SOA Yields Research Articles

Study on the reactions of n-pentanal and n-hexanal with Br atoms: Kinetics, gas-phase products, and SOA formation

Aldehydes are one of the most important oxygenated volatile organic compounds in the atmosphere, contributing to atmospheric radicals such as HO2 and RO2, which play a pivotal role in atmospheric chemistry and the oxidative capacity of the atmosphere. The kinetics, gas-phase products, and SOA formation of pentanal and n-hexanal initiated by Br atoms were studied in this work. The reaction rate constants of n-pentanal and n-hexanal reacting with Br atoms were determined to be (1.77 ± 0.25) × 10−11 cm3 molecule−1 s−1 and (2.07 ± 0.29) × 10−11 cm3 molecule−1 s−1 at 298 K and 760 Torr, respectively. Key gas-phase products, including smaller aldehydes, carboxylic acids, and alcohols, are known to contribute to atmospheric reactivity and have potential implications for air quality by promoting the formation of secondary pollutants and influencing the oxidative capacity of the atmosphere. The SOA yields were (5.24 ± 0.10)% and (6.00 ± 1.20)% for n-pentanal and n-hexanal, respectively, higher than the SOA yields of the aldehydes reaction initiated by OH radicals. The higher SOA yields observed from Br atoms oxidation compared to OH radical oxidation highlight the necessity to re-evaluate the role of halogen chemistry in atmospheric processes, particularly in regions where halogen compounds are prevalent. This research enhances our understanding of the atmospheric fate of aldehydes and highlights the importance of considering halogen-driven oxidation pathways in atmospheric models.

Chemical Differences between Phenolic Secondary Organic Aerosol Formed through Gas-Phase and Aqueous-Phase Reactions.

Phenolic compounds, which are significant emissions from biomass burning (BB), undergo rapid photochemical reactions in both gas and aqueous phases to form secondary organic aerosol, namely, gasSOA and aqSOA, respectively. The formation of gasSOA and aqSOA involves different reaction mechanisms, leading to different product distributions. In this study, we investigate the gaseous and aqueous reactions of guaiacol-a representative BB phenol-to elucidate the compositional differences between phenolic aqSOA and gasSOA. Aqueous-phase reactions of guaiacol produce higher SOA yields than gas-phase reactions (e.g., roughly 60 vs 30% at one half-life of guaiacol). These aqueous reactions involve more complex reaction mechanisms and exhibit a more gradual SOA evolution than their gaseous counterparts. Initially, gasSOA forms with high oxidation levels (O/C > 0.82), while aqSOA starts with lower O/C (0.55-0.75). However, prolonged aqueous-phase reactions substantially increase the oxidation state of aqSOA, making its bulk chemical composition closer to that of gasSOA. Additionally, aqueous reactions form a greater abundance of oligomers and high-molecular-weight compounds, alongside a more sustained production of carboxylic acids. AMS spectral signatures representative of phenolic gasSOA have been identified, which, together with tracer ions of aqSOA, can aid in the interpretation of field observation data on aerosol aging within BB smoke. The notable chemical differences between phenolic gasSOA and aqSOA highlighted in this study also underscore the importance of accurately representing both pathways in atmospheric models to better predict the aerosol properties and their environmental impacts.

Modeling daytime and nighttime secondary organic aerosol formation via multiphase reactions of biogenic hydrocarbons

Abstract. The daytime oxidation of biogenic hydrocarbons is attributed to both OH radicals and O3, while nighttime chemistry is dominated by the reaction with O3 and NO3 radicals. Here, daytime and nighttime patterns of secondary organic aerosol (SOA) originating from biogenic hydrocarbons were predicted under varying environmental conditions (temperature, humidity, sunlight intensity, NOx levels, and seed conditions) by using the UNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which comprises multiphase gas–particle partitioning and in-particle chemistry. The products originating from the atmospheric oxidation of three different hydrocarbons (isoprene, α-pinene, and β-caryophyllene) were predicted by using extended semi-explicit mechanisms for four major oxidants (OH, O3, NO3, and O(3P)) during day and night. The resulting oxygenated products were then classified into volatility–reactivity-based lumping species. The stoichiometric coefficients associated with lumping species were dynamically constructed under varying NOx levels, and they were applied to the UNIPAR SOA model. The predictability of the model was demonstrated by simulating chamber-generated SOA data under varying environments. For daytime SOA formation, both isoprene and α-pinene were dominated by the OH-radical-initiated oxidation showing a gradual increase in SOA yields with decreasing NOx levels. The nighttime isoprene SOA formation was processed mainly by the NO3-driven oxidation, yielding higher SOA mass than daytime at higher NOx level (isoprene / NOx < 5 ppb C ppb−1). At a given amount of ozone, the oxidation to produce the nighttime α-pinene SOA gradually transited from the NO3-initiated reaction to ozonolysis as NOx levels decreased. Nighttime α-pinene SOA yields were also significantly higher than daytime SOA yields, although the nighttime α-pinene SOA yields gradually decreased with decreasing NOx levels. β-Caryophyllene, which rapidly produced SOA with high yields, showed a relatively small variation in SOA yields from changes in environmental conditions (i.e., NOx levels, seed conditions, and sunlight intensity), and its SOA formation was mainly attributed to ozonolysis day and night. The daytime SOA formation was generally more sensitive to the aqueous reactions than the nighttime SOA because the daytime chemistry produced more highly oxidized multifunctional products. The simulation of α-pinene SOA in the presence of gasoline fuel, which can compete with α-pinene for the reaction with OH radicals in typical urban air, suggested more growth of α-pinene SOA by the enhanced ozonolysis path. We concluded that the oxidation of the biogenic hydrocarbon with O3 or NO3 radicals is a source of the production of a sizable amount of nocturnal SOA, despite the low emission at night.

Secondary Organic Aerosol Mass Yields from NO3 Oxidation of α-Pinene and Δ-Carene: Effect of RO2 Radical Fate.

Dark chamber experiments were conducted to study the SOA formed from the oxidation of α-pinene and Δ-carene under different peroxy radical (RO2) fate regimes: RO2 + NO3, RO2 + RO2, and RO2 + HO2. SOA mass yields from α-pinene oxidation were <1 to ∼25% and strongly dependent on available OA mass up to ∼100 μg m-3. The strong yield dependence of α-pinene oxidation is driven by absorptive partitioning to OA and not by available surface area for condensation. Yields from Δ-carene + NO3 were consistently higher, ranging from ∼10-50% with some dependence on OA for <25 μg m-3. Explicit kinetic modeling including vapor wall losses was conducted to enable comparisons across VOC precursors and RO2 fate regimes and to determine atmospherically relevant yields. Furthermore, SOA yields were similar for each monoterpene across the nominal RO2 + NO3, RO2 + RO2, or RO2 + HO2 regimes; thus, the volatility basis sets (VBS) constructed were independent of the chemical regime. Elemental O/C ratios of ∼0.4-0.6 and nitrate/organic mass ratios of ∼0.15 were observed in the particle phase for both monoterpenes in all regimes, using aerosol mass spectrometer (AMS) measurements. An empirical relationship for estimating particle density using AMS-derived elemental ratios, previously reported in the literature for non-nitrate containing OA, was successfully adapted to organic nitrate-rich SOA. Observations from an NO3- chemical ionization mass spectrometer (NO3-CIMS) suggest that Δ-carene more readily forms low-volatility gas-phase highly oxygenated molecules (HOMs) than α-pinene, which primarily forms volatile and semivolatile species, when reacted with NO3, regardless of RO2 regime. The similar Δ-carene SOA yields across regimes, high O/C ratios, and presence of HOMs, suggest that unimolecular and multistep processes such as alkoxy radical isomerization and decomposition may play a role in the formation of SOA from Δ-carene + NO3. The scarcity of peroxide functional groups (on average, 14% of C10 groups carried a peroxide functional group in one test experiment in the RO2 + RO2 regime) appears to rule out a major role for autoxidation and organic peroxide (ROOH, ROOR) formation. The consistently substantially lower SOA yields observed for α-pinene + NO3 suggest such pathways are less available for this precursor. The marked and robust regime-independent difference in SOA yield from two different precursor monoterpenes suggests that in order to accurately model SOA production in forested regions the chemical mechanism must feature some distinction among different monoterpenes.

Kinetic and mechanism of the reaction between Cl and several mono-methyl branched alkanes

Branched alkanes are ubiquitous in the troposphere and play an important role in the chemical processes. In this work, the rate constants and products for the reaction of Cl atoms with 3-methylhexane and 2-methylheptane were measured at room temperature (298 ± 0.2 K) and atmospheric pressure using a conventional relative rate method. The rate constants of 3-methylhexane and 2-methylheptane in units of cm3/(mol·sec) are (3.09 ± 0.31) × 10−10 and (3.67 ± 0.40) × 10−10, respectively. Furthermore, the corresponding atmospheric lifetime of the studied branched alkanes with Cl was 6.92-89.90 hours and 5.82-75.69 hours, respectively. The estimated atmospheric lifetimes indicated that the reaction with Cl atoms could be the most important atmospheric degradation pathway for 3-methylhexane and 2-methylheptane. Primary gas-phase products of the reactions were identified and quantified, and particle-phase products were also obtained. The atmosphere oxidation mechanism of Cl atoms with 3-methylhexane and 2-methylheptane is proposed. The SOA yields of 3-methylhexane and 2-methylheptane from the reaction of Cl atoms were determined to be 7.96% ± 0.89% and 13.35% ± 1.50% respectively. Overall, the results reveal that the primary loss process of branched alkanes is the reaction with Cl atoms, which impacts its degradation on a regional scale.

Using GECKO-A to derive mechanistic understanding of secondary organic aerosol formation from the ubiquitous but understudied camphene

Abstract. Camphene, a dominant monoterpene emitted from both biogenic and pyrogenic sources, has been significantly understudied, particularly in regard to secondary organic aerosol (SOA) formation. When camphene represents a significant fraction of emissions, the lack of model parameterizations for camphene can result in inadequate representation of gas-phase chemistry and underprediction of SOA formation. In this work, the first mechanistic study of SOA formation from camphene was performed using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A). GECKO-A was used to generate gas-phase chemical mechanisms for camphene and two well-studied monoterpenes, α-pinene and limonene, as well as to predict SOA mass formation and composition based on gas/particle partitioning theory. The model simulations represented observed trends in published gas-phase reaction pathways and SOA yields well under chamber-relevant photooxidation and dark ozonolysis conditions. For photooxidation conditions, 70 % of the simulated α-pinene oxidation products remained in the gas phase compared to 50 % for limonene, supporting model predictions and observations of limonene having higher SOA yields than α-pinene under equivalent conditions. The top 10 simulated particle-phase products in the α-pinene and limonene simulations represented 37 %–50 % of the SOA mass formed and 6 %–27 % of the hydrocarbon mass reacted. To facilitate comparison of camphene with α-pinene and limonene, model simulations were run under idealized atmospheric conditions, wherein the gas-phase oxidant levels were controlled, and peroxy radicals reacted equally with HO2 and NO. Metrics for comparison included gas-phase reactivity profiles, time-evolution of SOA mass and yields, and physicochemical property distributions of gas- and particle-phase products. The controlled-reactivity simulations demonstrated that (1) in the early stages of oxidation, camphene is predicted to form very low-volatility products, lower than α-pinene and limonene, which condense at low mass loadings; and (2) the final simulated SOA yield for camphene (46 %) was relatively high, in between α-pinene (25 %) and limonene (74 %). A 50 % α-pinene + 50 % limonene mixture was then used as a surrogate to represent SOA formation from camphene; while simulated SOA mass and yield were well represented, the volatility distribution of the particle-phase products was not. To demonstrate the potential importance of including a parameterized representation of SOA formation by camphene in air quality models, SOA mass and yield were predicted for three wildland fire fuels based on measured monoterpene distributions and published SOA parameterizations for α-pinene and limonene. Using the 50/50 surrogate mixture to represent camphene increased predicted SOA mass by 43 %–50 % for black spruce and by 56 %–108 % for Douglas fir. This first detailed modeling study of the gas-phase oxidation of camphene and subsequent SOA formation highlights opportunities for future measurement–model comparisons and lays a foundation for developing chemical mechanisms and SOA parameterizations for camphene that are suitable for air quality modeling.

Novel pathway of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; oxidation in the atmosphere: reactions with monoterpene ozonolysis intermediates and secondary organic aerosol

Abstract. Ozonolysis of monoterpenes is an important source of atmospheric biogenic secondary organic aerosol (BSOA). While enhanced BSOA formation has been associated with sulfate-rich conditions, the underlying mechanisms remain poorly understood. In this work, the interactions between SO2 and reactive intermediates from monoterpene ozonolysis were investigated under different humidity conditions (10 % vs. 50 %). Chamber experiments were conducted with ozonolysis of α-pinene or limonene in the presence of SO2. Limonene SOA formation was enhanced in the presence of SO2, while no significant changes in SOA yields were observed during α-pinene ozonolysis. Under dry conditions, SO2 primarily reacted with stabilized Criegee intermediates (sCIs) produced from ozonolysis, but at 50 % RH heterogeneous uptake of SO2 onto organic aerosol was found to be the dominant sink of SO2, likely owing to reactions between SO2 and organic peroxides. This SO2 loss mechanism to organic peroxides in SOA has not previously been identified in experimental chamber studies. Organosulfates were detected and identified using an electrospray ionization–ion mobility spectrometry–high-resolution time-of-flight mass spectrometer (ESI-IMS-TOF) when SO2 was present in the experiments. Our results demonstrate the synergistic effects between BSOA formation and SO2 oxidation through sCI chemistry and SO2 uptake onto organic aerosol and illustrate the importance of considering the chemistry of organic and sulfur-containing compounds holistically to properly account for their reactive sinks.

Time-resolved analysis of primary volatile emissions and secondary aerosol formation potential from a small-scale pellet boiler

Small-scale pellet boilers and stoves became popular as a wood combustion appliance for domestic heating in Europe, North America and Asia due to economic and environmental aspects. Therefore, an increasing contribution of pellet boilers to air pollution is expected despite their general high combustion efficiency. As emissions of primary organic aerosol (POA) and permanent gases of pellet boilers are well investigated, the scope of this study was to investigate the volatile organic emissions and the formation potential of secondary aerosols for this type of appliance. Fresh and aged emissions were analysed by a soot-particle aerosol time-of-flight mass spectrometry (SP-AMS) and the molecular composition of the volatile precursors with single-photon ionisation time-of-flight mass spectrometry (SPI-TOFMS) at different pellet boiler operation conditions. Organic emissions in the gas phase were dominated by unsaturated hydrocarbons while wood-specific VOCs, e.g. phenolic species or substituted furans, were only detected during the starting phase. Furthermore, organic emissions in the gas phase were found to correlate with fuel grade and combustion technology in terms of secondary air supply. Secondary organic aerosols of optimised pellet boiler conditions (OPT, state-of-the-art combustion appliance) and reduced secondary air supply (RSA, used as a proxy for pellet boilers of older type) were studied by simulating atmospheric ageing in a Potential Aerosol Mass (PAM) flow reactor. Different increases in OA mass (55% for OPT, 102% for RSA), associated with higher average carbon oxidation state and O:C, could be observed in a PAM chamber experiment. Finally, it was found that derived SOA yields and emission factors were distinctly lower than reported for log wood stoves.

An experimental kinetic study and products research of the reactions of O3 with a series of unsaturated alcohols

The gas-phase reactions of unsaturated alcohols with O3 were investigated in FEP Teflon film chamber at 298 K and 760 torr of atmosphere pressure. The rate constants of the reactions of C6-C8 alkenols with O3 were determined using both the absolute and the relative rate method, and the measured values were (5.96 ± 0.35) × 10−17 cm3 molecule−1 s−1 for (Z)-3-hexen-1-ol, (5.12 ± 0.30) × 10−17 cm3 molecule−1 s−1 for (Z)-3-hepten-1-ol, and (5.66 ± 0.52) × 10−17 cm3 molecule−1 s−1 for (Z)-3-octen-1-ol, respectively. The gas-phase products of these reactions mentioned above were detected using proton-transfer-reaction mass spectrum (PTR-MS). HOCH2CH2CHO, CH2CH2CHO, HCHO and CH3CHO were identified as the main gas products for (Z)-3-hexen-1-ol. HOCH2CH2CHO and CH3(CH2)2CHO dominated the gaseous products for (Z)-3-hepten-1-ol. And for (Z)-3-octen-1-ol, CH3(CH2)3CHO, CH3(CH2)2CHO and HOCH2CH2CHO were the main gaseous products. The SOA yields were monitored at the same time, which were 0.184 ± 0.013, 0.213 ± 0.017, 0.232 ± 0.021 for (Z)-3-hexen-1-ol, (Z)-3-hepten-1-ol and (Z)-3-octen-1-ol, respectively. The possible reaction mechanisms were proposed and discussed. The kinetic data presented here has been used to estimate their atmosphere lifetimes and the reaction reactivity. The atmosphere implication of these reactions has also been discussed.

SOA formation from naphthalene, 1-methylnaphthalene, and 2-methylnaphthalene photooxidation

The SOA yield and chemical characteristics of SOA formation from naphthalene and two methyl substituted naphthalenes, 1-methylnaphthalene and 2-methylnaphthalene, were studied for high NOx, low NOx, and ultra-low NOx conditions. The SOA yields are high compared to previous studies for all three PAHs precursors: 1-methylnaphthalene > 2-methylnaphthalene ∼ naphthalene for all atmospheric conditions studied. The SOA yields range from 0.03 to 0.60 for naphthalene, 0.21–1.52 for 1-methylnaphthalene, and 0.34–0.55 for 2-methylnaphthalene under high NOx with HONO (initial PAH:NO ratio = 0.03–0.17) conditions. The SOA yield ranges from 0.04 to 0.31 for naphthalene, 0.14–0.72 for 1-methylnaphthalene, and 0.06–0.49 for 2-methylnaphthalene under low NOx (initial PAH:NO ratio = 0.54–2.20) conditions. SOA yields were substantially greater than 1.0 under H2O2 (ultra low NOx) and low NOx + H2O2 conditions for all three PAH precursors. The system reactivity influenced by OH radicals, NOx levels, initial PAH/NO ratios, NO2/NO ratios, and all impacted the SOA formation from the PAH precursors. Fractal-like SOA is observed for the methylnaphthalene isomers during high NOx photooxidation experiments, implying that researchers studying SOA formation from this precursor must carefully account for particle shape or effective density. A m/z 104 (C7H4O+,104.026) peak, consistent with SOA products phthalic acid from earlier studies, was observed as a potential marker of PAH oxidation during HR-ToF-AMS analysis.

A chamber study of secondary organic aerosol formed by ozonolysis of α-pinene in the presence of nitric oxide

The amounts of SOA and reactive oxygen species (ROS) formed by the reaction of α-pinene with ozone (O3) in the presence of nitric oxide (NO) were studied by varying the ratio of O3 to NO in a chamber under conditions that simulate the indoor environment. The particle mass was measured using a tapered element oscillating microbalance (TEOM) and a scanning mobility particle sizer (SMPS) was used to obtain particle volume distributions. The concentrations of particle-bound reactive species (ROS) were quantitatively determined from collected filters by the measuring the oxidation of dichlorofluorescin (DCFH) to a fluorescent product. The measured fluorescent intensities were converted to equivalent H2O2 concentrations. Three sets of experiments using different concentration ratios of NO/O3 were conducted. Measured steady-state, particle-bound ROS concentrations ranged from 0.58 to 6.78 nmol/m3 of H2O2. The SOA yields were examined using Odum’s SOA yield model that relates the mass of SOA formed to the mass of hydrocarbon consumed. For the condition where NO and O3 were in a concentration ratio of 0.5, the SOA yield was the highest with 22 % of α-pinene converted to SOA. Under these experimental conditions, the highest concentration of nitrate radical in these experiments would have been produced.

Atmospheric reactivity of hydroxyl radicals with guaiacol (2-methoxyphenol), a biomass burning emitted compound: Secondary organic aerosol formation and gas-phase oxidation products

Methoxyphenols are low molecular weight semi-volatile polar aromatic compounds produced from the pyrolysis of wood lignin. The reaction of guaiacol (2-methoxyphenol) with hydroxyl radicals has been studied in the LPCA simulation chamber at (294 ± 2) K, atmospheric pressure, low relative humidity (RH < 1%) and under high-NOx conditions using CH3ONO as OH source. The aerosol production was monitored using a SMPS (Scanning Mobility Particle Sizer); the SOA yields were in the range from 0.003 to 0.87 and the organic aerosol formation can be expressed by a one-product gas/particle partitioning absorption model. Transmission (TEM) and Scanning (SEM) Electron Microscopy observations were performed to characterize the physical state of SOA produced from the OH reaction with guaiacol; they display both liquid and solid particles (in an amorphous state).GC-FID (Gas Chromatography – Flame Ionization Detection) and GC-MS (Gas Chromatography – Mass Spectrometry) analysis show the formation of nitroguaiacol isomers as main oxidation products in the gas- and aerosol-phases. In the gas-phase, the formation yields were (10 ± 2) % for 4-nitroguaiacol (1-hydroxy-2-methoxy-4-nitrobenzene; 4-NG) and (6 ± 2) % for 3- or 6-nitroguaiacol (1-hydroxy-2-methoxy-3-nitrobenzene or 1-hydroxy-2-methoxy-6-nitrobenzene; 3/6-NG; the standards are not commercially available so both isomers cannot be distinguished) whereas in SOA their yield were much lower (≤0.1%).To our knowledge, this work represents the first identification of nitroguaiacols as gaseous oxidation products of the OH reaction with guaiacol. As the reactivity of nitroguaiacols with atmospheric oxidants is probably low, we suggest using them as biomass burning emission gas tracers. The atmospheric implications of the guaiacol + OH reaction are also discussed.

Evaluation of recently-proposed secondary organic aerosol models for a case study in Mexico City

Abstract. Recent field studies have found large discrepancies in the measured vs. modeled SOA mass loadings in both urban and regional polluted atmospheres. The reasons for these large differences are unclear. Here we revisit a case study of SOA formation in Mexico City described by Volkamer et al. (2006), during a photochemically active period when the impact of regional biomass burning is minor or negligible, and show that the observed increase in OA/ΔCO is consistent with results from several groups during MILAGRO 2006. Then we use the case study to evaluate three new SOA models: 1) the update of aromatic SOA yields from recent chamber experiments (Ng et al., 2007); 2) the formation of SOA from glyoxal (Volkamer et al., 2007a); and 3) the formation of SOA from primary semivolatile and intermediate volatility species (P-S/IVOC) (Robinson et al., 2007). We also evaluate the effect of reduced partitioning of SOA into POA (Song et al., 2007). Traditional SOA precursors (mainly aromatics) by themselves still fail to produce enough SOA to match the observations by a factor of ~7. The new low-NOx aromatic pathways with very high SOA yields make a very small contribution in this high-NOx urban environment as the RO2·+NO reaction dominates the fate of the RO2· radicals. Glyoxal contributes several μg m−3 to SOA formation, with similar timing as the measurements. P-S/IVOC are estimated from equilibrium with emitted POA, and introduce a large amount of gas-phase oxidizable carbon that was not in models before. With the formulation in Robinson et al. (2007) these species have a high SOA yield, and this mechanism can close the gap in SOA mass between measurements and models in our case study. However the volatility of SOA produced in the model is too high and the O/C ratio is somewhat lower than observations. Glyoxal SOA helps to bring the O/C ratio of predicted and observed SOA into better agreement. The sensitivities of the model to some key uncertain parameters are evaluated.

An examination of oxidant amounts on secondary organic aerosol formation and aging

The effect of HO x radicals (OH and HO 2) and ozone (O 3) on aerosol formation and aging has been studied. Experiments were performed in presence as well as in absence of oxygen in a flow-through chamber at 299 K for three organic precursor gases, isoprene, α-pinene and m-xylene. The HO x source was the UV photolysis of humidified air or nitrogen and was measured with a GTHOS (Ground-based Tropospheric Hydrogen Oxides Sensor). The precursor gases concentration was monitored with an online GC-FID. The aerosol mass was then quantified by a Tapered Element Oscillating Microbalance (TEOM). Typical oxidant mixing ratios were (0–4.5) ppm for O 3, 200 pptv for OH and 3 ppbv for HO 2. A simple kinetics model is used to infer the aerosol production mechanism. In the present of O 3 (or O 2), the SOA yields were 0.46, 0.036 and 0.12 for α-pinene with an initial concentration of 100 ppbv (RH = 37%), isoprene with an initial concentration of 177 ppbv (RH = 50%) and m-xylene with an initial concentration of 100 ppbv (RH = 37%), respectively. When the chosen precursor gases reacted with HO x in the absence of O 3, the maximum SOA yields were significantly increased by factors of 1.6 for isoprene 1.1 for α-pinene, and 3 for m-xylene respectively. The comparison of the calculated and measured potential aerosol mass concentrations as function of time shows that presence of ozone or oxygen can influence the aerosol yield and the absence of ozone or oxygen in the system resulted in high concentrations of its organic aerosol products.

Photooxidation of α-pinene at high relative humidity in the presence of increasing concentrations of NO x

The photooxidation of ∼1 ppm α-pinene in the presence of increasing concentrations of NO 2 was studied in a Teflon ® chamber at 72–88% relative humidity and 296–304 K. The loss of α-pinene and formation of gas-phase products were followed using proton-transfer reaction mass spectrometry (PTR-MS). Gas-phase reaction products (and their yields) include formaldehyde (5±1%), formic acid (2.5±1.4%), methanol (0.6±0.3%), acetaldehyde (3.9±1.7%), acetic acid (8.6±1.9%), acetone (12±3%), pinonaldehyde (22±6%), and pinene oxide (0.9±0.1%). There was evidence of organic nitrates; and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3-dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition and density probed using single particle mass spectrometry (SPLAT II). SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and oxygenates having a density of 1.21±0.02 g cm −3, 20% larger than often assumed in calculating SOA yields. Three-wavelength light scattering measurements were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon.

Gas‐phase products and secondary aerosol yields from the photooxidation of 16 different terpenes

The photooxidation of isoprene, eight monoterpenes, three oxygenated monoterpenes, and four sesquiterpenes were conducted individually at the Caltech Indoor Chamber Facility under atmospherically relevant HC:NOx ratios to monitor the time evolution and yields of SOA and gas‐phase oxidation products using PTR‐MS. Several oxidation products were calibrated in the PTR‐MS, including formaldehyde, acetaldehyde, formic acid, acetone, acetic acid, nopinone, methacrolein + methyl vinyl ketone; other oxidation products were inferred from known fragmentation patterns, such as pinonaldehyde; and other products were identified according to their mass to charge (m/z) ratio. Numerous unidentified products were formed, and the evolution of first‐ and second‐generation products was clearly observed. SOA yields from the different terpenes ranged from 1 to 68%, and the total gas‐ plus particle‐phase products accounted for ∼50–100% of the reacted carbon. The carbon mass balance was poorest for the sesquiterpenes, suggesting that the observed products were underestimated or that additional products were formed but not detected by PTR‐MS. Several second‐generation products from isoprene photooxidation, including m/z 113, and ions corresponding to glycolaldehyde, hydroxyacetone, methylglyoxal, and hydroxycarbonyls, were detected. The detailed time series and relative yields of identified and unidentified products aid in elucidating reaction pathways and structures for the unidentified products. Many of the unidentified products from these experiments were also observed within and above the canopy of a Ponderosa pine plantation, confirming that many products of terpene oxidation can be detected in ambient air using PTR‐MS, and are indicative of concurrent SOA formation.

Impact of aerosol liquid water on secondary organic aerosol yields of irradiated toluene/propylene/NO x/(NH 4) 2SO 4/air mixtures

Laboratory experiments were conducted to assess whether the presence of liquid water on pre-existing submicron ammonium sulfate aerosols affects yields of condensible organic compounds. Toluene/propylene/NO x /air mixtures were irradiated in the presence of submicron ammonium sulfate aerosol for a series of relative humidities and ammonium sulfate concentrations, generating aerosols containing organic and inorganic constituents with liquid water concentrations ranging from 4 to 66 μg m −3. Organic aerosol carbon concentrations, measured on quartz filters, were corrected for uptake of gas-phase organic compounds and converted into mass concentrations of the organic species. Concentrations of the aerosol ammonium, nitrate, sulfate, and liquid water, collected on co-located Teflon filters, were determined along with the total mass concentrations. Reasonable mass balances were found for the aerosols from summing the liquid water and the organic and inorganic components and comparing the totals to gravimetric determinations. A regression analysis showed the secondary organic aerosol yields, that were measured at near ambient concentration levels, could be expressed as a linear function of the organic mass concentration for organic aerosol masses up to 25 μg m −3. The correlation coefficient of the regression did not improve by adding a term for aerosol liquid water concentrations in the regression. The results suggest that under these concentration conditions the presence of aerosol liquid water does not significantly increase or decrease SOA yields of the toluene oxidation products.