Aerosol Liquid Water Driven by Anthropogenic Nitrate - American

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Aerosol Liquid Water Driven by Anthropogenic Nitrate: Implications for Lifetimes of Water-Soluble Organic Gases and Potential for Secondary Organic Aerosol Formation Natasha Hodas,*,†,⊗ Amy P. Sullivan,‡ Kate Skog,§ Frank N. Keutsch,§ Jeffrey L. Collett, Jr.,‡ Stefano Decesari,∥ M. Cristina Facchini,∥ Annmarie G. Carlton,† Ari Laaksonen,⊥,# and Barbara J. Turpin† †

Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey 08901, United States Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523, United States § Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States ∥ Istituto di Scienze dell’Atmosfera e dell’Oceano, Consiglio Nazionale delle Ricerche, Via P. Gobetti, 101 40129, Bologna, Italy ⊥ Department of Applied Physics, University of Eastern Finland, POB 1627, 70211 Kuopio, Finland # Finnish Meteorological Institute, POB 503, 00101 Helsinki, Finland ‡

S Supporting Information *

ABSTRACT: Aerosol liquid water (ALW) influences aerosol radiative properties and the partitioning of gas-phase water-soluble organic compounds (WSOCg) to the condensed phase. A recent modeling study drew attention to the anthropogenic nature of ALW in the southeastern United States, where predicted ALW is driven by regional sulfate. Herein, we demonstrate that ALW in the Po Valley, Italy, is also anthropogenic but is driven by locally formed nitrate, illustrating regional differences in the aerosol components responsible for ALW. We present field evidence for the influence of controllable ALW on the lifetimes and atmospheric budgets of reactive organic gases and note the role of ALW in the formation of secondary organic aerosol (SOA). Nitrate is expected to increase in importance due to increased emissions of nitrate precursors, as well as policies aimed at reducing sulfur emissions. We argue that the impacts of increased particulate nitrate in future climate and air quality scenarios may be under predicted because they do not account for the increased potential for SOA formation in nitrate-derived ALW, nor do they account for the impacts of this ALW on reactive gas budgets and gas-phase photochemistry.

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INTRODUCTION Aerosol liquid water (ALW) is ubiquitous and important in many atmospheric processes. It is well-known that ALW influences aerosol optical properties, visibility, and radiative impacts.1−5 A substantial body of research now highlights the role that ALW can play in the formation of secondary organic aerosol (SOA) through the partitioning of gas-phase watersoluble organic compounds (WSOCg) to the condensed phase and subsequent aqueous-phase reactions to form oligomers, organosulfates, and nitrogen-containing organics through radical and nonradical reactions.6−18 Partitioning to ALW has been indicated as a possible sink for gas-phase glyoxal and was projected to explain 15−30% of the SOA mass generated in Mexico City.19 Increases in particle-phase WSOC (WSOCp) mass concentrations by as much as 52% have been observed under conditions favorable to higher ALW concentrations as compared to lower ALW periods.20 A modeling study recently demonstrated that a substantial fraction of ALW in the southeastern United States can be attributed to anthropogenic aerosol components.21 Due to an abundance of sulfate aerosol and high relative humidity, ALW © 2014 American Chemical Society

mass concentrations and the potential for WSOCg to partition to the condensed phase are high in the southeastern U.S. The authors argue that ALW could contribute to the high SOA concentrations in this region.21 While the focus of that study was on the southeastern U.S., a local region of high ALW was also found for Los Angeles, CA, where nitrate is a more substantial contributor to aerosol mass than sulfate.2,22 Nitrate is known to be hygroscopic,23 and several studies have explored the radiative impacts of nitrate and associated ALW in past, present, and future climates.1,3,4,24−26 These studies have shown that the direct-effect, top-of-the-atmosphere radiative forcing of nitrate could increase by as much as a factor of 7 by the year 2100 due to increased emissions of nitrate aerosol precursors, with the current radiative forcing of nitrate aerosol estimated to be −0.11 W/m2 (95% confidence interval −0.3, −0.03 W/m2).5 Particle-phase nitrate is formed when Received: Revised: Accepted: Published: 11127

May 22, 2014 August 11, 2014 September 5, 2014 September 5, 2014 dx.doi.org/10.1021/es5025096 | Environ. Sci. Technol. 2014, 48, 11127−11136

Environmental Science & Technology

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concentrations of gas-phase ammonia at SPC led to denuder breakthrough, and as a result, ammonium concentrations measured with the PILS were compromised. Ammonium concentrations were calculated assuming that the aerosol was neutral and that all measured nitrate and sulfate were in the form of ammonium nitrate and ammonium sulfate, respectively. This assumption is supported by ion balance calculations conducted for previous Po Valley sampling campaigns,36 as well as by the ion chromatographic analysis of impactor samples collected during the PEGASOS campaign (not discussed in the present manuscript). Sample integration time for the anion and cation measurements was 8 min, and a new chromatogram was generated every 30 min. For the organic acids, sample integration time was 16.5 min, and a new chromatogram was generated every 65 min. WSOCp sample integration time was 6 min. The Mad-LIP excites glyoxal with a 440 nm sapphire laser and utilizes the unique phosphorescent lifetime of glyoxal to provide high time resolution, in situ measurements of ambient gas-phase glyoxal concentrations.33 The Mad-LIP instrument was operated at a flow rate of approximately 19 standard Lmin−1. Phosphorescence counts were conducted each second and integrated over 30 s.33 Supporting measurements included RH and temperature, recorded hourly. Hourly average particle surface area was also calculated from particle number size distributions measured with a twin differential mobility particle sizer (DMPS) system. The twin-DMPS system consists of two DMPS operated in parallel, with one DMPS measuring particles 3−20 nm and the second measuring particles 15−600 nm.37 Aerosol sample and sheath flows were 1.5 and 10 L-min−1 and 1.0 and 6.7 L-min−1 for the first and second DMPS, respectively. Aerosol Liquid Water Calculations. Hourly ALW mass concentrations were calculated with the online versions of two equilibrium thermodynamic models: (1) the extended aerosol inorganics model (E-AIM)38 and (2) the aerosol inorganic− organic mixtures functional groups activity coefficients (AIOMFAC-web) model.39,40 For the E-AIM calculations, water uptake by the inorganic ions sulfate, nitrate, and ammonium was considered. As noted above, ammonium concentrations were calculated assuming that the aerosol at SPC was neutral and that all measured nitrate and sulfate were in the form of ammonium nitrate and ammonium sulfate, respectively. The formation of solids was suppressed, and thus, calculated water contents for aqueous solution droplets at RH values below their efflorescence points are representative of metastable conditions.30 ALW associated with ammonium, sulfate, and nitrate, as well as various carboxylic acids (oxalic, glutaric, maleic), was also calculated with AIOMFAC-web. AIOMFAC is a group-contribution model that considers the thermodynamic properties and water-uptake behavior of organic compounds based on their molecular structures.40 This detailed thermodynamic model predicts interactions between aerosol components, as well as the influence these interactions have on water uptake for mixed organic−inorganic particles.39,40

nitric acid is neutralized by a base such as ammonia, calcium, sodium, or amines, with the majority of atmospheric particulate nitrate in the form of ammonium nitrate.27,28 Anthropogenic activities that contribute to emissions of ammonium nitrate precursors include fossil fuel burning, agricultural activities, and biomass burning. It is expected that these activities will increase over the century.5 In addition to absolute changes in atmospheric nitrate concentrations, the relative importance of nitrate is expected to increase as a result of policies aimed at reducing sulfur emissions.4,5,29 Because nitrate is more hygroscopic than sulfate,30 such changes in aerosol composition can result in increases in ALW mass for a given relative humidity (RH). Herein, we present measurements of aerosol composition and ALW mass concentration calculations for a summer sampling campaign conducted in the Po Valley, Italy. We demonstrate that the ALW in this region can largely be attributed to anthropogenic nitrate aerosol and provide evidence that ALW serves as a sink for reactive organic gases. The potential role of nitrate-derived ALW in SOA formation and implications for gas-phase photochemistry are discussed. This work suggests that previous studies may have underestimated the impacts of increases in nitrate in future climate and emissions scenarios.

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EXPERIMENTAL SECTION Field Site. Measurements were conducted June 18 through July 10, 2012, at San Pietro Capofiume (SPC) in the Po Valley, Italy (44°39′N, 11°38′E) as part of the Pan-European GasAerosols Climate Interactions Study (PEGASOS; http:// pegasos.iceht.forth.gr/). The region immediately surrounding the site is characterized by widespread agricultural activities. Two major roadways A13 and A14 are approximately 15 and 25 km from SPC, respectively, and several smaller state and provincial roads surround SPC. Air quality in the region is also influenced by nearby cities Bologna (∼30 km to the southwest) and Ferrara (∼20 km to the north), as well as the long-range transport of regional pollutants. Measurements and Instrumentation. Among the measurements conducted at SPC were gas-phase glyoxal concentrations, measured by Madison laser-induced phosphorescence (Mad-LIP), particle species concentrations, measured with particle-into-liquid samplers (PILS) coupled with ion chromatographs (ICs), and WSOCp concentrations measured with a PILS coupled with a total organic carbon analyzer (TOC). Method details are provided elsewhere.31−34 Briefly, in the PILS, aerosol-containing ambient air is mixed rapidly with water-vapor-saturated air, resulting in supersaturated conditions and the activation of particles with diameters on the order of 10 nm and larger. Activated particles are collected on an inertial impactor. Purified water flows over the impaction plate, collecting the impacted droplets. In our measurements, anions, cations, and carboxylic acids in the aerosol-containing effluent were quantified with three dedicated ICs,31,32 and WSOCp was quantified with a TOC.34 All PILS were run at flow rates of 15 L-min−1. PM2.5 was selected with a 2.5 μm cut cyclone and denuders placed upstream of the PILS removed gas-phase compounds that could otherwise contribute to positive sampling artifacts. Particle collection efficiencies are high (>96%) and not expected to contribute substantially to uncertainty in species concentration measurements, with the exception of ammonium which can experience losses of approximately 12% due to volatilization.35 High ambient

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RESULTS AND DISCUSSION Local Formation of Nitrate Aerosol. The time series for measured nitrate, sulfate, and organic acid mass concentrations and modeled ALW mass concentrations are shown in Figure 1. Summary statistics for these variables, as well as measured gasphase glyoxal, RH, and temperature, are given in Table 1.

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dx.doi.org/10.1021/es5025096 | Environ. Sci. Technol. 2014, 48, 11127−11136


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that emissions of NOx and ammonia in this region are dominated by anthropogenic sources, with the majority of NOx attributable to vehicles, industrial combustion processes, and energy production and with ammonia largely associated with agricultural activities.44 In contrast, sulfate and organic aerosol do not show the same dependence on the mesoscale circulation and are associated with regional sources.36 Differences in fine aerosol composition across sampling locations, measured concurrently with Berner impactors during the summer 2012 campaign, also suggest local formation of particulate nitrate at SPC. Average nitrate concentrations measured at SPC were 88% higher than those measured at a sampling site in Bologna (generally upwind of SPC), whereas average sulfate concentrations were only 27% higher at SPC as compared to Bologna (data not shown here). Aerosol Liquid Water Calculations. Values of ALW calculated with AIOMFAC and E-AIM are in good agreement (Figure 1, Table 1; R2 > 0.999), despite the fact that organic acids were included in the AIOMFAC calculations and not the E-AIM calculations. Variability in ALW at SPC was driven primarily by nitrate concentrations. A comparison of the time series for nitrate and calculated ALW demonstrates that ALW values track closely with nitrate concentrations (Figure 1). Further, ALW is well correlated with nitrate (R2 = 0.61) but poorly correlated with sulfate (R2 = 0.08) and organic acids (R2 = 0.004; Figure 2). Thus, in contrast with the southeastern United States,21 variability in ALW concentrations in this region is driven by locally formed, anthropogenic nitrate aerosol.

Figure 1. Time series of particulate sulfate, nitrate, and organic acids (oxalic, maleic, glutaric) measured at San Pietro Capofiume, Italy, during the summer 2012 PEGASOS field campaign and aerosol liquid water calculated (ALW) with the E-AIM and AIOMFAC thermodynamic models. Times given are local time at San Pietro Capofiume (UTC+2).

Table 1. Summary Statistics of Variables Measured at San Pietro Capofiume, Italy, during the Summer 2012 PEGASOS Sampling Campaign and Aerosol Liquid Water (ALW) Calculated with the E-AIM and AIOMFAC Thermodynamic Models percentile

5th

25th

50th

75th

95th

nitrate (μg/m3) sulfate (μg/m3) organic acids (μg/m3) glyoxal (ppb) rel humidity (%) temp (°C) aerosol liquid water: E-AIM (μg/m3) aerosol liquid water: AIOMFAC (μg/m3)

0.60 1.74 0.09 0.03 29.0 20.23 0.56

0.78 2.23 0.13 0.05 40.3 22.70 1.36

1.09 2.71 0.34 0.06 54.6 26.64 2.86

1.98 3.25 0.42 0.07 68.7 30.45 5.36

5.04 4.08 0.56 0.10 84.4 34.36 19.75

0.74

1.43

2.90

5.21

18.51

Nitrate concentrations demonstrated a marked diurnal cycle with peaks in the early morning hours corresponding to time periods with the lowest temperatures and the highest RHs, as expected on the basis of the thermodynamics of ammonium nitrate formation and dissociation.41,42 Variability in nitrate concentrations on longer time scales is also evident in Figure 1, with lower concentrations occurring when SPC was influenced by easterly winds (e.g., June 26th and 27th) and higher concentrations when a southwesterly or westerly wind dominated. Sulfate and organic acid concentrations were more uniform across the sampling period, with the exception of a high sulfate event on July 6th (Figure 1). Standard deviations of species concentrations, normalized by the average concentration of each species, across the sampling campaign were 1.04, 0.49, and 0.31 for nitrate, organic acids, and sulfate, respectively. This is in agreement with the results of Crosier et al.,36 who also found elevated nitrate concentrations under westerly flow conditions but more uniform sulfate and organic aerosol concentrations. Surface westerly winds in the morning alternating with easterlies in afternoon hours are the typical circulation of the eastern Po Valley under anticyclonic conditions.43 Such a circulation pattern is responsible for the recirculation of the products of Po Valley emissions and clearly enhances the particulate nitrate in SPC. It is well documented

Figure 2. Calculated aerosol liquid water (ALW) and measured aerosol species concentrations: (a) nitrate, (b) sulfate, (c) organic acids (oxalic, maleic, glutaric). Dashed lines indicate linear regression models. Calculated values of ALW are well-correlated with nitrate concentrations (R2 = 0.61) but poorly correlated with sulfate (R2 = 0.08) and organic acids (R2 = 0.004).

In order to quantify the impact of ammonium nitrate on ALW concentrations, we recalculated ALW with ammonium nitrate removed from the system and compared those calculations to the initial ALW calculations. On average, ALW concentrations attributable to ammonium nitrate and ammonium sulfate were comparable at 2.36 and 2.75 μg/m3, respectively; however, nitrate was the main driver of high ALW events. One-hour maximum ALW associated with 11129



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ammonium nitrate was as high as 47.26 μg/m3, compared to 14.32 μg/m3 for ammonium sulfate. We also evaluated the enhancement in ALW attributable to locally formed ammonium nitrate by comparing ALW concentrations for the average particle compositions measured at the nitrate-rich SPC site and the upwind Bologna site, where average nitrate concentrations were 88% lower. For those calculations, average ambient conditions (RH and temperature) measured at SPC were used as inputs for both locations to isolate the effects of the ammonium nitrate enhancement. In further support of the importance of locally formed ammonium nitrate for ALW concentrations at SPC, total ALW concentrations were 42% higher for the nitrate-rich SPC particle composition and ALW associated with ammonium nitrate alone was 88% higher for SPC as compared to Bologna. Because the nitrate enhancement at SPC can be attributed to anthropogenic sources, we refer to the associated ALW as anthropogenic and controllable. A modeling study focused on secondary aerosol formation in the Po Valley found that nitric acid is the limiting factor to ammonium nitrate formation in ammonia-rich sites like SPC.45 However, as noted above, both NOx and ammonia emissions are associated with anthropogenic sources. Our results are in line with recent observations showing that particulate nitrate dominates over sulfates over vast areas in central Europe.46 High nitrate-to-sulfate ratios were somewhat unexpected for a Mediterranean region in the summer, when temperature is a limiting factor for the condensation of ammonium nitrate. According to our observations, the concentrations of particulate nitrate in the Po Valley in the summer are large enough to account for a major fraction of ALW and are the main driver of ALW in nocturnal and morning hours. Finally, it should be noted that in the central hours of the day, when the condensation of ammonium nitrate is inhibited at ground level, particulate nitrate can form and persist in the upper part of the boundary layer where temperatures are lower. This has been observed in the Po Valley (Curci et al., manuscript in preparation) and other regions influenced by anthropogenic emissions of ammonium nitrate precursors.47,48 There are limitations to using modeled ALW concentrations. First, not all aerosol species were considered in the ALW calculations. Sodium and chloride can contribute to water uptake and were measured at SPC. However, while measurable sodium concentrations were observed on 55% of sampling days, sodium and chloride accounted for less than 2% and 1% of the sum of measured particle species (sum of sulfate, nitrate, ammonium, sodium, chloride, and organic acids), respectively. It is possible that unmeasured organic aerosol components could alter aerosol hygroscopicity and, depending on the nature of these compounds, this could lead to over- or underestimates in ALW concentrations. The dominance of the inorganic aerosol components in driving ALW at SPC, however, is supported by the agreement between AIOMFAC and E-AIM, as organic acids were only taken into account in the AIOMFAC calculations. Both models have been evaluated against experimental data and have shown good agreement for the major contributors to particle mass considered here across a broad range of solute concentrations.39,49 Atmospheric Implications of ALW Associated with Anthropogenic Nitrate. Local Impacts. By increasing the dry aerosol mass and ALW mass concentrations (which further enhances the scattering efficiency of the aerosol), this locally formed nitrate influences the radiation balance and aerosol

optical properties in the Po Valley and regions influenced by outflow from the valley. Previous Po Valley sampling campaigns have found strong correlations between aerosol scattering coefficients, optical depths, and ammonium nitrate concentrations.36,45,50 A substantial fraction (∼40%) of the optical depth enhancement observed under nitrate-rich conditions was attributed to increased ALW.50 As we discuss below, however, the impacts of ALW associated with anthropogenic ammonium nitrate in the Po Valley extend beyond these direct radiative impacts. Herein, we demonstrate the impact of ALW at SPC on the gas-phase concentrations of a reactive, water-soluble compound, glyoxal. We argue that anthropogenic ALW contributes to SOA formation and perturbs gas-phase photochemistry by serving as a sink and reactive medium for a variety of organics. Glyoxal has been studied in both the laboratory and the field as a model compound for atmospherically relevant small, WSOCg that can contribute substantially to SOA through aqueousphase reactions.10,13−15,51−64 Figure 3 explores the relationship between gas-phase glyoxal concentrations measured at SPC and calculated ALW mass

Figure 3. Gas-phase glyoxal and calculated aerosol liquid water (ALW). The relationship between gas-phase glyoxal and ALW is best described by an exponential decrease in glyoxal concentrations with increasing ALW (solid lines). This relationship is modified by surface area, with a higher correlation between decreases in glyoxal concentrations and increased ALW observed at higher quartiles of aerosol surface area.

concentrations. We observed an exponential decrease in gasphase glyoxal with increasing ALW. Several studies have demonstrated that the partitioning of glyoxal to ALW is several orders of magnitude greater than for pure water or for the more dilute conditions characteristic of cloud droplets,61,65 which can be attributed to a “salting-in” effect66,67 and/or the formation of high molecular weight products (e.g., oligomers).10,68 Our results are also consistent with studies that have found an RH or ALW dependence of glyoxal uptake and SOA formation.11,52,69,70 It is important to note that the highest ALW concentrations corresponded to time periods at which temperatures and photochemistry, both of which influence gas-phase glyoxal concentrations, were at a minimum. It is possible that the observed relationship ALW-glyoxal actually reflects the relationship between glyoxal and temperature or photochemical 11130



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Figure 4. Gas-phase glyoxal concentrations as a function of solar zenith angle, used here as a surrogate for temperature and photochemical activity, under low (squares) and high (circles) aerosol liquid water (ALW) conditions. For SZA > ∼50°, at a given solar zenith angle, gas-phase concentrations of glyoxal were lower under high ALW conditions (third tertile of ALW) as compared to low ALW conditions (first tertile of ALW).

formation in wet aerosols because water provides the reaction medium and surface area affects the kinetics of precursor uptake.10,53,59,66,68 Kampf et al.,67 for example, observed kinetic limitations to hydration and oligomerization reactions of glyoxal at the high inorganic salt concentrations characteristic of aerosols as compared to cloud droplets. In addition, in the absence of condensed-phase production of OH radicals, aqueous-phase SOA formation is predicted to be limited by OH uptake from the gas phase and therefore exhibit a surface area dependence. Condensed phase OH production or recycling, on the other hand, could remove the surface area dependence and result in a dependence on ALW alone.63 A stagnation event at the beginning of the sampling campaign (June 18−22), during which WSOCp concentrations were approximately double that for the remainder of the campaign (4.00 versus 2.01 μg C/m 3 ), provided the opportunity to explore whether the elevated WSOC p (frequently used as a surrogate for SOA) could be explained, in part, by the partitioning of WSOCg to ALW and subsequent aqueous-phase reactions. As described by Carlton and Turpin,21 the potential for a given WSOCg species to partition to ALW (PWSOCP) can be calculated as follows, assuming instantaneous thermodynamic equilibrium

activity. To explore this further, we evaluated the relationship between gas-phase glyoxal concentrations and ALW at a fixed solar zenith angle (SZA), which we calculated with the National Oceanic and Atmospheric Administration Earth Systems Research Laboratory Solar Position Calculator (http://www. esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html). In other words, under similar photochemistry and temperature conditions, are glyoxal concentrations lower under higher ALW conditions? For SZA > ∼50°, at a given SZA (used here as a surrogate for both temperature and photochemical activity; Figure S1), gas-phase concentrations of glyoxal were lower under high ALW conditions (third tertile of ALW, > 4.07 μg/ m3) as compared to low ALW conditions (first tertile of ALW, < 1.75 μg/m3; Figure 4). The observed SZA threshold is likely due to the small number of high ALW data points at low SZA, when temperatures are higher and RH is lower. Volkamer et al.19 found that discrepancies between predicted and observed gas-phase glyoxal in Mexico City could be reconciled by accounting for partitioning to aerosols (i.e., ALW, aerosol surfaces, or organic matter [OM]). Because multiple factors in addition to SOA formation influence OM concentrations (e.g., primary sources, regional transport), we cannot expect SOA formation to result in correlations between gas-phase glyoxal and OM. However, it is possible that if decreases in glyoxal occurred merely because of changes in gasparticle partitioning (i.e., partitioning to OM), an inverse correlation between gas-phase glyoxal and OM concentrations would be observed. We found no correlations between gasphase glyoxal at SPC and particulate OM (R2 = 0.004). While there were no direct correlations between decreases in gasphase glyoxal and aerosol surface area (R2 = 0.02), interestingly, the ALW-glyoxal relationship was modified by surface area (Figure 3). We found moderate correlations between ALW and decreases in gas-phase glyoxal for the lower three quartiles of aerosol surface area (R2 = 0.27, 0.29, 0.36 for quartiles 1, 2, and 3 of surface area, respectively), but this correlation increased substantially for the upper quartile of surface area (R2 = 0.53; Figure 3). This is consistent with previous work that has shown that both bulk and surface processes can be important determinants of the reactive uptake of glyoxal and SOA

PWSOCp = [C J][C H2O] p

HJ HMGLY

ϕJ

(1)

where CJ is the concentration of WSOCg species J, CH2O is the ALW concentration, HJ is the Henry’s law constant for species J, HMGLY is the Henry’s law constant of methylglyoxal (3.2 × 1004 mol−1 atm−1), and ϕJ is the SOA-forming potential of species J. Hourly values of PWSOCP were calculated for glyoxal and correlations between ALW and WSOCp and between PWSOCP and WSOCp were evaluated. The solubility of compound J (in thise case glyoxal, HJ = 3.6 × 1005 mol−1 atm−1) is considered in terms of its ratio to the solubility of methyglyoxal because methylglyoxal is a reactive, water-soluble compound that, like glyoxal, has been identified as a potential aqueous SOA precursor. Values of ϕJ vary with multiple factors including aqueous-phase concentrations of oxidants and other 11131



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day emissions scenarios to between −0.95 and −1.28 W/m2 by the year 2100. Projected changes account only for the direct radiative effects of ammonium nitrate aerosol and the associated ALW. If our results for the Po Valley are robust, however, it suggests that SOA formed as a result of the reactive uptake of WSOCg in ALW could also increase under future emissions scenarios, altering the concentrations and properties of SOA. This has implications for climate forcing, air quality, and health and highlights the importance of incorporating aqueous SOA formation into chemical transport and climate models. While not studied here, global changes in ammonium nitrate concentrations also have the potential to alter cloud microphysics and optical properties by modifying the efficiency with which particles are activated to form cloud droplets.75 Indirect radiative effects might be of particular importance for ammonium nitrate as compared to nonvolatile hygroscopic aerosol components because the atmospheric processes that result in supersaturations with respect to water vapor (i.e., temperature changes in an ascending cloud parcel) are also likely to enhance gas-to-particle conversion of gaseous ammonium nitrate precursors. Note that changes in cloud properties could in turn influence the uptake of reactive gases into cloud droplets and the formation of SOA through cloud processing. In the following paragraphs, we consider the potential impacts of enhanced ALW associated with ammonium nitrate on reactive organic gas budgets and SOA concentrations. A rapidly expanding body of research is focused on elucidating the precursors, products, and mechanisms of SOA formation in wet aerosols with the aim of incorporating these processes into climate and chemical transport models. Such studies have demonstrated that water-soluble precursor gases (e.g., aldehydes, organic peroxides, epoxides, monoacids, alcohols, amides) are converted into oligomers, organosulfates, and nitrogen-containing organics through aqueous ammoniumcatalyzed, acid catalyzed, and radical-initiated reactions.7,56,76−82 Changes in aerosol hygroscopicity and the associated increases in ALW driven by predicted changes in sulfate and nitrate concentrations and mass fractions could result in substantial increases in SOA formed through these mechanisms. Previous studies provide evidence of the potential magnitude of this effect. Youn et al.,20 for example, observed a 52% increase in WSOCp concentrations and a 65% increase in the contribution of WSOCp to total particulate organic carbon loadings during the Sonoran desert monsoon season, which can be characterized by increases in both atmospheric moisture and aerosol hygroscopicity. Similarly, Hennigan et al.8 observed increases in WSOCp mass concentrations of between 10 and 25% under ambient conditions that favored high ALW concentrations. Both studies partially attributed these findings to increased partitioning of WSOCg to ALW and, possibly, subsequent aqueous-phase chemistry. Increased SOA burdens have direct implications for visibility and health and will have uncertain impacts on the radiative properties of atmospheric aerosol. In general, organic aerosol is assumed to have a negative radiative forcing;5 however, there is evidence that the aqueous processing of SOA precursors in the presence of nitrogen can produce light absorbing compounds.10,61,83,84 Further, the physiochemical properties of SOA formed through aqueous chemistry are different from those for SOA generated through gas-phase mechanisms. For example, the higher O:C ratios of aqueous SOA contributes to greater hygroscopicity,45,79,85−87

reactive species, ambient conditions, and aqueous-phase yields of low-volatility products. Carlton and Turpin21 evaluated the sensitivity of partitioning potential to uncertainty in this parameter by varying ϕJ between 0.1 and 1.0. In the present analysis, we calculated the partitioning potential assuming ϕJ = 1.0, while recognizing that ϕJ is uncertain. The time series of measured WSOCp concentrations, calculated ALW concentrations, and PWSOC are shown in Figure S2 (Supporting Information), with the inset highlighting the case period. This analysis provides support for the contribution of local aqueous SOA production to elevated WSOCp concentrations during the case period. Concentrations of ALW explained 49% of the variability in WSOCp concentrations (R2 = 0.49; Figure S3a, Supporting Information). When the temporal colocation of ALW and gas-phase glyoxal was taken into account through the use of the partitioning potential, correlations increased to R2 = 0.57 (Figure S3b, Supporting Information). Such correlations are expected only during the period of stagnant conditions, while regional sources are likely to be the dominant drivers of variability in particulate OM during other periods. While glyoxal concentrations at SPC were relatively low and would be expected to contribute only a small amount of SOA mass, glyoxal serves as a surrogate for other WSOCg. Notably, in a previous sampling campaign, Crosier et al.36 observed differences in organic aerosol composition in air masses that were and were not influenced by ammonium nitrate formed in the Po Valley. More specifically, the authors observed evidence of increases in carbonyl functional groups in the organic aerosol of nitrate-enriched air masses, consistent with expectations if SOA were to form through aqueous chemistry in ALW. Our results, in combination with the results of Crosier et al.,36 suggest that ALW associated with anthropogenic aerosol components can influence SOA production and properties in the Po Valley. This work presents field evidence for the uptake of reactive organic gases into ALW that is driven by anthropogenic nitrate. It demonstrates that nitrate can influence the lifetime and concentrations of water-soluble, reactive organic gases and provide a medium for aqueous SOA formation. As we discuss below, in addition to influencing SOA production, losses of reactive organics into anthropogenic water could also impact gas-phase photochemistry. Broader Atmospheric Implications. The global burden of nitrate aerosol is expected to increase over the century as a result of increases in fossil fuel burning, agricultural activities, and biomass burning.5 In addition, nitrate is expected to increase in importance relative to sulfate as a result of widespread policies aimed at reducing sulfur emissions.3,4,26 This reduction in sulfur further contributes to increased particulate nitrate loadings due to reduced competition for ammonium.4,5,26,71,72 Because nitrate is more hygroscopic than sulfate (κ = 0.67 and 0.61 for nitrate and sulfate, respectively),30,73 at a given relative humidity and aerosol loading, ALW concentrations will be larger for aerosol enriched in nitrate relative to sulfate. For example, Liao and Seinfeld3 estimate that as much as 92% of projected increases in ALW in Eastern China by 2100 can be attributed to increases in anthropogenic ammonium nitrate aerosol. Several studies have explored the global-scale radiative impacts of ammonium nitrate and associated ALW under present and future climate and emission scenarios.e.g.,1,4,26,74 These studies have predicted increases in the top-of-theatmosphere radiative forcing of anthropogenic ammonium nitrate from between −0.04 and −0.19 W/m2 under present11132



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There is geographic variability in the anthropogenic aerosol components responsible for controllable ALW. This work presents field evidence of a mechanism by which ALW driven by anthropogenic nitrate can influence the lifetimes and concentrations of WSOCg, the potential for SOA formation, and gas-phase photochemistry. ALW driven by anthropogenic nitrate is expected to increase in importance under future climate and emissions scenarios due to increased emissions of ammonium nitrate precursors and policies aimed at controlling sulfur. Such changes in emissions, emission controls, and aerosol composition are not geographically uniform. As a result, it is important to explore geographic variability in the aerosol components responsible for controllable ALW, as well as the ways in which differential pollution control strategies influence this variability. In addition to the direct radiative impacts of nitrate aerosol and associated ALW, studies investigating increased nitrate under future climate and emissions scenarios should consider changes in SOA concentrations and properties and changes in gas-phase photochemistry resulting from enhanced partitioning of WSOCg to the condensed phase and subsequent aqueous-phase reactions in and on particles. The ongoing effort to understand the precursors, products, and mechanisms of SOA formation through aqueous chemistry is key to the incorporation of these processes into global climate models and will aid in elucidating the broader impacts of increases in ammonium nitrate and ALW.

possibly resulting in further increases in ALW mass concentrations and changes in the cloud condensation nuclei properties of SOA. In addition to providing a medium for aqueous SOA formation, controllable ALW can serve as a sink for reactive organic gases that would otherwise engage in gas-phase photochemistry. For example, because the gas-phase photolysis of aldehydes is a substantial source of HO2, scavenging of aldehydes by ALW can reduce HO2 concentrations by as much as 91%.88 Faster reaction rates of aldehydes with OH in the aqueous phase compared to the gas phase can also increase the efficiency with which OH is converted to HOx.88 Changes in the HOx cycle due to increased partitioning of aldehydes to aerosols illustrate a possible mechanism by which increases in controllable ALW could alter the oxidizing capacity of the atmosphere. Reductions in gas-phase oxidant and reactive WSOCg concentrations could slow gas-phase photochemistry, with implications for SOA formed through gas-phase chemistry and vapor pressure based partitioning, ground-level ozone formation, and other photochemical processes. It is important to note that the changes in nitrate and sulfate precursor emissions discussed above are not geographically uniform.4 For example, increases in both NOx and ammonia emissions are expected for Asia,3−5 whereas emissions controls are successfully leading to decreases in NOx in parts of North America and Europe.4,89 Thus, we expect that heterogeneity in emissions and air quality management strategies will lead to geographic variability in the drivers of ALW and the fate of WSOCg in the atmosphere. For example, in regions in which ammonium nitrate formation is limited by nitric acid availability, it is possible that nitrate concentrations will decrease with decreased NOx emissions. It is also important to note that there is variability in projected emissions and concentrations of ammonium nitrate precursors, depending on the scenario and time frame considered, due to variability and uncertainty in emissions changes, socioeconomic factors, land use, and other factors that influence NOx and ammonia emissions. Another source of variability will be climate change itself, through the modifications of the global water cycle and, on local and regional scales, of temperature and humidity conditions. The Mediterranean area, for instance, is expected to experience a progressive decline of summer precipitation,90 which will have a negative feedback on ALW over land, and therefore on the potential for aqueous-phase SOA production. The frequency and duration of air stagnation and extreme pollution events might also be altered in a changing climate. A modeling study investigating the impact of climate change on extreme pollution events observed increases in the intensity of pollution episodes in regions influenced by anthropogenic sources of ammonium nitrate precursors.91 Further, it is important to note that multiple geographically and temporally varying factors including meteorological conditions, precursor concentrations and composition, oxidant concentrations, and particulate matter concentrations and composition influence aqueous SOA yields. Because WSOCg are ubiquitous, but ALW is not,21 we can speculate that the above-discussed impacts will be greatest in regions for which projected increases in ALW are large. Liao and Seinfeld3 predict increases in total atmospheric column burdens of ALW by between 20 and 90% for the Eastern United States, Europe, and Eastern China. As noted above, such projected increases in ALW can largely be attributed to increased aerosol nitrate burdens.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (626) 395-3195. Present Address ⊗

Division of Chemical Engineering, California Institute of Technology, Pasadena, CA 91125. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This material is based on work supported by the National Science Foundation under Grant Nos. AGS-1052611 (Rutgers University), AGS-1050052 (Colorado State University), and AGS-1051338 (University of WisconsinMadison). This work was made possible, in part, by the European Community’s Seventh Framework Programme (FP7) on the PEGASOS project (Grant Agreement 265148) and the ARPA Supersite Project. N.H. was supported by EPA STAR Fellowship Assistance Agreement no. FP 917336 and the Mid-Atlantic States Section of the Air and Waste Management Association (MASS-A&WMA) Air Pollution Education and Research Grant (APERG) Program. A.M.C. was supported by the USEPA (Grant No. 835041) and National Science Foundation (Grant No. AGS-1242155). The findings and conclusions presented here are expressed by the authors and do not necessarily reflect the views of the EPA. 11133



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Aerosol Liquid Water Driven by Anthropogenic Nitrate - American

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