Identification of organic acids in secondary organic aerosol and the corresponding gas phase from chamber experiments

Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters - pH- and Temperature-Dependent Kinetic Studies

The atmospheric α-pinene oxidation leads to three carboxylic acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA). In this study, the OH radical kinetics in the aqueous phase of these carboxylic acids were investigated at different temperatures and pH values of solutions. Activation parameters and the corresponding atmospheric lifetimes of the acids in the troposphere were derived. The overall second-order rate constants for the individual speciation forms of the acids (AH and A- for NPA; AH2, AH- and A2- for PA; and AH3, AH2-, AH2- and A3- for MBTCA) were determined. At 298 K, the rate constants for reactions of protonated forms (AHx) of NPA, PA, and MBTCA with •OH, were (1.5 ± 0.2) × 109 L mol-1 s-1, (2.4 ± 0.1) × 109 L mol-1 s-1, and (4.1 ± 0.6) × 108 L mol-1 s-1, respectively. For the fully deprotonated forms (Ax-) of studied acids, the second-order rate constants were (2.2 ± 0.2) × 109 L mol-1 s-1, (2.8 ± 0.1) × 109 L mol-1 s-1, and (10.2 ± 0.7) × 108 L mol-1 s-1 at 298 K, respectively. It was found that the reactions of NPA and PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction rate depends on pH more strongly at elevated temperatures (>298 K). The atmospheric lifetimes of the acids considered due to their reactivity with •OH were calculated for different model scenarios at a temperature of 283 K and pH = 2 in the aqueous phase. For this purpose, liquid water content (LWC) was used for aerosols and clouds under storm conditions and at various aqueous-phase concentrations of OH radicals. The lifetimes decreased with increasing LWC (from 10-12 m3 m-3 in aerosol to 10-5 m3 m-3 in storms), indicating that the acids undergo significant aqueous processing under realistic atmospheric conditions. Besides, the aerosol systems appeared less effective in removing PA and NPA, with lifetimes ranging from hundreds of days to tens and hundreds of hours, respectively. Clouds were more effective, with lifetimes ranging from tens of hours to a single second or less. MBTCA, which dissolves better in water, was effectively removed in all systems, with the longest lifetime of approximately 90 min.

Real-Time Identification of Aerosol-Phase Carboxylic Acid Production Using Extractive Electrospray Ionization Mass Spectrometry

Comprehensive identification of aerosol sources and their constituent organic compounds requires aerosol-phase molecular-level characterization with a high time resolution. While real-time chemical characterization of aerosols is becoming increasingly common, information about functionalization and structure is typically obtained from offline methods. This study presents a method for determining the presence of carboxylic acid functional groups in real time using extractive electrospray ionization mass spectrometry based on measurements of [M - H + 2Na]+ adducts. The method is validated and characterized using standard compounds. A proof-of-concept application to α-pinene secondary organic aerosol (SOA) shows the ability to identify carboxylic acids even in complex mixtures. The real-time capability of the method allows for the observation of the production of carboxylic acids, likely formed in the particle phase on short time scales (<120 min). Our research explains previous findings of carboxylic acids being a significant component of SOA and a quick decrease in peroxide functionalization following SOA formation. We show that the formation of these acids is commensurate with the increase of dimers in the particle phase. Our results imply that SOA is in constant evolution through condensed-phase processes, which lower the volatility of the aerosol components and increase the available condensed mass for SOA growth and, therefore, aerosol mass loading in the atmosphere. Further work could aim to quantify the effect of particle-phase acid formation on the aerosol volatility distributions.

Dissociation constants of relevant secondary organic aerosol components in the atmosphere

The presented studies focus on measuring the determination of the acidity constant (pKa) of relevant secondary organic aerosol components. For our research, we selected important oxidation products (mainly carboxylic acids) of the most abundant terpene compounds, such as α-pinene, β-pinene, β-caryophyllene, and δ-3-carene. The research covered the synthesis and determination of the acidity constant of selected compounds. We used three methods to measure the acidity constant, i.e., 1H NMR titration, pH-metric titration, Bates-Schwarzenbach spectrophotometric method. Moreover, the pKa values were calculated with Marvin 21.17.0 software to compare the experimentally derived values with those calculated from the chemical structure. pKa values measured with 1H NMR titration ranged from 3.51 ± 0.01 for terebic acid to 5.18 ± 0.06 for β-norcaryophyllonic acid. Moreover, the data determined by the 1H NMR method revealed a good correlation with the data obtained with the commonly used potentiometric and UV-spectroscopic methods (R2 = 0.92). In contrast, the comparison with in silico results exhibits a relatively low correlation (R2Marvin = 0.66). We found that most of the values calculated with the Marvin Program are lower than experimental values obtained with pH-metric titration with an average difference of 0.44 pKa units. For di- and tricarboxylic acids, we obtained two and three pKa values, respectively. A good correlation with the literature values was observed, for example, Howell and Fisher (1958) used pH-metric titration and measured pKa1 and pKa2 to be 4.48 and 5.48, while our results are 4.24 ± 0.10 and 5.40 ± 0.02, respectively.

Stepwise hydrations of anhydride tuned by hydrogen bonds: rotational study on maleic anhydride-(H2O)1-3

The rotational spectra of maleic anhydride-(H₂O)_1-3 have been investigated for the first time by using pulsed jet Fourier transform microwave spectroscopy with complementary computational analyses. The experimental evidence points out that water tends to self-aggregate with hydrogen bonds and form homodromic cycles. Differences in bond lengths and charge distribution between the two carbonyl sites have been observed upon stepwise hydrations, which might further introduce a selectivity on the nucleophilic attack sites of hydrolysis. This study provides an important insight into the incipient solvation process (microsolvation) of maleic anhydride in water by understanding the cooperation and rearrangement of intermolecular hydrogen bonds in its stepwise hydrates.

Computational chemistry of cluster: Understanding the mechanism of atmospheric new particle formation at the molecular level

New particle formation (NPF), which exerts significant influence over human health and global climate, has been a hot topic and rapidly expands field of research in the environmental and atmospheric chemistry recent years. Generally, NPF contains two processes: formation of critical nucleus and further growth of the nucleus. However, due to the complexity of the atmospheric nucleation, which is a multicomponent process, formation of critical clusters as well as their growth is still connected to large uncertainties. Detection limits of instruments in measuring specific gaseous aerosol precursors and chemical compositions at the molecular level call for computational studies. Computational chemistry could effectively compensate the deficiency of laboratory experiments as well as observations and predict the nucleation mechanisms. We review the present theoretical literatures that discuss nucleation mechanism of atmospheric clusters. Focus of this review is on different nucleation systems involving sulfur-containing species, nitrogen-containing species and iodine-containing species. We hope this review will provide a deep insight for the molecular interaction of nucleation precursors and reveal nucleation mechanism at the molecular level.

OH Group Orientation Leads to Organosulfate Formation at the Liquid Aerosol Surface

Organosulfates (OSs) are well-known and ubiquitous constituents of atmospheric aerosol particles and have been used as secondary organic aerosol markers in many field studies. Hence, it is imperative to understand the formation of OS species in the atmosphere. Recently, hydroxy acids (HAs) and hydroxy acid sulfates have been extensively detected in the atmospheric environment. However, the reaction mechanism of HAs to form OSs is much less understood. In this work, we have mainly investigated the reaction of typical α-HAs, including glycolic acid (GA) and lactic acid (LA), and SO₃ at the liquid aerosol surface using quantum chemistry calculations and Born-Oppenheimer molecular dynamics simulations. The OH group orientation of α-HAs at the air-water interface is found to exert a significant impact on the formation of OSs. The OH group pointing to the gas phase is obviously beneficial to the formation of OSs. Two key factors are discovered important to the reaction of α-HAs adsorbed on the liquid surface with SO₃: (a) the exposure position of the active site to the gas phase and (b) the reactivity of the exposed site to the attracted SO₃ molecule. Moreover, we found that the air-water interface exerts a significant influence on the physicochemical behaviors of GA and LA, especially on their OH group orientation, and thus leads to their different properties for the SO₃ colliding reaction. The presented reaction mechanism provides a new feasible pathway for the production of OSs at the liquid aerosol surface, which may have important impacts on the formation of organic aerosols.

Second-Order Kinetic Rate Coefficients for the Aqueous-Phase Sulfate Radical (SO4•-) Oxidation of Some Atmospherically Relevant Organic Compounds

The sulfate anion radical (SO₄^•–) is a reactive oxidant formed in the autoxidation chain of sulfur dioxide, among other sources. Recently, new formation pathways toward SO₄^•– and other reactive sulfur species have been reported. This work investigated the second-order rate coefficients for the aqueous SO₄^•– oxidation of the following important organic aerosol compounds (k_SO4): 2-methyltetrol, 2-methyl-1,2,3-trihydroxy-4-sulfate, 2-methyl-1,2-dihydroxy-3-sulfate, 1,2-dihydroxyisoprene, 2-methyl-2,3-dihydroxy-1,4-dinitrate, 2-methyl-1,2,4-trihydroxy-3-nitrate, 2-methylglyceric acid, 2-methylglycerate, lactic acid, lactate, pyruvic acid, pyruvate. The rate coefficients of the unknowns were determined against that of a reference in pure water in a temperature range of 298–322 K. The decays of each reagent were measured with nuclear magnetic resonance (NMR) and high-performance liquid chromatography–high-resolution mass spectrometry (HPLC-HRMS). Incorporating additional SO₄^•– reactions into models may aid in the understanding of organosulfate formation, radical propagation, and aerosol mass sinks.

Characterization of dicarboxylic acids, oxoacids, and α-dicarbonyls in PM2.5 within the urban boundary layer in southern China: Sources and formation pathways

Low-molecular-weight dicarboxylic acids, which are important components of secondary organic aerosols, have been extensively studied in recent years. Many studies have focused on ground-level observations and literature reports on the vertical distribution of the organic aerosols within the urban boundary layer are limited. In this study, the vertical profiles of dicarboxylic acids and related organic compounds (DCRCs) in PM_2.5 were investigated at altitudinal levels (ground level and 488 m above the ground level) at the Canton Tower in Guangzhou, southern China, to elucidate their primary sources and secondary formation processes. The concentrations of DCRCs at ground level were generally higher than those at 488 m. Oxalic acid (C₂) was the most abundant species, followed by succinic acid (C₄) and malonic acid (C₃) at both heights. The higher ratio of DCRCs-bound carbon to organic carbon (i.e., DCRCs-C/OC) at 488 m (4.8 ± 1.2%) relative to that at ground level (2.7 ± 0.5%) indicated a higher degree of aerosol aging at 488 m. The abundance of C₂ was increased and the conversion of C₄ to C₃ was enhanced due to the photochemical oxidation of its homologues during long-range transport periods. The increase in C₂ was associated with in-cloud processes during pollution periods. Principal component analysis showed that DCRCs were mainly derived from atmospheric secondary processing and biomass burning was also an important source of long-chain carboxylic acids during autumn in Guangzhou. Our results illustrate that secondary processing and biomass burning play prominent roles in controlling the abundance of DCRCs. Furthermore, DCRCs are affected by air masses from regional areas, oxidation of their precursors via vertical transport and in-cloud processes.

Determination of highly polar compounds in atmospheric aerosol particles at ultra-trace levels using ion chromatography Orbitrap mass spectrometry

A method using ion chromatography coupled to high-resolution Orbitrap mass spectrometry was developed to quantify highly-polar organic compounds in aqueous filter extracts of atmospheric particles. In total, 43 compounds, including short-chain carboxylic acids, terpene-derived acids, organosulfates, and inorganic anions were separated within 33 min by a KOH gradient. Ionization by electrospray was maximized by adding 100 µL min^-1 isopropanol as post-column solvent and optimizing the ion source settings. Detection limits (S/N ≥ 3) were in the range of 0.075-25 μg L^-1 and better than previously reported for 22 compounds. Recoveries of extraction typically range from 85 to 117%. The developed method was applied to three ambient samples, including two arctic flight samples, and one sample from Melpitz, a continental backround research site. A total of 32 different compounds were identified for all samples. From the arctic flight samples, organic tracers could be quantified for the first time with concentrations ranging from 0.1 to 17.8 ng m^-3 . Due to the minimal sample preparation, the beneficial figures of merit, and the broad range of accessible compounds, including very polar ones, the new method offers advantages over existing ones and enables a detailed analysis of organic marker compounds in atmospheric aerosol particles.

The effects of humidity and ammonia on the chemical composition of secondary aerosols from toluene/NOx photo-oxidation

The secondary aerosol formation mechanism in the presence of ammonia (NH₃), is poorly understood, especially under high relative humidity (RH) conditions. In this study, a total of seven experiments were conducted from toluene/NOx photo-oxidation in the presence/absence of NH₃ under dry (~7% RH) and wet (>60% RH) conditions in a ~3 m³ smog chamber. A series of instruments including gas analysers, scanning mobility particle sizer (SMPS), aerosol mass spectrometry (HR-ToF-AMS) etc. were applied to measure the NOx and O₃ concentrations, the mass concentration and chemical composition of secondary aerosol. It was found that NH₃ could enhance the mass loading of secondary aerosol, especially under wet condition. However, the presence of NH₃ or increasing RH did not have a significant influence on SOA yield. The organic aerosol mass spectrum from AMS showed that the most abundant fragment was at m/z = 44, which was mainly from the fragmentation of carboxylic acids. Compared to the absence of NH₃, the fraction of fragment at m/z = 44 and O:C was higher in the presence of NH₃, regardless of dry or wet conditions. The highest O:C value of 0.71-0.75 was observed in the presence of NH₃ under wet condition, suggesting there could be a synergetic effect between the high RH and the presence of NH₃, which jointly contributed to the photochemical aging process of SOA. The N:C increased in the presence of NH₃ under both dry and wet conditions, which might be attributed to the carboxylates and organic nitrates formed from the reaction between NH₃ and carboxylic acids. The results implied that SOA modelling should consider the role of NH₃ and water vapour, which might fill the gap of O:C between laboratory studies and field measurements.

Evidence for lactone formation during infrared multiple photon dissociation spectroscopy of bromoalkanoate doped salt clusters

Reaction mechanisms of organic molecules in a salt environment are of fundamental interest and are potentially relevant for atmospheric chemistry, in particular sea-salt aerosols. Here, we found evidence for lactone formation upon infrared multiple photon dissociation (IRMPD) of non-covalent bromoalkanoate complexes as well as bromoalkanoate embedded in sodium iodide clusters. The mechanism of lactone formation from bromoalkanoates of different chain lengths is studied in the gas phase with and without salt environment by a combination of IRMPD and quantum chemical calculations. IRMPD spectra are recorded in the 833-3846 cmT¹ range by irradiating the clusters with tunable laser systems while they are stored in the cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The measurements of the binary complex Br(CH2)_mCOOH·Br(CH2)_mCOO^- for m = 4 indicate valerolactone formation without salt environment while lactone formation is hindered for longer chain lengths. When embedded in sodium iodide clusters, butyrolactone formation from 4-bromobutyrate seems to take place already during formation of the doped clusters in the electrospray process, evidenced by the infrared (IR) signature of the lactone. In contrast, IRMPD spectra of sodium iodide clusters containing 5-bromovalerate contain signatures for both valerate as well as valerolactone. In both cases, however, a neutral fragment corresponding to the mass of valerolactone is eliminated, indicating that ring formation can be activated by IR light in the salt cluster. Quantum chemical calculations show that already complexation with one sodium ion significantly increases the barrier for lactone formation for all chain lengths. IRMPD of sodium iodide clusters doped with neutral bromoalkanoic acid molecules proceeds by elimination of HI or desorption of the intact acid molecule from the cluster.

Study of polar organic compounds in airborne particulate matter of a coastal urban city

Two classes of polar organic compounds, dicarboxylic acids (DCAs) and sugars/sugar anhydrides (S/SAs), were measured in airborne particulate matter in the area of Thessaloniki, northern Greece. The target compounds were measured simultaneously in two particle fractions PM₁₀ and PM_2.5 during cold and warm periods by employing extraction in an ultrasonic bath with a mixture of MeOH/DCM (1:2 v/v), derivatization with BSTFA-TMCS and GC-MS for analysis. At both fractions, phthalic was the predominant carboxylic acid during cold season and a-ketoglutaric acid in warm season, followed by maleic and malic. Levoglucosan was the dominant sugar anhydride during the cold and arabitol during the warm season. In total, the distribution of DCAs seemed to favor the PM_2.5 particle fraction, probably due to anthropogenic emissions and photochemical formation. The relative contribution of DCAs to PM_2.5 fraction was 0.9-3.2% in cold and 0.9-7.0% in warm period. Regarding S/SAs, levoglucosan was also predominantly distributed in fine particles, with relative contribution to this fraction 0.1-6.3% in cold and <0.65% in warm season, suggesting impact of biomass burning emissions. In contrast, arabitol, fructose, and glucose were mainly found in coarse fraction, possibly due to their biogenic origin. Negative correlation of target compounds with temperature and total solar radiation suggested the contribution of seasonal dependant local sources. Positive relationship with NO and NO₂ oxidants and relative humidity showed secondary formation of polar compounds or enhanced gas-to-particle conversion.

The relative importance of tailpipe and non-tailpipe emissions on the oxidative potential of ambient particles in Los Angeles, CA

This study examines the associations between the oxidative potential of ambient PM2.5 and PM0.18, measured by means of the dithiothreitol (DTT) assay, and their chemical constituents and modeled sources. Particulate matter (PM) samples were collected from 2012-2013 in Central Los Angeles (LA) and 2013-2014 in Anaheim, California, USA. Detailed chemical analyses of the PM samples, including carbonaceous species, inorganic elements and water-soluble ions, were conducted. Univariate analysis indicated a high correlation (R > 0.60) between the DTT activity and the concentrations of carbonaceous species at both sites. The strongest correlations were observed between DTT and organic tracers of primary vehicle tailpipe emissions including polycyclic aromatic hydrocarbons (PAHs) and hopanes as well as EC, with higher correlations for PM0.18versus PM2.5 components. Moreover, metals and trace elements (e.g., Ba, Cu, Fe, Mn, Pb and Sb) in both size ranges were also associated with DTT activity. Multiple linear regression (MLR) analysis was performed on DTT activity and PM sources identified by a Molecular Marker-Chemical Mass Balance (MM-CMB) model (i.e. major carbonaceous sources: vehicle tailpipe emissions, wood smoke, primary biogenic and secondary organic carbon) together with other typical sources of ambient PM (i.e. crustal material, vehicular abrasion, secondary ions and sea salt). Overall, our findings illustrate the relative importance of different traffic sources on the oxidative potential of ambient PM. Despite major reductions of tailpipe emissions, the lack of similar reductions (and possibly an increase) in non-tailpipe emissions makes them an important source of traffic-related PM in Los Angeles and their increasing role in the overall PM toxicity raises concerns for public health.

Quantum Chemical Calculations Resolved Identification of Methylnitrocatechols in Atmospheric Aerosols

Methylnitrocatechols (MNCs) are secondary organic aerosol (SOA) tracers and major contributors to atmospheric brown carbon; however, their formation and aging processes in atmospheric waters are unknown. To investigate the importance of aqueous-phase electrophilic substitution of 3-methylcatechol with nitronium ion (NO2(+)), we performed quantum calculations of their favorable pathways. The calculations predicted the formation of 3-methyl-5-nitrocatechol (3M5NC), 3-methyl-4-nitrocatechol (3M4NC), and a negligible amount of 3-methyl-6-nitrocatechol (3M6NC). MNCs in atmospheric PM2 samples were further inspected by LC/(-)ESI-MS/MS using commercial as well as de novo synthesized authentic standards. We detected 3M5NC and, for the first time, 3M4NC. In contrast to previous reports, 3M6NC was not observed. Agreement between calculated and observed 3M5NC/3M4NC ratios cannot unambiguously confirm the electrophilic mechanism as the exclusive formation pathway of MNCs in aerosol water. However, the examined nitration by NO2(+) is supported by (1) the absence of 3M6NC in the ambient aerosols analyzed and (2) the constant 3M5NC/3M4NC ratio in field aerosol samples, which indicates their common formation pathway. The magnitude of error one could make by incorrectly identifying 3M4NC as 3M6NC in ambient aerosols was also assessed, suggesting the importance of evaluating the literature regarding MNCs with special care.

High abundances of dicarboxylic acids, oxocarboxylic acids, and α-dicarbonyls in fine aerosols (PM2.5) in Chengdu, China during wintertime haze pollution

Daytime and nighttime fine aerosol (PM2.5) samples were collected during a haze episode in January 2013 within the urban area of Chengdu, southwest China. Aerosol samples were analyzed for low-molecular-weight homologous dicarboxylic acids, oxocarboxylic acids and α-dicarbonyls, as well as organic carbon and elemental carbon. Concentration ranges of diacids, oxoacids, and α-dicarbonyls were 1,400-5,250, 272-1,380, and 88-220 ng m(-3), respectively. Molecular distributions of diacids (mean 3,388 ± 943 ng m(-3)) were characterized by a predominance of oxalic acid (C2; 1,373 ± 427 ng m(-3)), followed by succinic (C4), terephthalic (tPh), and phthalic (Ph) acids. Such high levels of tPh and Ph were different from those in other Asian cities where malonic acid (C3) is the second or third highest species, mostly owing to significant emissions from coal combustion and uncontrolled waste incineration. High contents of diacids, oxoacids, and α-dicarbonyls were detected on hazy days, suggesting an enhanced emission and/or formation of these organics during such a weather condition. Concentrations of unsaturated aliphatic diacids (e.g., maleic acid) and phthalic acids were higher in nighttime than in daytime. Good positive correlations of C2 with C3, C4, ketomalonic (kC3), pyruvic (Pyr), and glyoxylic (ɷC2) acids in daytime suggest secondary production of C2 via the photooxidation of longer chain diacids and ɷC2. This study demonstrated that both primary emissions and secondary production are important sources of dicarboxylic acids and related compounds in atmospheric aerosols in the Sichuan Basin.

Method for characterization of low molecular weight organic acids in atmospheric aerosols using ion chromatography mass spectrometry

The structural composition of PM2.5 monitored in the atmosphere is usually divided by the analysis of organic carbon, black (also called elemental) carbon, and inorganic salts. The characterization of the chemical composition of aerosols represents a significant challenge to analysts, and studies are frequently limited to determination of aerosol bulk properties. To better understand the potential health effects and combined interactions of components in aerosols, a variety of measurement techniques for individual analytes in PM2.5 need to be implemented. The method developed here for the measurement of organic acids achieves class separation of aliphatic monoacids, aliphatic diacids, aromatic acids, and polyacids. The selective ion monitoring capability of a triple quadropole mass analyzer was frequently capable of overcoming instances of incomplete separations. Standard Reference Material (SRM) 1649b Urban Dust was characterized; 34 organic acids were qualitatively identified, and 6 organic acids were quantified.

Limits of detection for the determination of mono- and dicarboxylic acids using gas and liquid chromatographic methods coupled with mass spectrometry

The chromatographic separation and instrumental limits of detection (LODs) were obtained for a broad range of C(1)-C(18) monocarboxylic (MCAs) and C(2)-C(14) dicarboxylic acids (DCAs) employing either chemical derivatization followed by gas chromatography-mass spectrometry and flame ionization detection (GC-MS/FID) or direct analysis with liquid chromatography high resolution MS and tandem MS (LC-MS). Suitability, efficiency and stability of reaction products for several derivatization agents used for esterification (BF(3)/butanol), and trimethysilylation, including trimethylsilyl-N-N-dimethylcarbamate (TMSDMC) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) were evaluated. The lowest limits of detection for the majority of compounds below 10 pg (with the exception of acetic acid) were obtained for derivatization with BF(3)/butanol followed by GC-MS in the total ion current (TIC) mode. Further improvements were achieved when applying either selected ion monitoring (SIM), which decreased the LODs to 1-4 pg or a combination of SIM and TIC (SITI) (2-5 pg). GC-FID provided LODs comparable to those obtained by GC-MS TIC. Both trimethylsilylation (followed by GC-MS) and direct LC-MS/MS analysis yielded LODs of 5-40 pg for most of the acids. For volatile acids the LODs were higher, e.g., 25 and 590 ng for TMSDMC and BSTFA derivatized formic acid, respectively, whereas the LC-MS methods did not allow for the analysis of formic acid at all.

Cellular responses after exposure of lung cell cultures to secondary organic aerosol particles

The scope of this work was to examine in vitro responses of lung cells to secondary organic aerosol (SOA) particles, under realistic ambient air and physiological conditions occurring when particles are inhaled by mammals, using a novel particle deposition chamber. The cell cultures included cell types that are representative for the inner surface of airways and alveoli and are the target cells for inhaled particles. The results demonstrate that an exposure to SOA at ambient-air concentrations of about 10(4) particles/cm(3) for 2 h leads to only moderate cellular responses. There is evidence for (i) cell type specific effects and for (ii) different effects of SOA originating from anthropogenic and biogenic precursors, i.e. 1,3,5-trimethylbenzene (TMB) and alpha-pinene, respectively. There was no indication for cytotoxic effects but for subtle changes in cellular functions that are essential for lung homeostasis. Decreased phagocytic activity was found in human macrophages exposed to SOA from alpha-pinene. Alveolar epithelial wound repair was affected by TMB-SOA exposure, mainly because of altered cell spreading and migration at the edge of the wound. In addition, cellular responses were found to correlate with particle number concentration, as interleukin-8 production was increased in pig explants exposed to TMB-SOA with high particle numbers.

Recent developments in the mass spectrometry of atmospheric aerosols

Atmospheric aerosol particles consist of a highly complex mixture of thousands of different compounds. Mass spectrometric techniques are well suited for the analysis of these particles, with each method of analysis having specific advantages and disadvantages. On-line techniques offer high time resolution and thus allow for the investigation of rapidly changing signals. They typically measure either single particles or the average non-refractory submicrometer aerosol. Off-line techniques are often coupled to chromatography or another technique separating for a specific property, which enhances their resolving power. Ultra-high resolution mass spectrometry allows for an unambiguous assignment of the elemental composition throughout the majority of the mass range typically measured in ambient aerosol samples, i.e. up to about m/z 400-600. The quantitative determination of individual compounds, or of classes of compounds, remains an important, but often unresolved, topic. Examples of applications of various mass spectrometric techniques are presented, both from laboratory and field studies.

Fundamental and overtone vibrational spectra of gas-phase pyruvic acid

Pyruvic acid (CH(3)COCOOH) is an important keto acid present in the atmosphere. In this study, the vibrational spectroscopy of gas-phase pyruvic acid has been investigated with special emphasis on the overtone transitions of the OH-stretch, with Delta v(OH) = 2, 4, 5. Assignments were made to fundamental and combination bands in the mid-IR. The two lowest energy rotational conformers of pyruvic acid are clearly observed in the spectrum. The lowest energy conformer possesses an intramolecular hydrogen bond, while the next lowest rotational conformer does not. This difference is clearly seen in the spectra of the OH vibrational overtone transitions, and it is reflected in the anharmonicities of the OH-stretching modes for each conformer. The spectra of the OH-stretching vibration for both conformers were investigated to establish the effect of the hydrogen bond on frequency, intensity, and line width.

Molecular size evolution of oligomers in organic aerosols collected in urban atmospheres and generated in a smog chamber

Only a minor fraction of the total organic aerosol mass can be resolved on a molecular level. High molecular weight compounds in organic aerosols have recently gained much attention because this class of compound potentially explains a major fraction of the unexplained organic aerosol mass. These compounds have been identified with different mass spectrometric methods, and compounds with molecular masses up to 1000 Da are found in secondary organic aerosols (SOA) generated from aromatic and terpene precursors in smog chamber experiments. Here, we apply matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to SOA particles from two biogenic precursors, alpha-pinene and isoprene. Similar oligomer patterns are found in these two SOA systems, but also in SOA from trimethylbenzene, an anthropogenic SOA precursor. However, different maxima molecular sizes were measured for these three SOA systems. While oligomers in alpha-pinene and isoprene have sizes mostly below 600-700 Da, they grow up to about 1000 Da in trimethylbenzene-SOA. The final molecular size of the oligomers is reached early during the particle aging process, whereas other particle properties related to aging, such as the overall acid concentration or the oligomer concentration, increase continuously over a much longer time scale. This kinetic behavior of the oligomer molecular size growth can be explained by a chain growth kinetic regime. Similar oligomer mass patterns were measured in aqueous extracts of ambient aerosol samples (measured with the same technique). Distinct differences between summer and winter were observed. In summer a few single mass peaks were measured with much higher intensity than in winter, pointing to a possible difference in the formation processes of these compounds in winter and summer.

Modeling secondary organic aerosol formation via multiphase partitioning with molecular data

A new model for atmospheric secondary organic aerosol (SOA) is presented for biogenic compounds. It is based to the extent possible on experimental molecular SOA data, and it is compatible with any existing gas-phase chemical kinetic mechanism. Six SOA precursors or groups of precursors are used to represent biogenic monoterpenes and sesquiterpenes. SOA formation is modeled using five SOA surrogates to represent classes of compounds with different partitioning properties, e.g., hydrophobicity, aqueous solubility, acid dissociation, and saturation vapor pressure. Model simulations are evaluated against smog chamber data for SOA yields and some adjustments are made to uncertain stoichiometric coefficients and saturation vapor pressure parameters to improve model performance. The model is applied undertypical atmospheric conditions to exemplify the effect of relative humidity on SOA formation and the relative contributions of hydrophilic and hydrophobic SOA.

Photoinduced Oligomerization of Aqueous Pyruvic Acid

The 320 nm-band photodecarboxylation of aqueous pyruvic acid (PA), a representative of the alpha-oxocarboxylic acids widely found in the atmospheric aerosol, yields 2,3-dimethyltartaric (A) and 2-(3-oxobutan-2-yloxy)-2-hydroxypropanoic (B) acids, rather than 3-hydroxy-2-oxobutanone as previously reported. A and B are identified by liquid chromatography with UV and ESI-MS detection, complemented by collisionally induced dissociation and 2H and 13C isotope labeling experiments. The multifunctional ether B gives rise to characteristic delta approximately 80 ppm 13C NMR resonances. Product quantum yields are proportional to [PA](a + [PA])(-1) in the range [PA] = 5-100 mM. CO2(g) release rates are halved, while A and B are suppressed by the addition of >1.5 mM TEMPO. A and B are only partially quenched in air-saturated solutions. These observations are shown to be consistent with an oligomerization process initiated by a bimolecular reaction between 3PA and PA producing ketyl, CH3C(OH)C(O)OH, and acetyl, CH3C(O)*, radicals, rather than by the unimolecular decomposition of 3PA into 1-hydroxyethylidene, 3HO(CH3)C: (+CO2), or [CH(3)C(O)* + *C(O)OH] pairs. A arises from the dimerization of ketyl radicals, while B ensues the facile decarboxylation of the C8beta-ketoacid formed by association of acetyl radicals with the ketyl radical adduct of PA. Since the radical precursors to A and B are scavenged by O2 with a low probability per encounter (k(sc) approximately 1 x 10(6) M(-1) s(-1)), PA is able to accrete into multifunctional polar species in aerated aqueous media under solar illumination.

Real-Time Measurement of Oligomeric Species in Secondary Organic Aerosol with the Aerosol Time-of-Flight Mass Spectrometer

Real-time detection of oligomers in secondary organic aerosols has been carried out with an aerosol time-of-flight mass spectrometer sampling particles generated in a smog chamber. The photooxidation products of 1,3,5-trimethylbenzene and NOx were studied over a range of initial 1,3,5-trimethylbenzene concentrations (137-1180 ppb), while keeping the 1,3,5-trimethylbenzene to NOx ratio nearly constant. The photooxidation products of a mixture of alpha-pinene (initial concentration 191 ppb), 1,3,5-trimethylbenzene (60 ppb), and NOx were also investigated. In both systems, ions were observed in the single-particle mass spectra up to 750 Da; the species observed differed in the two systems. These high-mass ions occur with characteristic spacing of 14 and 16 Da, indicative of oligomeric species. The results obtained agree well with off-line (matrix-assisted) laser desorption/ionization mass spectrometry results. The real-time capabilities of the aerosol time-of-flight mass spectrometer make it possible to investigate the temporal development of the oligomers with 5-min time resolution and also demonstrate that there are certain ions within the oligomer population that occur in nearly all of the particles and with relatively high signal intensity, suggesting that these ions have higher stability or that the species are formed preferentially.

Determination of stable carbon isotopes of organic acids and carbonaceous aerosols in the atmosphere

A wet oxidation method for the compound-specific determination of stable carbon isotopes (delta(13)C) of organic acids in the gas and aerosol phase, as well as of water-soluble organic carbon (WSOC), is presented. Sampling of the organic acids was done using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to an ion chromatography (IC) system. The method allows for compound-specific stable carbon isotope analysis by collecting different fractions of organic acids at the end of the IC system using a fraction collector. delta(13)C analyses of organic acids were conducted by oxidizing the organic acids with sodium persulfate at a temperature of 100 degrees C and determining the delta(13)C value of the resulting carbon dioxide (CO(2)) with an isotope ratio mass spectrometer. In addition, analysis of delta(13)C of the WSOC was performed for particulate carbon collected on aerosol filters. The WSOC was extracted from the filters using ultrapure water (MQ water), and the dissolved organic carbon was oxidized to CO(2) using the oxidation method. The wet oxidation method has an accuracy of 0.5 per thousand with a precision of +/-0.4 per thousand and provides a quantitative result for organic carbon with a detection limit of 150 ng of carbon.

Secondary organic aerosol formation by irradiation of 1,3,5-trimethylbenzene-NOx-H2O in a new reaction chamber for atmospheric chemistry and physics

A new environmental reaction smog chamber was built to simulate particle formation and growth similar to that expected in the atmosphere. The organic material is formed from nucleation of photooxidized organic compounds. The chamber is a 27 m3 fluorinated ethylene propylene (FEP) bag suspended in a temperature-controlled enclosure. Four xenon arc lamps (16 kW total) are used to irradiate primary gas components for experiments lasting up to 24 h. Experiments using irradiations of 1,3,5-trimethylbenzene-NOx-H2O at similar input concentrations without seed particles were used to determine particle number and volume concentration wall loss rates of 0.209+/-0.018 and 0.139+/-0.070 h(-1), respectively. The particle formation was compared with and without propene.

Secondary organic aerosols from anthropogenic and biogenic precursors

Secondary organic aerosol (SOA) formation from the photooxidation of an anthropogenic (1,3,5-trimethylbenzene) and a biogenic (alpha-pinene) precursor was investigated at the new PSI smog chamber. The chemistry of the gas phase was followed by proton transfer reaction mass spectrometry, while the aerosol chemistry was investigated with aerosol mass spectrometry, ion chromatography, laser desorption ionization mass spectrometry, and infrared spectroscopy, along with volatility and hygroscopicity studies. Evidence for oligomer formation for SOA from both precursors was given by an increasing abundance of compounds with a high molecular weight (up to 1000 Da) and by an increasing thermal stability with increasing aging time. The results were compared to data obtained from ambient aerosol samples, revealing a number of similar features.