Isoprene Secondary Organic Aerosol Research Articles

Chemical characterization of secondary organic aerosol at a rural site in the southeastern US: insights from simultaneous high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and FIGAERO chemical ionization mass spectrometer (CIMS) measurements

Abstract. The formation and evolution of secondary organic aerosol (SOA) were investigated at Yorkville, GA, in late summer (mid-August to mid-October 2016). The organic aerosol (OA) composition was measured using two online mass spectrometry instruments, the high-resolution time-of-flight aerosol mass spectrometer (AMS) and the Filter Inlet for Gases and AEROsols coupled to a high-resolution time-of-flight iodide-adduct chemical ionization mass spectrometer (FIGAERO-CIMS). Through analysis of speciated organics data from FIGAERO-CIMS and factorization analysis of data obtained from both instruments, we observed notable SOA formation from isoprene and monoterpenes during both day and night. Specifically, in addition to isoprene epoxydiol (IEPOX) uptake, we identified isoprene SOA formation from non-IEPOX pathways and isoprene organic nitrate formation via photooxidation in the presence of NOx and nitrate radical oxidation. Monoterpenes were found to be the most important SOA precursors at night. We observed significant contributions from highly oxidized acid-like compounds to the aged OA factor from FIGAERO-CIMS. Taken together, our results showed that FIGAERO-CIMS measurements are highly complementary to the extensively used AMS factorization analysis, and together they provide more comprehensive insights into OA sources and composition.

Characteristics of fresh and aged volatile organic compounds from open burning of crop residues

Open burning of crop residues is a major source of volatile organic compounds (VOCs), which contribute substantially to the formation of secondary organic aerosols (SOAs) in the atmosphere. An integrated system of combustion chamber coupled with potential aerosol mass (PAM) reactor was used to demonstrate the emission characteristics of fresh and aged VOCs (corresponding to 2- and 7-day atmospheric aging) from the burning of rice, maize, and wheat straws. The average emission factor (EF) of quantified non-methane VOCs (NMVOCs) emitted from the straw (fresh) was 1.82 ± 0.41 g/kg and wheat straw had the highest EFs. The EF residues of quantified NMVOCs decreased considerably after photo-oxidation in PAM. Stronger oxidation condition (7-day aging) produced a 57.2% decline in NMVOC EFs, compared with 42.3% decline during 2-day atmospheric aging. The largest declines were observed in the alkenes group: 82.6% and 66.2% after 7- and 2-day aging, respectively, which is consistent with their high reactivity toward oxidation with ozone and hydroxyl radical (OH). Aromatic compounds mainly reacted with OH, and their EFs decreased 59.1% on average. Alkanes were much less reactive, and their EFs only decreased an average of 29.8% after the oxidation processes. Considerable SOAs formation was observed in the fine particulate matter (PM2.5) filter samples collected after the oxidation of isoprene, benzene and toluene. The moderate to strong correlations between isoprene and isoprene-derived SOAs, between benzene and toluene with nitrophenols, and between toluene and aromatic acids demonstrate that the VOCs were degraded in the reactions with oxidative radicals, producing active contributors to SOAs formations.

Role of functional groups in reaction kinetics of dithiothreitol with secondary organic aerosols

The toxicity of organic aerosols has been largely ascribed to the generation of reactive oxygen species, which could subsequently induce oxidative stress in biological systems. The reaction of DTT with redox-active species in PM has been generally assumed to be pseudo-first order, with the oxidative potential of PM being represented by the DTT consumption per minute of reaction time per μg of PM. Although catalytic reactive species such as transition metals and quinones are long believed to be the main contributors of DTT responses, the role of non-catalytic DTT reactive species such as organic hydroperoxides (ROOH) and electron-deficient alkenes (e.g., conjugated carbonyls) in DTT consumption has been recently highlighted. Thus, understanding the reaction kinetics and mechanisms of DTT consumption by various PM components is required to interpret the oxidative potential measured by DTT assays more accurately. In this study, we measured the DTT consumptions over time and characterized the reaction products using model compounds and secondary organic aerosols (SOA) with varying initial concentrations. We observed that the DTT consumption rates linearly increased with both initial DTT and sample concentrations. The overall reaction order of DTT with non-catalytic reactive species and SOA in this study is second order. The reactions of DTT with different functional groups have significantly different rate constants. The reaction rate constant of isoprene SOA with DTT is mainly determined by the concentration of ROOH. For toluene SOA, both ROOH and electron-deficient alkenes may dominate its DTT reaction rates. These results provide some insights into the interpretation of DTT-based aerosol oxidative potential and highlight the need to study the toxicity mechanism of ROOH and electron-deficient alkenes in PM for future work.

Heterogeneous oxidation of isoprene SOA and toluene SOA tracers by ozone

Secondary Organic aerosols (SOA) are important components of PM 2.5. In order to control the heavy haze pollution, it is essential to find out the contributions of main SOA precursors. Nowadays, the tracer-based method has been widely used in analyzing the contributions of the precursors to SOA. However, it is not well known that whether the SOA tracers can be oxidized or how the instability of the SOA tracers would influence the accuracy of the tracer-based method for source apportionment.In this paper, the heterogeneous oxidation experiments of SOA tracers produced from isoprene and toluene as well as their mixtures under different conditions were conducted in a 2 m3 indoor chamber. The relative rate constants approach was used to determine the effective rate constants of the ozone reactions of the tracers. Concentrations of 2-Methyl Erythritol, a tracer of isoprene SOA, and 2, 3-Dihydroxy-4-oxopentanoic Acid, a tracer of toluene SOA, were analyzed using GC-MS. The effects of different seed aerosols and initial VOC0/NO on the heterogeneous oxidation of the tracers were investigated. The effects of co-existing components in the SOA produced from the mixture of isoprene and toluene on the heterogeneous oxidation of the tracers by ozone were also studied.

Structural Characterization of Lactone-Containing MW 212 Organosulfates Originating from Isoprene Oxidation in Ambient Fine Aerosol.

Isoprene (C5H8) is the main non-methane hydrocarbon emitted into the global atmosphere. Despite intense research, atmospheric transformations of isoprene leading to secondary organic aerosol (SOA) are still not fully understood, including its multiphase chemical reactions. Herein, we report on the detailed structural characterization of atmospherically relevant isoprene-derived organosulfates (OSs) with a molecular weight (MW) of 212 (C5H8SO7), which are abundantly present in both ambient fine aerosol (PM2.5) and laboratory-generated isoprene SOA. The results obtained from smog chamber-generated isoprene SOA and aqueous-phase laboratory experiments coupled to the S(IV)-autooxidation chemistry of isoprene, 3-methyl-2(5H)-furanone, and 4-methyl-2(5H)-furanone, allowed us for the first time to fully elucidate the isomeric structures of the MW 212 OSs. By applying liquid chromatography interfaced to electrospray ionization high-resolution mass spectrometry, we firmly confirmed six positional isomers of the MW 212 OSs in PM2.5 collected from different sites in Europe and the United States. Our results also show that despite the low solubility of isoprene in water, aqueous-phase or multiphase chemistry can play an important role in the formation of OSs from isoprene. Possible formation mechanisms for the MW 212 OSs are also tentatively proposed.

The Cooling Rate- and Volatility-Dependent Glass-Forming Properties of Organic Aerosols Measured by Broadband Dielectric Spectroscopy.

Glass transitions of secondary organic aerosols (SOA) from liquid/semisolid to solid phase states have important implications for aerosol reactivity, growth, and cloud formation properties. In the present study, glass transition temperatures (Tg) of isoprene SOA components, including isoprene hydroxy hydroperoxide (ISOPOOH), isoprene-derived epoxydiols (IEPOX), 2-methyltetrols, and 2-methyltetrol sulfates, were measured at atmospherically relevant cooling rates (2-10 K/min) by thin film broadband dielectric spectroscopy. The results indicate that 2-methyltetrol sulfates have the highest glass transition temperature, while ISOPOOH has the lowest glass transition temperature. By varying the cooling rate of the same compound from 2 to 10 K/min, the Tg of these compounds increased by 4-5 K. This temperature difference leads to a height difference of 400-800 m in the atmosphere for the corresponding updraft induced cooling rates, assuming a hygroscopicity value (κ) of 0.1 and relative humidity less than 95%. The Tg of the organic compounds was found to be strongly correlated with volatility, and a semiempirical formula between glass transition temperatures and volatility was derived. The Gordon-Taylor equation was applied to calculate the effect of relative humidity (RH) and water content at five mixing ratios on the Tg of organic aerosols. The model shows that Tg could drop by 15-40 K as the RH changes from <5 to 90%, whereas the mixing ratio of water in the particle increases from 0 to 0.5. These results underscore the importance of chemical composition, updraft rates, and water content (RH) in determining the phase states and hygroscopic properties of organic particles.

A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol

Abstract. Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone (O3), the hydroxyl radical (OH), nitrogen oxide radicals (NOx), carbon monoxide (CO), oxygenated and nitrated organic compounds, and secondary organic aerosol (SOA). We analyze these effects in box models and in the global GEOS-Chem chemical transport model using the new reduced Caltech isoprene mechanism (RCIM) condensed from a recently developed explicit isoprene oxidation mechanism. We find many similarities with previous global models of isoprene chemistry along with a number of important differences. Proper accounting of the isomer distribution of peroxy radicals following the addition of OH and O2 to isoprene influences the subsequent distribution of products, decreasing in particular the yield of methacrolein and increasing the capacity of intramolecular hydrogen shifts to promptly regenerate OH. Hydrogen shift reactions throughout the mechanism lead to increased OH recycling, resulting in less depletion of OH under low-NO conditions than in previous mechanisms. Higher organonitrate yields and faster tertiary nitrate hydrolysis lead to more efficient NOx removal by isoprene and conversion to inorganic nitrate. Only 20 % of isoprene-derived organonitrates (excluding peroxyacyl nitrates) are chemically recycled to NOx. The global yield of formaldehyde from isoprene is 22 % per carbon and less sensitive to NO than in previous mechanisms. The global molar yield of glyoxal is 2 %, much lower than in previous mechanisms because of deposition and aerosol uptake of glyoxal precursors. Global production of isoprene SOA is about one-third from each of the following: isoprene epoxydiols (IEPOX), organonitrates, and tetrafunctional compounds. We find a SOA yield from isoprene of 13 % per carbon, much higher than commonly assumed in models and likely offset by SOA chemical loss. We use the results of our simulations to further condense RCIM into a mini Caltech isoprene mechanism (Mini-CIM) for less expensive implementation in atmospheric models, with a total size (108 species, 345 reactions) comparable to currently used mechanisms.

Low-volatility compounds contribute significantly to isoprene secondary organic aerosol (SOA) under high-NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; conditions

Abstract. Recent advances in our knowledge of the gas-phase oxidation of isoprene, the impact of chamber walls on secondary organic aerosol (SOA) mass yields, and aerosol measurement analysis techniques warrant reevaluating SOA yields from isoprene. In particular, SOA from isoprene oxidation under high-NOx conditions forms via two major pathways: (1) low-volatility nitrates and dinitrates (LV pathway) and (2) hydroxymethyl-methyl-α-lactone (HMML) reaction on a surface or the condensed phase of particles to form 2-methyl glyceric acid and its oligomers (2MGA pathway). These SOA production pathways respond differently to reaction conditions. Past chamber experiments generated SOA with varying contributions from these two unique pathways, leading to results that are difficult to interpret. This study examines the SOA yields from these two pathways independently, which improves the interpretation of previous results and provides further understanding of the relevance of chamber SOA yields to the atmosphere and regional or global modeling. Results suggest that low-volatility nitrates and dinitrates produce significantly more aerosol than previously thought; the experimentally measured SOA mass yield from the LV pathway is ∼0.15. Sufficient seed surface area at the start of the reaction is needed to limit the effects of vapor wall losses of low-volatility compounds and accurately measure the complete SOA mass yield. Under dry conditions, substantial amounts of SOA are formed from HMML ring-opening reactions with inorganic ions and HMML organic oligomerization processes. However, the lactone organic oligomerization reactions are suppressed under more atmospherically relevant humidity levels, where hydration of the lactone is more competitive. This limits the SOA formation potential from the 2MGA pathway to HMML ring-opening reactions with water or inorganic ions under typical atmospheric conditions. The isoprene SOA mass yield from the LV pathway measured in this work is significantly higher than previous studies have reported, suggesting that low-volatility compounds such as organic nitrates and dinitrates may contribute to isoprene SOA under high-NOx conditions significantly more than previously thought and thus deserve continued study.

Seasonal characteristics of biogenic secondary organic aerosols at Mt. Wuyi in Southeastern China: Influence of anthropogenic pollutants

Thirteen secondary organic aerosol (SOA) tracers of isoprene, monoterpenes and sesquiterpenes were measured for PM2.5 aerosols collected at the summit of Mt. Wuyi (1139 m, a.s.l.), to investigate their seasonality and formation mechanism. Concentrations of the isoprene and monoterpene SOA tracers were much higher in summer than those in other seasons. In contrast, β-caryophyllinic acid was found to be the lowest in summer. Concentrations of those BSOA tracers showed a positive correlation with temperature (R2 = 0.52-0.70), and a negative correlation with relative humidity (R2 = 0.43-0.78). Moreover, thermodynamic model (i.e., ISORROPIA-II) calculation results showed that acidity conditions are favorable for BSOA formation. Robust linear correlations between the BSOA tracers and anthropogenic pollutants such as SO2 (R2 = 0.53-0.7) and NO2 (R2 = 0.37-0.54) were observed for all the samples, suggesting that SO2 and NOx can enhance BSOA production in the remote mountain area of southeast China, which is related to an acid-catalyzed heterogeneous chemistry. Moreover, we also found a significant correlation between the concentrations of the BSOA tracers and levoglucosan especially for β-caryophyllinic acid, indicating that biomass burning plumes from the distant lowland regions could influence the production of BSOA in the mountain free troposphere. Our results clearly demonstrated that anthropogenic emissions in China could enhance BSOA formation in the distant mountain regions.

Biogenic emissions and land–atmosphere interactions as drivers of the daytime evolution of secondary organic aerosol in the southeastern US

Abstract. The interactions between biogenic volatile organic compounds (BVOCs), like isoprene and monoterpenes, and anthropogenic emissions of nitrogen and sulfur oxides lead to high concentrations of secondary organic aerosol (SOA) in the southeastern United States. To improve our understanding of SOA formation, we study the diurnal evolution of SOA in a land–atmosphere coupling context based on comprehensive surface and upper air observations from a characteristic day during the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign. We use a mixed layer model (MXLCH-SOA) that is updated with new chemical pathways and an interactive land surface scheme that describes both biogeochemical and biogeophysical couplings between the land surface and the atmospheric boundary layer (ABL) to gain insight into the drivers of the daytime evolution of biogenic SOA. MXLCH-SOA reproduces observed BVOC and surface heat fluxes, gas-phase chemistry, and ABL dynamics well, with the exception of isoprene and monoterpene mixing ratios measured close to the land surface. This is likely due to the fact that these species do not have uniform profiles throughout the atmospheric surface layer due to their fast reaction with OH and incomplete mixing near the surface. The flat daytime evolution of the SOA concentration is caused by the dampening of the increase due to locally formed SOA by entrainment of SOA-depleted air from the residual layer. SOA formation from isoprene through the intermediate species isoprene epoxydiols (IEPOXs) and isoprene hydroxyhydroperoxides (ISOPOOHs) is in good agreement with the observations, with a mean isoprene SOA yield of 1.8 %. However, SOA from monoterpenes, oxidised by OH and O3, dominates the locally produced SOA (69 %), with a mean monoterpene SOA yield of 10.7 %. Isoprene SOA is produced primarily through OH oxidation via ISOPOOH and IEPOX (31 %). Entrainment of aged SOA from the residual layer likely contributes to the observed more oxidised oxygenated organic aerosol (MO-OOA) factor. A sensitivity analysis of the coupled land surface–boundary layer–SOA formation system to changing temperatures reveals that SOA concentrations are buffered under increasing temperatures: a rise in BVOC emissions is offset by decreases in OH concentrations and the efficiency with which SVOCs partition into the aerosol phase.

Chemical composition of isoprene SOA under acidic and non-acidic conditions: effect of relative humidity.

The effect of acidity and relative humidity on bulk isoprene aerosol parameters has been investigated in several studies; however, few measurements have been conducted on individual aerosol compounds. The focus of this study has been the examination of the effect of acidity and relative humidity on secondary organic aerosol (SOA) chemical composition from isoprene photooxidation in the presence of nitrogen oxide (NOx). A detailed characterization of SOA at the molecular level was also investigated. Experiments were conducted in a 14.5 m3 smog chamber operated in flow mode. Based on a detailed analysis of mass spectra obtained from gas chromatography–mass spectrometry of silylated derivatives in electron impact and chemical ionization modes, ultra-high performance liquid chromatography/electrospray ionization/time-of-flight high-resolution mass spectrometry, and collision-induced dissociation in the negative ionization modes, we characterized not only typical isoprene products but also new oxygenated compounds. A series of nitroxy-organosulfates (NOSs) were tentatively identified on the basis of high-resolution mass spectra. Under acidic conditions, the major identified compounds include 2-methyltetrols (2MT), 2-methylglyceric acid (2mGA), and 2MT-OS. Other products identified include epoxydiols, mono- and dicarboxylic acids, other organic sulfates, and nitroxy- and nitrosoxy-OS. The contribution of SOA products from isoprene oxidation to PM2.5 was investigated by analyzing ambient aerosol collected at rural sites in Poland. Methyltetrols, 2mGA, and several organosulfates and nitroxy-OS were detected in both the field and laboratory samples. The influence of relative humidity on SOA formation was modest in non-acidic-seed experiments and stronger under acidic seed aerosol. Total secondary organic carbon decreased with increasing relative humidity under both acidic and non-acidic conditions. While the yields of some of the specific organic compounds decreased with increasing relative humidity, others varied in an indeterminate manner from changes in the relative humidity.

Drought Impacts on Secondary Organic Aerosol: A Case Study in the Southeast United States.

Secondary organic aerosol (SOA) is a significant component of fine particulate matter, and it has increased during past drought periods in the U.S. Here, we use the Community Multiscale Air Quality (CMAQ) model to characterize the complex effects of drought on SOA through a case study comparing a drought period (June 2011) and a wet period (June 2013) over the southeast U.S. The model simulates a 68% (1.7 μg/m3) higher SOA concentration at the surface during drought and attributes 98% of this increase to biogenic SOA. Through model sensitivity simulations, the SOA increase associated with drought is attributed to 54% from accelerated gas-phase reactions oxidizing volatile organic compounds (VOCs) to SOA, 45% from higher emissions of biogenic VOCs, 18% from enhanced acid-catalyzed production of isoprene SOA in aerosol water due to changing sulfate, 3% from enhanced in-cloud aqueous phase chemistry. Because the higher SOA levels overwhelm the reduced precipitation, there is an increase in wet deposition flux in the drought month which offsets 20% of the total SOA increase. If anthropogenic emissions are held constant, anthropogenic SOA is 51% higher during drought, highlighting the importance of meteorological impacts on chemistry.

Secondary organic aerosol of PM2.5 in a mountainous forest area in southeastern China: Molecular compositions and tracers implication

Secondary organic aerosol (SOA) plays an important role in global climate change and air quality. PM2.5 (particles with aerodynamic diameters ≤2.5 μm) samples were collected at a mountainous forest site (Mt. Wuyi) in southeastern China between November 2015 and July 2016. Fourteen PM2.5-bound SOA tracers, including isoprene, α/β‑pinene, β‑caryophyllene, and toluene, were measured using the gas-chromatography–mass-spectrometry method. The total concentrations of the isoprene, α/β‑pinene, β‑caryophyllene, and toluene SOA tracers were 45.28 ± 65.52, 30.66 ± 24.44, 5.99 ± 7.25, and 0.62 ± 0.72 ng m−3, respectively. The isoprene SOA tracers exhibited the highest concentration (145.97 ± 53.78 ng m−3) and accounted for 76 ± 9% of the total concentration of SOA tracers in summer. In fall-winter, the mass fraction of 2‑methylglyceric acid was significantly enhanced because of the lower temperature and higher NOx level. As later-generation products of α/β‑pinene tracers, high proportions of 3‑hydroxyglutaric acid and 3‑methyl‑1,2,3 butanetricarboxylic acid were observed on Mt. Wuyi, suggesting that the aerosols were highly oxidized. Biomass burning events affected by local and regional sources were identified by analyzing typical SOA tracers. Significant positive correlation (R2 = 0.74) was found between the β‑caryophyllene tracer and levoglucosan. The average concentration of secondary organic carbon (SOC) as estimated from SOA tracers was 1.46 μgC m−3. The isoprene SOC accounted for 70% of the total SOC in summer, whereas the β‑caryophyllene SOC was the predominant component in winter. Meanwhile, the estimated toluene SOC accounted for 11.6% of the total SOC during the study period. The study helps understanding the characteristics and the formation of SOA in a mountainous forest area of southeastern China.

Reactive Oxygen Species Formed by Secondary Organic Aerosols in Water and Surrogate Lung Fluid.

Reactive oxygen species (ROS) play a central role in adverse health effects of air pollutants. Respiratory deposition of fine air particulate matter can lead to the formation of ROS in epithelial lining fluid, potentially causing oxidative stress and inflammation. Secondary organic aerosols (SOA) account for a large fraction of fine particulate matter, but their role in adverse health effects is unclear. Here, we quantify and compare the ROS yields and oxidative potential of isoprene, β-pinene, and naphthalene SOA in water and surrogate lung fluid (SLF). In pure water, isoprene and β-pinene SOA were found to produce mainly OH and organic radicals, whereas naphthalene SOA produced mainly H2O2 and O2•-. The total molar yields of ROS of isoprene and β-pinene SOA were 11.8% and 8.2% in water and decreased to 8.5% and 5.2% in SLF, which can be attributed to ROS removal by lung antioxidants. A positive correlation between the total peroxide concentration and ROS yield suggests that organic (hydro)peroxides may play an important role in ROS formation from biogenic SOA. The total molar ROS yields of naphthalene SOA was 1.7% in water and increased to 11.3% in SLF. This strong increase is likely due to redox reaction cycles involving environmentally persistent free radicals (EPFR) or semiquinones, antioxidants, and oxygen, which may promote the formation of H2O2 and the adverse health effects of anthropogenic SOA from aromatic precursors.

Secondary Organic Aerosol Formation From Isoprene Epoxides in the Pearl River Delta, South China: IEPOX‐ and HMML‐Derived Tracers

AbstractAtmospheric photooxidation of isoprene forms isoprene epoxydiols (IEPOX) and hydroxymethel‐methyl‐α‐lactone (HMML) via hydroperoxyl radical (HO2) channel and NO/NO2 channel, respectively. Reactive uptake of these epoxides onto particles produces isoprene secondary organic aerosols (iSOA). Currently, there is little information regarding these two epoxides during iSOA formation in polluted regions. In this study, iSOA tracers from IEPOX and HMML were measured from summer to fall in the heavily polluted Pearl River Delta (PRD) region. The total concentration of the iSOA tracers ranged from 5.77 to 466 ng m−3. Isoprene SOA tracers correlated well with sulfate (p &lt; 0.01), indicating that the abundant sulfate in the PRD plays an important role in iSOA formation. A kinetic model of IEPOX loss showed that 58% of IEPOX could undergo ring‐opening reactions under the polluted PRD conditions in summer. This leads to high levels of IEPOX‐derived SOA tracers in the PRD. High temperature in the PRD (&gt;22 °C) suppresses the production of HMML, likely as a result of fast decomposition of HMML's precursor under high temperatures. Thus, the HMML‐derived tracers had lower levels than the IEPOX‐derived SOA tracers during the whole campaign. The ratios of the IEPOX‐derived tracers to the HMML‐derived SOA tracers in summer were ~3 times higher than those in fall. This seasonal trend may be explained by the relative high isoprene/NOx ratio, temperature, and fast heterogeneous reaction of IEPOX in summer. Our study shows that in highly polluted regions like PRD, reduction in SO2 emission can significantly reduce iSOA formation.

Typical polar organic aerosol tracers in PM2.5 over the North China Plain: Spatial distribution, seasonal variations, contribution and sources

PM2.5 samples were collected simultaneously at three urban sites including Beijing (BJ), Tianjin (TJ), Shijiazhuang (SJZ) and one background site Xinglong (XL) from June 2014 till April 2015. Typical polar organic aerosol tracers including 2-methylglyceric acid, 2-methyltetrols, 3-hydroxyglutaric acid, anhydrous saccharides and phthalic acid were analyzed by gas chromatography-mass spectrometry. Both Isoprene secondary organic aerosol (SOA) tracers and pinene SOA tracers had higher mass concentrations at background site than those at urban sites. But they showed different seasonal variations. The former had the highest mass concentration in summer while the latter had the highest mass concentration in spring. Anhydrous saccharides and phthalic acids at urban sites had higher mass concentrations than those at background site and showed higher mass concentration in autumn and winter, respectively. Corresponding estimated secondary organic carbons (SOCs) showed similar variation trends with tracers. In general, biogenic sources had more influence on background site while urban sites were more affected by anthropogenic sources.

Aerosol composition and the contribution of SOA formation over Mediterranean forests

Abstract. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx), a series of aerosol and gas-phase measurements were deployed aboard the SAFIRE ATR42 research aircraft in summer 2014. The present study focuses on the four flights performed in late June early July over two forested regions in the south of France. We combine in situ observations and model simulations to aid in the understanding of secondary organic aerosol (SOA) formation over these forested areas in the Mediterranean and to highlight the role of different gas-phase precursors. The non-refractory particulate species measured by a compact aerosol time-of-flight mass spectrometer (cToF-AMS) were dominated by organics (60 to 72 %) followed by a combined contribution of 25 % by ammonia and sulfate aerosols. The contribution from nitrate and black carbon (BC) particles was less than 5 % of the total PM1 mass concentration. Measurements of non-refractory species from off-line transmission electron microscopy (TEM) showed that particles have different mixing states and that large fractions (35 %) of the measured particles were organic aerosol containing C, O, and S but without inclusions of crystalline sulfate particles. The organic aerosol measured using the cToF-AMS contained only evidence of oxidized organic aerosol (OOA), without a contribution of fresh primary organic aerosol. Positive matrix factorization (PMF) on the combined organic–inorganic matrices separated the oxidized organic aerosol into a more-oxidized organic aerosol (MOOA), and a less-oxidized organic aerosol (LOOA). The MOOA component is associated with inorganic species and had higher contributions of m∕z 44 than the LOOA factor. The LOOA factor is not associated with inorganic species and correlates well with biogenic volatile organic species measured with a proton-transfer-reaction mass spectrometer, such as isoprene and its oxidation products (methyl vinyl ketone, MVK; methacroleine, MACR; and isoprene hydroxyhydroperoxides, ISOPOOH). Despite a significantly high mixing ratio of isoprene (0.4 to 1.2 ppbV) and its oxidation products (0.2 and 0.8 ppbV), the contribution of specific signatures for isoprene epoxydiols SOA (IEPOX-SOA) within the aerosol organic mass spectrum (m∕z 53 and m∕z 82) were very weak, suggesting that the presence of isoprene-derived SOA was either too low to be detected by the cToF-AMS, or that SOA was not formed through IEPOX. This was corroborated through simulations performed with the Polyphemus model showing that although 60 to 80 % of SOA originated from biogenic precursors, only about 15 to 32 % was related to isoprene (non-IEPOX) SOA; the remainder was 10 % sesquiterpene SOA and 35 to 40 % monoterpene SOA. The model results show that despite the zone of sampling being far from industrial or urban sources, a total contribution of 20 to 34 % of the SOA was attributed to purely anthropogenic precursors (aromatics and intermediate or semi-volatile compounds). The measurements obtained during this study allow us to evaluate how biogenic emissions contribute to increasing SOA concentrations over Mediterranean forested areas. Directly comparing these measurements with the Polyphemus model provides insight into the SOA formation pathways that are prevailing in these forested areas as well as processes that need to be implemented in future simulations.

Predicting the glass transition temperature and viscosity of secondary organic material using molecular composition

Abstract. Secondary organic aerosol (SOA) accounts for a large fraction of submicron particles in the atmosphere. SOA can occur in amorphous solid or semi-solid phase states depending on chemical composition, relative humidity (RH), and temperature. The phase transition between amorphous solid and semi-solid states occurs at the glass transition temperature (Tg). We have recently developed a method to estimate Tg of pure compounds containing carbon, hydrogen, and oxygen atoms (CHO compounds) with molar mass less than 450 g mol−1 based on their molar mass and atomic O : C ratio. In this study, we refine and extend this method for CH and CHO compounds with molar mass up to ∼ 1100 g mol−1 using the number of carbon, hydrogen, and oxygen atoms. We predict viscosity from the Tg-scaled Arrhenius plot of fragility (viscosity vs. Tg∕T) as a function of the fragility parameter D. We compiled D values of organic compounds from the literature and found that D approaches a lower limit of ∼ 10 (±1.7) as the molar mass increases. We estimated the viscosity of α-pinene and isoprene SOA as a function of RH by accounting for the hygroscopic growth of SOA and applying the Gordon–Taylor mixing rule, reproducing previously published experimental measurements very well. Sensitivity studies were conducted to evaluate impacts of Tg, D, the hygroscopicity parameter (κ), and the Gordon–Taylor constant on viscosity predictions. The viscosity of toluene SOA was predicted using the elemental composition obtained by high-resolution mass spectrometry (HRMS), resulting in a good agreement with the measured viscosity. We also estimated the viscosity of biomass burning particles using the chemical composition measured by HRMS with two different ionization techniques: electrospray ionization (ESI) and atmospheric pressure photoionization (APPI). Due to differences in detected organic compounds and signal intensity, predicted viscosities at low RH based on ESI and APPI measurements differ by 2–5 orders of magnitude. Complementary measurements of viscosity and chemical composition are desired to further constrain RH-dependent viscosity in future studies.

Growth Kinetics and Size Distribution Dynamics of Viscous Secondary Organic Aerosol.

Low bulk diffusivity inside viscous semisolid atmospheric secondary organic aerosol (SOA) can prolong equilibration time scale, but its broader impacts on aerosol growth and size distribution dynamics are poorly understood. Here, we present quantitative insights into the effects of bulk diffusivity on the growth and evaporation kinetics of SOA formed under dry conditions from photooxidation of isoprene in the presence of a bimodal aerosol consisting of Aitken (ammonium sulfate) and accumulation (isoprene or α-pinene SOA) mode particles. Aerosol composition measurements and evaporation kinetics indicate that isoprene SOA is composed of several semivolatile organic compounds (SVOCs), with some reversibly reacting to form oligomers. Model analysis shows that liquid-like bulk diffusivities can be used to fit the observed evaporation kinetics of accumulation mode particles but fail to explain the growth kinetics of bimodal aerosol by significantly under-predicting the evolution of the Aitken mode. In contrast, the semisolid scenario successfully reproduces both evaporation and growth kinetics, with the interpretation that hindered partitioning of SVOCs into large viscous particles effectively promotes the growth of smaller particles that have shorter diffusion time scales. This effect has important implications for the growth of atmospheric ultrafine particles to climatically active sizes.

Dithiothreitol activity by particulate oxidizers of SOA produced from photooxidation of hydrocarbons under varied NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; levels

Abstract. When hydrocarbons (HCs) are atmospherically oxidized, they form particulate oxidizers, including quinones, organic hydroperoxides, and peroxyacyl nitrates (PANs). These particulate oxidizers can modify cellular materials (e.g., proteins and enzymes) and adversely modulate cell functions. In this study, the contribution of particulate oxidizers in secondary organic aerosols (SOAs) to the oxidative potential was investigated. SOAs were generated from the photooxidation of toluene, 1,3,5-trimethylbenzene, isoprene, and α-pinene under varied NOx levels. Oxidative potential was determined from the typical mass-normalized consumption rate (reaction time t = 30 min) of dithiothreitol (DTTt), a surrogate for biological reducing agents. Under high-NOx conditions, the DTTt of toluene SOA was 2–5 times higher than that of the other types of SOA. Isoprene DTTt significantly decreased with increasing NOx (up to 69 % reduction by changing the HC ∕ NOx ratio from 30 to 5). The DTTt of 1,3,5-trimethylbenzene and α-pinene SOA was insensitive to NOx under the experimental conditions of this study. The significance of quinones to the oxidative potential of SOA was tested through the enhancement of DTT consumption in the presence of 2,4-dimethylimidazole, a co-catalyst for the redox cycling of quinones; however, no significant effect of 2,4-dimethylimidazole on modulation of DTT consumption was observed for all SOA, suggesting that a negligible amount of quinones was present in the SOA of this study. For toluene and isoprene, mass-normalized DTT consumption (DTTm) was determined over an extended period of reaction time (t = 2 h) to quantify their maximum capacity to consume DTT. The total quantities of PANs and organic hydroperoxides in toluene SOA and isoprene SOA were also measured using the Griess assay and the 4-nitrophenylboronic acid assay, respectively. Under the NOx conditions (HC ∕ NOx ratio: 5–36 ppbC ppb−1) applied in this study, the amount of organic hydroperoxides was substantial, while PANs were found to be insignificant for both SOAs. Isoprene DTTm was almost exclusively attributable to organic hydroperoxides, while toluene DTTm was partially attributable to organic hydroperoxides. The DTT assay results of the model compound study suggested that electron-deficient alkenes, which are abundant in toluene SOA, could also modulate DTTm.