Yields and molecular composition of gas phase and secondary organic aerosol from the photooxidation of the volatile consumer product benzyl alcohol: formation of highly oxygenated and hydroxy nitroaromatic compounds

Recently, volatile chemical products (VCPs) have been increasingly recognized as important precursors for secondary organic aerosol (SOA) and ozone in urban areas. However, their atmospheric chemistry, physical transformation, and their impact on climate, environment and human health remain poorly understood. Here, the yields and chemical composition at the molecular level of gas and particle phase products originating from the photooxidation of one of these VCPs, benzyl alcohol (BnOH), is reported. The SOA was generated in the presence of seed aerosol from nebulized ammonium sulfate solution in a 14.5 m3 smog chamber operated in flow mode. More than 50 organic compounds containing nitrogen and/or up to seven oxygen atoms were identified by mass spectrometry. While a detailed non-targeted analysis has been made, our primary focus has been to examine highly oxygenated and nitro-aromatic compounds. The major components include ring-opening products with high oxygen to carbon ratio (e.g., malic acid, tartaric acids, arabic acid, trihydroxy-oxo-pentanoic acids, and pentaric acid), and ring-retaining products (e.g., benzaldehyde, benzoic acid, catechol, 3-nitrobenzyl alcohol, 4-nitrocatechol, 2-hydroxy-5-nitrobenzyl alcohol, 2-nitrophloroglucinol, 3,4-dihydroxy-5-nitrobenzyl alcohol). The presence of some of these products in the gas and particle phases simultaneously provides evidence of their gas/particle partitioning. These oxygenated oxidation products made dominant contributions to the SOA particle composition in both low and high NOx systems. Yields, organic mass to organic carbon ratio, and proposed reaction schemes for selected compounds are provided. The aerosol yield was 5.2% for BnOH/H2O2 at SOA concentration of 52.9 µg m-3 and ranged between 1.7-8.1 % for BnOH/NOx at SOA concentration of 40.0-119.5 µg m-3.

Aging of α-Pinene Secondary Organic Aerosol by Hydroxyl Radicals in the Aqueous Phase: Kinetics and Products

The reaction of hydroxyl radicals (OH) with a water-soluble fraction of the α-pinene secondary organic aerosol (SOA) was investigated using liquid chromatography coupled with negative electrospray ionization mass spectrometry. The SOA was generated by the dark ozonolysis of α-pinene, extracted into the water, and subjected to chemical aging by the OH. Bimolecular reaction rate coefficients (k_OH) for the oxidation of terpenoic acids by the OH were measured using the relative rate method. The unaged SOA was dominated by the cyclobutyl-ring-retaining compounds, primarily cis-pinonic, cis-pinic, and hydroxy-pinonic acids. Aqueous oxidation by the OH resulted in the removal of early-stage products and dimers, including well-known oligomers with MW = 358 and 368 Da. Furthermore, a 2- to 5-fold increase in the concentration of cyclobutyl-ring-opening products was observed, including terpenylic and diaterpenylic acids and diaterpenylic acid acetate as well as some of the newly identified OH aging markers. At the same time, results obtained from the kinetic box model showed a high degree of SOA fragmentation following the reaction with the OH, which indicates that non-radical reactions occurring during the evaporation of water likely contribute to the high yields of terpenoic _aqSOAs reported previously. The estimated atmospheric lifetimes showed that in clouds, terpenoic acids react with the OH exclusively in the aqueous phase. Aqueous OH aging of the α-pinene SOA results in a 10% increase of the average O/C ratio and a 3-fold decrease in the average k_OH value, which is likely to affect the cloud condensation nuclei activity of the _aqSOA formed after the evaporation of water.

Peroxy Radical Processes and Product Formation in the OH Radical-Initiated Oxidation of α-Pinene for Near-Atmospheric Conditions

α-Pinene, C₁₀H₁₆, represents one of the most important biogenic emissions into the atmosphere. The formation of RO₂ radicals HO-C₁₀H₁₆O_x, x = 2-6, and their closed-shell products from the OH + α-pinene reaction has been measured for close to atmospheric reaction conditions in the presence of NO with concentrations of (1.7-490) × 10⁹ molecules cm^-3. Main closed-shell products are substances with the composition C₁₀H₁₆O₂ and C₁₀H₁₆O₄, most likely carbonyls, obtained with molar yields in the range 0.42-0.45 and 0.17-0.19, respectively, for NO concentrations >5 × 10¹⁰ molecules cm^-3. The corresponding total product yields amount to 0.75-0.81, indicating efficient product detection by the mass spectrometric method applied. All stated molar yields represent lower limit values affected with an uncertainty of [Formula: see text]. Kinetic and product analysis consistently revealed the suppression of the formation of highly oxygenated organic molecules (HOMs) by a factor of 2-2.2 for the highest NO concentration used. The findings of this study provide insights into the RO₂ radical processes of the OH + α-pinene reaction for atmospheric conditions and give an overview about the first-generation products.

Rapid production of highly oxidized molecules in isoprene aerosol via peroxy and alkoxy radical isomerization pathways in low and high NOx environments: Combined laboratory, computational and field studies

Recently, we identified seven novel hydroxy-carboxylic acids resulting from gas-phase reactions of isoprene in the presence of nitrogen oxides (NO_x), ozone (O₃), and/or hydroxyl radicals (OH). In the present study, we provide evidence that hydroxy-carboxylic acids, namely methyltartaric acids (MTA) are: (1) reliable isoprene tracers, (2) likely produced via rapid peroxy radical hydrogen atom (H) shift reactions (autoxidation mechanism) and analogous alkoxy radical H shifts in low and high NO_x environments respectively and (3) representative of aged ambient aerosol in the low NO_x regime. Firstly, MTA are reliable tracers of isoprene aerosol because they have been identified in numerous chamber experiments involving isoprene conducted under a wide range of conditions and are absent in the oxidation of mono- and sesquiterpenes. They are also present in numerous samples of ambient aerosol collected during the past 20 years at several locations in the U.S. and Europe. Furthermore, MTA concentrations measured during a year-long field study in Research Triangle Park (RTP), NC in 2003 show a seasonal trend consistent with isoprene emissions and photochemical activity. Secondly, an analysis of chemical ionization mass spectrometer (CIMS) data of several chamber experiments in low and high NO_x environments show that highly oxidized molecules (HOMs) derived from isoprene that lead to MTAs may be produced rapidly and considered as early generation isoprene oxidation products in the gas phase. Density functional theory calculations show that rapid intramolecular H shifts involving peroxy and alkoxy radicals possess low barriers for methyl-hydroxy-butenals (MHBs) that may represent precursors for MTA. From these results, a viable rapid H shift mechanism is proposed to occur that produces isoprene derived HOMs like MTA. Finally, an analysis of the mechanism shows that autoxidation-like pathways in low and high NO_x may produce HOMs in a few OH oxidation steps like commonly detected methyl tetrol (MT) isoprene tracers. The ratio of MTA/MT in isoprene aerosol is also shown to be significantly greater in field versus chamber samples indicating the importance of such pathways in the atmosphere even for smaller hydrocarbons like isoprene.

Organic Molecular Tracers in PM2.5 at Urban Sites during Spring and Summer in Japan: Impact of Secondary Organic Aerosols on Water-Soluble Organic Carbon

To understand the characteristics of secondary organic aerosols (SOAs) and estimate their impact on water-soluble organic carbon (WSOC) in urban areas in Japan, we measured 17 organic tracers using gas chromatography–mass spectrometry from particulate matter with an aerodynamic diameter smaller than 2.5 μm collected at five urban sites in Japan during spring and summer. Most anthropogenic, monoterpene-derived, and isoprene-derived SOA tracers showed meaningful correlations with potential ozone in both these seasons. These results indicate that oxidants play an important role in SOAs produced during both seasons in urban cities in Japan. WSOC was significantly affected by anthropogenic and monoterpene-derived SOAs during spring and three SOA groups during summer at most of the sites sampled. The total estimated secondary organic carbons (SOCs), including mono-aromatic, di-aromatic, monoterpene-derived, and isoprene-derived SOCs, could explain the WSOC fractions of 39–63% in spring and 46–54% in summer at each site. Notably, monoterpene-derived and mono-aromatic SOCs accounted for most of the total estimated SOCs in both spring (85–93%) and summer (75–82%) at each site. These results indicate that SOAs significantly impact WSOC concentrations during both these seasons at urban sites in Japan.

Time-resolved characterization of organic compounds in PM2.5 collected at Oki Island, Japan, affected by transboundary pollution of biomass and non-biomass burning from Northeast China

To evaluate the transboundary pollution of organic aerosols from Northeast Asia, a highly time-resolved measurement of organic compounds was performed in March 2019 at Oki Island located in Japan, which is a remote site and less affected by local anthropogenic sources. PM_2.5, water-soluble organic carbon (WSOC) concentrations, and WSOC fraction in PM_2.5 showed high values on March 22-23 (high-WSOC period (HWSOC)) when the air mass passed through the area where many fire spots were detected in Northeast China. Biomass burning tracers showed higher concentration, especially levoglucosan exceeded 1 μg/m³ during the HWSOC than the low-WSOC period (LWSOC). Notably, high time-resolved measurements of biomass burning tracers and back trajectory analysis during HWSOC revealed a difference in the variation of lignin pyrolyzed compounds and anhydrous sugars on 22 and 23 March. The air mass passed to different areas in Northeast China in which fire spots were detected, such as the eastern area on the 22nd and the western area on the 23rd. Almost-organic compounds also showed high concentration and strong correlations with levoglucosan and sulfate during HWSOC. Moreover, low-carbon dicarboxylic acids (e.g., adipic acid) and secondary products from anthropogenic volatile organic compounds (e.g., 2,3-dihydroxy-4-oxopentanoic, phthalic, 5-nitrosalicylic acids), also showed a strong correlation with sulfate ions during the HWSOC and LWSOC, respectively. These higher concentrations and strong correlations with levoglucosan and sulfate during the HWSOC propose that their generation could be enhanced by biomass burning. The ratios of organics (e.g., levoglucosan/mannnosan, pinic/3-methylbutane-1,2,3-tricarboxylic acids) suggest that the high concentrations of PM_2.5 and WSOC observed during the HWSOC were caused by aged organic aerosols that originated from the combustion of herbaceous plants transported from Northeast China. Our findings indicate that biomass combustion in Northeast China could significantly affect the chemical compositions and the characterization of organic aerosols in downwind regions of Northeast China.

Organic Hydroxy Acids as Highly Oxygenated Molecular (HOM) Tracers for Aged Isoprene Aerosol

Highly oxygenated molecules (HOMs) are a class of compounds associated with secondary organic aerosols exhibiting high oxygen to carbon (O:C) ratios and often originating from the oxidation of biogenic compounds. Here, the photooxidation and ozonolysis of isoprene were examined under a range of conditions to identify HOM tracers for aged isoprene aerosol. The HOM tracers were identified as silylated derivatives by gas chromatography-mass spectrometry and by detecting their parent compounds by liquid chromatography-high resolution mass spectrometry. In addition to the previously observed methyltetrols and 2-methylglyceric acid, seven tracer compounds were identified, including 2-methyltartronic acid (MTtA), 2-methylerythronic acid (2MeTrA), 3-methylerythronic acid (3MeTrA), 2-methylthreonic acid (2MTrA), 3-methylthreonic acid (3MTrA), erythro-methyltartaric acid (e-MTA), and threo-methyltartaric acid (t-MTA). The molecular structures were confirmed with authentic standards synthesized in the laboratory. The presence of some of these HOMs in the gas and particle phases simultaneously provides evidence of their gas/particle partitioning. To determine the contributions of aged isoprene products to ambient aerosols, we analyzed ambient PM2.5 samples collected in the southeastern United States in summer 2003 and at two European monitoring stations located in Zielonka and Godów (Poland). Our findings show that methyltartaric acids (MTA) and 2- and 3-methylthreonic acids (and their stereoisomers) are representative of aged isoprene aerosol because they occur both in the laboratory chamber aerosol obtained and in ambient PM2.5. On the basis of gas chromatography-mass spectrometry (GC-MS) analysis, their concentrations were found to range from 0.04 ng for 3-methylthreonic acid to 6.3 ng m-3 for methyltartaric acid at the southeast site in Duke Forest, NC, USA.

Effect of relative humidity and the presence of aerosol particles on the α-pinene ozonolysis

The α-pinene ozonolysis under the different environmental conditions were observed in a smog chamber. The second-order rate constant (k) was determined to be (7.25±0.06)×10-¹⁷cm³/(molecule·sec) under 20% of relative humidity (RH) and room temperature. RH showed a marked influence on the α-pinene ozonolysis. The value of k increased with RH increase, which was 1.6 times faster at RH=80% than that at RH=20%. Additionally, the value of k apparently changed in the presence of the aerosol particles. The diesel soot increased the k value. The fly ash prohibited the reaction, however, H₂SO₄-treated fly ash promoted the reaction. The information of products gained using FT-IR and SPAMS showed that pinonic acid, 10-hydroxy-pinonic acid and pinic acid could be generated during the α-pinene ozonolysis. Water molecules could take part in the formation of the products, and play a vital role in the degradation of α-pinene. The atmospheric residence time calculation showed that the ozonolysis in the atmosphere is an important way of the α-pinene consumption as compared to that reacted with OH during daytime. The results suggested that the degradation of α-pinene via the ozonization in the atmosphere may be affected greatly by RH, as well as the presence of aerosol particles. The ozonolysis reaction may be an important way of the α-pinene consumption during daytime.

Physical and Chemical Properties of 3-Methyl-1,2,3-butanetricarboxylic Acid (MBTCA) Aerosol

The properties and the chemical fate of later generation products of the oxidation of biogenic organic compounds are mostly unknown. The properties of fresh MBTCA aerosol, a later generation product of the oxidation of monoterpenes in the atmosphere, were determined combining an aerosol mass spectrometer (AMS), a thermodenuder, and a scanning mobility particle sizer. Based on its AMS spectrum m/z 141.055 (C₇H₉O₃⁺) could be used as an MBTCA signature. The MBTCA particle density was 1.43 ± 0.04 g cm^-3, its saturation concentration was (1.8 ± 1.3) × 10^-3 μg m^-3 at 298 K, and its vaporization enthalpy was 150 ± 15 kJ mol^-1. After OH radical exposure (∼1.2 days) and UV illumination the average aerosol O:C ratio decreased from 0.72 to 0.58-0.64 suggesting net fragmentation. Our findings suggest that the reactions of MBTCA with OH lead to CO₂ loss with or without an oxygen addition.

Identification and quantification of carbonyl-containing α-pinene ozonolysis products using O-tert-butylhydroxylamine hydrochloride

The yields of carbonyl-containing reaction products from the ozonolysis of α-pinene have been investigated using concentrations of ozone found in the indoor environment ([O₃] ≤ 100 ppb). An impinger was used to collect gas-phase oxidation products in water, where the derivatization agent O-tert-butylhydroxylamine hydrochloride (TBOX) and gas chromatography-mass spectrometry were used to identify carbonyl-containing species. Seven carbonyl-containing products were observed. The yield of the primary product, pinonaldehyde was measured to be 76 %. Using cyclohexane as a hydroxyl radical (OH) scavenger, the yield of pinonaldehyde decreased to 46 %, indicating the influence secondary OH radicals have on α-pinene ozonolysis products. Furthermore, the use of TBOX, a small molecular weight derivatization agent, allowed for the acquisition of the first mass spectral data of oxopinonaldehyde, a tricarbonyl reaction product of α-pinene ozonolysis. The techniques described herein allow for an effective method for the collection and identification of terpene oxidation products in the indoor environment.

Chemical Characterization of Gas- and Particle-Phase Products from the Ozonolysis of α-Pinene in the Presence of Dimethylamine

Amines are recognized as key compounds in new particle formation (NPF) and secondary organic aerosol (SOA) formation. In addition, ozonolysis of α-pinene contributes substantially to the formation of biogenic SOAs in the atmosphere. In the present study, ozonolysis of α-pinene in the presence of dimethylamine (DMA) was investigated in a flow tube reactor. Effects of amines on SOA formation and chemical composition were examined. Enhancement of NPF and SOA formation was observed in the presence of DMA. Chemical characterization of gas- and particle-phase products by high-resolution mass spectrometric techniques revealed the formation of nitrogen containing compounds. Reactions between ozonolysis reaction products of α-pinene, such as pinonaldehyde or pinonic acid, and DMA were observed. Possible reaction pathways are suggested for the formation of the reaction products. Some of the compounds identified in the laboratory study were also observed in aerosol samples (PM₁) collected at the SMEAR II station (Hyytiälä, Finland) suggesting that DMA might affect the ozonolysis of α-pinene in ambient conditions.

Chemical characterization of α-pinene secondary organic aerosol constituents using gas chromatography, liquid chromatography, and paper spray-based mass spectrometry techniques

RATIONALE

Despite ample research into the atmospheric oxidation of α-pinene, an important precursor to biogenic secondary organic aerosol formation, the identification of its reaction products, specifically organic nitrates, which impact atmospheric NOx concentrations, is still incomplete. This negatively impacts our understanding of α-pinene oxidation chemistry and its relation to air quality.

METHODS

Photochemical chamber experiments were conducted in conjunction with mass spectrometric techniques, including gas chromatography/mass spectrometry (GC/MS), high-performance liquid chromatography/time-of-flight (HPLC/TOF), and paper spray ionization MS, to investigate products from the OH radical initiated oxidation of α-pinene under high NOx conditions.

RESULTS

Over 30 compounds were tentatively identified, including those newly detected from photochemical chamber studies of α-pinene oxidation, pinocamphenol, fencholenic aldehyde, and α-pinene-derived nitrate isomers. α-Pinene-derived hydroxynitrate isomers were successfully detected using chromatographic methods, demonstrating, for the first time, the identification of individual first-generation organic nitrate products derived from α-pinene. The application of paper spray ionization to particle-phase compounds collected on filters represents a novel method for the direct analysis of filter samples at ambient pressure and temperature.

CONCLUSIONS

The use of HPLC/TOF and paper spray ionization methods to identify previously unobserved α-pinene-derived products helps lower the uncertainty in α-pinene oxidation chemistry and provides new platforms that can be used to identify and quantify important atmospheric compounds that relate to air quality in a complex sample matrix, such as ambient aerosol particles. Additionally, the use of paper spray ionization for direct filter analysis is a fast, relatively inexpensive sample preparation technique that can be used to reduce sample manipulation from solvent-induced reactions. Copyright © 2016 John Wiley & Sons, Ltd.

Assessing the potential for diol and hydroxy sulfate ester formation from the reaction of epoxides in tropospheric aerosols

Polyols and sulfate esters have recently been identified in the secondary organic aerosol (SOA) formed in the photooxidation of biogenic hydrocarbons both in the laboratory and under ambient atmospheric conditions. In the present study, the potential role of the reactions of epoxides in SOA to form diols and hydroxy sulfate esters is explored. Nuclear magnetic resonance methods were used to monitor the bulk reaction kinetics of the epoxide hydrolysis reactions for a number of simple epoxides. The experiments were carried out at various acid concentrations in order to confirm the acid-catalysis rate order and to determine the second-order rate constants for such reactions in aerosols under the previously studied laboratory conditions and under ambient atmospheric conditions. The measured rate constants depended systematically on the carbon substitution nature of the epoxide ring, with the tertiary epoxides characterized by the largest rate constants. The hydroxy sulfate yield was observed to depend linearly on the total sulfate concentration, with yields as high as 30% observed at high sulfate concentrations. Due to the large values of the observed rate constants, these reactions are expected to be efficient even for mostly neutralized tropospheric SOA, let alone the much more acidic SOA particles previously studied in laboratory experiments. Therefore, the epoxide hydrolysis mechanism appears to be a kinetically feasible route to the formation of the diols and hydroxy sulfate esters observed in the SOA resulting from the photooxidation of biogenic hydrocarbons.

Comprehensive two-dimensional liquid chromatography–time-of-flight mass spectrometry in the analysis of acidic compounds in atmospheric aerosols

A novel method utilising comprehensive two-dimensional liquid chromatography interfaced to electrospray ionisation time-of-flight mass spectrometry was developed for the determination of organic acids in atmospheric aerosols. The system was applied to the analysis of methanolic extracts of filters from a high volume sampler. The enhanced separation power of two-dimensional separation was demonstrated in the analysis of both rural and urban samples. Quantification was performed for compounds for which standards were available. Limit of detection was 2-200 ng/ml. Average reproducibility of retention times in each dimensions was 0.1%, and average reproducibility of peak areas was 8% (10 microg/ml, n=3).

Secondary organic carbon and aerosol yields from the irradiations of isoprene and alpha-pinene in the presence of NOx and SO2

A laboratory study was carried out to investigate the secondary organic carbon (SOC) yields of alpha-pinene and isoprene in the presence of SO2, which produces acidic aerosol in the system. Experiments were based on irradiating each hydrocarbon (HC) with NOx in a 14.5 m3 smog chamber operated in the dynamic mode. The experimental design consisted of several multi-part experiments for each HC. In the first part of each experiment, an HC/NOx irradiation was conducted in the absence of SO2 and was followed by irradiations with the addition of SO2 in subsequent parts. Filter-based analyses for organic carbon were made using a thermal-optical approach either with an off-line instrument or in situ with an automated instrument. For isoprene in the absence of SO2, the SOC yield was approximately 0.001, a value consistent with earlier work from this laboratory. With the addition of up to 200 ppb SO2, the yield increased by a factor of 7. For alpha-pinene in the absence of SO2, the SOC yield of the irradiated mixture was found to average 0.096 from two experiments. With SO2 in the system, the SOC yield increased on average to 0.132. These results suggest that SO2, and by inference acidic aerosol, may play a role in increasing the yield of SOC from the photooxidation products of biogenic hydrocarbons or by the direct uptake of biogenic hydrocarbons onto acidic aerosol.

Investigation of alpha-pinene + ozone secondary organic aerosol formation at low total aerosol mass

We present a method for measuring secondary organic aerosol (SOA) production at low total organic mass concentration (COA) using proton-transfer reaction mass spectrometry (PTR-MS). PTR-MS provides high time resolution measurements of gas-phase organic species and, coupled with particle measurements, allows for the determination of aerosol yield in real time. This approach facilitates the measurement of aerosol production at low COA; in fact aerosol mass fractions can be measured during alpha-pinene consumption as opposed to only at the completion of gas-phase chemistry. The high time resolution data are consistent with both the partitioning theory of Pankow (Atmos. Environ. 1994, 28,185 and 189) and the previous experimental measurements. Experiments including the effect of UV illumination and NOx reveal additional features of alpha-pinene + ozone product photochemistry and volatility. The high time resolution data also elucidate aerosol production from alpha-pinene ozonolysis at COA < 10 microg m(-3) and show that extrapolations of current partitioning models to conditions of low COA significantly underestimate SOA production under dark, low-NOx conditions. However, extrapolations of current models overestimate SOA production under illuminated, higher-NOx conditions typical of polluted regional air masses.

Kinetic mechanism for predicting secondary organic aerosol formation from the reaction of d-limonene with ozone

A semi-explicit mechanism of d-limonene was developed and tested against experimental results obtained from large outdoor Teflon film chambers at the University of North Carolina (UNC) smog chamber facility. The model couples gas-phase reactions with partitioning processes and possible particle-phase reactions. The model not only tracks the gas-phase ozonolysis reaction of d-limonene, but also provides a reasonable prediction of the secondary aerosol mass production under different conditions. Limononaldehyde was the major identified product, followed by limona-ketone, referred to here as keto-limonene, keto-limononaldehyde, limononic acid, and keto-limononic acid. Identified particle-phase products accounted for about 60% of the observed particle mass in the initial stages of the reaction. Model sensitivity was tested and discussed with respect to effects of temperature, humidity, water uptake, and reactant concentrations.

Secondary organic aerosol production from terpene ozonolysis. 2. Effect of NOx concentration

We report secondary organic aerosol (SOA) yields from the ozonolysis of alpha-pinene in the presence of NO and NO2. Experimental conditions are characterized by the [VOC]0/ [NOx]0 ratio (ppbC/ppb), which varies from approximately 1 to approximately 300. SOA yield is constant for [VOC]0/[NOx]0 > approximately 15 and decreases dramatically (by more than a factor of 4) as [VOC]0/[NOx]0 decreases. Aerosol production is completely suppressed in the presence of NO for [VOC]0/[NOx]0 < or = 4.5. Fouriertransform IR analysis of filter samples reveals that nitrate-containing species contribute significantly to the total aerosol mass at low [VOC]0/[NOx]0. Yield reduction is a result of the formation of a more volatile product distribution as [VOC]0/[NOx]0 decreases; we propose that the change in the product distribution is driven by changes in the gas-phase chemistry as NOx concentration increases. We also present two-product model parameters to describe aerosol production from the alpha-pinene/0/NOx system under both high- and low-NOx conditions.

Secondary organic aerosol production from terpene ozonolysis. 1. Effect of UV radiation

We report secondary organic aerosol (SOA) yields from the ozonolysis of alpha-pinene under both dark and UV-illuminated conditions. Exposure to UV light reduces SOA yield by 20-40%, with a maximum reduction in yield coinciding with a minimum in the amount of terpene consumed (15-30 ppb). The data are consistent with a constant absolute reduction in the yield of approximately 0.03. Gas chromatography mass spectrometry analysis of filter samples indicates that the major products found in alpha-pinene SOA include organic acids (e.g., pinic acid), keto acids (e.g., pinonic acid), and hydroxy keto acids (e.g., 10-hydroxypinonic acid). Analysis of filter-based results suggests that yield reduction is a result of the formation of a more volatile product distribution when experiments are conducted in the presence of UV light. These results implythat previous "dark bag" experiments may overestimate SOA generation from monoterpenes and also that SOA generation in the atmosphere may depend significantly on actinic flux.

Continuous Real-Time Analysis of Products from the Reaction of Some Monoterpenes with Ozone Using Atmospheric Sampling Glow Discharge Ionization Coupled to a Quadrupole Ion Trap Mass Spectrometer

An on-line technique has been demonstrated for the analysis of photochemical oxidation reaction products. The technique is based on the direct introduction of gas and particulate oxidation products into a custom-built atmospheric sampling glow discharge ionization source (ASGDI) coupled to a quadrupole ion trap mass spectrometer (QITMS). Operational parameters of the ASGDI system were investigated to determine their influence on the ion signal for the analysis of oxidation products in real time. These parameters include the discharge current, ion accumulation time, and type of reagent gas. Reference mass spectra from standards were generated for a variety of biogenic compounds and terpene reaction products containing keto, hydroxy, aldehyde, carboxylic acid, or epoxy groups to better understand the fragmentation that occurs in the glow discharge ion source. Results are presented for ozonolysis reactions of four biogenic monoterpenes (alpha-pinene, beta-pinene, D-limonene, Delta(3)-carene) monitored with the ASGDI quadrupole ion trap to demonstrate the ability to obtain real-time measurements. The reaction products identified with ASGDI-QITMS correspond to those products identified with other techniques, including on-line atmospheric pressure chemical ionization techniques. Efficient differentiation of multifunctional products including mono-/di-/hydroxy-/keto-carboxylic acid and keto-/hydroxy-aldehyde was possible by use of the MS/MS capability of the instrument.

Atmospheric Organic Aerosol Production by Heterogeneous Acid-Catalyzed Reactions

Exploratory evidence from our laboratories shows that acidic surfaces on atmospheric aerosols lead to very real and potentially multifold increases in secondary organic aerosol (SOA) mass and build-up of stabilized nonvolatile organic matter as particles age. One possible explanation for these heterogeneous processes are the acid-catalyzed (e.g., H2SO4 and HNO3) reactions of atmospheric multifunctional organic species (e.g., multifunctional carbonyl compounds) that are accommodated onto the particle phase from the gas phase. Volatile organic hydrocarbons (VOCs) from biogenic sources (e.g., terpenoids) and anthropogenic sources (aromatics) are significant precursors for multifunctional organic species. The sulfur content of fossil fuels, which is released into the atmosphere as SO2, results in the formation of secondary inorganic acidic aerosols or indigenous acidic soot particles (e.g., diesel soot). The predominance of SOAs contributing to PM2.5 (particulate matter, that is, 2.5 microm or smaller than 2.5 microm), and the prevalence of sulfur in fossil fuels suggests that interactions between these sources could be considerable. This study outlines a systematic approach for exploring the fundamental chemistry of these particle-phase heterogeneous reactions. If acid-catalyzed heterogeneous reactions of SOA products are included in next-generation models, the predicted SOA formation will be much greater and have a much larger impact on climate-forcing effects than we now predict. The combined study of both organic and inorganic acids will also enable greater understanding of the adverse health effects in biological pulmonary organs exposed to particles.

Atmospheric secondary aerosol formation by heterogeneous reactions of aldehydes in the presence of a sulfuric acid aerosol catalyst

Particle growth by the heterogeneous reaction of aldehydes was evaluated in 0.5 m3 Teflon film bags under darkness in the presence of background seed aerosols. The aldehydes used were as follows: glyoxal, butanal, hexanal, octanal, and decanal. To study acid catalyst effects on aldehyde heterogeneous reactions, one of the Teflon bags was initially filled with seed aerosols composed of ammonium sulfate-aerosol acidified with sulfuric acid. These results were compared to particle growth reactions that contained only ammonium sulfate as a background seed aerosol. The gas-phase aldehydes were then added to the Teflon bags. In selected experiments, 1-decanol was also added to the Teflon bags with aldehydes to clarify particle growth via a heterogeneous hemiacetal/acetal formation in the presence/absence of an acid catalyst. The particle size distribution and growth were measured using a scanning mobility particle sizer (TSI-SMPS), and the results were applied to predicting aerosol growth and size distribution changes by condensation and heterogeneous reactions. Aerosols created from the heterogeneous reactions of aldehydes were collected directly on an ungreased zinc selenide (ZnSe) FTIR disk (25 mm in diameter) by impaction. The ZnSe disks were directly analyzed for product functional groups inthe aerosol phase using a Fourier transform infrared (FTIR) spectrometer with a deuterated triglycine sulfate (DTGS) detector. Aerosol growth by heterogeneous aldehyde reactions proceeds via a hydration, polymerization process, and hemiacetal/acetal formation from the reaction of aldehydes with alcohols. These aldehyde heterogeneous reactions were accelerated in the presence of an acid catalyst, H2SO4, and led to higher aerosol yields than when H2SO4 was not present in the seed aerosol. The FTIR spectra obtained from the growing aerosol, also illustrated aldehyde group transformation in the particle phase as a function of the heterogeneous reaction. It was concluded that aldehydes, which can be produced by atmospheric photochemical reactions, can significantly contribute on secondary aerosol formation through heterogeneous reactions in the presence of an acid catalyst.