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Characterization of organic coatings on hygroscopic salt particles and their atmospheric impacts
Atmospheric Environment 44 (2010) 1209e1218
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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Characterization of organic coatings on hygroscopic salt particles and their atmospheric impacts Alla Zelenyuk a, *, Michael J. Ezell b, Véronique Perraud b, Stanley N. Johnson b, Emily A. Bruns b, Yong Yu b,1, Dan Imre c, M. Liz Alexander a, Barbara J. Finlayson-Pitts b a b c
Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA Department of Chemistry, University of California, Irvine, CA 92697-2025, USA Imre Consulting, Richland, WA 99352, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2009 Received in revised form 22 November 2009 Accepted 29 November 2009
The photooxidation of a-pinene in the presence of NO2, with and without added NaNO3 seed particles, has been studied in a large-diameter flow tube. Particles formed by homogeneous nucleation and by condensation on the pre-existing seeds were sampled at various stages of the reaction, dried using four diffusion dryers, size selected at different mobility diameters (dm) using a differential mobility analyzer (DMA), and characterized with a single particle mass spectrometer (SPLAT II). It was found that homogeneously nucleated particles are spherical, have a density (r) of 1.25 0.02 g cm3 (2s) and contain a significant amount of organic nitrates. The mass spectra of the low volatility products condensed on the NaNO3 seed particles were found to be virtually the same as in the case of homogeneous nucleation. The data show that the presence of even a submonolayer of organics on the NaNO3 particles causes water retention that leads to a decrease in particle density and that the amount of water retained increases with organic coating thickness. Thicker coatings appear to inhibit water evaporation from the particle seeds altogether. This suggests that in the atmosphere, where low volatility organics are plentiful, some hygroscopic salts will retain water and have different densities and refractive indices than expected in the absence of the organic coating. This water retention combined with the organic shell on the particles can potentially impact light scattering by these particles and activity as cloud condensation nuclei (CCN), as well as heterogeneous chemistry and photochemistry on the particles. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Secondary organic aerosols Single particle mass spectrometry Density
1. Introduction Field measurements have persistently shown that organic compounds make up a significant fraction of the composition of atmospheric aerosols, accounting for 20e90% of the total atmospheric dry aerosol mass (Kanakidou et al., 2005). The vast majority of these organics are secondary organic aerosols (SOA) formed through the oxidation of volatile organic compounds (VOCs) that are emitted into the atmosphere by a variety of anthropogenic and biogenic sources. The amount of measured SOA is higher than what is predicted on the basis of the observed VOCs and current models, by as much as a factor of 10 (Volkamer et al., 2006). These findings and the realization that organics play an important role in
* Corresponding author. Tel.: þ1 509 371 6155. E-mail address: [email protected] (A. Zelenyuk). 1 Currently at California Air Resources Board, 9528 Telstar Avenue, El Monte, CA 91731, USA. 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.11.047
determining the properties of atmospheric particles, including their optical properties as well as their cloud condensation nuclei (CCN) and ice nuclei (IN) activity, provide impetus for intensive research aimed at developing better understanding of SOA and their impacts (Johnson and Marston, 2008; Kroll and Seinfeld, 2008). A common component of particles over the oceans and in coastal regions is sea salt, which consists primarily of NaCl (Finlayson-Pitts and Pitts, 2000). During transport in air, the chloride is typically replaced by nitrate via reactions with oxides of nitrogen, and in some cases, the particles become primarily NaNO3 (Gard et al., 1998). In addition these marine aerosol were shown to have organic coating that are formed either during sea-salt particle generation (Tervahattu et al., 2002) or by condensation of low volatility compounds formed in air by processes described above. These coated particles are expected to have a wide range of coating thicknesses that lead to a wide spectrum of aerosol properties and behavior (Zelenyuk et al., 2007; Varutbangkul et al., 2006), the characterization of which is best carried out by single particle analysis. As we show here, SPLAT II, a single particle mass
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spectrometer, makes it possible to measure the single particle size, density, shape, and internal composition of these complex particle systems (Zelenyuk et al., 2009b, 2007, 2008a,b). This paper presents the results of an application of SPLAT II to study SOA formed by homogeneous nucleation during the NOx photooxidation of a-pinene and by condensation on NaNO3 seed particles. In the latter case, nitrate ion photooxidation of the parent organic at the particle surface may also contribute to the SOA formation (Yu et al., 2008b). Coating NaNO3 particles with SOA is shown to lead to increased water retention, which is detected and quantified here on the basis of particle densities. This approach makes it possible to detect the effect of organic coatings that are too thin to be observed in the mass spectra. The data show that when the organic coating thickness approaches w10 monolayers, water evaporation from the hygroscopic seeds is virtually stopped. This impact of organic coatings on water retention is very important for determining particle properties that are central to light scattering and CCN activity, and hence to climate change. While the present studies focus on NaNO3, the results could be relevant to atmospheric particles in general that consist of hygroscopic salts and organics.
2. Experimental The experiments described here were performed in a recently developed slow-flow, large-diameter flow system, which is described in detail in a separate publication (Ezell et al., submitted for publication). Briefly, the flow tube is a stainless-steel cylinder of 0.46 m diameter with a 1.2 m long mixing section upstream of a 6.1 m reaction and sampling section, having a total volume of 1.2 m3 and a surface to volume ratio of 10 m1. A typical flow rate of 20 L min1 corresponds to a residence time of w50 min in the reaction section. Five sets of sampling ports (P1eP5) are located at equal intervals along the length of the reaction section accommodating a suite of sampling instruments used to characterize gases and particles. Air-cooled interior fluorescent UV lamps (Phillips TL 100W/01 FS72 ultraviolet-B, light emission centered at 311 nm) are used to induce photochemical reactions. SOA particles were formed by photolyzing NO2 in the presence of a-pinene and particle formation was followed under conditions of both homogeneous nucleation and in the presence of sodium nitrate seed particles. The NO2 and a-pinene concentrations were 1.8 and 1.0 ppm, respectively, while the NaNO3 seed particle concentrations and relative humidity (RH) were varied. Nitrogen dioxide (Praxair, 5069 ppm in air) was diluted to 1.8 ppm using the purified air and mixed with 1 ppm of a-pinene (Aldrich, 99% purity, further purified by passing through a column of alumina) in the 1.2 m long inlet/mixing section. NOy concentrations were monitored using a chemiluminescence trace gas NOx analyzer (Thermo Model 42C). The a-pinene concentration was followed by collecting a sample in a 10 mL loop and analyzing by GCeMS (Hewlett-Packard 5890 Series II GC and a 5971A MSD). Sodium nitrate particles were generated by aerosolizing a water solution of NaNO3 (Fisher Scientific, >99.5% purity) with an atomizer (TSI Model 3076), neutralized, and added to the inlet/ mixing section along with either dry or humidified particle-free air. The size distributions of the particles in the flow tube at each of the ports were measured using a scanning mobility particle sizer (SMPS, TSI Model 3936). The experiments were carried out under dry conditions (RH < 6%) and a total number NaNO3 number concentration of 2.0 105 particles cm3 (Run 1) or 2.5 104 particles cm3 (Run 3), and under high RH conditions (RH ¼ 71%) and a total NaNO3 number concentration of 3.0 104 particles cm3 (Run 2).
Compositions and vacuum aerodynamic diameters (dva) of individual particles were measured using SPLAT II, which is described in detail elsewhere (Zelenyuk et al., 2009b). SPLAT II uses an aerodynamic lens to form a low divergence particle beam and transport the particles into the vacuum system with extremely high efficiencies (Zelenyuk et al., 2009a). The aerodynamic lens imparts on each particle a velocity that is a narrow function of the particle's dva (Zelenyuk et al., 2009b). Each particle is detected by light scattering at two optical detection stages that are spaced 10.5 cm apart. Particle time of flight (PTOF) between the two stages is used to determine particle velocity, which is a function of its dva. A particle detection event and its PTOF are then used to generate a size-dependent trigger to fire the ultraviolet (UV) excimer laser as described in detail elsewhere (Zelenyuk et al., 2009a,b). Individual particle mass spectra are acquired using an angular reflectron timeof-flight mass spectrometer (TOF-MS) and digitized by an A/D converter. To measure individual particle density and sphericity, particles were first passed at low flow rate (0.3 L min1) through four inline diffusion dryers (TSI Model 3062) to a differential mobility analyzer (DMA, TSI Model 3081) used to select particles with known mobility diameters dm. The vacuum aerodynamic diameters, dva of these particles were measured by SPLAT II to yield information on particle sphericity and high precision particle densities, as described in detail elsewhere (Zelenyuk et al., 2009b, 2008c). Note that throughout the study all density and mass spectral measurements were made after the particles were dried, a procedure shown to be important for obtaining reproducible and interpretable data because of effects of water content on particle sizing (Zelenyuk et al., 2006). 3. Results and discussion The focus of this work is on the properties of SOA-coated NaNO3 particles. However, to make the interpretation of data on these complex particles possible, it is important to study first the properties of particles formed by NOx photooxidation of a-pinene in the absence of seed particles as well as the properties of NaNO3 particles alone. The sections below treat these simpler systems first, followed by the case of SOA-coated NaNO3 seed particles. 3.1. NOx photooxidation of a-pinene As reported in previous studies (Yu et al., 2008a and references therein), NOx photooxidation of a-pinene leads to SOA formation because O3, OH, and NO3 (formed from NOx photochemistry) react with a-pinene to form a complex mixture of low vapor pressure organic products, some of which are organic nitrates (Bruns et al., submitted for publication; Kroll and Seinfeld, 2008; Yu et al., 2008a,b). Fig. 1 shows observed density distributions of DMA classified SOA particles, formed by homogeneous nucleation in typical NOx photooxidation experiments. As the figure shows, at all five ports of the flow tube, the dva size distributions of SOA particles have line-widths between 5.5% and 6.5%, providing direct evidence that these, homogeneously nucleated particles are spherical (Zelenyuk et al., 2008c). The measured particle densities are between 1.22 g cm3 and 1.26 g cm3, with particles smaller than 200 nm having lower density, and larger particles with a density of 1.25 0.02 g cm3. Fig. 2 shows an average mass spectrum of homogeneously nucleated SOA particles from the NOx photooxidation of a-pinene. The mass spectrum is characteristic of oxygenated organics, with the most intense peak at m/z ¼ 30 (NOþ) signifying the presence of organic nitrates in these particles (Bruns et al., submitted for publication). A comparison between the mass spectra of particles
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Fig. 1. Measured density distribution of SOA particles formed by homogeneous nucleation. The narrow line-widths indicate that these particles are spherical. P1eP5 are the port numbers, the legend indicates particle mobility diameters, and the solid green area demarks an average density distribution of all but the first two distributions.
with different sizes, densities, and different ports reveals that the mass spectra are remarkably similar. Aside from the high intensity peak at m/z ¼ 30, these mass spectra are also rather similar to that of particles formed by the ozonolysis of a-pinene (data not shown). 3.2. Sodium nitrate particles Fig. 3 shows typical steady-state data for the geometric mean diameter (GMD) of a polydisperse distribution of NaNO3 particles as they travel down the flow tube from P1 to P5 (blue circles). It is clear that there is no significant change in the GMD with time/port for pure NaNO3 particles in the absence of added organics. Fig. 4 shows the mass spectrum of 100 nm NaNO3 particles. The mass spectra of NaNO3 particles are very reproducible, size independent, and show virtually no dependence on the presence of minute amounts of water in the particles. The most intense peak is Naþ at m/z ¼ 23, followed by a peak at m/z ¼ 62 due to Na2Oþ. The NOþ peak at m/z ¼ 30 is clearly visible, but is not one of the more intense peaks.
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Fig. 3. Geometric mean diameter for NaNO3 particles as a function of time/port without added organics (filled blue circles) and in the presence of SOA (filled green circles). Error bars represent the statistical 2s value from 2 to 5 experimental repetitive measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
While the mass spectra of NaNO3 particles are straight forward, the density of NaNO3 in small particle form presents a more complex picture. A number of studies have demonstrated that when small particles composed of sodium nitrate and other salts such as ammonium nitrate, calcium nitrate, and ammonium bisulfate are deliquesced, it is virtually impossible to transform them back to the crystalline phase (Cai et al., 2006; Hoffman et al., 2004; Tang and Munkelwitz, 1994; Zelenyuk et al., 2005). This is true even when the particles are in vacuum and the RH is reduced to nearly zero. Instead they form “glassy” amorphous phases that continuously absorb and lose water in response to increasing or decreasing RH. For NaNO3 we have termed this glassy state as the “anhydrous phase”. The density of NaNO3 in this state is 2 10 g cm3 which is lower than the density of crystalline NaNO3, 2.26 g cm3 (Cai et al., 2006; Zelenyuk et al., 2005, 2007). When thoroughly dried, the dva distribution of DMA classified anhydrous NaNO3 particles shows narrow line-width consistent with spherical particles of uniform composition. To reproducibly reach this anhydrous phase, a fraction of the NaNO3 aerosol flow is first mixed with dry air at a ratio of at least
Fig. 2. Mass spectrum of SOA particles formed by homogeneous nucleation. The spectra in the higher mass regions have been multiplied by the factors shown.
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Fig. 4. Mass spectrum of 100 nm sodium nitrate particles. The relative intensities above 100 amu have been multiplied by a factor of 100.
15:1 and allowed to equilibrate. The diluted and partially dried aerosol is passed through four inline diffusion dryers at a flow rate 0.3 L min1 and classified by a dry DMA. Fig. 5a illustrates an example of the measured density distributions of NaNO3 particles in this “glassy” anhydrous phase. The data show narrow dva linewidths that are consistent with the DMA settings and a reproducible, size independent density of 2.10 0.02 g cm3 as expected for anhydrous NaNO3. In contrast, if NaNO3 particles or the DMA sheath flow are not adequately dried, observed densities can be complex, size dependent, irreproducible, and clearly dependent on the drying procedure. Fig. 5b provides an example in which NaNO3 particles from an atomizer were only partially dried by four inline diffusion dryers at a flow rate of w0.3 L min1 and classified by DMA. Despite having passed through four inline dryers, the line-shapes are wide and complex, with two distinct density modes: one at w2.10 g cm3 and the other at w1.96 g cm3. Our experience shows that incompletely dried NaNO3 particles always exhibit lower densities, and that larger particles are more difficult to dry. For example, in Fig. 5b the density of 102 nm particles peaks at w2.06 g cm3, that of 154 nm particles exhibits bimodal density distribution, and the density of 206 nm particles peaks at w1.96 g cm3. It is not uncommon to find that two distinct density modes are simultaneously present. The high density mode typically peaks between 2.05 g cm3 and 2.10 g cm3, and the low density mode peaks between 1.90 g cm3 and 1.96 g cm3. As long as dried NaNO3 particles are free of impurities their density are never lower than 1.90 g cm3. However, as we will show below, once NaNO3 particles are exposed to organics, they become even harder to dry and their density drops below 1.90 g cm3. 3.3. Photooxidation of a-pinene by NOx in the presence of NaNO3 particles All runs in this experiment began with photooxidation of apinene by NOx to form homogeneously nucleated SOA particles. Once the system was stabilized, NaNO3 particles were introduced into the flow tube to provide surfaces on which low vapor pressure organics condensed. Fig. 3 (green circles) shows that there is a significant increase in the geometric mean diameter of particles from P1 to P5 when the products of the photooxidation of a-pinene by NOx condense on the NaNO3 seed particles. In addition, there is a contribution from the direct oxidation of a-pinene at the NaNO3 particle surface due to the production of OH radicals in the NO 3
photolysis as reported earlier (Yu et al., 2008a,b). Since depth profiling experiments (Zelenyuk et al., 2008b) were not carried out in this study, the exact morphological distribution of SOA and NaNO3, both of which are hydrophilic liquids, within the particles was not determined. Consequently, we refer here to SOA as a coat, but the data interpretation is consistent with either SOA being partially dissolved in NaNO3 or SOA acting as a non-interacting coating on top of the NaNO3 seed particles. Fig. 6 shows the measured density distributions of 102 nm dried, SOA-coated NaNO3 particles at increasing reaction times/port number during Run 1. To illustrate the reproducibility of repeated measurements, two traces recorded at P5 w45 min apart are shown. Three distinct peaks are apparent, the first at w2.04 g cm3 similar to that of anhydrous NaNO3, and two others at w1.80 g cm3, and w1.55 g cm3. Significant numbers of particles with density w2.04 g cm3 are found only at P0 and P1. The density of particles in the intermediate mode decreases from w1.82 g cm3 to w1.79 g cm3 as the particles are transported along the flow tube; and most importantly, the fraction of low density particles at w1.55 g cm3 increases as the particles move from P0 to P5. Given that the density of pure NaNO3 is always greater than 1.90 g cm3, the two lower density modes must represent particles that have been impacted by the presence of organics. Their lower densities might be due to water and/or an organic component that lowers the overall particle density. No detectable amounts of organics are found in the mass spectra of particles that are in the two high density peaks, but the mass spectra of particles in the third low density mode do show the presence of organics (data not shown). The relatively low intensities of the mass spectral peaks that correspond to organics (e.g., m/z ¼ 43, 55, 59.) are consistent with the small amount of SOA that are expected to be found in these particles in Run 1. This is not the case in Runs 2 and 3, presented below, in which the concentration of NaNO3 particles was 10 times lower and as a result NaNO3 particles acquired thicker organic coatings. Fig. 7 shows measured density distributions of 154 nm, 206 nm, and 240 nm particles sampled at different ports, during Runs 2 and 3. The 154 nm particles sampled at P1 have a wide distribution of densities, from that of nearly pure anhydrous NaNO3 (grey shaded area) to that of nearly pure SOA (green shaded area). Fig. 7a and d shows the changes in the measured density distributions of 154 nm particles at different reaction ports, showing that the fraction of particles with lower density increases along the flow tube. In addition, Fig. 7b, c, e and f shows that the larger, 206 nm and 240 nm particles are dominated by low density particles at all ports.
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shows, in an expanded intensity scale, the mass spectra of particles with selected densities averaged overall particle sizes for Run 2. An examination of the individual particle mass spectral data shows a simple pattern in which particles of equal density but with different particle sizes show very similar mass spectra. To facilitate the comparison, the mass spectra in this figure are normalized by their total integrated intensities. As expected, the decrease in density that is due to the presence of organics is accompanied by an increase in the mass spectral peaks that correspond to organics (e.g., m/z ¼ 43, 55, 59.). Fig. 8b is another view of the same mass spectra except that, in order to accentuate differences, the “ratio” mass spectra (Ratio MS), obtained as follows, are plotted:
RatioMSðrÞ ¼
Fig. 5. Density distributions of: (a) anhydrous NaNO3 particles dried in dilution/mixing chamber followed by four diffusion dryers; (b) NaNO3 particles dried with four diffusion dryers.
In short, Fig. 7 shows the presence of a wide range of densities for all particle sizes. Each of the density distributions in Fig. 7 shows, on the low density side, a relatively sharp edge that approaches the density of pure SOA particles. However, since every one of the individual particle mass spectra of these low density particles shows the presence of NaNO3, the possibility that the lowest density particles were formed by homogeneous nucleation can be dismissed. The density distributions indicate that NaNO3 particles become coated with SOA and that the coating thickness increases with reaction time/port. This is confirmed by the dependence of the individual particles mass spectra on the particle density. Fig. 8a
MScoatedSNðrÞ MSSN MSSN
(1)
In Equation (1), MScoatedSN(r) is the mass spectrum of SOA-coated NaNO3 particles with density r and MSN is the mass spectrum of pure NaNO3 particles. This view provides a clearer picture for the low intensity peaks, in which the high density mass spectra (dark lines) that correspond to nearly pure NaNO3 have essentially no signal intensity, while lower density mass spectra exhibit a systematic increase as the particle density decreases. The systematic trend of increasing intensities of organic peaks for particles with decreasing densities in Fig. 8a and b shows that even the low intensity mass spectral peaks are highly reproducible. Analysis of Run 3 data yields virtually identical results. Fig. 9 is an overview of the entire experiment, laying out, at the top, the experimental setup and providing below a schematic representation of the transformations that sodium nitrate particles undergo as they become coated with SOA in the flow tube, and are dried with four diffusion driers and classified by the DMA at 154 nm. The density and mass spectral data presented above clearly show that NaNO3 particles emerge from the flow tube with a wide range of coating thicknesses that must reflect a corresponding range of processes. To ease the discussion, four regions are defined based on particle final density: Region A between 1.90 and 2.10 g cm3, Region B between 1.70 and 1.90 g cm3, Region C between 1.40 and 1.70 g cm3, and Region D below 1.40 g cm3. Particles belonging to Region A have densities and mass spectra consistent with pure NaNO3. At the upstream end of the flow tube at 71% RH, these particles are aqueous solution droplets which, based on the known thermodynamics, are a 51 wt% solution of NaNO3 with density 1.43 g cm3 (Tang and Munkelwitz, 1994; Tang et al., 1997). Before drying and shrinking to produce amorphous, anhydrous 154 nm NaNO3 particles with density of 1.90e2.10 g cm3, these particles must have been 224 nm at 71% RH. Particles in Region B, with final particle densities between 1.70 and 1.90 g cm3, cannot be anhydrous NaNO3, but because their mass spectra show no detectable organics their lower density is most likely due to increased water retention caused by very small amounts of organics. Fig. 9 indicates that these particles are coated with less than a monolayer of organics and lose most of their water on passing through the dryers, but retain enough to lower the overall particle density below 1.9 g cm3. Particles with 154 nm diameter and with density lower than 1.40 g cm3 belong to Region D. These particles have significant SOA weight fraction and, given that the NaNO3/H2O core density at 71% RH is 1.43 g cm3, we must conclude that in this region the observed decrease in density from 1.4 to 1.25 g cm3 is dominated by an increase in SOA coating thickness. Finally, Region C, with measured densities in the range of 1.40e1.70 g cm3, corresponds to a NaNO3/H2O core with SOA
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Fig. 6. Density distributions of 102 nm NaNO3 particles exposed to SOA (Run 1) at different ports. The grey shaded area represents anhydrous amorphous sodium nitrate and the green shaded area marks the density of pure SOA.
coatings that are thinner than those observed in Region D, but sufficiently thick to be detectable in the individual particle mass spectra (Fig. 8). The decrease in the overall density in this region is due to a combination of increased water retention (the major factor) that is coupled to the increased organic coating thickness. We summarize that when NaNO3/H2O particles are coated with SOA the particles final densities reflect the presence of low density SOA and the fact that water evaporation dynamics is strongly coupled to the presence of organics. This leads, as we will show below, to a simple pattern, in which higher organic content induces more water to be retained. In the section below we use the measured densities and mass spectral peak intensities to calculate the relationship between particle compositions and densities. 3.4. Calculating the relationship between particle density and composition The relative NaNO3 and SOA mass spectral peak intensities can be used to calculate the SOA weight fraction (WFSOA), defined in Equation (2) below. ISOA and ISN are calculated by summing the intensities of the organics and NaNO3 peaks, respectively, and applying a relative calibration factor (K) that is based on
measurements for pure SOA and pure NaNO3 particles and reflects the fact that NaNO3 has higher ionization efficiency than SOA:
WFSOA h
P mass of SOA K ISOA P ¼ P ISOA þ ISN mass of SOA þ mass of SN K
(2)
The calculated WFSOA are plotted in Fig. 10a (green filled circles) as a function of particle density, in which Regions A through D that were discussed in the section above are also indicated. The calculated WFSOA remain below the detection limit for particles with densities above w1.7 g cm3 (Regions A and B), increases slowly as the particle density decreases to w1.4 g cm3 (Region C) and rapidly increases as the density approaches w1.25 g cm3, which is the density of pure SOA. It is important to note that the calculated relationship between WFSOA and particle density is entirely based on mass spectral intensities. An attempt to fit observed particle density by using a ternary NaNO3/H2O/SOA mixture with the calculated WFSOA and a constant water content shows that it cannot be accomplished. In other words, it is important to account for the fact that water retention by these particles depends on the extent to which they are coated with SOA. To this end we consider the NaNO3/H2O core density, and by inference, the amount of water retained by the SOAcoated NaNO3 particles to be dependent on the weight fraction of
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Fig. 7. Density distributions of NaNO3 particles coated with SOA in Run 2 (aec) and Run 3 (def). The solid green and grey areas represent the density of pure SOA and NaNO3 particles, respectively.
Fig. 8. Evolution of the mass spectra of SOA-coated NaNO3 particles (Run 2) as a function of particle density. (a) Average normalized mass spectra (see text); (b) “ratio” mass spectra (see Equation (1) in the text). Particle densities and their corresponding colors are indicated in the legend.
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Fig. 9. Top: Schematic of the experimental system that includes the flow tube, diffusion dryers, DMA and SPLAT II. Center: the evolution of particles that after drying are 154 nm in diameter, in four defined regions, A through D and their corresponding densities. See text for detailed explanation.
SOA. It amounts to treating these particles as a ternary mixture of SOA/NaNO3/H2O, with the NaNO3/H2O composition being a function of the amount of SOA present. The goal is to find an empirical function that fits the relationship between the observed WFSOA and particle densities shown in Fig. 10a. In a previous study of the properties of surfactant coated hygroscopic particles (Zelenyuk et al., 2007), it was also found that organic coatings as thin as a monolayer induce water retention by the hygroscopic core. The present study shows that this phenomenon is not limited to surfactants and extends to the mixture of compounds found in SOA. For SOA-coated NaNO3 particles, we construct a simple function that describes the changes in core density due to water retention that is coupled to the particle organic content. We find that to fit all the experimental data points presented in Fig. 10a (green filled circles), the density of the NaNO3/H2O core must first rapidly decrease to 1.46 g cm3 as the organic coating thickness increases, and then remain relatively constant for particles with thicker SOA layers and densities below w1.4 g cm3 (Region D). Equation (3) directly relates the core density to the organic volume fraction (VFSOA), where the total particle volume includes NaNO3, water, and SOA:
rcore ¼ 1:46 þ 0:4*expð 20*VFSOA Þ
(3)
According to Equation (3) the core density exponentially decreases from w1.9 g cm3, the lowest observed density of anhydrous sodium nitrate to 1.46 g cm3 as the organic content increases and then remains constant at even higher SOA fraction. This functional form is empirically derived to fit the relationship between the organic content (WFSOA) and the measured overall particle density that was calculated on the basis of the mass spectral intensity and does not therefore include the retained water. For each VFSOA value, from 0 to 1, a core density (rcore) is calculated using Equation (3). The combination of VFSOA, the calculated core density, the
measured SOA density of 1.25 g cm3, and the assumption of volume additivity makes it possible to calculate the overall particle density, the NaNO3 mass in the NaNO3/H2O core, the mass of SOA in the particle, and according to Equation (2) the WFSOA for each VFSOA value. The blue line in Fig. 10a represents the calculated WFSOA as a function of particle density using Equation (3). The good agreement with the mass spectrally based WFSOA provides support for Equation (3). In Fig. 10b we show the results (solid black line) of using Equation (3) to calculate the relationship between the modeled NaNO3/H2O core density and the overall particle density. The dotted vertical lines in Fig. 10a and b mark the particle densities that, according to the model, contain these core compositions. Fig. 10a and b indicate that the cores of particles in Region B contain on average 1e2 H2O molecules per NaNO3, while the particles in Region D contain 4 water molecules per NaNO3. For illustrative purposes, we also calculated for 154 nm particles the corresponding core diameters (Fig. 10a) and organic coating thickness as a function of particle density (Fig. 10c). Fig. 10c shows that as the particle density decreases, the coating thickness increases slowly from essentially zero to 8 nm in Region C, followed by a rapid increase in Region D. To reiterate, in Regions A and B the decreasing particle densities are due to increasing water retention which in Region B is enhanced by the presence of a submonolayer of organics. In Region C the density decreases due to both increasing coating thickness and water retention; and in Region D, the density decreases because of the increase in the organic content. For Region C it is possible to get a rough estimate of the number of SOA monolayers (ML) on the particles assuming that the surface density of an ML is between 2 and 3 1014 molecules cm2 and that the molecular mass of SOA is w300 g mol1. As shown by the green shaded area (Fig. 10c), the SOA coating thickness in Region C changes from submonolayer at particle density of w1.7 g cm3 to w10 ML at a density of w1.4 g cm3 where Region D begins.
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It is intriguing that the data in Region D are consistent with a core of constant composition and core density of 1.46 g cm3, which is nearly the same as the density of 1.43 g cm3 calculated for the initial NaNO3 particle in equilibrium with water vapor at 71% RH. This suggests that the thicker layers of low volatility organics formed in the photooxidation effectively “seal off” the core as depicted in Region D of Fig. 9, so that little water is removed on passing through the dryers. It is also interesting that the number of water molecules per NaNO3 in this core, 4:1, is in the range reported for the number of water molecules in the first hydration shell around a nitrate ion where the waters are more strongly bound to the ion (Megyes et al., 2009; Zhao et al., 2009 and references therein). However, in highly concentrated salt solutions, ion pairing between the anion and cation can weaken the watereanion interactions (Megyes et al., 2009). Polar compounds such as pinonic and pinic acids are products of a-pinene oxidation and as part of the organic coating may draw Naþ closer to the surface and away from NO 3 , increasing the stability of waterenitrate ion clusters. Such an effect of organic acids may also play a role in the unusual effect on water retention observed for even submonolayers of organic products. While the analysis here has focused on Run 2, the results for Run 3 at low relative humidity are very similar to those for Run 2. This was initially surprising in that there was very little water vapor in the flow tube so that one might expect the NaNO3/H2O particles to contain much less water initially. However, it is important to keep in mind that the particles emerge from the atomizer at w100% RH. The fact that they display similar behavior to that for Run 2 at high RH suggests that the particles rapidly gain an organic coating that prevents the NaNO3/H2O particles coming to equilibrium with the dry air in the flow tube. 4. Conclusions SPLAT II was coupled with a DMA to characterize the shape, composition and density of SOA particles formed by homogeneous nucleation and by condensation on NaNO3 seed particles during NOx photooxidation of a-pinene. The particles formed by homogeneous nucleation were found to be spherical and their density was determined to be 1.25 0.02 g cm3. The individual particle mass spectra clearly show the presence of organic nitrates. When NaNO3 particles are introduced into the reaction chamber, they serve as seed particles on which the low volatility oxidation products condense. Both the acquired mass spectra and measured individual particle densities indicate an increase in coating thickness with reaction time/port, but at all reaction stages, a relatively wide distribution of coating thicknesses exist. The mass spectra of the organic fraction are virtually indistinguishable from the mass spectra of particles formed by homogeneous nucleation. A detailed analysis of the relationship between particle composition and densities shows that water retention by the NaNO3 core is strongly impacted by the presence of organic coatings and that this effect can be observed even at submonolayer SOA coatings. The organic coating retards evaporation of water from the core, resulting in decreasing densities of the NaNO3/H2O core from w2.10e1.90 g cm3 for anhydrous NaNO3 to w1.46 g cm3 for the
Fig. 10. (a) SOA weight fraction as a function of particle density (green filled circles). The blue line is the fit to the data assuming composition dependent water retention. Open blue triangles show the calculated core diameters for particles with a final diameter of 154 nm; (b) modeled core density used to fit the data in (a). Filled red squares mark different stoichiometric NaNO3:H2O compositions; (c) calculated SOA coating thickness for 154 nm particles (black line), and estimated number of SOA monolayers in Region C (green shaded area). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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salt/water core coated with about w10 monolayers of organics. As the amount of organics on the particle increases, it virtually seals off the salt/water core so that no further evaporation of water occurs. At this point, increasing coating thickness results in decreasing overall densities of particles, approaching that of homogeneously nucleated SOA particles. In the atmosphere where low volatility organics are plentiful, hygroscopic salts such as sodium nitrate (commonly found in processed sea-salt particles), ammonium nitrate, ammonium bisulfate and calcium nitrate are expected to take up these organics, resulting in increased water retention and particle densities and refractive indices different from those in the absence of organics. Furthermore, these important particle properties will also be affected by the formation of an organic shell around the aqueous core. Such effects can impact light scattering and CCN activity, as well as heterogeneous chemistry and photochemistry associated with these particles. Acknowledgments We are grateful to the US Department of Energy Office of Biological and Environmental Research (Grant No. DE-FG0205ER64000), the Office of Basic Energy Sciences, Chemical Sciences Division, the National Science Foundation through AirUCI, an Environmental Molecular Science Institute (Grant No. CHE0431312), and the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the US Department of Energy's Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL), for support of this work. PNNL is operated by the US Department of Energy by Battelle Memorial Institute under contract no. DEAC06-76RL0 1830. References Bruns, E.A., Perraud, V., Zelenyuk, A., Ezell, M.J., Johnson, S.N., Yu, Y., Imre, D., Finlayson-Pitts, B.J., Alexander, M.L. Comparison of FTIR and particle mass spectrometry for the measurement of particulate organic nitrates. Environmental Science and Technology, submitted for publication. Cai, Y., Zelenyuk, A., Imre, D., 2006. A high resolution study of the effect of morphology on the mass spectra of single PSL particles with Na-containing layers and nodules. Aerosol Science and Technology 40, 1111e1122. Ezell, M.J., Johnson, S.N., Yu, Y., Perraud, V., Bruns, E.A., Alexander, M.L., Zelenyuk, A., Dabdub, D., Finlayson-Pitts, B.J. A new aerosol flow system for photochemical and thermal studies of tropospheric aerosols. Aerosol Science & Technology, submitted for publication. Finlayson-Pitts, B.J., Pitts Jr., J.N., 2000. Chemistry of the Upper and Lower Atmosphere e Theory, Experiments, and Applications. Academic Press, San Diego. Gard, E.E., Gross, D.S., Hughes, L.S., Allen, J.O., Morrical, B.D., Fergenson, D.P., Diemers, T., Galli, M.E., Johnson, R.J., Cass, G.R., Prather, K.A., 1998. Direct observation of heterogeneous chemistry in the atmosphere. Science 279, 1184e1187. Hoffman, R.C., Laskin, A., Finlayson-Pitts, B.J., 2004. Sodium nitrate particles: physical and chemical properties during hydration and dehydration, and implications for aged sea salt aerosols. Journal of Aerosol Science 35, 869e887. Johnson, D., Marston, G., 2008. The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere. Chemical Society Reviews 37, 699e716. Kanakidou, M., Seinfeld, J.H., Pandis, S.N., Barnes, I., Dentener, F.J., Facchini, M.C., Van Dingenen, R., Ervens, B., Nenes, A., Nielsen, C.J., Swietlicki, E., Putaud, J.P.,
Balkanski, Y., Fuzzi, S., Horth, J., Moortgat, G.K., Winterhalter, R., Myhre, C.E.L., Tsigaridis, K., Vignati, E., Stephanou, E.G., Wilson, J., 2005. Organic aerosol and global climate modelling: a review. Atmospheric Chemistry and Physics 5, 1053e1123. Kroll, J.H., Seinfeld, J.H., 2008. Chemistry of secondary organic aerosol: formation and evolution of low-volatility organics in the atmosphere. Atmospheric Environment 42, 3593e3624. Megyes, T., Balint, S., Peter, E., Grosz, T., Bako, I., Krienke, H., Bellissent-Funel, M.C., 2009. Solution structure of NaNO3 in water: diffraction and molecular dynamics simulation study. Journal of Physical Chemistry B 113, 4054e4064. Tang, I.N., Munkelwitz, H.R., 1994. Water activities, densities, and refractive-indexes of aqueous sulfates and sodium-nitrate droplets of atmospheric importance. Journal of Geophysical Research e Atmospheres 99, 18801e18808. Tang, I.N., Tridico, A.C., Fung, K.H., 1997. Thermodynamic and optical properties of sea salt aerosols. Journal of Geophysical Research e Atmospheres 102, 23269e23275. Tervahattu, H., Juhanoja, J., Kupiainen, K., 2002. Identification of an organic coating on marine aerosol particles by TOF-SIMS. Journal of Geophysical Research 107, 4319. doi:10.1029/2001JD001403. Varutbangkul, V., Brechtel, F.J., Bahreini, R., Ng, N.L., Keywood, M.D., Kroll, J.H., Flagan, R.C., Seinfeld, J.H., Lee, A., Goldstein, A.H., 2006. Hygroscopicity of secondary organic aerosols formed by oxidation of cycloalkenes, monoterpenes, sesquiterpenes, and related compounds. Atmospheric Chemistry and Physics 6, 2367e2388. Volkamer, R., Jimenez, J.L., San Martini, F., Dzepina, K., Zhang, Q., Salcedo, D., Molina, L.T., Worsnop, D.R., Molina, M.J., 2006. Secondary organic aerosol formation from anthropogenic air pollution: rapid and higher than expected. Geophysical Research Letters 33, L17811. Yu, Y., Ezell, M.J., Zelenyuk, A., Imre, D., Alexander, L., Ortega, J., D'Anna, B., Harmon, C.W., Johnson, S.N., Finlayson-Pitts, B.J., 2008a. Photooxidation of a-pinene at high relative humidity in the presence of increasing concentrations of NOx. Atmospheric Environment 42, 5044e5060. Yu, Y., Ezell, M.J., Zelenyuk, A., Imre, D., Alexander, L., Ortega, J., Thomas, J.L., Gogna, K., Tobias, D.J., D'Anna, B., Harmon, C.W., Johnson, S.N., FinlaysonPitts, B.J., 2008b. Nitrate ion photochemistry at interfaces: a new mechanism for oxidation of alpha-pinene. Physical Chemistry Chemical Physics 10, 3063e3071. Zelenyuk, A., Cai, Y., Chieffo, L., Imre, D., 2005. High precision density measurements of single particles: the density of metastable phases. Aerosol Science and Technology 39, 972e986. Zelenyuk, A., Imre, D., Cuadra-Rodriguez, L.A., 2006. Evaporation of water from particles in the aerodynamic lens inlet: an experimental study. Analytical Chemistry 78, 6942e6947. Zelenyuk, A., Imre, D., Cuadra-Rodriguez, L.A., Ellison, B., 2007. Measurements and interpretation of the effect of a soluble organic surfactant on the density, shape and water uptake of hygroscopic particles. Journal of Aerosol Science 38, 903e923. Zelenyuk, A., Imre, D., Han, J.H., Oatis, S., 2008a. Simultaneous measurements of individual ambient particle size, composition, effective density, and hygroscopicity. Analytical Chemistry 80, 1401e1407. Zelenyuk, A., Yang, J., Imre, D., Choi, E., 2009a. Achieving size independent hit-rate in single particle mass spectrometry. Aerosol Science and Technology 43, 305e310. Zelenyuk, A., Yang, J., Imre, D., Choi, E., 2009b. SPLAT II: an aircraft compatible, ultra-sensitive, high precision instrument for in-situ characterization of the size and composition of fine and ultrafine particles. Aerosol Science and Technology 43, 411e424. Zelenyuk, A., Yang, J., Song, C., Zaveri, R.A., Imre, D., 2008b. “Depth-profiling” and quantitative characterization of the size, composition, shape, density, and morphology of fine particles with SPLAT, a single-particle mass spectrometer. Journal of Physical Chemistry A 112, 669e677. Zelenyuk, A., Yang, J., Song, C., Zaveri, R.A., Imre, D., 2008c. A new real-time method for determining particles' sphericity and density: application to secondary organic aerosol formed by ozonolysis of alpha-pinene. Environmental Science & Technology 42, 8033e8038. Zhao, L.J., Zeng, Q.X., Zhang, Y.H., 2009. State of water in supersaturated nitrate aerosols disclosed by the Raman difference spectra. Journal of Physical Chemistry A 113, 215e220.
Contents lists available at ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Characterization of organic coatings on hygroscopic salt particles and their atmospheric impacts Alla Zelenyuk a, *, Michael J. Ezell b, Véronique Perraud b, Stanley N. Johnson b, Emily A. Bruns b, Yong Yu b,1, Dan Imre c, M. Liz Alexander a, Barbara J. Finlayson-Pitts b a b c
Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA Department of Chemistry, University of California, Irvine, CA 92697-2025, USA Imre Consulting, Richland, WA 99352, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2009 Received in revised form 22 November 2009 Accepted 29 November 2009
The photooxidation of a-pinene in the presence of NO2, with and without added NaNO3 seed particles, has been studied in a large-diameter flow tube. Particles formed by homogeneous nucleation and by condensation on the pre-existing seeds were sampled at various stages of the reaction, dried using four diffusion dryers, size selected at different mobility diameters (dm) using a differential mobility analyzer (DMA), and characterized with a single particle mass spectrometer (SPLAT II). It was found that homogeneously nucleated particles are spherical, have a density (r) of 1.25 0.02 g cm3 (2s) and contain a significant amount of organic nitrates. The mass spectra of the low volatility products condensed on the NaNO3 seed particles were found to be virtually the same as in the case of homogeneous nucleation. The data show that the presence of even a submonolayer of organics on the NaNO3 particles causes water retention that leads to a decrease in particle density and that the amount of water retained increases with organic coating thickness. Thicker coatings appear to inhibit water evaporation from the particle seeds altogether. This suggests that in the atmosphere, where low volatility organics are plentiful, some hygroscopic salts will retain water and have different densities and refractive indices than expected in the absence of the organic coating. This water retention combined with the organic shell on the particles can potentially impact light scattering by these particles and activity as cloud condensation nuclei (CCN), as well as heterogeneous chemistry and photochemistry on the particles. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Secondary organic aerosols Single particle mass spectrometry Density
1. Introduction Field measurements have persistently shown that organic compounds make up a significant fraction of the composition of atmospheric aerosols, accounting for 20e90% of the total atmospheric dry aerosol mass (Kanakidou et al., 2005). The vast majority of these organics are secondary organic aerosols (SOA) formed through the oxidation of volatile organic compounds (VOCs) that are emitted into the atmosphere by a variety of anthropogenic and biogenic sources. The amount of measured SOA is higher than what is predicted on the basis of the observed VOCs and current models, by as much as a factor of 10 (Volkamer et al., 2006). These findings and the realization that organics play an important role in
* Corresponding author. Tel.: þ1 509 371 6155. E-mail address: [email protected] (A. Zelenyuk). 1 Currently at California Air Resources Board, 9528 Telstar Avenue, El Monte, CA 91731, USA. 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.11.047
determining the properties of atmospheric particles, including their optical properties as well as their cloud condensation nuclei (CCN) and ice nuclei (IN) activity, provide impetus for intensive research aimed at developing better understanding of SOA and their impacts (Johnson and Marston, 2008; Kroll and Seinfeld, 2008). A common component of particles over the oceans and in coastal regions is sea salt, which consists primarily of NaCl (Finlayson-Pitts and Pitts, 2000). During transport in air, the chloride is typically replaced by nitrate via reactions with oxides of nitrogen, and in some cases, the particles become primarily NaNO3 (Gard et al., 1998). In addition these marine aerosol were shown to have organic coating that are formed either during sea-salt particle generation (Tervahattu et al., 2002) or by condensation of low volatility compounds formed in air by processes described above. These coated particles are expected to have a wide range of coating thicknesses that lead to a wide spectrum of aerosol properties and behavior (Zelenyuk et al., 2007; Varutbangkul et al., 2006), the characterization of which is best carried out by single particle analysis. As we show here, SPLAT II, a single particle mass
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spectrometer, makes it possible to measure the single particle size, density, shape, and internal composition of these complex particle systems (Zelenyuk et al., 2009b, 2007, 2008a,b). This paper presents the results of an application of SPLAT II to study SOA formed by homogeneous nucleation during the NOx photooxidation of a-pinene and by condensation on NaNO3 seed particles. In the latter case, nitrate ion photooxidation of the parent organic at the particle surface may also contribute to the SOA formation (Yu et al., 2008b). Coating NaNO3 particles with SOA is shown to lead to increased water retention, which is detected and quantified here on the basis of particle densities. This approach makes it possible to detect the effect of organic coatings that are too thin to be observed in the mass spectra. The data show that when the organic coating thickness approaches w10 monolayers, water evaporation from the hygroscopic seeds is virtually stopped. This impact of organic coatings on water retention is very important for determining particle properties that are central to light scattering and CCN activity, and hence to climate change. While the present studies focus on NaNO3, the results could be relevant to atmospheric particles in general that consist of hygroscopic salts and organics.
2. Experimental The experiments described here were performed in a recently developed slow-flow, large-diameter flow system, which is described in detail in a separate publication (Ezell et al., submitted for publication). Briefly, the flow tube is a stainless-steel cylinder of 0.46 m diameter with a 1.2 m long mixing section upstream of a 6.1 m reaction and sampling section, having a total volume of 1.2 m3 and a surface to volume ratio of 10 m1. A typical flow rate of 20 L min1 corresponds to a residence time of w50 min in the reaction section. Five sets of sampling ports (P1eP5) are located at equal intervals along the length of the reaction section accommodating a suite of sampling instruments used to characterize gases and particles. Air-cooled interior fluorescent UV lamps (Phillips TL 100W/01 FS72 ultraviolet-B, light emission centered at 311 nm) are used to induce photochemical reactions. SOA particles were formed by photolyzing NO2 in the presence of a-pinene and particle formation was followed under conditions of both homogeneous nucleation and in the presence of sodium nitrate seed particles. The NO2 and a-pinene concentrations were 1.8 and 1.0 ppm, respectively, while the NaNO3 seed particle concentrations and relative humidity (RH) were varied. Nitrogen dioxide (Praxair, 5069 ppm in air) was diluted to 1.8 ppm using the purified air and mixed with 1 ppm of a-pinene (Aldrich, 99% purity, further purified by passing through a column of alumina) in the 1.2 m long inlet/mixing section. NOy concentrations were monitored using a chemiluminescence trace gas NOx analyzer (Thermo Model 42C). The a-pinene concentration was followed by collecting a sample in a 10 mL loop and analyzing by GCeMS (Hewlett-Packard 5890 Series II GC and a 5971A MSD). Sodium nitrate particles were generated by aerosolizing a water solution of NaNO3 (Fisher Scientific, >99.5% purity) with an atomizer (TSI Model 3076), neutralized, and added to the inlet/ mixing section along with either dry or humidified particle-free air. The size distributions of the particles in the flow tube at each of the ports were measured using a scanning mobility particle sizer (SMPS, TSI Model 3936). The experiments were carried out under dry conditions (RH < 6%) and a total number NaNO3 number concentration of 2.0 105 particles cm3 (Run 1) or 2.5 104 particles cm3 (Run 3), and under high RH conditions (RH ¼ 71%) and a total NaNO3 number concentration of 3.0 104 particles cm3 (Run 2).
Compositions and vacuum aerodynamic diameters (dva) of individual particles were measured using SPLAT II, which is described in detail elsewhere (Zelenyuk et al., 2009b). SPLAT II uses an aerodynamic lens to form a low divergence particle beam and transport the particles into the vacuum system with extremely high efficiencies (Zelenyuk et al., 2009a). The aerodynamic lens imparts on each particle a velocity that is a narrow function of the particle's dva (Zelenyuk et al., 2009b). Each particle is detected by light scattering at two optical detection stages that are spaced 10.5 cm apart. Particle time of flight (PTOF) between the two stages is used to determine particle velocity, which is a function of its dva. A particle detection event and its PTOF are then used to generate a size-dependent trigger to fire the ultraviolet (UV) excimer laser as described in detail elsewhere (Zelenyuk et al., 2009a,b). Individual particle mass spectra are acquired using an angular reflectron timeof-flight mass spectrometer (TOF-MS) and digitized by an A/D converter. To measure individual particle density and sphericity, particles were first passed at low flow rate (0.3 L min1) through four inline diffusion dryers (TSI Model 3062) to a differential mobility analyzer (DMA, TSI Model 3081) used to select particles with known mobility diameters dm. The vacuum aerodynamic diameters, dva of these particles were measured by SPLAT II to yield information on particle sphericity and high precision particle densities, as described in detail elsewhere (Zelenyuk et al., 2009b, 2008c). Note that throughout the study all density and mass spectral measurements were made after the particles were dried, a procedure shown to be important for obtaining reproducible and interpretable data because of effects of water content on particle sizing (Zelenyuk et al., 2006). 3. Results and discussion The focus of this work is on the properties of SOA-coated NaNO3 particles. However, to make the interpretation of data on these complex particles possible, it is important to study first the properties of particles formed by NOx photooxidation of a-pinene in the absence of seed particles as well as the properties of NaNO3 particles alone. The sections below treat these simpler systems first, followed by the case of SOA-coated NaNO3 seed particles. 3.1. NOx photooxidation of a-pinene As reported in previous studies (Yu et al., 2008a and references therein), NOx photooxidation of a-pinene leads to SOA formation because O3, OH, and NO3 (formed from NOx photochemistry) react with a-pinene to form a complex mixture of low vapor pressure organic products, some of which are organic nitrates (Bruns et al., submitted for publication; Kroll and Seinfeld, 2008; Yu et al., 2008a,b). Fig. 1 shows observed density distributions of DMA classified SOA particles, formed by homogeneous nucleation in typical NOx photooxidation experiments. As the figure shows, at all five ports of the flow tube, the dva size distributions of SOA particles have line-widths between 5.5% and 6.5%, providing direct evidence that these, homogeneously nucleated particles are spherical (Zelenyuk et al., 2008c). The measured particle densities are between 1.22 g cm3 and 1.26 g cm3, with particles smaller than 200 nm having lower density, and larger particles with a density of 1.25 0.02 g cm3. Fig. 2 shows an average mass spectrum of homogeneously nucleated SOA particles from the NOx photooxidation of a-pinene. The mass spectrum is characteristic of oxygenated organics, with the most intense peak at m/z ¼ 30 (NOþ) signifying the presence of organic nitrates in these particles (Bruns et al., submitted for publication). A comparison between the mass spectra of particles
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Fig. 1. Measured density distribution of SOA particles formed by homogeneous nucleation. The narrow line-widths indicate that these particles are spherical. P1eP5 are the port numbers, the legend indicates particle mobility diameters, and the solid green area demarks an average density distribution of all but the first two distributions.
with different sizes, densities, and different ports reveals that the mass spectra are remarkably similar. Aside from the high intensity peak at m/z ¼ 30, these mass spectra are also rather similar to that of particles formed by the ozonolysis of a-pinene (data not shown). 3.2. Sodium nitrate particles Fig. 3 shows typical steady-state data for the geometric mean diameter (GMD) of a polydisperse distribution of NaNO3 particles as they travel down the flow tube from P1 to P5 (blue circles). It is clear that there is no significant change in the GMD with time/port for pure NaNO3 particles in the absence of added organics. Fig. 4 shows the mass spectrum of 100 nm NaNO3 particles. The mass spectra of NaNO3 particles are very reproducible, size independent, and show virtually no dependence on the presence of minute amounts of water in the particles. The most intense peak is Naþ at m/z ¼ 23, followed by a peak at m/z ¼ 62 due to Na2Oþ. The NOþ peak at m/z ¼ 30 is clearly visible, but is not one of the more intense peaks.
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Fig. 3. Geometric mean diameter for NaNO3 particles as a function of time/port without added organics (filled blue circles) and in the presence of SOA (filled green circles). Error bars represent the statistical 2s value from 2 to 5 experimental repetitive measurements. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
While the mass spectra of NaNO3 particles are straight forward, the density of NaNO3 in small particle form presents a more complex picture. A number of studies have demonstrated that when small particles composed of sodium nitrate and other salts such as ammonium nitrate, calcium nitrate, and ammonium bisulfate are deliquesced, it is virtually impossible to transform them back to the crystalline phase (Cai et al., 2006; Hoffman et al., 2004; Tang and Munkelwitz, 1994; Zelenyuk et al., 2005). This is true even when the particles are in vacuum and the RH is reduced to nearly zero. Instead they form “glassy” amorphous phases that continuously absorb and lose water in response to increasing or decreasing RH. For NaNO3 we have termed this glassy state as the “anhydrous phase”. The density of NaNO3 in this state is 2 10 g cm3 which is lower than the density of crystalline NaNO3, 2.26 g cm3 (Cai et al., 2006; Zelenyuk et al., 2005, 2007). When thoroughly dried, the dva distribution of DMA classified anhydrous NaNO3 particles shows narrow line-width consistent with spherical particles of uniform composition. To reproducibly reach this anhydrous phase, a fraction of the NaNO3 aerosol flow is first mixed with dry air at a ratio of at least
Fig. 2. Mass spectrum of SOA particles formed by homogeneous nucleation. The spectra in the higher mass regions have been multiplied by the factors shown.
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Fig. 4. Mass spectrum of 100 nm sodium nitrate particles. The relative intensities above 100 amu have been multiplied by a factor of 100.
15:1 and allowed to equilibrate. The diluted and partially dried aerosol is passed through four inline diffusion dryers at a flow rate 0.3 L min1 and classified by a dry DMA. Fig. 5a illustrates an example of the measured density distributions of NaNO3 particles in this “glassy” anhydrous phase. The data show narrow dva linewidths that are consistent with the DMA settings and a reproducible, size independent density of 2.10 0.02 g cm3 as expected for anhydrous NaNO3. In contrast, if NaNO3 particles or the DMA sheath flow are not adequately dried, observed densities can be complex, size dependent, irreproducible, and clearly dependent on the drying procedure. Fig. 5b provides an example in which NaNO3 particles from an atomizer were only partially dried by four inline diffusion dryers at a flow rate of w0.3 L min1 and classified by DMA. Despite having passed through four inline dryers, the line-shapes are wide and complex, with two distinct density modes: one at w2.10 g cm3 and the other at w1.96 g cm3. Our experience shows that incompletely dried NaNO3 particles always exhibit lower densities, and that larger particles are more difficult to dry. For example, in Fig. 5b the density of 102 nm particles peaks at w2.06 g cm3, that of 154 nm particles exhibits bimodal density distribution, and the density of 206 nm particles peaks at w1.96 g cm3. It is not uncommon to find that two distinct density modes are simultaneously present. The high density mode typically peaks between 2.05 g cm3 and 2.10 g cm3, and the low density mode peaks between 1.90 g cm3 and 1.96 g cm3. As long as dried NaNO3 particles are free of impurities their density are never lower than 1.90 g cm3. However, as we will show below, once NaNO3 particles are exposed to organics, they become even harder to dry and their density drops below 1.90 g cm3. 3.3. Photooxidation of a-pinene by NOx in the presence of NaNO3 particles All runs in this experiment began with photooxidation of apinene by NOx to form homogeneously nucleated SOA particles. Once the system was stabilized, NaNO3 particles were introduced into the flow tube to provide surfaces on which low vapor pressure organics condensed. Fig. 3 (green circles) shows that there is a significant increase in the geometric mean diameter of particles from P1 to P5 when the products of the photooxidation of a-pinene by NOx condense on the NaNO3 seed particles. In addition, there is a contribution from the direct oxidation of a-pinene at the NaNO3 particle surface due to the production of OH radicals in the NO 3
photolysis as reported earlier (Yu et al., 2008a,b). Since depth profiling experiments (Zelenyuk et al., 2008b) were not carried out in this study, the exact morphological distribution of SOA and NaNO3, both of which are hydrophilic liquids, within the particles was not determined. Consequently, we refer here to SOA as a coat, but the data interpretation is consistent with either SOA being partially dissolved in NaNO3 or SOA acting as a non-interacting coating on top of the NaNO3 seed particles. Fig. 6 shows the measured density distributions of 102 nm dried, SOA-coated NaNO3 particles at increasing reaction times/port number during Run 1. To illustrate the reproducibility of repeated measurements, two traces recorded at P5 w45 min apart are shown. Three distinct peaks are apparent, the first at w2.04 g cm3 similar to that of anhydrous NaNO3, and two others at w1.80 g cm3, and w1.55 g cm3. Significant numbers of particles with density w2.04 g cm3 are found only at P0 and P1. The density of particles in the intermediate mode decreases from w1.82 g cm3 to w1.79 g cm3 as the particles are transported along the flow tube; and most importantly, the fraction of low density particles at w1.55 g cm3 increases as the particles move from P0 to P5. Given that the density of pure NaNO3 is always greater than 1.90 g cm3, the two lower density modes must represent particles that have been impacted by the presence of organics. Their lower densities might be due to water and/or an organic component that lowers the overall particle density. No detectable amounts of organics are found in the mass spectra of particles that are in the two high density peaks, but the mass spectra of particles in the third low density mode do show the presence of organics (data not shown). The relatively low intensities of the mass spectral peaks that correspond to organics (e.g., m/z ¼ 43, 55, 59.) are consistent with the small amount of SOA that are expected to be found in these particles in Run 1. This is not the case in Runs 2 and 3, presented below, in which the concentration of NaNO3 particles was 10 times lower and as a result NaNO3 particles acquired thicker organic coatings. Fig. 7 shows measured density distributions of 154 nm, 206 nm, and 240 nm particles sampled at different ports, during Runs 2 and 3. The 154 nm particles sampled at P1 have a wide distribution of densities, from that of nearly pure anhydrous NaNO3 (grey shaded area) to that of nearly pure SOA (green shaded area). Fig. 7a and d shows the changes in the measured density distributions of 154 nm particles at different reaction ports, showing that the fraction of particles with lower density increases along the flow tube. In addition, Fig. 7b, c, e and f shows that the larger, 206 nm and 240 nm particles are dominated by low density particles at all ports.
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shows, in an expanded intensity scale, the mass spectra of particles with selected densities averaged overall particle sizes for Run 2. An examination of the individual particle mass spectral data shows a simple pattern in which particles of equal density but with different particle sizes show very similar mass spectra. To facilitate the comparison, the mass spectra in this figure are normalized by their total integrated intensities. As expected, the decrease in density that is due to the presence of organics is accompanied by an increase in the mass spectral peaks that correspond to organics (e.g., m/z ¼ 43, 55, 59.). Fig. 8b is another view of the same mass spectra except that, in order to accentuate differences, the “ratio” mass spectra (Ratio MS), obtained as follows, are plotted:
RatioMSðrÞ ¼
Fig. 5. Density distributions of: (a) anhydrous NaNO3 particles dried in dilution/mixing chamber followed by four diffusion dryers; (b) NaNO3 particles dried with four diffusion dryers.
In short, Fig. 7 shows the presence of a wide range of densities for all particle sizes. Each of the density distributions in Fig. 7 shows, on the low density side, a relatively sharp edge that approaches the density of pure SOA particles. However, since every one of the individual particle mass spectra of these low density particles shows the presence of NaNO3, the possibility that the lowest density particles were formed by homogeneous nucleation can be dismissed. The density distributions indicate that NaNO3 particles become coated with SOA and that the coating thickness increases with reaction time/port. This is confirmed by the dependence of the individual particles mass spectra on the particle density. Fig. 8a
MScoatedSNðrÞ MSSN MSSN
(1)
In Equation (1), MScoatedSN(r) is the mass spectrum of SOA-coated NaNO3 particles with density r and MSN is the mass spectrum of pure NaNO3 particles. This view provides a clearer picture for the low intensity peaks, in which the high density mass spectra (dark lines) that correspond to nearly pure NaNO3 have essentially no signal intensity, while lower density mass spectra exhibit a systematic increase as the particle density decreases. The systematic trend of increasing intensities of organic peaks for particles with decreasing densities in Fig. 8a and b shows that even the low intensity mass spectral peaks are highly reproducible. Analysis of Run 3 data yields virtually identical results. Fig. 9 is an overview of the entire experiment, laying out, at the top, the experimental setup and providing below a schematic representation of the transformations that sodium nitrate particles undergo as they become coated with SOA in the flow tube, and are dried with four diffusion driers and classified by the DMA at 154 nm. The density and mass spectral data presented above clearly show that NaNO3 particles emerge from the flow tube with a wide range of coating thicknesses that must reflect a corresponding range of processes. To ease the discussion, four regions are defined based on particle final density: Region A between 1.90 and 2.10 g cm3, Region B between 1.70 and 1.90 g cm3, Region C between 1.40 and 1.70 g cm3, and Region D below 1.40 g cm3. Particles belonging to Region A have densities and mass spectra consistent with pure NaNO3. At the upstream end of the flow tube at 71% RH, these particles are aqueous solution droplets which, based on the known thermodynamics, are a 51 wt% solution of NaNO3 with density 1.43 g cm3 (Tang and Munkelwitz, 1994; Tang et al., 1997). Before drying and shrinking to produce amorphous, anhydrous 154 nm NaNO3 particles with density of 1.90e2.10 g cm3, these particles must have been 224 nm at 71% RH. Particles in Region B, with final particle densities between 1.70 and 1.90 g cm3, cannot be anhydrous NaNO3, but because their mass spectra show no detectable organics their lower density is most likely due to increased water retention caused by very small amounts of organics. Fig. 9 indicates that these particles are coated with less than a monolayer of organics and lose most of their water on passing through the dryers, but retain enough to lower the overall particle density below 1.9 g cm3. Particles with 154 nm diameter and with density lower than 1.40 g cm3 belong to Region D. These particles have significant SOA weight fraction and, given that the NaNO3/H2O core density at 71% RH is 1.43 g cm3, we must conclude that in this region the observed decrease in density from 1.4 to 1.25 g cm3 is dominated by an increase in SOA coating thickness. Finally, Region C, with measured densities in the range of 1.40e1.70 g cm3, corresponds to a NaNO3/H2O core with SOA
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Fig. 6. Density distributions of 102 nm NaNO3 particles exposed to SOA (Run 1) at different ports. The grey shaded area represents anhydrous amorphous sodium nitrate and the green shaded area marks the density of pure SOA.
coatings that are thinner than those observed in Region D, but sufficiently thick to be detectable in the individual particle mass spectra (Fig. 8). The decrease in the overall density in this region is due to a combination of increased water retention (the major factor) that is coupled to the increased organic coating thickness. We summarize that when NaNO3/H2O particles are coated with SOA the particles final densities reflect the presence of low density SOA and the fact that water evaporation dynamics is strongly coupled to the presence of organics. This leads, as we will show below, to a simple pattern, in which higher organic content induces more water to be retained. In the section below we use the measured densities and mass spectral peak intensities to calculate the relationship between particle compositions and densities. 3.4. Calculating the relationship between particle density and composition The relative NaNO3 and SOA mass spectral peak intensities can be used to calculate the SOA weight fraction (WFSOA), defined in Equation (2) below. ISOA and ISN are calculated by summing the intensities of the organics and NaNO3 peaks, respectively, and applying a relative calibration factor (K) that is based on
measurements for pure SOA and pure NaNO3 particles and reflects the fact that NaNO3 has higher ionization efficiency than SOA:
WFSOA h
P mass of SOA K ISOA P ¼ P ISOA þ ISN mass of SOA þ mass of SN K
(2)
The calculated WFSOA are plotted in Fig. 10a (green filled circles) as a function of particle density, in which Regions A through D that were discussed in the section above are also indicated. The calculated WFSOA remain below the detection limit for particles with densities above w1.7 g cm3 (Regions A and B), increases slowly as the particle density decreases to w1.4 g cm3 (Region C) and rapidly increases as the density approaches w1.25 g cm3, which is the density of pure SOA. It is important to note that the calculated relationship between WFSOA and particle density is entirely based on mass spectral intensities. An attempt to fit observed particle density by using a ternary NaNO3/H2O/SOA mixture with the calculated WFSOA and a constant water content shows that it cannot be accomplished. In other words, it is important to account for the fact that water retention by these particles depends on the extent to which they are coated with SOA. To this end we consider the NaNO3/H2O core density, and by inference, the amount of water retained by the SOAcoated NaNO3 particles to be dependent on the weight fraction of
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Fig. 7. Density distributions of NaNO3 particles coated with SOA in Run 2 (aec) and Run 3 (def). The solid green and grey areas represent the density of pure SOA and NaNO3 particles, respectively.
Fig. 8. Evolution of the mass spectra of SOA-coated NaNO3 particles (Run 2) as a function of particle density. (a) Average normalized mass spectra (see text); (b) “ratio” mass spectra (see Equation (1) in the text). Particle densities and their corresponding colors are indicated in the legend.
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Fig. 9. Top: Schematic of the experimental system that includes the flow tube, diffusion dryers, DMA and SPLAT II. Center: the evolution of particles that after drying are 154 nm in diameter, in four defined regions, A through D and their corresponding densities. See text for detailed explanation.
SOA. It amounts to treating these particles as a ternary mixture of SOA/NaNO3/H2O, with the NaNO3/H2O composition being a function of the amount of SOA present. The goal is to find an empirical function that fits the relationship between the observed WFSOA and particle densities shown in Fig. 10a. In a previous study of the properties of surfactant coated hygroscopic particles (Zelenyuk et al., 2007), it was also found that organic coatings as thin as a monolayer induce water retention by the hygroscopic core. The present study shows that this phenomenon is not limited to surfactants and extends to the mixture of compounds found in SOA. For SOA-coated NaNO3 particles, we construct a simple function that describes the changes in core density due to water retention that is coupled to the particle organic content. We find that to fit all the experimental data points presented in Fig. 10a (green filled circles), the density of the NaNO3/H2O core must first rapidly decrease to 1.46 g cm3 as the organic coating thickness increases, and then remain relatively constant for particles with thicker SOA layers and densities below w1.4 g cm3 (Region D). Equation (3) directly relates the core density to the organic volume fraction (VFSOA), where the total particle volume includes NaNO3, water, and SOA:
rcore ¼ 1:46 þ 0:4*expð 20*VFSOA Þ
(3)
According to Equation (3) the core density exponentially decreases from w1.9 g cm3, the lowest observed density of anhydrous sodium nitrate to 1.46 g cm3 as the organic content increases and then remains constant at even higher SOA fraction. This functional form is empirically derived to fit the relationship between the organic content (WFSOA) and the measured overall particle density that was calculated on the basis of the mass spectral intensity and does not therefore include the retained water. For each VFSOA value, from 0 to 1, a core density (rcore) is calculated using Equation (3). The combination of VFSOA, the calculated core density, the
measured SOA density of 1.25 g cm3, and the assumption of volume additivity makes it possible to calculate the overall particle density, the NaNO3 mass in the NaNO3/H2O core, the mass of SOA in the particle, and according to Equation (2) the WFSOA for each VFSOA value. The blue line in Fig. 10a represents the calculated WFSOA as a function of particle density using Equation (3). The good agreement with the mass spectrally based WFSOA provides support for Equation (3). In Fig. 10b we show the results (solid black line) of using Equation (3) to calculate the relationship between the modeled NaNO3/H2O core density and the overall particle density. The dotted vertical lines in Fig. 10a and b mark the particle densities that, according to the model, contain these core compositions. Fig. 10a and b indicate that the cores of particles in Region B contain on average 1e2 H2O molecules per NaNO3, while the particles in Region D contain 4 water molecules per NaNO3. For illustrative purposes, we also calculated for 154 nm particles the corresponding core diameters (Fig. 10a) and organic coating thickness as a function of particle density (Fig. 10c). Fig. 10c shows that as the particle density decreases, the coating thickness increases slowly from essentially zero to 8 nm in Region C, followed by a rapid increase in Region D. To reiterate, in Regions A and B the decreasing particle densities are due to increasing water retention which in Region B is enhanced by the presence of a submonolayer of organics. In Region C the density decreases due to both increasing coating thickness and water retention; and in Region D, the density decreases because of the increase in the organic content. For Region C it is possible to get a rough estimate of the number of SOA monolayers (ML) on the particles assuming that the surface density of an ML is between 2 and 3 1014 molecules cm2 and that the molecular mass of SOA is w300 g mol1. As shown by the green shaded area (Fig. 10c), the SOA coating thickness in Region C changes from submonolayer at particle density of w1.7 g cm3 to w10 ML at a density of w1.4 g cm3 where Region D begins.
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It is intriguing that the data in Region D are consistent with a core of constant composition and core density of 1.46 g cm3, which is nearly the same as the density of 1.43 g cm3 calculated for the initial NaNO3 particle in equilibrium with water vapor at 71% RH. This suggests that the thicker layers of low volatility organics formed in the photooxidation effectively “seal off” the core as depicted in Region D of Fig. 9, so that little water is removed on passing through the dryers. It is also interesting that the number of water molecules per NaNO3 in this core, 4:1, is in the range reported for the number of water molecules in the first hydration shell around a nitrate ion where the waters are more strongly bound to the ion (Megyes et al., 2009; Zhao et al., 2009 and references therein). However, in highly concentrated salt solutions, ion pairing between the anion and cation can weaken the watereanion interactions (Megyes et al., 2009). Polar compounds such as pinonic and pinic acids are products of a-pinene oxidation and as part of the organic coating may draw Naþ closer to the surface and away from NO 3 , increasing the stability of waterenitrate ion clusters. Such an effect of organic acids may also play a role in the unusual effect on water retention observed for even submonolayers of organic products. While the analysis here has focused on Run 2, the results for Run 3 at low relative humidity are very similar to those for Run 2. This was initially surprising in that there was very little water vapor in the flow tube so that one might expect the NaNO3/H2O particles to contain much less water initially. However, it is important to keep in mind that the particles emerge from the atomizer at w100% RH. The fact that they display similar behavior to that for Run 2 at high RH suggests that the particles rapidly gain an organic coating that prevents the NaNO3/H2O particles coming to equilibrium with the dry air in the flow tube. 4. Conclusions SPLAT II was coupled with a DMA to characterize the shape, composition and density of SOA particles formed by homogeneous nucleation and by condensation on NaNO3 seed particles during NOx photooxidation of a-pinene. The particles formed by homogeneous nucleation were found to be spherical and their density was determined to be 1.25 0.02 g cm3. The individual particle mass spectra clearly show the presence of organic nitrates. When NaNO3 particles are introduced into the reaction chamber, they serve as seed particles on which the low volatility oxidation products condense. Both the acquired mass spectra and measured individual particle densities indicate an increase in coating thickness with reaction time/port, but at all reaction stages, a relatively wide distribution of coating thicknesses exist. The mass spectra of the organic fraction are virtually indistinguishable from the mass spectra of particles formed by homogeneous nucleation. A detailed analysis of the relationship between particle composition and densities shows that water retention by the NaNO3 core is strongly impacted by the presence of organic coatings and that this effect can be observed even at submonolayer SOA coatings. The organic coating retards evaporation of water from the core, resulting in decreasing densities of the NaNO3/H2O core from w2.10e1.90 g cm3 for anhydrous NaNO3 to w1.46 g cm3 for the
Fig. 10. (a) SOA weight fraction as a function of particle density (green filled circles). The blue line is the fit to the data assuming composition dependent water retention. Open blue triangles show the calculated core diameters for particles with a final diameter of 154 nm; (b) modeled core density used to fit the data in (a). Filled red squares mark different stoichiometric NaNO3:H2O compositions; (c) calculated SOA coating thickness for 154 nm particles (black line), and estimated number of SOA monolayers in Region C (green shaded area). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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salt/water core coated with about w10 monolayers of organics. As the amount of organics on the particle increases, it virtually seals off the salt/water core so that no further evaporation of water occurs. At this point, increasing coating thickness results in decreasing overall densities of particles, approaching that of homogeneously nucleated SOA particles. In the atmosphere where low volatility organics are plentiful, hygroscopic salts such as sodium nitrate (commonly found in processed sea-salt particles), ammonium nitrate, ammonium bisulfate and calcium nitrate are expected to take up these organics, resulting in increased water retention and particle densities and refractive indices different from those in the absence of organics. Furthermore, these important particle properties will also be affected by the formation of an organic shell around the aqueous core. Such effects can impact light scattering and CCN activity, as well as heterogeneous chemistry and photochemistry associated with these particles. Acknowledgments We are grateful to the US Department of Energy Office of Biological and Environmental Research (Grant No. DE-FG0205ER64000), the Office of Basic Energy Sciences, Chemical Sciences Division, the National Science Foundation through AirUCI, an Environmental Molecular Science Institute (Grant No. CHE0431312), and the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the US Department of Energy's Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL), for support of this work. PNNL is operated by the US Department of Energy by Battelle Memorial Institute under contract no. DEAC06-76RL0 1830. References Bruns, E.A., Perraud, V., Zelenyuk, A., Ezell, M.J., Johnson, S.N., Yu, Y., Imre, D., Finlayson-Pitts, B.J., Alexander, M.L. Comparison of FTIR and particle mass spectrometry for the measurement of particulate organic nitrates. Environmental Science and Technology, submitted for publication. Cai, Y., Zelenyuk, A., Imre, D., 2006. A high resolution study of the effect of morphology on the mass spectra of single PSL particles with Na-containing layers and nodules. Aerosol Science and Technology 40, 1111e1122. Ezell, M.J., Johnson, S.N., Yu, Y., Perraud, V., Bruns, E.A., Alexander, M.L., Zelenyuk, A., Dabdub, D., Finlayson-Pitts, B.J. A new aerosol flow system for photochemical and thermal studies of tropospheric aerosols. Aerosol Science & Technology, submitted for publication. Finlayson-Pitts, B.J., Pitts Jr., J.N., 2000. Chemistry of the Upper and Lower Atmosphere e Theory, Experiments, and Applications. Academic Press, San Diego. Gard, E.E., Gross, D.S., Hughes, L.S., Allen, J.O., Morrical, B.D., Fergenson, D.P., Diemers, T., Galli, M.E., Johnson, R.J., Cass, G.R., Prather, K.A., 1998. Direct observation of heterogeneous chemistry in the atmosphere. Science 279, 1184e1187. Hoffman, R.C., Laskin, A., Finlayson-Pitts, B.J., 2004. Sodium nitrate particles: physical and chemical properties during hydration and dehydration, and implications for aged sea salt aerosols. Journal of Aerosol Science 35, 869e887. Johnson, D., Marston, G., 2008. The gas-phase ozonolysis of unsaturated volatile organic compounds in the troposphere. Chemical Society Reviews 37, 699e716. Kanakidou, M., Seinfeld, J.H., Pandis, S.N., Barnes, I., Dentener, F.J., Facchini, M.C., Van Dingenen, R., Ervens, B., Nenes, A., Nielsen, C.J., Swietlicki, E., Putaud, J.P.,
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