low-NOx Conditions Research Articles

Molecular dynamics investigation of IEPOX chemical behavior at the interface and in the bulk phase of acidic aerosols

Isoprene epoxydiol (IEPOX) is an important reactive gas-phase intermediate produced by the photooxidation of isoprene under low NOx conditions, playing a key role in the formation of secondary organic aerosols (SOA). Previous studies have mostly focused on the liquid-phase reactions of IEPOX within aerosols; however, interfacial heterogeneous chemical reactions are equally important in SOA formation. This study systematically explores the reaction mechanisms of IEPOX at the acidic aerosol interface and in the bulk phase using classical molecular dynamics (MD) and ab initio molecular dynamics simulations (AIMD). The study found that the free energy of IEPOX at the aerosol interface significantly decreases, indicating that interfacial heterogeneous chemical reactions are indispensable for the formation of IEPOX-derived SOA. The research reveals the formation pathways of 2-methyltetrols (2-MTO) and 1,3,4-trihydroxy-3-methylbutan-2-yl sulfates (2-MTOOS), finding that the protonation of the epoxy O atom and the cleavage of the C–O bond are the rate-controlling steps, while the nucleophilic addition is a spontaneous process. Through multiple sets of simulations, it was observed that the formation frequency of 2-MTO at the acidic aerosol interface and in the bulk phase reached 53.8%, significantly higher than the 30.8% of 2-MTOOS, which is consistent with field observation data. Additionally, through metadynamics (MTD) simulations, it was suggested that IEPOX could undergoes acid-catalyzed ring-opening reactions at the interface, potentially followed by the transfer of H atoms from primary alcohols into the aerosol, leading to the possible formation of the intermediate product 3-methylbut-3-ene-1,2,4-triol (one of the proposed structures of C5-alkene triols). These findings provide new insights into the formation mechanism of IEPOX-derived SOA and offer a scientific basis for future studies on their physicochemical properties and atmospheric fate.

Analytical optical methods for measuring organic peroxides and hydroperoxides: An evaluation

Hydrogen peroxide, organic peroxides and hydroperoxides exhibit high reactivity and play a pivotal role in atmospheric chemistry. These compounds are formed during the oxidation of volatile organic compounds in both gaseous and aqueous phases, particularly under low NOx conditions. Their significant contribution to the mass of secondary organic aerosols (SOA) is well-documented, and they are believed to have significant health implications. Several spectrophotometric methods have been employed to measure SOA-bound peroxides in laboratory samples, but systematic comparisons are lacking. In this study, we have assessed the advantages and limitations of these methods, including the traditional and microwave-assisted iodometric methods, the 4-nitrophenyl boronic acid assay (NPBA), and the Fenton reaction-assisted ferrous-xylenol (FOX2) assay. Besides, a comprehensive evaluation of these methodologies was conducted for the first time across a substantial cohort of commercial peroxides and hydroperoxides, employing diverse solvents (namely, water, 1-propanol, acetonitrile, methanol and chloroform) to provide broader insights compared to previous work. Ultimately, the four methods were applied and compared for peroxide determination in laboratory-generated SOA resulting from gas-phase ozonolysis of α-pinene. This study opens new opportunities for future mechanistic investigation into SOA formation and reactivity.

A comprehensive evaluation of enhanced temperature influence on gas and aerosol chemistry in the lamp-enclosed oxidation flow reactor (OFR) system

Abstract. Oxidation flow reactors (OFRs) have been extensively utilized to examine the formation of secondary organic aerosol (SOA). However, the UV lamps typically employed to initiate the photochemistry in OFRs can result in an elevated reactor temperature when their implications are not thoroughly evaluated. In this study, we conducted a comprehensive investigation into the temperature distribution within an Aerodyne potential aerosol mass OFR (PAM-OFR) and then examined the subsequent effects on flow and chemistry due to lamp heating. A lamp-induced temperature increase was observed, which was a function of lamp-driving voltage, number of lamps, lamp types, OFR residence time, and positions within the PAM-OFR. Under typical PAM-OFR operational conditions (e.g., < 5 d of equivalent atmospheric OH exposure under low-NOx conditions), the temperature increase typically ranged from 1–5 °C. Under extreme (but less frequently encountered) conditions, the heating could reach up to 15 °C. The influences of the increased temperature over ambient conditions on the flow distribution, gas, and condensed-phase chemistry within PAM-OFR were evaluated. Our findings indicate that the increase in temperature altered the flow field, resulting in a diminished tail on the residence time distribution and corresponding oxidant exposure due to faster recirculation. According to simulation results from a radical chemistry box model, the variation in absolute oxidant concentration within PAM-OFR due to temperature increase was minimal (< 5 %). The temperature influences on seed organic aerosol (OA) and newly formed secondary OA were also investigated, suggesting that an increase in temperature can impact the yield, size, and oxidation levels of representative biogenic and anthropogenic SOA types. Recommendations for temperature-dependent SOA yield corrections and PAM-OFR operating protocols that mitigate lamp-induced temperature enhancement and fluctuations are presented. We recommend blowing air around the reactor's exterior with fans during PAM-OFR experiments to minimize the temperature increase within PAM-OFR. Temperature increases are substantially lower for OFRs utilizing less powerful lamps compared to the Aerodyne version.

Production of oxygenated volatile organic compounds from the ozonolysis of coastal seawater

Abstract. Dry deposition of ozone (O3) to the ocean surface and the ozonolysis of organics in the sea surface microlayer (SSML) are potential sources of volatile organic compounds (VOCs) to the marine atmosphere. We use a gas chromatography system coupled to a Vocus proton-transfer-reaction time-of-flight mass spectrometer to determine the chemical composition and product yield of select VOCs formed from ozonolysis of coastal seawater collected from Scripps Pier in La Jolla, California. Laboratory-derived results are interpreted in the context of direct VOC vertical flux measurements made at Scripps Pier. The dominant products of laboratory ozonolysis experiments and the largest non-sulfur emission fluxes measured in the field correspond to Vocus CxHy+ and CxHyOz+ ions. Gas chromatography (GC) analysis suggests that C5–C11 oxygenated VOCs, primarily aldehydes, are the largest contributors to these ion signals. In the laboratory, using a flow reactor experiment, we determine a VOC yield of 0.43–0.62. In the field at Scripps Pier, we determine a maximum VOC yield of 0.04–0.06. Scaling the field and lab VOC yields for an average O3 deposition flux and an average VOC structure results in an emission source of 10.7 to 167 Tg C yr−1, competitive with the DMS source of approximately 20.3 Tg C yr−1. This study reveals that O3 reactivity to dissolved organic carbon can be a significant carbon source to the marine atmosphere and warrants further investigation into the speciated VOC composition from different seawater samples and the reactivities and secondary organic aerosol (SOA) yields of these molecules in marine-relevant, low NOx conditions.

Oxygenated organic molecules produced by low-NOx photooxidation of aromatic compounds: contributions to secondary organic aerosol and steric hindrance

Abstract. Oxygenated organic molecules (OOMs) produced by the oxidation of aromatic compounds are key components of secondary organic aerosol (SOA) in urban environments. The steric effects of substitutions and rings and the role of key reaction pathways in altering the OOM distributions remain unclear because of the lack of systematic multi-precursor study over a wide range of OH exposure. In this study, we conducted flow-tube experiments and used the nitrate adduct time-of-flight chemical ionization mass spectrometer (NO3--TOF-CIMS) to measure the OOMs produced by the photooxidation of six key aromatic precursors under low-NOx conditions. For single aromatic precursors, the detected OOM peak clusters show an oxygen atom difference of one or two, indicating the involvement of multi-step auto-oxidation and alkoxy radical pathways. Multi-generation OH oxidation is needed to explain the diverse hydrogen numbers in the observed formulae. In particular, for double-ring precursors at higher OH exposure, multi-generation OH oxidation may have significantly enriched the dimer formulae. The results suggest that methyl substitutions in precursor lead to less fragmented OOM products, while the double-ring structure corresponds to less efficient formation of closed-shell monomeric and dimeric products, both highlighting significant steric effects of precursor molecular structure on the OOM formation. Naphthalene-derived OOMs however have lower volatilities and greater SOA contributions than the other-type of OOMs, which may be more important in initial particle growth. Overall, the OOMs identified by the NO3--TOF-CIMS may have contributed up to 30.0 % of the measured SOA mass, suggesting significant mass contributions of less oxygenated, undetected semi-volatile products. Our results highlight the key roles of progressive OH oxidation, methyl substitution and ring structure in the OOM formation from aromatic precursors, which need to be considered in future model developments to improve the model performance for organic aerosol.

Atmospheric oxidation of (E)-β-farnesene initiated by hydroxyl radical: New formation mechanism of HOMs

(E)-β-farnesene is one of the acyclic sesquiterpenes, whose atmospheric oxidation has been determined as a significant contributor to SOA formation due to the formed highly oxidized multifunctional molecules (HOMs). In this work, the atmospheric oxidation of (E)-β-farnesene initiated by OH radical was comprehensively investigated using quantum chemical calculation. For the initial reactions, OH addition reactions are dominant over H atom abstraction reactions because of lower free energy barriers. The first-generation products involving acetone, (E)− 4-methyl-8-methylenedeca-4,9-dienal, 4-methylenehex-5-enal (P3), 6-methylhept-5-en-2-one (P4), formaldehyde, (E)− 7,11-dimethyldodeca-1,6,10-trien-3-one and (E)− 6,10-dimethyl-2-methyleneundeca-5,9-dienal are produced from the subsequent reactions of the OH-(E)-β-farnesene adducts with NO and HO2. The second-generation products, such as 4-oxopentanal, 4-methylenehex-5-enal, (E)− 4-methyl-8-oxodeca-4,9-dienal, formaldehyde and (E)− 4-methyl-8-methylenenon-4-enedial, are also determined from the successive oxidations of P2 and P3 under high and low NOx conditions. For the formation of HOMs, the H atom shift is the bottleneck for the autooxidation mechanism while our newly proposed mechanism, i.e. the multiple H abstractions by OH radical and reactions with O2 and HO2, is a more thermodynamically favorable formation pathway of the HOMs. Due to the intricate oxidation mechanisms and products, it is difficult to comprise the oxidation mechanisms of sesquiterpenes (including (E)-β-farnesene) in the atmospheric chemical models to thoroughly evaluate their contributions to SOA formation. We expect that this comprehensive oxidation mechanism of (E)-β-farnesene from a theoretical perspective would be conducive to clarifying the atmospheric fate of (E)-β-farnesene and even sesquiterpenes.

Characterization of products formed from the oxidation of toluene and m-xylene with varying NOx and OH exposure

Aromatic volatile organic compounds (VOCs) are an important precursor of secondary organic aerosol (SOA) in the urban environment. SOA formed from the oxidation of anthropogenic VOCs can be substantially more abundant than biogenic SOA and has been shown to account for a significant fraction of fine particulate matter in urban areas. A potential aerosol mass (PAM) chamber was used to investigate the oxidised products from the photo-oxidation of m-xylene and toluene. The experiments were carried out with OH radical as oxidant in both high- and low-NOx conditions and the resultant aerosol samples were collected using quartz filters and analysed by GC × GC-TOFMS. Results show the oxidation products derived from both precursors included ring-retaining and -opening compounds (unsaturated aldehydes, unsaturated ketones and organic acids) with a high number of ring-opening compounds observed from toluene oxidation. Glyoxal and methyl glyoxal were the major ring-cleavage products from both oxidation systems, indicating that a bicyclic route plays an important role in their formation. SOA yields were higher for both precursors under high-NOx (toluene: 0.111; m-xylene: 0.124) than at low-NOx (toluene: 0.089; m-xylene: 0.052), likely linked to higher OH concentrations during low-NOx experiments which may lead to higher degree of fragmentation. DHOPA (2,3-dihydroxy-4-oxo-pentanoic acid), a known tracer of toluene oxidation, was observed in both oxidation systems. The mass fraction of DHOPA in SOA from toluene oxidation was about double the value reported previously, but it should not be regarded as a tracer solely for oxidation of toluene as m-xylene oxidation gave a similar relative yield.

Isothermal evaporation of α-pinene secondary organic aerosol particles formed under low NOx and high NOx conditions

Abstract. Many recent secondary organic aerosol (SOA) studies mainly focus on biogenic SOA particles formed under low NOx conditions and thus are applicable to pristine environments with minor anthropogenic influence. Although interactions between biogenic volatile organic compounds and NOx are important in, for instance, suburban areas, there is still a lack of knowledge about the volatility and processes controlling the evaporation of biogenic SOA particles formed in the presence of high concentrations of NOx. Here we provide detailed insights into the isothermal evaporation of α-pinene SOA particles that were formed under low NOx and high NOx conditions to investigate the evaporation process and the evolution of particle composition during the evaporation in more detail. We coupled Filter Inlet for Gases and AEROsols-Chemical Ionization Mass Spectrometer (FIGAERO-CIMS) measurements of the molecular composition and volatility of the particle phase with isothermal evaporation experiments conducted under a range of relative humidity (RH) conditions from low RH (<7 % RH) to high RH (80 % RH). Very similar changes were observed in particle volatility at any set RH during isothermal evaporation for the α-pinene SOA particles formed under low NOx and high NOx conditions. However, there were distinct differences in the initial composition of the two SOA types, possibly due to the influence of NOx on the RO2 chemistry during SOA formation. Such compositional differences consequently impacted the primary type of aqueous-phase processes in each type of SOA particle in the presence of particulate water.

Source and variability of formaldehyde (HCHO) at northern high latitudes: an integrated satellite, aircraft, and model study

Abstract. Here we use satellite observations of formaldehyde (HCHO) vertical column densities (VCD) from the TROPOspheric Monitoring Instrument (TROPOMI), aircraft measurements, combined with a nested regional chemical transport model (GEOS-Chem at 0.5×0.625∘ resolution), to better understand the variability and sources of summertime HCHO in Alaska. We first evaluate GEOS-Chem with in-situ airborne measurements during the Atmospheric Tomography Mission 1 (ATom-1) aircraft campaign. We show reasonable agreement between observed and modeled HCHO, isoprene, monoterpenes and the sum of methyl vinyl ketone and methacrolein (MVK+MACR) in the continental boundary layer. In particular, HCHO profiles show spatial homogeneity in Alaska, suggesting a minor contribution of biogenic emissions to HCHO VCD. We further examine the TROPOMI HCHO product in Alaska in summer, reprocessed by GEOS-Chem model output for a priori profiles and shape factors. For years with low wildfire activity (e.g., 2018), we find that HCHO VCDs are largely dominated by background HCHO (58 %–71 %), with minor contributions from wildfires (20 %–32 %) and biogenic VOC emissions (8 %–10 %). For years with intense wildfires (e.g., 2019), summertime HCHO VCD is dominated by wildfire emissions (50 %–72 %), with minor contributions from background (22 %–41 %) and biogenic VOCs (6 %–10 %). In particular, the model indicates a major contribution of wildfires from direct emissions of HCHO, instead of secondary production of HCHO from oxidation of larger VOCs. We find that the column contributed by biogenic VOC is often small and below the TROPOMI detection limit, in part due to the slow HCHO production from isoprene oxidation under low NOx conditions. This work highlights challenges for quantifying HCHO and its precursors in remote pristine regions.

Reactions of C12–C14 n-Alkylcyclohexanes with Cl Atoms: Kinetics and Secondary Organic Aerosol Formation

Long-chain alkanes are a type of intermediate volatility organic compound (IVOC) in the atmosphere and a potential source of secondary organic aerosols (SOAs). C12-C14n-alkylcyclohexanes are important compositions of IVOCs, with considerable concentrations and emission rates. The reaction rate constants and SOA formation of the reactions of C12-C14n-alkylcyclohexanes with Cl atoms were investigated in the present study. The reaction rate constants of the long-chain alkanes obtained via the relative-rate method at 298 ± 0.2 K (in units of ×10-10 cm3 molecule-1 s-1) were as follows: khexylcyclohexane = 5.11 ± 0.28, kheptylcyclohexane = 5.56 ± 0.30, and koctylcyclohexane = 5.74 ± 0.31. The gas-phase products of the reactions were identified as mainly small molecules of aldehydes, ketones, and acids. The particle-phase products were mostly monomers and oligomers, but there were still trimers even under high-NOx conditions. Moreover, under high-NOx conditions (urban atmosphere), the SOA yields of hexylcyclohexane are higher than that under low-NOx conditions (remote atmosphere), indicating that more attention should be given to the SOA formation of Cl-initiated n-alkylcyclohexane oxidations in polluted regions. This research can further clarify the oxidation processes and SOA formation of n-alkylcyclohexanes in the atmosphere.

Secondary organic aerosol formation from photooxidation of C3H6 under the presence of NH3: Effects of seed particles

Frequently-occurred secondary organic aerosols (SOAs) under low-NOx conditions contribute to the winter haze episodes and remain unclear in the abundant presence of NH3. Here, the effects of CaCl2 seed particles on the photooxidation of low-molecular-weight C3H6 with co-existing NO2 and NH3 were highlighted and investigated through a chamber-simulation study equipped with high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR–ToF–MS). The influences of NH3 are often overestimated to exclusively enhance SOA yields under a low-[NO2]0 condition. Instead, the seeds played a central role in the heterogeneous formation of SOAs in this reaction with two orders of magnitudes higher than that in the absence of seeds at relative humidity (RH) of 82%. Interestedly, the O3 production was unchanged whether the seeds existed or not, small changes in the production of O3 were observed whether the seeds existed or not, indicating that the gas-phase conversions of C3H6 and NOx into C1–C3 oxygenated volatile organic compounds (OVOCs) and nitrogen-containing compounds (NOCs) were not affected by seed particles. Given that the ensuing formation of these low-volatile compounds was condensed into nucleation on the seeds, the explosive growth of C3H6 SOAs was then stimulated in the addition of NH3. Besides NO2 photolysis, the producing O3 was related to the formation of secondary carbonyls such as formaldehyde and then was consumed in the ·OH generation of approximately 3.40 × 10−12 molecules cm−3. This study provides a new insight to better understand the new gas-to-particle formation mechanisms when the haze pollution outbreaks in the complex air mixture.

Modelling the gas–particle partitioning and water uptake of isoprene-derived secondary organic aerosol at high and low relative humidity

Abstract. This study presents a characterization of the hygroscopic growth behaviour and effects of different inorganic seed particles on the formation of secondary organic aerosols (SOAs) from the dark ozone-initiated oxidation of isoprene at low NOx conditions. We performed simulations of isoprene oxidation using a gas-phase chemical reaction mechanism based on the Master Chemical Mechanism (MCM) in combination with an equilibrium gas–particle partitioning model to predict the SOA concentration. The equilibrium model accounts for non-ideal mixing in liquid phases, including liquid–liquid phase separation (LLPS), and is based on the AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) model for mixture non-ideality and the EVAPORATION (Estimation of VApour Pressure of ORganics, Accounting for Temperature, Intramolecular, and Non-additivity effects) model for pure compound vapour pressures. Measurements from the Cosmics Leaving Outdoor Droplets (CLOUD) chamber experiments, conducted at the European Organization for Nuclear Research (CERN) for isoprene ozonolysis cases, were used to aid in parameterizing the SOA yields at different atmospherically relevant temperatures, relative humidity (RH), and reacted isoprene concentrations. To represent the isoprene-ozonolysis-derived SOA, a selection of organic surrogate species is introduced in the coupled modelling system. The model predicts a single, homogeneously mixed particle phase at all relative humidity levels for SOA formation in the absence of any inorganic seed particles. In the presence of aqueous sulfuric acid or ammonium bisulfate seed particles, the model predicts LLPS to occur below ∼ 80 % RH, where the particles consist of an inorganic-rich liquid phase and an organic-rich liquid phase; however, this includes significant amounts of bisulfate and water partitioned to the organic-rich phase. The measurements show an enhancement in the SOA amounts at 85 % RH, compared to 35 % RH, for both the seed-free and seeded cases. The model predictions of RH-dependent SOA yield enhancements at 85 % RH vs. 35 % RH are 1.80 for a seed-free case, 1.52 for the case with ammonium bisulfate seed, and 1.06 for the case with sulfuric acid seed. Predicted SOA yields are enhanced in the presence of an aqueous inorganic seed, regardless of the seed type (ammonium sulfate, ammonium bisulfate, or sulfuric acid) in comparison with seed-free conditions at the same RH level. We discuss the comparison of model-predicted SOA yields with a selection of other laboratory studies on isoprene SOA formation conducted at different temperatures and for a variety of reacted isoprene concentrations. Those studies were conducted at RH levels at or below 40 % with reported SOA mass yields ranging from 0.3 % up to 9.0 %, indicating considerable variations. A robust feature of our associated gas–particle partitioning calculations covering the whole RH range is the predicted enhancement of SOA yield at high RH (> 80 %) compared to low RH (dry) conditions, which is explained by the effect of particle water uptake and its impact on the equilibrium partitioning of all components.

Variability in Aromatic Aerosol Yields under Very Low NOx Conditions at Different HO2/RO2 Regimes.

Current chemical transport models generally use a constant secondary organic aerosol (SOA) yield to represent SOA formation from aromatic compounds under low NOx conditions. However, a wide range of SOA yields (10 to 42%) from m-xylene under low NOx conditions is observed in this study. The chamber HO2/RO2 ratio is identified as a key factor explaining SOA yield variability: higher SOA yields are observed for runs with a higher HO2/RO2 ratio. The RO2 + RO2 pathway, which can be increasingly significant under low NOx and HO2/RO2 conditions, shows a lower SOA-forming potential compared to the RO2 + HO2 pathway. While the traditional low-NOx chamber experiments are commonly used to represent the RO2 + HO2 pathway, this study finds that the impacts of the RO2 + RO2 pathway cannot be ignored under certain conditions. We provide guidance on how to best control for these two pathways in conducting chamber experiments to best obtain SOA yield curves and quantify the contributions from each pathway. On the global scale, the chemical transport model GEOS-Chem is used to identify regions characterized by lower surface HO2/RO2 ratios, suggesting that the RO2 + RO2 pathway is more likely to prove significant to overall SOA yields in those regions. Current models generally do not consider the RO2 + RO2 impacts on aromatic SOA formation, but preliminary sensitivity tests with updated SOA yield parameters based on such a pathway suggest that without this consideration, some types of SOA may be overestimated in regions with lower HO2/RO2 ratios.

Atmospheric Fate of the CH3SOO Radical from the CH3S + O2 Equilibrium.

The atmospheric oxidation mechanisms of reduced sulfur compounds are of great importance in the biogeochemical sulfur cycle. The CH3S radical represents an important intermediate in these oxidation processes. Under atmospheric conditions, CH3S will predominantly react with O2 to form the peroxy radical CH3SOO. The formed CH3SOO has two competing unimolecular reaction pathways: isomerization to CH3SO2, which further decomposes into CH3 and SO2, or a hydrogen shift followed by HO2 loss, leading to CH2S. Previous theoretical calculations have suggested that CH2S formation should be the dominant pathway, in disagreement with existing experimental results. Our large active space multireference configuration interaction calculations agree with the experimental results that the formation of CH3 and SO2 is the dominant route and the formation of CH2S and HO2 can, at most, be a minor pathway. We support the calculations with new experiments starting from the OH + CH3SH reaction for CH3S formation under low NOx conditions and find a SO2 yield of 0.86 ± 0.18 within our reaction time of 7.9 s. Model simulations of our experiments show that the SO2 yield converges to 0.98. This combined theoretical and experimental study thus furthers the understanding of the general oxidation mechanisms of sulfur compounds in the atmosphere.

Gas-Phase Chlorine Radical Oxidation of Alkanes: Effects of Structural Branching, NOx, and Relative Humidity Observed during Environmental Chamber Experiments

Chlorine-initiated oxidation of alkanes has been shown to rapidly form secondary organic aerosol (SOA) at higher yields than OH-alkane reactions. However, the effects of alkane volatile organic compound precursor structure and the reasons for the differences in SOA yield from OH-alkane reactions remain unclear. In this work, we investigated the effects of alkane molecular structure on oxidation by chlorine radical (Cl) and resulting formation of SOA through a series of laboratory chamber experiments, utilizing data from an iodide chemical ionization mass spectrometer and an aerosol chemical speciation monitor. Experiments were conducted with linear, branched, and branched cyclic C10 alkane precursors under different NOx and RH conditions. Observed product fragmentation patterns during the oxidation of branched alkanes demonstrate the abstraction of primary hydrogens by Cl, confirming a key difference between OH- and Cl-initiated oxidation of alkanes and providing a possible explanation for higher SOA production from Cl-initiated oxidation. Low-NOx conditions led to higher SOA production. SOA formed from butylcyclohexane under low NOx conditions contained higher fractions of organic acids and lower volatility molecules that were less prone to oligomerization relative to decane SOA. Branched alkanes produced less SOA, and branched cycloalkanes produced more SOA than linear n-alkanes, consistent with past work on OH-initiated reactions. Overall, our work provides insights into the differences between Cl- and OH-initiated oxidation of alkanes of different structures and the potential significance of Cl as an atmospheric oxidant.

Secondary Organic Aerosol Formation of Fleet Vehicle Emissions in China: Potential Seasonality of Spatial Distributions.

Vehicle emissions are an important source of urban particular matter. To investigate the secondary organic aerosol (SOA) formation potential of real-world vehicle emissions, we exposed on-road air in Beijing to hydroxyl radicals generated in an oxidation flow reactor (OFR) under high-NOx conditions on-board a mobile laboratory and characterized SOA and their precursors with a suite of state-of-the-art instrumentation. The OFR produced 10-170 μg m-3 of SOA with a maximum SOA formation potential of 39-50 μg m-3 ppmv-1 CO that occurred following an integrated OH exposure of (1.3-2.0) × 1011 molecules cm-3 s. The results indicate relatively shorter photochemical ages for maximum SOA production than previous OFR results obtained under low-NOx conditions. Such timescales represent the balance of functionalization and fragmentation, possibly resulting in different spatial distributions of SOA in different seasons as the oxidant level changes. The detected precursors may explain as much as 13% of the observed SOA with the remaining plausibly contributed by the oxidation of undetected intermediate-volatility organic compounds. Extrapolation of the results suggests an annual SOA production rate of 0.78 Tg yr-1 from mobile gasoline sources in China, highlighting the importance of effective regulation of gaseous vehicular precursors to improve air quality in the future.

Effect of NOx on secondary organic aerosol formation from the photochemical transformation of allyl acetate

Smog chamber experiments were conducted to explore the photochemical degradation of allyl acetate (AA) in the presence of SO2 and to assess the role of NOx on the secondary organic aerosol (SOA) formation potential. A one-product gas-particle partitioning absorption model was used to express the formed organic aerosol. The experimental results showed a clear dependence of SOA formation potential on the NOx level. The particle mass concentrations at low NOx conditions (AA/NOx > 3) were commonly higher than those at high NOx conditions (AA/NOx < 3). The SOA yields increased with the increasing NOx concentration under low NOx conditions and decreased with the increasing NOx concentration under high NOx conditions due to the different reaction pathways of RO2 radicals. Gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) analysis suggested that organic nitrates and organic sulfates were formed during photochemistry experiments. Based on detected products, the mechanisms of AA photooxidation were proposed. The results of this study show the different effects of NOx concentrations on promoting and inhibiting aerosol formation, and highlight the pronounced formation of SOA from the photooxidation of AA in polluted environment.

Chemical characterisation of benzene oxidation products under high- and low-NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; conditions using chemical ionisation mass spectrometry

Abstract. Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban volatile organic compound (VOC) emissions that contribute to the formation of secondary organic aerosol (SOA). Benzene is one of the most abundant species emitted from vehicles, biomass burning and industry. An iodide time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the Jülich Plant Atmosphere Chamber as part of a series of experiments examining benzene oxidation by OH under high- and low-NOx conditions, where a range of organic oxidation products were detected. The nitrate scheme detects many oxidation products with high masses, ranging from intermediate volatile organic compounds (IVOCs) to extremely low volatile organic compounds (ELVOCs), including C12 dimers. In comparison, very few species with C≥6 and O≥8 were detected with the iodide scheme, which detected many more IVOCs and semi-volatile organic compounds (SVOCs) but very few ELVOCs and low volatile organic compounds (LVOCs). A total of 132 and 195 CHO and CHON oxidation products are detected by the iodide ToF-CIMS in the low- and high-NOx experiments respectively. Ring-breaking products make up the dominant fraction of detected signal and 21 and 26 of the products listed in the Master Chemical Mechanism (MCM) were detected. The time series of highly oxidised (O≥6) and ring-retaining oxidation products (C6 and double-bond equivalent = 4) equilibrate quickly, characterised by a square form profile, compared to MCM and ring-breaking products which increase throughout oxidation, exhibiting sawtooth profiles. Under low-NOx conditions, all CHO formulae attributed to radical termination reactions of first-generation benzene products, and first-generation auto-oxidation products are observed. Several N-containing species that are either first-generation benzene products or first-generation auto-oxidation products are also observed under high-NOx conditions. Hierarchical cluster analysis finds four clusters, of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio, indicating it is sensitive to NO concentration and thus likely to contain NO addition products and alkoxy-derived termination products. This cluster has the highest average carbon oxidation state (OSC‾) and the lowest average carbon number. Where nitrogen is present in a cluster member of cluster 2, the oxygen number is even, as expected for alkoxy-derived products. In contrast, cluster 1 shows no dependency on the NO2/NOx ratio and so is likely to contain more NO2 addition and peroxy-derived termination products. This cluster contains fewer fragmented species, as the average carbon number is higher and OSC‾ lower than cluster 2, and more species with an odd number of oxygen atoms. This suggests that clustering of time series which have features pertaining to distinct chemical regimes, for example, NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.

Impact of NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; on secondary organic aerosol (SOA) formation from &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-pinene and &amp;lt;i&amp;gt;β&amp;lt;/i&amp;gt;-pinene photooxidation: the role of highly oxygenated organic nitrates

Abstract. The formation of organic nitrates (ONs) in the gas phase and their impact on mass formation of secondary organic aerosol (SOA) was investigated in a laboratory study for α-pinene and β-pinene photooxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NOx, and therein the role of highly oxygenated multifunctional molecules (HOMs). We observed that with increasing NOx concentration (a) the portion of HOM organic nitrates (HOM-ONs) increased, (b) the fraction of accretion products (HOM-ACCs) decreased, and (c) HOM-ACCs contained on average smaller carbon numbers. Specifically, we investigated HOM organic nitrates (HOM-ONs), arising from the termination reactions of HOM peroxy radicals with NOx, and HOM permutation products (HOM-PPs), such as ketones, alcohols, or hydroperoxides, formed by other termination reactions. Effective uptake coefficients γeff of HOMs on particles were determined. HOMs with more than six O atoms efficiently condensed on particles (γeff&gt;0.5 on average), and for HOMs containing more than eight O atoms, every collision led to loss. There was no systematic difference in γeff for HOM-ONs and HOM-PPs arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOMs: as functional groups in HOMs arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character of the final termination group. As a consequence, the suppressing effect of NOx on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups. According to their γeff all HOM-ONs with more than six O atoms will contribute to organic bound nitrate (OrgNO3) in the particulate phase. However, the fraction of OrgNO3 stored in condensable HOMs with molecular masses &gt; 230 Da appeared to be substantially higher than the fraction of particulate OrgNO3 observed by aerosol mass spectrometry. This result suggests losses of OrgNO3 for organic nitrates in particles, probably due to hydrolysis of OrgNO3 that releases HNO3 into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO3 alone could not explain the observed suppressing effect of NOx on particle mass formation from α-pinene and β-pinene. Instead we can attribute most of the reduction in SOA mass yields with increasing NOx to the significant suppression of gas phase HOM-ACCs, which have high molecular mass and are potentially important for SOA mass formation at low-NOx conditions.

Effects of NOx and SO2 on the secondary organic aerosol formation from the photooxidation of 1,3,5-trimethylbenzene: A new source of organosulfates

1,3,5-Trimethylbeneze (TMB) is an important constituent of anthropogenic volatile organic compounds that contributes to the formation of secondary organic aerosol (SOA). A series of chamber experiments were performed to probe the effects of NOx and SO2 on SOA formation from TMB photooxidation. The molecular composition of TMB SOA was investigated by ultra-high performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). We found that the SOA yield increases notably with elevated NOx concentrations under low-NOx condition ([TMB]0/[NOx]0 > 10 ppbC ppb−1), while an opposite trend is observed in high-NOx experiments ([TMB]0/[NOx]0 < 10 ppbC ppb−1). The increase in SOA yield in low-NOx regime is attributed to the increase of NOx-induced OH concentrations. The formation of low-volatility species might be suppressed, thereby leading to a lower SOA yield in high-NOx conditions. Moreover, SOA formation was promoted in experiment with SO2 addition. Multifunctional products containing carbonyl, acid, alcohol, and nitrate functional groups were characterized in TMB/NOx photooxidation, whereas several organosulfates (OSs) and nitrooxy organosulfates were identified in TMB/NOx/SO2 photooxidation based on HR-Q-TOFMS analysis. The formation mechanism relevant to the detected compounds in SOA were proposed. Based on our measurements, the photooxidation of TMB in the presence of SO2 may be a new source of OSs in the atmosphere. The results presented here also deepen the understanding of SOA formation under relatively complex polluted environments.