High Secondary Organic Aerosol Yields Research Articles

Unpacking the diversity of monoterpene oxidation pathways via nitrooxy–alkyl radical ring-opening reactions and nitrooxy–alkoxyl radical bond scissions

Terrestrial vegetation emits vast quantities of monoterpenes to the atmosphere. These compounds, once oxidized, can contribute to the formation and growth of secondary organic aerosol (SOA) particles. However, studies report widely different SOA yields from atmospheric oxidation of different monoterpenes, despite their structural similarities. The NO3-radical-initiated oxidation of α-pinene for instance, leads to minimal SOA yields, whereas with Δ-carene a high SOA yield is observed. A previous study indicated that their oxidation mechanisms diverge after formation of a nitrooxy–alkoxyl radical intermediate, whose C–C bond scission reactions can either lead to early termination of the oxidative chain, thus limiting the yield of condensable vapors, or further propagate it, leading to low-volatility products. In this study, we employ computational methods to investigate these reactions in the NO3-radical oxidation of five other monoterpenes: limonene, sabinene, β-pinene, α-thujene and camphene. Additionally, we explore the possibility of rearrangement via ring-opening of the nitrooxy-alkyl radical adducts produced immediately after NO3 radical attack. Our calculations predict that alkyl radical rearrangement is dominant over O2-addition for sabinene, minor but competitive for α-thujene and β-pinene, and negligible for camphene. These rearrangements can induce further propagation of the oxidative chain, and thus higher SOA yields. Concerning alkoxyl radical C–C bond scissions, our results indicate that endocyclic nitrate species (derived from limonene and α-thujene) react preferentially via the channel leading to oxidative chain termination, whereas exocyclic nitrate species (sabinene, β-pinene, and camphene) react preferentially via channels leading to further propagation.

Combined Smog Chamber/Oxidation Flow Reactor Study on Aging of Secondary Organic Aerosol from Photooxidation of Aromatic Hydrocarbons.

Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter (PM2.5), and their physicochemical properties can be significantly changed in the aging process. In this study, we used a combination consisting of a smog chamber (SC) and oxidation flow reactor (OFR) to investigate the continuous aging process of gas-phase organic intermediates and SOA formed from the photooxidation of toluene, a typical aromatic hydrocarbon. Our results showed that as the OH exposure increased from 2.6 × 1011 to 6.3 × 1011 molecules cm-3 s (equivalent aging time of 2.01-4.85 days), the SOA mass concentration (2.9 ± 0.05-28.7 ± 0.6 μg cm-3) and corrected SOA yield (0.073-0.26) were significantly enhanced. As the aging process proceeds, organic acids and multiple oxygen-containing oxidation products are continuously produced from the photochemical aging process of gas-phase organic intermediates (mainly semi-volatile and intermediate volatility species, S/IVOCs). The multigeneration oxidation products then partition to the aerosol phase, while functionalization of SOA rather than fragmentation dominated in the photochemical aging process, resulting in much higher SOA yield after aging compared to that in the SC. Our study indicates that SOA yields as a function of OH exposure should be considered in air quality models to improve SOA simulation, and thus accurately assess the impact on SOA properties and regional air quality.

Secondary organic aerosol formation from mixed volatile organic compounds: Effect of RO2 chemistry and precursor concentration

Secondary organic aerosol (SOA) plays a significant role in contributing to atmospheric fine particles, as well as in global air quality and climate. However, the current understanding of the atmospheric formation of SOA and its simulation is still highly uncertain due to the complexity of its precursor VOCs. In our study, SOA formation in different mixed VOC scenarios was investigated using a 30 m3 indoor smog chamber. By comparing SOA formation in individual VOC scenarios, it was found that SOA yield from anthropogenic VOCs (AVOCs) can be positively (+83.9%) affected by coexisting AVOCs, while inhibited (−51.4%) by the presence of isoprene, via the OH scavenging effect. The cross-reactions of peroxyl radical (RO2) generated from different AVOCs were proved to be the main contributor (up to 39.0%) to SOA formation, highlighting the importance of RO2 + RʹO2 reactions in mixed VOC scenarios. Meanwhile, the formation of gas-phase organic intermediates of different volatility categories from the RO2 reactions was also affected by the precursor concentration, and a higher SOA yield was found at lower precursor concentrations due to the larger contribution of intermediates with lower volatility. Our study provides new insights into SOA formation by considering the interactions between intermediate products from mixed VOCs.

A lumped species approach for the simulation of secondary organic aerosol production from intermediate-volatility organic compounds (IVOCs): application to road transport in PMCAMx-iv (v1.0)

Abstract. Secondary organic aerosol (SOA) is formed in the atmosphere through the oxidation and condensation of organic compounds. Intermediate-volatility compounds (IVOCs), compounds with effective saturation concentration (C∗) at 298 K between 103 and 106 µg m−3, have high SOA yields and can be important SOA precursors. The first efforts to simulate IVOCs in chemical transport models (CTMs) used the volatility basis set (VBS), a highly parametrized scheme that oversimplifies their chemistry. In this work we propose a more detailed approach for simulating IVOCs in CTMs, treating them as lumped species that retain their chemical characteristics. Specifically, we introduce four new lumped species representing large alkanes, two lumped species representing polyaromatic hydrocarbons (PAHs) and one species representing large aromatics, all in the IVOC range. We estimate IVOC emissions from road transport using existing estimates of volatile organic compound (VOC) emissions and emission factors of individual IVOCs from experimental studies. Over the European domain, for the simulated period of May 2008, estimated IVOC emissions from road transport were about 21 Mmol d−1, a factor of 8 higher than emissions used in previous VBS applications. The IVOC emissions from diesel vehicles were significantly higher than those from gasoline ones. SOA yields under low-NOx and high-NOx conditions for the lumped IVOC species were estimated based on recent smog chamber studies. Large cyclic alkane compounds have both high yields and high emissions, making them an important, yet understudied, class of IVOCs.

Secondary organic aerosol formation from the oxidation of decamethylcyclopentasiloxane at atmospherically relevant OH concentrations

Abstract. Decamethylcyclopentasiloxane (D5, C10H30O5Si5) is measured at parts per trillion (ppt) levels outdoors and parts per billion (ppb) levels indoors. Primarily used in personal care products, its outdoor concentration is correlated to population density. Since understanding the aerosol formation potential of volatile chemical products is critical to understanding particulate matter in urban areas, the secondary organic aerosol yield of D5 was studied under a wide range of OH concentrations and, correspondingly, OH exposures using both batch-mode chamber and continuously run flow tube experiments. These results were comprehensively analyzed and compared to two other secondary organic aerosol (SOA) yield datasets from literature. It was found that the SOA yield from the oxidation of D5 is extremely dependent on either the OH concentration or exposure. For OH concentrations of ≲ 107 molec.cm-3 or OH exposures of ≲ 2 × 1011 molec.scm-3, the SOA yield is largely < 5 % and usually ∼ 1 %. This is significantly lower than SOA yields previously reported. Using a two-product absorptive partitioning model for the upper bound SOA yields, the stoichiometric mass fraction and absorptive partitioning coefficients are, for the first product, α1 = 0.056 and KOM,1 = 0.022 m3 µg−1; for the second product, they are α2 = 7.7 and KOM,2 = 4.3 × 10−5 m3 µg−1. Generally, there are high SOA yields (> 90 %) at OH mixing ratios of 5 × 109 molec.cm-3 or OH exposures above 1012 molec.scm-3.

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.

Significant secondary organic aerosol production from aqueous-phase processing of two intermediate volatility organic compounds

Intermediate volatility organic compounds (IVOCs) recently have been identified as an important group of air pollutants, and they can contribute significantly to the secondary organic aerosol (SOA) formation via gas-phase reactions. However, significance of the SOA from aqueous oxidation of IVOCs was largely unknown. Here, we investigated the aqueous aging of two model IVOCs of naphthoquinone (NAQ) and phenanthrene (PHE) at a staring concentration of 10 μM. Both species were found to have high SOA yields (∼50%) against hydroxyl radical photochemical oxidation. PHE can be slowly oxidized under dark conditions (∼10% in 8 h) while NAQ cannot, but it can photo-degrade itself under simulated sunlight at a high rate (∼56% in 1 h). A group of oxygenated products were formed and its concentrations increased continuously throughout hydroxyl radical-mediated aging of both precursors. Aqueous aging led to enhancements of light absorption, especially for NAQ, and the oxygenated products were likely light chromophores. Our findings highlight the possible significance of SOA from aqueous oxidation of IVOCs. Future investigations on a broad spectrum of IVOCs and inclusion of such SOA formation pathway into climate and air quality models should be considered.

SOA and gas phase organic acid yields from the sequential photooxidation of seven monoterpenes

We investigate secondary organic aerosol (SOA) and gas phase organic acid yields from the sequential photooxidation of seven monoterpene isomers (α-pinene, β-pinene, limonene, sabinene, terpinolene, α-terpinene, and γ-terpinene) using an Oxidative Flow Reactor under dry conditions. SOA yields were highest for terpinolene (33% at 5.7 days of aging), followed by sabinene, β-pinene, α-pinene, limonene, γ-terpinene, and α-terpinene. Isomers with exocyclic double bonds (i.e. immediately adjacent to a ring) had higher SOA yields than those with endocyclic double bonds, or with double bonds that are unattached to a ring structure. SOA yields increased with OH exposure, highlighting the limitation of using single values for SOA yield in modeling studies, and the need for isomer-specific SOA parameterizations. SOA yields were adequately fit by a one-product model and in broad agreement with previous studies. SOA yields linearly increased with organic aerosol mass concentration, possibly the result of high OH loading and short residence times in the flow reactor. Gas phase yields of formic, acetic, butyric, and methacrylic acid (or their isomers) followed very different patterns as a function of OH exposure from SOA yields, and were poorly correlated with OH rate constants. These observations suggest that higher volatility (fragmentation) products of monoterpene photooxidation were produced and lost on different timescales from the production and condensation of lower-volatility (functionalization) products. Formic acid yields ranged from 0.06 to 9.3% across all OH exposures. Formic acid yields from γ-terpinene (0.36–3%) monotonically increased with OH exposure, unlike the other monoterpene isomers, which exhibited initial increases in formic acid yields with OH exposure, followed by decreases at higher OH concentrations. This difference in organic acid yield trends is consistent with the distribution of carbon-containing species identified by chemical ionization mass spectrometry.

Influence of NH3 on secondary organic aerosols from the ozonolysis and photooxidation of α-pinene in a flow reactor

This study presents detailed characterizations of a newly-developed flow reactor including (1) residence time distribution measurements, (2) relative humidity (RH) and temperature control, and (3) OH radical exposure range (i.e., atmospheric aging time). Hydroxyl (OH) radical exposures ranged from 8.20 × 1010 to 7.22 × 1011 molecules cm−3 s (0.5–4.9 d of atmospheric aging). In this study, the effects of NH3 gas on the secondary organic aerosol (SOA) formation of α-pinene by dark ozonolysis and photooxidation were investigated using the newly-developed flow reactor. For both dark α-pinene ozonolysis and photooxidation, higher SOA yields were observed in the presence of NH3 than in the absence of NH3. At RH of ∼50%, the SOA yield for ozonolysis and photooxidation in the presence of NH3 increased by 23% and 15% relative to those in the absence of NH3. Similar effects were observed at lower and higher RH conditions. Fourier transform infrared spectroscopy analysis confirmed the presence of nitrogen-containing functional groups in SOA formed in the presence of NH3. The α-pinene SOA formed in the presence of NH3 showed higher absorption and fluorescence for UV-visible radiation than those formed in the absence of NH3.

Secondary organic aerosol formation from acyclic, monocyclic, and polycyclic alkanes.

A large number of organic species emitted into the atmosphere contain cycloalkyl groups. While cyclic species are believed to be important secondary organic aerosol (SOA) precursors, the specific role of cyclic moieties (particularly for species with multiple or fused rings) remains uncertain. Here we examine the yields and composition of SOA formed from the reaction of OH with a series of C10 (cyclo)alkanes, with 0-3 rings, in order to better understand the role of multiple cyclic moieties on aerosol formation pathways. A chamber oxidation technique using high, sustained OH radical concentrations was used to simulate long reaction times in the atmosphere. This aging technique leads to higher yields than in previously reported chamber experiments. Yields were highest for cyclic and polycyclic precursors, though yield exhibited little dependence on number of rings. However, the oxygen-to-carbon ratio of the SOA was highest for the polycyclic precursors. These trends are consistent with aerosol formation requiring two generations of oxidation and 3-4 oxygen-containing functional groups in order to condense. Cyclic, unbranched structures are protected from fragmentation during the first oxidation step, with C-C bond scission instead leading to ring opening, efficient functionalization, and high SOA yields. Fragmentation may occur during subsequent oxidation steps, limiting yields by forming volatile products. Polycyclic structures can undergo multiple ring opening reactions, but do not have markedly higher yields, likely due to enhanced fragmentation in the second oxidation step. By contrast, C-C bond scission for the linear and branched structures leads to fragmentation prior to condensation, resulting in low SOA yields. The results highlight the key roles of multigenerational chemistry and susceptibility to fragmentation in the formation and evolution of SOA.

Development and characterization of an exposure generation system to investigate the health effects of particles from fresh and aged traffic emissions

Atmospheric photochemical reactions of vehicular primary emissions result in the formation of secondary organic aerosol (SOA). This is the first study that has investigated the toxicity of secondary particles based on fleet vehicular emissions. We developed methods for photochemical oxidation of traffic primary emissions to produce mixtures of primary and/or secondary particles suitable for animal exposures. The exposure generation system produced test atmospheres of primary (P), aged primary plus SOA (P + SOA), or SOA particles suitable for animal exposures. The system consists of (1) a sampling system to extract the traffic emissions from the plenum of a highway tunnel ventilation stack, (2) a photochemical chamber to simulate atmospheric aging, and (3) a nonselective diffusion denuder to remove gaseous pollutants prior to exposure. In the presence of traffic primary particles (P + SOA), a longer mean residence time resulted in a higher SOA yield. Higher baseline plenum primary particle mass concentration resulted in lower SOA yield. In the absence of primary particles (SOA), higher plenum gas concentrations resulted in higher SOA yield. Secondary aerosol was largely organic but contained some nitrate and sulfate. Formation of secondary aerosol is influenced significantly by reaction of primary gases with ·OH. The system (1) provides adequate flow and stable chamber output of P, P + SOA, and SOA for characterization and animal exposures and (2) generates reproducible exposure atmospheres of P, P + SOA, and SOA, all at consistent mass concentrations.

Emission of sunscreen salicylic esters from desert vegetation and their contribution to aerosol formation

Abstract. Biogenic volatile organic compounds (BVOC) produced by plants are known to have an important role in atmospheric chemistry. However, our knowledge of the range of BVOCs produced by different plant processes is still expanding, and there remain poorly understood categories of BVOCs. In this study, emissions of a novel class of BVOC emissions were investigated in a desert region. Our study considered 8 species of common desert plants: blackbrush (Coleogyne ramosissima), desert willow (Chilopsis linearis), mesquite (Prosopis glandulosa), mondel pine (Pinus eldarica), pinyon pine (Pinus monophylla), cottonwood (Populus deltoides), saguaro cactus (Carnegiea gigantea) and yucca (Yucca baccata). The measurements focused on BVOCs with relatively high molecular weight (>C15) and/or an oxygenated functional group. Significantly high emission rates of two salicylic esters were found for blackbrush, desert willow and mesquite with emission rates of 3.1, 1.0 and 4.8μgC dwg−1 h−1, respectively (dwg; dry weight of the leaves in gram). The salicylic esters were identified as 2-ethylhexenyl salicylate (2-EHS) and 3,3,5-trimethylcyclohexenyl salicylate (homosalate) and are known as effective ultraviolet (UV) absorbers. We propose that the plants derive a protective benefit against UV radiation from the salicylic esters and that the emission process is driven by the physical evaporation of the salicylic esters due to the high ambient temperatures. In addition, the salicylic esters are predicted to be an effective precursor of secondary organic aerosol (SOA) because they probably produce oxidation products that can condense onto the aerosol phase. We estimated the contribution of the sunscreen esters themselves and their oxidation products on the SOA formation for the Las Vegas area using a BVOC emission model. The contribution was estimated to reach 50% of the biogenic terpenoid emission in the landscapes dominated by desert willow and mesquite and 13% in the Las Vegas area. The contributions to biogenic SOA are likely to be higher due to the potentially high SOA yields of these compounds.

Secondary Organic Aerosol from Sesquiterpene and Monoterpene Emissions in the United States

Emissions of volatile organic compounds (VOC) from vegetation are believed to be a major source of secondary organic aerosol (SOA), which in turn comprises a large fraction of fine particulate matter in many areas. Sesquiterpenes are a class of biogenic VOC with high chemical reactivity and SOA yields. Sesquiterpenes have only recently been quantified in emissions from a wide variety of plants. In this study, a new sesquiterpene emission inventory is used to provide input to the Models-3 Community Multiscale Air Quality (CMAQ) model. CMAQ is used to estimate the contribution of sesquiterpenes and monoterpenes to SOA concentrations over the contiguous United States. The gas-particle partitioning module of CMAQ was modified to include condensable products of sesquiterpene oxidation and to update values of the enthalpy of vaporization. The resulting model predicts July monthly average surface concentrations of total SOA in the eastern U.S. ranging from about 0.2-0.8 microg m(-3). This is roughly double the amount of SOA produced in this region when sesquiterpenes are not included. Even with sesquiterpenes included, however, the model significantly underpredicts surface concentrations of particle-phase organic matter compared to observed values. Treating all SOA as capable of undergoing polymerization increases predicted monthly average surface concentrations in July to 0.4-1.2 microg m(-3), in closer agreement with observations. Using the original enthalpy of vaporization value in CMAQ in place of the values estimated from the recent literature results in predicted SOA concentrations of about 0.3-1.3 microg m(-3).

Effect of NOx on Secondary Organic Aerosol Concentrations

The secondary organic aerosol (SOA) module in PMCAMx, a three-dimensional chemical transport model, has been updated to incorporate NOx-dependent SOA yields. Under low-NOx conditions, the RO2 radicals react with other peroxy radicals to form a distribution of products with lower volatilities, resulting in higher SOA yields. At high-NOx conditions, the SOA yields are lower because aldehydes, ketones, and nitrates dominate the product distribution. Based on recent laboratory smog chamber experiments, high-NOx SOA parametrizations were created using the volatility basis-set approach.The organic aerosol (OA) concentrations in the Eastern US are simulated for a summer episode, and are compared to the available ambient measurements. Changes in NOx levels result in changes of both the oxidants (ozone, OH radical, etc.) and the SOA yields during the oxidation of the corresponding organic vapors. The NOx dependent SOA parametrization predicts a maximum average SOA concentration of 5.2 microg m(-3) and a domain average concentration of 0.6 microg m(-3). As the NOx emissions are reduced by 25%, the domain average SOA concentration does not significantly change, but the response is quite variable spatially. However, the predicted average SOA concentrations increase in northern US cities by around 3% but decrease in the rural southeast US by approximately 5%. A decrease of the average biogenic SOA by roughly 0.5 microg m(-3) is predicted for the southeast US for a 50% reduction in NOx emissions.

β‐caryophyllinic acid: An atmospheric tracer for β‐caryophyllene secondary organic aerosol

The chemical compositions of ambient PM2.5 samples, collected in Research Triangle Park, North Carolina, USA, and a sample of secondary organic aerosol, formed by irradiating a mixture of the sesquiterpene (SQT), β‐caryophyllene, and oxides of nitrogen in a smog chamber, were chemically analyzed using derivative‐based GC‐MS methods. The analyses showed the presence of an oxidized compound, tentatively identified as β‐caryophyllinic acid, in both the ambient PM2.5 field samples and in the smog chamber sample. The seasonal concentrations of β‐caryophyllinic acid in the ambient PM2.5 samples were 0.5, 0.9, 7.0, and 0.5 ng m−3 during the winter, spring, summer and fall respectively. To our knowledge, this is the first time that an oxidation product of a sesquiterpene, a hydrocarbon with high secondary organic aerosol yields and emitted from plants and trees in significant quantities, has been detected in ambient PM2.5 samples.

Estimates of Anthropogenic Secondary Organic Aerosol Formation in Houston, Texas Special Issue ofAerosol Science and Technologyon Findings from the Fine Particulate Matter Supersites Program

Secondary Organic Aerosol (SOA) formation due to precursor emissions from anthropogenic sources in the Houston/Galveston (HG) area was estimated by multiplying the anthropogenic emissions of SOA precursors by fractional aerosol coefficients (FAC). The analysis indicated that area and nonroad mobile sources contributed 56% of the aerosol precursor emissions, while mobile and point sources contributed 27% and 16%, respectively. However, due to high SOA yields of the precursors emitted by point sources, especially emissions of terpenes from pulp and paper processing and emissions of aromatics, point source emissions resulted in 53% of the projected SOA from anthropogenic sources in the Houston-Galveston (HG) area. Estimated SOA formation rates were consistent with average concentrations of particle-phase organic carbon in the Houston-Galveston area.

Temperature dependence of secondary organic aerosol formation by photo-oxidation of hydrocarbons

Photo-oxidation experiments on hydrocarbons were performed with a temperature-controlled smog chamber to study the temperature dependence of secondary organic aerosol (SOA) formation. A higher SOA yield was obtained at lower temperature and with a higher concentration of SOA generated. The relationship of SOA yield to temperature and SOA concentration is expressed by a gas/particle partitioning absorption model considered with temperature dependence. Under the condition of the same SOA concentration, the SOA yield at 283 K was approximately twice that at 303 K. It has been clarified experimentally that temperature is one of the most important factors in SOA formation. The experiments were performed not only with three aromatic hydrocarbons (toluene, m-xylene and 1,2,4-trimethylbenzene) and one biogenic alkene (α-pinene), but also with one alkane (n-undecane) on which few experiments for SOA formation have been performed. n-Undecane indicates a lower SOA yield than any other hydrocarbon investigated in this study.