et al., 2003; Merrill and Kim, 2004), and the air sampled

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Fig. 1. Cruise track where shipboard aerosol measurements were made from Hawaii to Japan. The number on the red line represents the day of the year.

on this transect should be a mixture of marine and continental polluted air from Asia. Aerosols were collected on polycarbonate tracketched membrane filters of 0.40-mm pore size using a Programmable Streaker Sampler (PIXE International). The inlet used was on a 6 m sampling mast, 18 m above the ocean surface. A split of the isokinetic inlet flow was used for the Streaker filter samples, 2  8 mm areas for each discrete sample distributed radially on a 90 mm circular stage. The stage was advanced every 2 h. The ship’s inlet is known to be 95% efficient for particles of 6.5 mm in aerodynamic diameter. The transmission efficiency of such particles to the Streaker was not measured, although a significant number of particles with geometric diameters 410 mm are present on the Streaker filter samples. Flow to the Streaker was 1 standard liter per minute and monitored with a calibrated digital flowmeter. The ship had a computer-controlled moveable aerosol inlet with sector control mounted forward on the ship to avoid contamination from the ship plume (Quinn et al., 2004). 2.2. Sample analyses Samples were analyzed for individual-particle quantification with an automated JEOL 5800

SEM at Arizona State University (ASU) following the procedures of Anderson et al. (1992, 1996). Backscattered electron (BSE) images were acquired using an annular, split-ring, semi-conductor detector mounted 10 mm above the sample. Operating conditions were 15-keV accelerating voltage and 400-pA beam current with spectral acquisition times of 60 s. Kalman frame-averaged 1024  1024 pixel BSE images were acquired at 2000  magnification, with a pixel size of 50 nm. The images were segmented using a threshold that separated background from particles and the lowest size limit of 200 nm geometric diameter was used in order to eliminate particles consisting of very few pixels. For each particle found in this way, an X-ray spectrum was acquired with a thin-window NORAN Pioneer energy dispersive spectrometer (EDS). Reference spectra were fitted to particle spectra using NORAN’s Voyager-4 software and elemental weight percents were calculated using a standard ZAF matrix correction procedure. There are several simplifications using the SEM procedures. First, the use of flat-sample corrections introduces some systematic error into the composition of particles. However, this error has little effect on characterization of particle types. Second, measured C, N, and O peaks are not quantified in the

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Table 1 Locations of R/V Ron Brown at the mid-point of each collection of the 10 samples selected in this study and sources of arriving air mass at each location Sample ID

Day of year (mid-point)a

Latitude (1N)

Longitude (1E)

Trajectories sources of arriving air mass

RB3-11

81.2

31.7

177.2

500 m: N. Pacific 2500 m: Mongolia/N. China 5500 m: Southern China

RB4-11

83.2

32.3

168.4

500 m: N. Pacific marine air 2500 m: Mongolia/N. China 5500 m: N. China

RB4-23

84.2

33.2

164.6

500 m: N. Pacific 2500 m: Mongolia/N. China 5500 m: Mongolia/N.China

RB4-28

85.0

34.4

162.4

500 m: Mongolia/N. China 2500 m: Mongolia/N. China 5500 m: Western/N. China

RB5-4

85.7

33.0

159.1

500 m: Russian/Japan 2500 m: Mongolia/N. China 5500 m: Western/N. China

RB5-20

87.2

32.8

154.4

500 m: Russian/N. China 2500 m: Mongolia/N. China 5500 m: South Asia

RB5-29

88.1

32.9

149.5

500 m: Mongolia/China/Japan 2500 m: Mongolia/N. China 5500 m: India/South Asia

RB6-9

89.1

33.0

144.5

500 m: Russian/Japan 2500 m: Russian/N. China 5500 m: Russian/N. China

RB6-21

91.0

33.2

139.8

500 m: Russian/Korea 2500 m: N. Pacific/Japan 5500 m: Russian/N. China

RB6-29

91.9

33.0

137.8

500 m: N. Pacific/Japan 2500 m: Russian/N. China 5500 m: Russian/Japan

a

The mid-point of each sampling period.

spectral fitting procedure due to C and O interference from the polycarbonate filter, and thus speciation of most compounds is inferred instead of being directly determined. Third, the BSE threshold for particle detection was set so that small particles (ranging from 200 to 300 nm) with average atomic numbers close to that of the polycarbonate filter would be deliberately undercounted. This would undercount carbon-rich particles, but also greatly reduces artifacts from the filter such as the edges of holes. In previous work, when we have examined such particles in the transmission electron microscope, they are mostly soot as examined from ACE1 and ACE-2 by Posfai et al. (1999). Soot particles

o300 nm are always undercounted by the SEM methods, regardless of how the BSE threshold is set. A total of 11,482 individual aerosol particles from 10 samples were examined using the SEM technique. In addition, a FESEM, Hitachi-4700-II, at the Center for Solid State Electronics Research at ASU was also used for qualitative examination of aerosol particles with high resolution. We randomly selected a frame from a filter at low magnification and then studied each particle within this frame at high magnification. Secondary electron images (SEI) and chemical analyses were obtained with a Hitachi4700-II FESEM. Analyses were performed at a 15 keV accelerating voltage using EDS.

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The clustering methods employed for SEM data are described by Anderson et al. (1996). The basis for non-hierarchical cluster analysis was the Forgey k-means algorithm. A similarity measure that represents the angle between vectors from the origin to two points in multi-dimensional composition space was used (Killeen et al., 1981). 3. Results and discussion 3.1. Complexity of aerosol particles One feature of dust particles from Asia is that they are often mixed with anthropogenic substances, both internally by chemical reactions and externally by aggregation, which may affect the overall aerosol radiative forcing (Gao and Anderson, 2001; Conant et al., 2003). Fig. 2 shows FESEM images of relatively simple particles from samples collected at 341N, 1621E during March 25–26, 2001, an episode of strong Asian dust transport. These images illustrate the diversity of aerosol types over the North Pacific far from Asia. Fig. 2a shows four typical dust particles; all these particles are non-spherical and have rough surfaces. Fig. 2b shows the chemical nature of four individual particles, which are relatively abundant in this region during the observation period. Three major sources for these particles are: desert, anthropogenic, and marine. There are two dust particles in Fig. 2b, one is crystalline silica (quartz) and the other is feldspar (the Na-rich variety albite). Quartz is a common mineral in all soils, but the unaltered feldspar is more common in loess than in heavily weathered soils. The large, relatively compact soot particle in Fig. 2b represents soot of a range of morphologies and sizes present in these samples, reflecting the influence of anthropogenic sources, in particular fossil fuel burning, on aerosol composition over this oceanic region. Additional soot from biomass burning may be present, but the minor element composition of examined soot suggests fossil fuel sources are dominant. The features of soot particles observed during this work on the mid-latitude of North Pacific are consistent with those observed from the southern Sea of Japan during the same ACE-Asian experiments, where soot of similar range of morphologies and sizes existed based on a 2-h sample on the Ron Brown (Quinn et al., 2004). Quinn et al. (2004) found that several types of soot particles are in aggregates with dust, and dust-soot

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aggregates were also observed in samples collected at high elevations on the NCAR C-130 (Clarke et al., 2004). The complex of such aggregates may have different optical effects compared with if particles are homogeneous. On the other hand, such aggregates are thought to be removed from the atmosphere faster than homogeneous particles, due to their large size. Surprisingly, the dust-soot aggregates as shown in Fig. 2c and d commonly exist in the samples collected during this study at the locations far away from Asia but impacted by air masses originated from East Asia shown by air mass back trajectories. This finding suggests that soot can be externally mixed with dust and transported long distance far away from the sources. Results from cruise-based measurement in the central North Pacific reveal that black carbon and non-sea-salt sulfate of the Asian origin also co-exist and their concentrations correlate well, suggesting the combustion of sulfur-containing materials as the major source of these two aerosol species over the North Pacific (Kaneyasu and Murayama, 2000). Recent findings based on the empirical assessment of the frequency and intensity of dust transport from Asia to North America reveal that a mixture of dust and combustion products dominates the Asian aerosols that regularly impacts several elevated sites in the western United States (VanCuren, 2003). Therefore, aerosols from deserts and from fossil fuel combustion in Asia may contribute to the alteration of the natural composition of the atmosphere not only over the North Pacific but also North America. It is interesting to see the spherical flyash particles shown in Fig. 2c from the sample collected during this cruise over the mid-latitude of the North Pacific. These flyash particles have similar composition to that of natural aluminosilicate minerals, in particularly, smectitic clays. These silicate cenospheres are hollow glass particles, a distinctive indicator of coal combustion. Although they are present in these samples, they comprise only a tiny fraction of the total silicate particles that are dominated by soil dust. Results obtained from this transect clearly indicate that aggregates of dust and anthropogenic substances do survive and get transported long distances over the remote ocean. Highly complex mixtures of dust associated with other aerosol species were commonly found in other ocean regions downwind of the major continental sources (Quinn and Bates, 2005). Such complex nature of aerosol chemical composition through the global marine atmosphere certainly presents more

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Fig. 2. Secondary electron images (SEI) of some aerosol particles found in the marine atmospheric boundary layer over the North Pacific: (a) typical dust particles; numbers in these images indicate the mineral names: 1—clay perhaps admixed with ilmenite because of high Ti and Fe; 2—clay containing Fe; 3—clay containing Fe and admixed with S-bearing phase; 4—clay containing Fe and Cl; 5—albite; 6— mixture of carbon and CaSO4; 7—NaCl plus minor silicate; 8—mixture of SiO2 and NaCl; (b) four types of particles with respect to their different chemical nature: sea salt (NaCl) and its reaction product (Na2SO4), silica (SiO2), albite, and soot (the inset boxes on the upper right corner show the EDS spectra associated with these particles); (c) aggregates of flyash, clay minerals, and soot; (d) aggregates of clay minerals, gypsum (CaSO4), and soot.

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Fig. 2. (Continued)

challenge on the estimation of aerosol radiative forcing over the ocean. In addition to natural mineral dust and anthropogenically derived aerosols, sea-salt particles are also abundant in the marine atmospheric boundary layer over this region, illustrated by a crystal of NaCl with Na2SO4 at its side as shown in Fig. 2b. 3.2. Particle types by compositional categories Table 2 presents a summary of particle types and their relative number abundance determined by cluster analysis. A cluster composition (i.e., the cluster centroid) represents the average of all particles that fall within the boundaries of the cluster in compositional space and reflects either the composition of one species or a common aggregation of multiple species. As the samples collected on this transect are under the strong influence of Asian dust transport and may likely bear some degrees of similarity in chemical and physical properties, the particles from all ten samples discussed are then used in the calculation of cluster compositions. The approach of combining all particles collected on this transect can reveals the characteristics of aerosol properties as a whole for this narrow latitudinal band, and this approach also provides better statistical results with sufficient data points for calculations, which has been widely used by many investigators working with aerosol characterization (Anderson et al., 1996; Reid et al., 2003; Gao and Anderson, 2001). The composition of clusters is normalized in atomic fractions of elements analyzed. The order of the elements in atomic fraction in Table 2 follows the order of their abundance in the Earth’s crust. Defined clusters represent at least 0.4% of the total number of particles examined. Cluster number assignment is arbitrary. Analysis of

assigned particles resulted in 28 cluster types (labeled C1–C28). Grouping of clusters from all samples collected at the 10 location results in six major particle types as indicated in Table 2, with the most enriched element highlighted in bold. A brief discussion of them is as follows. 3.2.1. Silica-rich particles Eight clusters in this group show high Si content (C1–C8), accounting for 16% of the total particles examined. This group is mainly comprised Si–Al–Na, Si–Al–K, and Si–Fe–Al particles, and the chemical composition of particles is similar to that of the coarse dust particles observed over the Taklamakan desert in western North China (Iwasaka et al., 2003). However, since particles of this group are composed of common minerals, it is difficult to infer their sources only from their compositional data. Also, the presence of unaltered feldspar suggests at least some of the dust may be from the Loess Plateau. The diversity of particles in this group is reflected by the fact that each cluster differs from the others in chemical composition and Fig. 3 highlights this feature. Such diversity is probably due to the aggregation of two or more minerals. The mineralogy of dust from typical loess deposits includes quartz (SiO2), feldspars, micas, clays (several groups of aluminosilicates with ranges of composition), carbonates (especially calcite, CaCO3), and a number of minor minerals (Pye, 1987). More extensively weathered dust generally contains less feldspar and mica but more clay minerals. One of the complicating factors in these samples is that silicate cenospheres from coal combustion (a distinctive component of ‘‘flyash’’) have compositions similar to natural silicate minerals, so it was necessary to manually examine the samples to determine silicate cenospheres although

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Table 2 Individual-particle types determined by clustering and their relative abundance Cluster

Type

Cluster number

Number concentration (%)

Atomic fraction Si

Al

Fe

Ca

Na

K

Mg

S

Cl

A

Si-rich

C1 C2 C3 C4 C5 C6 C7 C8

2.2 1.9 1.8 4.4 2.2 1.5 0.7 1.7

0.50 0.70 0.41 0.41 0.32 0.31 0.27 0.34

0.14 0.026 0.16 0.21 0.17 0.15 0.14 0.16

0.06 0.035 0.056 0.085 0.21 0.096 0.090 0.094

– – 0.029 – – – 0.12 –

0.064 0.098 0.15 0.079 0.083 0.14 0.11 0.13

0.043 – – 0.091 0.043 0.061 – 0.055

0.032 0.026 0.034 0.048 0.10 0.075 0.062 0.055

0.096 0.052 0.042 0.037 0.036 – 0.12 0.13

– 0.022 – – – 0.083 – –

B

Ca-rich

C10 C12 C13

0.4 1.3 1.5

0.065 0.049 0.014

– – –

0.031 – –

0.59 0.40 0.31

0.091 0.12 0.23

– 0.018 0.030

0.047 0.028 0.044

0.030 0.30 0.31

0.056 0.024 0.033

C

Fe-rich

C11

0.6

0.054

0.024

0.69

–

0.077

–

0.032

0.042

0.020

D

S-rich

C18 C21 C25 C29

2.4 2.1 3.2 0.1

– – 0.003 –

– – – –

– – – 0.25

0.033 0.21 0.004 0.009

0.35 0.24 0.31 0.32

0.007 0.053 0.033 0.009

0.21 0.097 0.035 0.067

0.36 0.24 0.61 0.34

0.033 0.14 – –

E

Cl-rich

C15 C16 C19

31.1 3.6 3.0

– – –

– – –

– – –

0.015 0.077 0.16

0.40 0.33 0.25

– 0.024 0.045

0.019 0.056 0.093

0.024 0.10 0.18

0.53 0.40 0.27

F

Na-rich

C9 C14 C17 C20 C22 C23 C24 C26 C27 C28

0.6 2.3 1.6 3.0 5.0 3.6 2.1 4.5 9.1 2.4

0.19 – – – – – – – – –

0.065 – – 0.003 – – 0.027 – – –

– – – – – – – – – –

0.043 0.14 0.005 0.017 – 0.016 – 0.012 0.021 –

0.32 0.42 0.41 0.42 0.54 0.62 0.26 0.77 0.48 0.63

0.034 0.009 0.19 – 0.017 0.007 0.032 0.011 – 0.006

0.072 0.090 0.13 0.13 0.067 0.095 0.23 0.063 0.048 0.25

0.21 0.28 0.24 0.18 0.35 0.25 0.24 0.10 0.053 0.035

– 0.042 0.020 0.20 0.007 0.010 0.20 0.037 0.39 0.065

Note: (a) The compositions are in atomic fractions, taking into account only those elements analyzed; (b) the percents of the clusters are for all ten samples combined; (c) the cluster numbers are strictly arbitrary; (d) –: the atomic fraction below 0.001.

they account for only a minor fraction of the total silicates, at most a few percent. On the other hand, Fe is a minor element in this group with atomic fraction ranging from 0.035 to 0.21. These concentrations are consistent with much of the Fe being within the structure of aluminosilicates such as the clays, although from manual SEM observation it is clear that at least some of the Fe in these particles is due to aggregation of Fe oxide with silicates. The extent of Fe oxide aggregation has not been quantified, but qualitatively it appears to be minor compared to structural Fe within aluminosilicates. 3.2.2. Calcium-rich particles Clusters C10, C12, and C13 have calcium as their major constituent element. Both C12 and C13 are

also associated with high S contents and their Ca:S ratio is close to 1:1, probably occurring as CaSO4. The primary natural source of calcium sulfate is gypsum, a typical mineral in the earth crust but generally much less abundant than CaCO3. Therefore, another possible formation mechanism for CaSO4 is through reactions of carbonates with sulfuric acid derived from either anthropogenic sources or marine biogenic sources during longrange transport. In East Asia, particularly in China, the anthropogenic emissions of sulfur are dominated by the combustion of high-sulfur and lowgrade coal. Earlier and recent studies have provided evidence of the heterogeneous nature of dust particles with respect of reactions with sulfur species over East Asia and the western North Pacific

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C1

C5

C2

C6

C3

C7

C4

C8

0.4

0.2

0.0

0.6

Composition (atomic fraction)

0.4

0.2

0.0 0.6

0.4

0.2

0.0 0.6

0.4

0.2

0.0 Mg Al

Si

Fe Ca

S

Na Cl

K

Mg Al

Si

Fe Ca

S

Na Cl

K

Element Fig. 3. Complex chemical composition of Si-rich particles as indicated by eight clusters (C1–C8).

(Okada et al., 1990; Parungo et al., 1996; Gao and Anderson, 2001). In addition, through manual SEM observation we have also observed Ca nitrate, although it is a qualitative identification based on the presence of an N peak and lack of a C peak in the spectrum (the lighter elements such as C, N, and O are very difficult to quantify from EDS spectra). Ca nitrate is

much less abundant than Ca sulfate. In the automated analysis, Ca nitrate and carbonate cannot be distinguished, while Ca sulfate is easily identified. Nowadays in China, with the increasing use of automobiles and associated NOx emissions, the interaction of dust particles with certain nitrogen species have become apparent. A recent investigation (Laskin et al., 2005) indicates that

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dust particles from Asian sources, such as from loess from China (containing high levels of calcium carbonate), could react with nitric acid to form calcium nitrate particles. Ooki and Uematsu (2005) found that the concentration peaks of NO 3 and mineral dust particles co-exist in the coarse mode of all aerosols they collected from western North Pacific region during the high-dust season. Their further in situ measurements also show that HNO3 reacts with mineral dust particles much more efficiently than SO2. The high adsorption of HNO3 on the surface of mineral dust particles could certainly increase the hygroscopic efficiency of dust and thus tremendously affect their radiative properties. 3.2.3. Fe-rich particles C11 represents a group of particles with Fe relative abundance as high as 0.69, far more dominant than any other elements in the group (such as Na, Si, and S), although they only account for 0.6% of the total particles examined. This type of particles could be either flyash from coal combustion or of natural origin. However, based on the facts that these particles are lack of significant amount of common crustal elements and that the median diameters of the particles are in submicrometer range (0.41–0.76 mm), these Fe-rich particles are likely from coal combustion, which are transported with Asian dust outflow over the North Pacific. An earlier 3-year time series measurement of selected trace elements at Beijing shows that atmospheric concentrations of Fe and Sb vary in the same phase and Sb is primarily derived from coal combustion, suggesting that coal combustion may emits Fe into the atmosphere, altering the natural cycle of this element (Gao et al., 1997). At a coastal site in the East China Sea, Fe is found enriched in fine aerosol particles, likely produced by anthropogenic sources, which can be transported long distance (Gao and Anderson, 2001). Therefore, fossil fuel combustion over East Asian, in addition to emitting soot, may also serve as a contributor of certain trace elements such as Fe found over the North Pacific. The importance of these fine Fe-rich particles is that with large surface areas, they provide more reaction sites for many heterogeneous reactions involving SO2, NOy, etc., that may enhance Fe solubility and then its bioavailability to phytoplankton in the surface ocean, as the solubility of aerosol Fe is found to be dependent of particle sizes. More than 80% of the total soluble

Fe (II) was found associated with fine particles over the Arabian Sea (Siefert et al., 1999). Similar results on the Fe (II) content and particle-size relationships were also obtained by observations in other oceanic regions and modeling (Luo et al., 2005). Therefore, the input of aerosol Fe of both desert and fossil fuel combustion from East Asia may be an important source of nutrient Fe to fertilize the surface waters of the North Pacific basin, enhancing the biogeochemical processes in that oceanic region. 3.2.4. Sulfur-rich particles Three clusters (C18, C21, and C25) have compositions with significant amounts of S and Na, plus combinations of Mg, Ca, Cl, S, and K. Sodium sulfate, with other variable minor elements, is a common product of the reaction between sea salt and sulfuric acid. Another S-containing compound is ammonium sulfate, observed as a major component in the fine mode aerosol particles over the coastal and open ocean regions (Anderson et al., 1992, 1996; Zhang et al., 2000; Quinn and Bates, 2005). Some of S-rich particles may have originated as ammonium bisulfate, but the analysis methods cannot distinguish sulfate from bisulfate even if the bisulfate were to survive until analysis. C25 may be reacted sea salt aggregated with ammonium sulfate, which could produce Na:S ratios of 2:1. However, there are some Na-salts of organic acids, such as methanesulfonic acid, that could produce Na:S ratios deviated from 2:1. Certainly the methods used cannot determine the actual source of the S. 3.2.5. Chlorine-rich particles Three clusters (C15, C16, and C19) have compositions dominated by Cl and Na, with sulfur as the third major element. Clusters C15 is composed of Na and Cl with minor S, Mg, and Ca, reflecting a typical elemental composition of sea-salt aerosol, which accounts for 82% of the particles in this group. C16 and C19 are aggregates that include sea salt, perhaps with some extent of reaction with sulfate or nitrate. One should keep in mind that clustering results depend upon the choice of clustering parameters, but if clustering finds evidence of multi-phase aggregation then that aggregation is present. 3.2.6.. Sodium-rich particles This group of particles includes 10 clusters, with high content of Na. Seven clusters in this group (C9, C14, C17, C22–C24, and C26) are enriched in S,

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suggesting the majority of these clusters be dominated by Na2SO4, probably formed through the reactions between NaCl and sulfuric acid. The deficiency of S relative to Na in C20, C27, and C28 suggests the possible presence of nitrate. The combined total numbers of Cl-rich and Na-rich particles accounts for 72% of the total particles examined, indicating that sea salt is a dominant type of particles among aerosols in this part of the marine atmospheric boundary layer, not surprising considering the typical presence of whitecaps on the ocean surface during the cruise. 3.3. Particle shapes indicated by particle circularity We use particle circularity as the shape parameter to characterize the degree of the shape complexity of particles. The circularity is defined as C ¼ L2 ð4pAÞ1 , where C is circularity (non-dimensional), L is particle perimeter (mm), and A is the two-dimensional area of the particle in the SEM image (mm2). A circle has a circularity of 1.0, and a square has a circularity of 1.27. As the shape gets more complex, the circularity value increases; the circularity should not be o1.0. Digital images consist of pixels, and the calculated circularity of particles with few pixels tends to be less than its true value; hence, a lower threshold for particle size (0.4 mm in this case) is needed in looking at shape distributions. The number of particles o0.4 mm that have been limited in this way accounts for only a small portion of the total particles considered; in the case of Si-rich particles, this group of particles account for o10% of the total. Fig. 4 shows the circularity distributions for three types of particles: Si-rich, Ca-rich, and S-rich particles by percentile plots. The latter two likely have some involvement with dust particles. The Srich particles may have deliquesced, so that their dry shapes may not be indicative of their shapes under ambient conditions; these particles could be from gas-to-particle conversion or may be sea-salt components. Particles with circularity o1.0 while with size 40.4 mm have been placed in the bin centered on 1.0. Therefore, the majority of the remaining particles from all the locations have their circularity values 41.0 despite the possible bias toward lower circularity caused by the calculation problem discussed above. The circularity of dust particles as represented by Si-rich type (Fig. 4a) ranges from 1

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up to 4.5 (although the maximum value does not show up in this percentile plot), with the median values from 1.12 to 1.24 across the ten locations, suggesting the non-spherical nature of dust particles over the North Pacific. The highest median circularity values of dust particles were found to be 1.22 at 1591E (day 87) and 1.24 at 1681E (day 91), and high population of dust particles were also observed at these two locations, with corresponding numbers of dust particles being 254 and 268, respectively. High degrees of shape complexity of dust particles certainly indicate strong influence of continental sources over this part of the open ocean. Under typical ambient relative humidity (RH) conditions in the marine atmosphere, ammonium sulfate, sea salt, and reacted sea salt would be deliquesced. However, many types of particles would not do so and their complex shapes would be maintained even at high ambient RH. Such particles normally contain one or more components that are not deliquescent, such as dust particles that are coated with the substances having marine or pollution origin (Okada et al., 1990; Parungo et al., 1996; Zhang et al., 2000). Among the particle types that could deliquesce are ammonium sulfate particles (or bisulfate, which is indistinguishable from sulfate by SEM analysis). These types of particles tend to have simple shapes, that is, with circularity close to one, even when they are dry. Sea salt and its reaction products, however, can crystallize into irregular shapes, because multiple solid phases with different crystal habits may present. These may contribute to the complexity of particle shapes of marine origin, which has been reflected by relative high circularity with S-rich particles seen in this study (median values: 1.14–1.28 across the ten location; Fig. 4c). The shapes of aerosol particles are found to affect their optical properties. Kalashnikova and Sokolic (2002) applied circularity data and reconstructed a composition–shape–size of a mixture of dust particles to calculate the optical properties, and they demonstrated that the presence of shape-edge, angular-type particles results in various differences in the scattering phase function, asymmetry parameter, optical depth and single scattering albedo compared to those of the volume-equivalent spheres to ellipses. Quinn et al. (2004) compared the measured and calculated aerosol absorption coefficients during ACE-Asia and concluded that the differences between the two approaches are beyond reasonable uncertainty limits, indicating the inability

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4 Si-rich particles 3 254 2

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Fig. 4. Particle shapes indicated by the degree of particle circularity from samples collected at 10 locations: (a) Si-rich particles; (b) Ca-rich particles; and (c) S-rich particles. The minimum particle cut-off size was arbitrarily set at 0.4 mm as particles smaller than this value may not have enough pixels to be calculated for their true values. The data distributions here are represented by percentile box plots. The bottom and top of each box represent 5% and 95% of the data. Three lines are drawn inside each box. The middle solid line represents the median value of the data (50%), while the lower and upper dashed lines represent 25% and 75% of the data, respectively. The number at the top of each box indicates the number of particles examined from the sample at the given location.

of Mie theory and the assumption of internally mixed homogeneous spheres to predict absorption by Asian aerosols. An early study by Yang et al. (2000) also reported the low backscattering by nonspherical particles. Thus, the presence of complex

aggregates, significant numbers of particles with non-deliquescent components, and varying distributions of irregular particle shapes could strongly affect the optical properties of the aerosols over the mid-latitude of the North Pacific.

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3.4. Particle size distributions The size distributions of individual particles from this transect are presented in Fig. 5, demonstrated by three typical particle types: the Si-rich, Ca-rich, and S-rich particles. The particle size is operationally defined and calculated by the square root of

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particle area. The average diameters of Si-rich particles (containing mainly Si and Al) at these locations range from 0.86 to 1.38 mm (Fig. 5a), and the overall average diameter combining all particles of this type is 1.1 mm with a standard deviation of 0.66. However, the maximum diameters of dust particles at these locations range from 2.5 to 5.4 mm,

6 Si-rich particles

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Fig. 5. Size distributions of particles from samples collected at ten locations: (a) Si-rich particles; (b) Ca-rich particles; and (c) S-rich particles. The data distributions here are represented by box plots. The lower and upper horizontal bars associated with each box represent 5% and 95% of the data. Three solid horizontal lines in each box mean: the middle line represents the median value of the data (50%), while the lower and upper lines represent 25% and 75% of the data, respectively. The open circles above the boxes are outliers.

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indicating that large dust particles do get transported long distances from their sources. Clearly, the size distributions of dust particles have a wide spectrum and the occurrence of large dust particles is not uncommon over the ocean during dust transport events. This is particularly true during the period of days 86–87 when a significant number of large Si-rich particles were observed around the area of 331N, 1591E, when the ship encountered the continentally influenced air. Air mass back trajectories calculated at 6-h intervals at altitudes of 500, 2500, and 5500 m confirm that during that period all air masses that arrived in this area passed over the Asian continent, in particular the 500 and 2500 m trajectories flowing from the western north China and Mongolia dust source regions (http:// www.arl.noaa.gov/ready/hysplit4.html). The Si-rich particles observed from this study are typical dust particles similar to those observed over the dust sources in Asia (Iwasaka et al., 2003). Previous studies also demonstrate that dust associated with other aerosol gas-phase species were transported from Asia over the North Pacific, consistent with the results from this cruise (Duce et al., 1980; Jaffe et al., 1999; Perry et al., 1999; Huebert et al., 2003). These results demonstrate that the marine atmospheric boundary layer over the mid-latitude of the North Pacific is under strong influence of dust originated from Asia during the spring time. One mechanism driving the outflow of air masses from Asia appears to be the mid-latitude cyclones that export dust and air pollutants from the Asian boundary layer into the free troposphere over the western North Pacific (Bey et al., 2001). These substances of Asian origins can be transported effectively in westerly winds over the North Pacific, although the strengths of the outflows and transport may vary on different time scales (Liang et al., 2005). Therefore, this region as a whole can be strongly impacted by these meteorological processes (Merrill and Kim, 2004; Liang et al., 2005). The average sizes of the Ca-rich particles range from 0.59 to 0.99 mm at the locations on this transect (overall mean: 0.6970.40 mm), smaller than those of Si-rich particles. This type of particles in the atmosphere is of either continental or marine origin. They may have undergone reactions involving S species from either pollution sources, such as coal burning, or from marine biogenic sources, such as dimethylsulfide (DMS); the S source cannot be determined by the methods used, but the high concentrations of continental dust and pollutant

particles would suggest a continental origin for much of the S. The average sizes of S-rich particles at these locations range from 0.21 to 0.67 mm (overall mean: 0.4470.21 mm). This type of particles is primarily dominated by submicrometer particles, which are heterogeneous reaction products or seawater components. 4. Conclusions Analyses of individual aerosol particles collected over the North Pacific during ACE-Asia in the spring 2001 lead to the following conclusions; (1) transported Asian continental aerosols in the marine boundary have three main groups of dust particles: Si-rich, Ca-rich, and Fe-rich particles, and the dominant type is Si-rich particles. On average, the mineral particles are highly heterogeneous, both internally and externally; aggregates of dust with pollution-derived substances are common. (2) Almost all dust particles are non-spherical, having circularities with a range from 1 up to 4.5 and the median values from 1.12 to 1.24, indicating the non-spherical nature of dust particles in this region far away from the continent, which could affect dust optical properties. (3) The majority of dust particles in this region are dominated by particles with median diameters from 0.67 to 1.26 mm. However, dust particles with diameters of 5 mm or larger do exist during intense dust transport events. (4) The existence of large, relatively compact soot particles over the mid-latitude of the North Pacific observed during this study is a strong evidence of the influence of fossil fuel sources in Asia. In addition, a small number of aerosol particles of highly enriched with Fe were found, likely from fossil fuel sources as well. Aerosol Fe of both desert and fossil fuel combustion from East Asia may contribute to the total nutrient Fe to fertilize the surface waters of the North Pacific basin. Therefore, the transport of Asian dust associated with species of fossil fuel burning in the spring may play an important role in altering the natural composition of aerosols over the North Pacific. Acknowledgments Support for this work was provided by the US National Science Foundation (ATM-0002330 to Y.G., ATM-0002513 to J.R.A.) and NASA EOS Interdisciplinary Science Program (NNG04G091G to Y.G.). We thank L. Russell and M. Rivera for

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assistance with sampling. This work would not have become possible without the dedicated work by the crew of the NOAA R/V Ronald H. Brown at sea. The authors are also grateful for the constructive comments provided during the review process. References Anderson, J.R., Buseck, P.R., Saucy, D.A., Pacyna, J., 1992. Characterization of individual fine-fraction particles from the Arctic aerosol at Spitsbergen, May–June, 1987. Atmospheric Environment 26A, 1747–1762. Anderson, J.R., Buseck, P.R., Patterson, T.L., Arimoto, R., 1996. Characterization of the Bermuda tropospheric aerosol by combined individual-particle and bulk-aerosol analysis. Atmospheric Environment 30, 319–338. Arimoto, R., Zhang, X.Y., Huebert, B.J., Kang, C.H., Savoie, D.L., Prospero, J.M., Sage, S.K., Schloesslin, C.A., Khaing, H.M., Oh, S.N., 2004. Chemical composition of atmospheric aerosols from Zhenbeitai, China, and Gosan, South Korea, during ACE-Asia. Journal of Geophyscal Research 109, D19S04. Bates, T.S., Quinn, P.K., Coffman, D.J., Covert, D.S., Miller, T.L., Johnson, J.E., Carmichael, G.R., Uno, I., Guazzotti, S.A., Sodeman, D.A., Prather, K.A., Rivera, M., Russell, L.M., Merrill, J.T., 2004. Marine boundary layer dust and pollutant transport associated with the passage of a frontal system over eastern Asia. Journal of Geophysical Research 109, D19S19. Bey, I., Jacob, D.J., Logan, J.A., Yantosca, R.M., 2001. Asian chemistry outflow to the Pacific: origin, pathways, and budgets. Journal of Geophysical Research 106, 23,097–23,114. Bishop, J.K., Davis, R.E., Sherman, J.T., 2002. Robotic observations of dust storm enhancement of carbon biomass in the North Pacific. Science 298, 817–821. Carmichael, G.R., Zhang, Y., Chen, L.L., Hong, M.S., Ueda, H., 1996. Seasonal variation of aerosol composition at Cheju Island, Korea. Atmospheric Environment 30, 2407–2416. Clarke, A.D., Shnozuka, K., Kapustin, V.N., Howell, S., Huebert, B., Doherty, S., Anderson, T., Covert, D., Anderson, J., Hua, X., Moore II, K.G., McNaughton, C., Carmichael, G., Weber, R., 2004. Size distributions and mixtures of dust and black carbon aerosol in Asian outflow: physiochemistry and optical properties. Journal of Geophysical Research 109, D15S09. Conant, W., Seinfeld, J.H., Wand, J., Carmichael, G.R., Tang, Y., Uno, I., Flatau, P.J., Markowicz, K.M., Quinn, P.K., 2003. A model for the radiative forcing during ACE-Asia derived from CIRPAS twin otter and R/V Ronald H. Brown data and comparison with observations. Journal of Geophysical Research 108, D23,8661. de Gouw, J.A., Cooper, O.R., Warneke, C., Hudson, P.K., Fehsenfeld, F.C., Holloway, J.S., Hu¨bler, G., Nicks Jr., D.K., Nowak, J.B., Parrish, D.D., Ryerson, T.B., Atlas, E.L., Donnelly, S.G., Schauffler, S.M., Stroud, V., Johnson, K., Carmichael, G.R., Streets, D.G., 2004. Chemical composition of air masses transported from Asia to the US west coast during ITCT 2K2: fossil fuel combustion versus biomassburning signatures. Journal of Geophysical Research 109, D23.

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Dentener, F.J., Carmichael, G.R., Zhang, Y., Lelieveld, J., Crutzen, P.J., 1996. Role of mineral aerosol as a reactive surface in the global troposphere. Journal of Geophysical Research 101, 22,869–22,889. Duce, R.A., Unni, C.K., Bay, B.J., Prospero, J.M., Merrill, J.T., 1980. Long-range atmospheric transport of soil dust from Asia to the tropical North Pacific: temporal variability. Science 209, 1522–1524. Gao, Y., Anderson, J., 2001. Characteristics of Chinese aerosols determined by individual-particle analyses. Journal of Geophysical Research 106, 18,037–18,045. Gao, Y., Arimoto, R., Duce, R.A., Zhang, X.Y., Zhang, G.Y., An, Z.S., Chen, L.Q., Zhou, M.Y., Gu, D.Y., 1997. Temporal and spatial distributions of dust and its deposition to the China Sea. Tellus 49B, 172–189. Huebert, B.J., Bates, T., Russell, P.B., Shi, G., Kim, Y.J., Kawamura, K., Carmichael, G., Nakajima, T., 2003. An overview of ACE-Asia: strategies for quantifying the relationships between Asian aerosols and their climatic impacts. Journal of Geophysical Research 108, 8633. Iwasaka, Y., Shi, G.-Y., Yamada, M., Matsuki, A., Trochkine, D., Kim, Y.S., Zhang, S.T., Nagatani, A., Shibata, T., Nagatani, M., Nakata, H., Shen, Z., Li, G., Chen, B., 2003. Importance of dust particles in the free troposphere over the Taklamakan desert: electron microscopic experiments of particles collected with a balloonborne particle impactor at Dunhuang, China. Journal of Geophysical Research 108, D23,8644. Jaffe, D., Anderson, T., Covert, D., Kotchenruther, R., Trost, B., Danielson, J., Simpson, W., Berntsen, T., Karlsdottir, S., Blake, D., Harris, J., Carmichael, G., 1999. Transport of Asian air pollution to North America. Geophysical Research Letters 26, 711–714. Kalashnikova, O.V., Sokolic, I.N., 2002. Importance of shapes and compositions of wind-blown dust particles for remote sensing at solar wavelengths. Geophysical Research Letters 29 (10), 1398. Kaneyasu, N., Murayama, S., 2000. High concentrations of black carbon over middle latitudes in the North Pacific Ocean. Journal of Geophysical Research 105, 19,881–19,890. Kato, N., Akimoto, H., 1992. Anthropogenic emission of SO2 and NOx in Asia: emission inventories. Atmospheric Environment 26, 2992–3017. Killeen, T.J., Eastwood, D., Hendrick, M.S., 1981. Oil-matching by using a simple vector model for fluorescence spectra. Talanta 28, 1–6. Laskin, A., Wietsma, T.W., Krueger, B.J., Grassian, V.H., 2005. Heterogeneous chemistry of individual mineral dust particles with nitric acid: a combined CCSEM/EDX, ESEM, and ICP-MS study. Journal of Geophysical Research 110 (D10208). Liang, Q., Jaegle, L., Wallace, J.M., 2005. Meteorological indices for Asian outflow and transpacific transport on daily to interannual timescales. Journal of Geophysical Research 110, D18308. Luo, C., Mahowald, N.M., Meskhidze, N., Chen, Y., Siefert, R.L., Baker, A.R., Johansen, A.M., 2005. Estimation of iron solubility from observations and a global aerosol model. Journal of Geophysical Research 110 D23307, doi:10.1029/ 2005JD006059. Matsumoto, K., Uyama, Y., Hayano, T., Tanimoto, H., Uno, I., Uematsu, M., 2003. Chemical properties and outflow patterns

ARTICLE IN PRESS 7922

Y. Gao et al. / Atmospheric Environment 41 (2007) 7907–7922

of anthropogenic and dust particles on Rishirl Island during the Asian Pacific Regional aerosol characterization experiment (ACE-Asia). Journal of Geophysical Research 108 (D23). Merrill, J.T., Kim, J., 2004. Meteorological events and transport patterns in ACE-Asia. Journal of Geophysical Research 109, D19S18. Okada, K., Naruse, H., Yanaka, T., Nemoto, O., Iwasaka, Y., Wu, P.M., Ono, A., Duce, R.A., Uematsu, M., Merrill, J., 1990. X-ray spectrometry of individual Asian dust-storm particles over the Japanese Islands and the North Pacific Ocean. Atmospheric Environment 24, 1369–1378. Ooki, A., Uematsu, M., 2005. Chemical interactions between mineral dust particles and acid gases during Asian dust events. Journal of Geophysical Research 110, D03201. Parungo, F., Kim, Y., Zhu, C., Harris, J., Schnell, R., Li, X., Young, D., Fang, X., Yan, P., Yu, X., Zhou, M., Chen, Z., Qian, F., Park, K., 1996. Asian Dust Storms and their Effects on Radiation and Climate, Part II. STC Technical Report No. 2959. Science and Technology Corp., Hampton, VA, 80pp. Perry, K.D., Cahill, T.A., Schnell, R.C., Harris, J.M., 1999. Long-range transport of anthropogenic aerosols to the NOAA baseline station at Mauna Poa Obersatory, Hawaii. Journal of Geophysical Research 104, 18,521–18,533. Perry, K., Cliff, S.S., Jimenez-Cruz, M.P., 2004. Evidence for hygroscopic mineral dust particles from the Intercontinental transport and chemistry transformation experiment. Journal of Geophysical Research 109, D23S28. Posfai, M., Anderson, J.R., Buseck, P.R., Sievering, H., 1999. Soot and sulfate aerosol particles in the remote marine troposphere. Journal of Geophysical Research 104, 21,685–21,693. Prospero, J.M., Ginoux, P., Torres, O., Nicholson, S.E., Gill, T.E., 2002. Environmental characterization of global sources of atmospheric soil dust identified with the NIMBUS 7 total ozone mapping spectrometer (TOMS) absorbing aerosol product. Review of Geophysics 40 (1), 1002. Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, London. Quinn, P.K., Bates, T.S., 2005. Regional aerosol properties: Comparisons of boundary layer measurements from ACE 1, ACE 2, Aerosols99, INDOEX, ACE Asia, TARFOX, and NEAQS. Journal of Geophysical Research 110, D14202. Quinn, P.K., Coffman, D.J., Bates, T.S., Welton, E.J., Covert, D.S., Miller, T.L., Johnson, J.E., Maria, S., Russell, L., Arimoto, R., Carrico, C.M., Rood, M.J., Anderson, J., 2004. Aerosol optical properties measured on board the Ronald H. Brown during ACE-Asia as a function of aerosol chemical composition and source region. Journal of Geophysical Research 109 D19S01, doi:10.1029/2003JD004010. Reid, E.A., Reid, J.S., Meier, M.M., Dunlap, M.R., Cliff, S.S., Broumas, A., Perry, K., Maring, H., 2003. Characterization of African dust transport to Puerto Rica by individual particle and size segregated bulk analysis. Journal of Geophysical Research, 108.

Siefert, R.L., Johansen, A.M., Hoffmann, M.R., 1999. Chemical characterization of ambient aerosol collected during the southwest monsoon and intermonsoon seasons over the Arabian Sea: labile-Fe(II) and other trace metals. Journal of Geophysical Research 104, 3511–3526. Sokolik, I.N., Toon, O.B., 1999. Incorporation of mineralogical composition into models of the radiative properties of mineral aerosol from UV to IR wavelengths. Journal of Geophysical Research 104, 9423–9444. Sullivan, R.C., Guazzotti, S.A., Sodeman, D.A., Prather, K.A., 2007. Direct observations of the atmospheric processing of Asian mineral dust. Atmospheric Chemistry and Physics 7, 1213–1236. Tang, Y., Carmichael, G.R., Kurata, G., Uno, I., Weber, R.J., Song, C.-H., Guttikunda, S.K., Woo, J.-H., Streets, D.G., Wei, C., Clarke, A.D., Huebert, B., Anderson, T.L., 2004. Impacts of dust on regional tropospheric chemistry during the ACE-Asia experiment: a model study with observations. Journal of Geophysical Research 109, D19S21. Tegen, I., Lacis, A.A., Fung, I., 1996. The influence on climate forcing of mineral aerosols from disturbed soils. Nature 380, 419–422. Uematsu, M., Duce, R.A., Prospero, J.M., 1985. Deposition of atmospheric mineral particles in the North Pacific Ocean. Journal of Atmospheric Chemistry 3, 123–138. VanCuren, R.A., 2003. Asian aerosols in North America: extracting the chemical composition and mass concentration of the Asian continental aerosol plume from long-term aerosol records in the western United States. Journal of Geophysical Research 108 (D20), 4623. VanCuren, R.A., Cahill, T.A., 2002. Asian aerosols in North America frequency and concentration of fine dust. Journal of Geophysical Research 107, D24,4804. VanCuren, R.A., Cliff, S.S., Perry, K.D., Jimenez-Cruz, M., 2005. Asian continental aerosol persistence above the marine boundary layer over the eastern North Pacific: continuous aerosol measurements from Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2). Journal of Geophysical Research 110, D09S90. Yang, P., Liou, K.N., Mishchenko, M.I., Gao, B.-C., 2000. Efficient finite-difference time-domain scheme for light scattering by dielectric particles: application to aerosols. Applied Optics 39, 3727–3737. Zhang, D.E., 1984. Synoptic-climate studies of dust fall in China since historic times. Zhongguo Kexue Bao XXVII, 825–836. Zhang, Y., Sunwoo, Y., Kotamarthi, V., Carmichael, G.R., 1994. Photochemical oxidant processes in the presence of dust: an evaluation of the impact of dust on particulate nitrate and ozone formation. Journal of Applied Meteorology 33, 813–824. Zhang, D., Shi, G.Y., Iwasaka, Y., Hu, M., 2000. Mixture of sulfate and nitrate in coastal atmospheric aerosols: individual particle studies in Qingdao (361040 N, 1201210 E), China. Atmospheric Environment 34, 2669–2679. Zhao, D.W., Xiong, J.L., Xu, Y., Chan, W.H., 1988. Acid rain in southwestern China. Atmospheric Environment 22, 349–458.