Semi-volatile Organic Aerosol Research Articles

Sources of organic gases and aerosol particles and their roles in nighttime particle growth at a rural forested site in southwest Germany

Abstract. The composition, sources and chemical transformation of volatile organic compounds (VOCs) and organic aerosol (OA) particles were investigated during July–August 2021 at a rural forested site in southwest Germany, 10 km north of the city of Karlsruhe. VOCs and semi-volatile OA particles were measured with a proton-transfer-reaction mass spectrometer coupled to the CHemical Analysis of aeRosols ONline particle inlet (CHARON–PTR-MS). The CHARON-measured OA mass accounted for 62 ± 18 % on average of the total OA mass (4.2 ± 2.8 µg m−3) measured concurrently with an aerosol mass spectrometer (AMS). The total concentrations of measured VOCs ranged from 7.6 to 88.9 ppb with an average of 31.2 ± 13.4 ppb. Positive matrix factorization (PMF) was used to identify major source factors of VOCs and OA particles. Three types of oxygenated VOC (OVOC), namely aromatic OVOCs, biogenic OVOCs and aged OVOCs, contributed on average 11 % ± 9 %, 37 % ± 29 % and 29 % ± 21 % of total VOC concentrations, respectively. The results of AMS–PMF indicated substantial contributions of oxygenated organic compounds to OA particle mass. Three secondary OA (SOA) factors determined by CHARON–PMF analysis, namely aromatic SOA (5 % ± 7 %), daytime biogenic SOA (17 % ± 17 %) and nighttime biogenic SOA (28 % ± 21 %), consistently showed high contributions to the total CHARON-measured OA mass. Nighttime particle growth was observed regularly in this area, which was mainly attributed to the semi-volatile organic compounds and organic nitrates formed from the oxidation of monoterpenes and sesquiterpenes. This study presents the major sources, real-time transformations of VOCs and OA particles, and nighttime particle formation characteristics for central European forested areas.

Characterization of volatile organic compounds and submicron organic aerosol in a traffic environment

Abstract. Urban air consists of a complex mixture of gaseous and particulate species from anthropogenic and biogenic sources that are further processed in the atmosphere. This study investigated the characteristics and sources of volatile organic compounds (VOCs) and submicron organic aerosol (OA) in a traffic environment in Helsinki, Finland, in late summer. The anthropogenic VOCs (aVOCs; aromatic hydrocarbons) and biogenic VOCs (bVOCs; terpenoids) relevant for secondary-organic-aerosol formation were analyzed with an online gas chromatograph mass spectrometer, whereas the composition and size distribution of submicron particles was measured with a soot particle aerosol mass spectrometer. This study showed that aVOC concentrations were significantly higher than bVOC concentrations in the traffic environment. The largest aVOC concentrations were measured for toluene (campaign average of 1630 ng m−3) and p/m xylene (campaign average of 1070 ng m−3), while the dominating bVOC was α-pinene (campaign average of 200 ng m−3). For particle-phase organics, the campaign-average OA concentration was 2.4 µg m−3. The source apportionment analysis extracted six factors for OA. Three OA factors were related to primary OA sources – traffic (24 % of OA, two OA types) and a coffee roastery (7 % of OA) – whereas the largest fraction of OA (69 %) consisted of oxygenated OA (OOA). OOA was divided into less oxidized semi-volatile OA (SV-OOA; 40 % of OA) and two types of low-volatility OA (LV-OOA; 30 %). The focus of this research was also on the oxidation potential of the measured VOCs and the association between VOCs and OA in ambient air. Production rates of the oxidized compounds (OxPR) from the VOC reactions revealed that the main local sources of the oxidation products were O3 oxidation of bVOCs (66 % of total OxPR) and OH radical oxidation of aVOCs and bVOCs (25 % of total OxPR). Overall, aVOCs produced a much smaller portion of the oxidation products (18 %) than bVOCs (82 %). In terms of OA factors, SV-OOA was likely to originate from biogenic sources since it correlated with an oxidation product of monoterpene, nopinone. LV-OOA consisted of highly oxygenated long-range or regionally transported OA that had no correlation with local oxidant concentrations as it had already spent several days in the atmosphere before reaching the measurement site. In general, the main sources were different for VOCs and OA in the traffic environment. Vehicle emissions impacted both VOC and OA concentrations. Due to the specific VOCs attributed to biogenic emissions, the influence of biogenic emissions was more clearly detected in the VOC concentrations than in OA. In contrast, the emissions from the local coffee roastery had a distinctive mass spectrum for OA, but they could not be seen in the VOC measurements due to the measurement limitations for the large VOC compounds. Long-range transport increased the OA concentration and oxidation state considerably, while its effect was observed less clearly in the VOC measurements due to the oxidation of most VOC in the atmosphere during the transport. Overall, this study revealed that in order to properly characterize the impact of different emission sources on air quality, health, and climate, it is of importance to describe both gaseous and particulate emissions and understand how they interact as well as their phase transfers in the atmosphere during the aging process.

Mass Absorption Efficiency of PM1 in Mexico City during ACU15

From January to March 2015, an atmospheric aerosol measurement campaign, “Aerosoles en Ciudad Universitaria 2015” (ACU15), was carried out in Mexico City to determine the particles’ optical properties and chemical composition. Two photoacoustic spectrometers measured the scattering and absorption coefficient at two different wavelengths. The average absorption coefficient at 532 nm was 12.71 ± 9.48 Mm−1 and at 870 nm was 10.35 ± 7.36 Mm−1. The average scattering coefficient was 65.63 ± 47.12 Mm−1 (532 nm) and 21.12 ± 14.24 Mm−1 (870 nm). The chemical composition was determined via an aerosol chemical speciation monitor. The organic aerosol fraction represented 53% of the total PM1 and was made up of 63% low volatile (4.64 µg m−3), 22% hydrogenated (1.90 µg m−3), and 15% semi-volatile organics (1.54 µg m−3). The correlation coefficient of chemical species (NO3−, NH4+, SO42−, low-volatile, and semi-volatile organics) and optical properties was 0.92. The multilinear regression showed a good agreement among chemical species and optical properties (r > 0.7). The mass absorption coefficient calculated for the measuring site at 870 nm was MAE870 = 5.8 m2 g−1, instead of the default 4.74 m2 g−1. Furthermore, based on the median AAE, the 532 nm MAE532 resulting from the multiple linear regression (MLR) showed the following coefficients: 7.70 m2 g−1 (eBC), 0.22 m2 g−1 (HOA), and 0.16 m2 g−1 (LV–OOA). The coefficients of MLR were: 7.08 m2 g−1 (eBC), 5.83 m2 g−1 (NO3−), 5.69 m2 g−1 (low volatile organic aerosol), 2.78 m2 g−1 (SO42−), 2.40 m2 g−1 (hydrocarbon-like organic aerosol), and 1.04 m2 g−1 (semi volatile organic aerosol).

Measurement report: Distinct size dependence and diurnal variation in organic aerosol hygroscopicity, volatility, and cloud condensation nuclei activity at a rural site in the Pearl River Delta (PRD) region, China

Abstract. Organic aerosol (OA) has a significant contribution to cloud formation and hence climate change. However, high uncertainties still exist in its impact on global climate, owing to the varying physical properties affected by the complex formation and aging processes. In this study, the hygroscopicity, volatility, cloud condensation nuclei (CCN) activity, and chemical composition of particles were measured using a series of online instruments at a rural site in the Pearl River Delta (PRD) region of China in fall 2019. During the campaign, the average hygroscopicity of OA (κOA) increased from 0.058 at 30 nm to 0.09 at 200 nm, suggesting a higher oxidation state of OA at larger particle sizes, supported by a higher fraction of extremely low volatility OA (ELVOA) for larger size particles. Significantly different diurnal patterns of κOA were observed between Aitken mode particles and accumulation mode particles. For Aitken mode particles (30–100 nm), the κOA values showed daily minima (0.02–0.07) during daytime, while the accumulation mode exhibited a daytime peak (∼ 0.09). Coincidently, a daytime peak was observed for both aged biomass burning organic aerosol (aBBOA) and less oxygenated organic aerosol (LOOA) based on source apportionment, which was attributed to the aging processes and gas–particle partitioning through photochemical reactions. In addition, the fraction of semi-volatile OA (SVOA) was higher at all measured sizes during daytime than during nighttime. These results indicate that the formation of secondary OA (SOA) through gas–particle partitioning can generally occur at all diameters, while the aging processes of pre-existing particles are more dominated in the accumulation mode. Furthermore, we found that applying a fixed κOA value (0.1) could lead to an overestimation of the CCN number concentration (NCCN) up to 12 %–19 % at 0.1 %–0.7 % supersaturation (SS), which was more obvious at higher SS during daytime. Better prediction of NCCN could be achieved by using size-resolved diurnal κOA, which indicates that the size dependence and diurnal variations in κOA can strongly affect the NCCN at different SS values. Our results highlight the need for accurately evaluating the atmospheric evolution of OA at different size ranges and their impact on the physicochemical properties and hence climate effects.

Source apportionment of secondary organic aerosols in the Pearl River Delta region: Contribution from the oxidation of semi-volatile and intermediate volatility primary organic aerosols

Secondary organic aerosols (SOAs) are some of the most important components of aerosol over south China, especially, in the Pearl River Delta (PRD). However, traditional SOA simulations usually result in severe underestimates because of the limited understanding of SOA formation. Furthermore, the source contribution features of SOAs in the PRD region remain poorly understood. In this study, a 1-D Volatility Basis-Set (VBS) scheme, and a 1.5-D VBS scheme are implemented in the Comprehensive Air Quality Model with Extensions (CAMx) to improve the poor model performance with the traditional SOA scheme, and a 1-year simulation of SOAs over the PRD region are conducted for each scheme. A comparison of the simulated results with the observational data reveals that the 1-D VBS scheme shows better model performance for SOA simulation in both absolute concentrations and time series agreement, which suggests that the VBS scheme effectively reduces the underestimation in SOA simulations. The missing processes [oxidation of intermediate-volatility organic compound (IVOC)/semi-volatility organic compound (SVOC)] contribute to SOA concentration of 4–7 μg/m3, (relatively as high as about 400–500%) at the center of the PRD region and of 1–2 μg/m3 (relatively about 100–300%) in the rural areas of the PRD region.By implementing the SOA source apportionment technology (SSAT) with 1-D VBS scheme in CAMx, the source contributions from geographic source regions and source types are discussed. The results at a rural supersite (HKUST) with SOA observation data show that the super-regional transport accounts for 56.5%–76.9% of SOA concentration, PRD region contribution accounts for 13.1%–32.7% of SOA concentration. Of the SOAs from the PRD region, mobile emission takes the largest contribution among the emissions categories, which suggests that the anthropogenic emission significantly contribute to the SOAs over the PRD region. Our results also indicate that the applications of VBS models and SSAT algorithm provide new insights into policy implications for improving local and regional air quality.

Simulating secondary organic aerosol in a regional air quality model using the statistical oxidation model – Part 3: Assessing the influence of semi-volatile and intermediate-volatility organic compounds and NO<sub><i>x</i></sub>

Abstract. Semi-volatile and intermediate-volatility organic compounds (SVOCs and IVOCs) from anthropogenic sources are likely to be important precursors of secondary organic aerosol (SOA) in urban airsheds, yet their treatment in most models is based on limited and obsolete data or completely missing. Additionally, gas-phase oxidation of organic precursors to form SOA is influenced by the presence of nitric oxide (NO), but this influence is poorly constrained in chemical transport models. In this work, we updated the organic aerosol model in the UCD/CIT (University of California at Davis/California Institute of Technology) chemical transport model to include (i) a semi-volatile and reactive treatment of primary organic aerosol (POA), (ii) emissions and SOA formation from IVOCs, (iii) the NOx influence on SOA formation, and (iv) SOA parameterizations for SVOCs and IVOCs that are corrected for vapor wall loss artifacts during chamber experiments. All updates were implemented in the statistical oxidation model (SOM) that simulates the oxidation chemistry, thermodynamics, and gas–particle partitioning of organic aerosol (OA). Model treatment of POA, SVOCs, and IVOCs was based on an interpretation of a comprehensive set of source measurements available up to the year 2016 and resolved broadly by source type. The NOx influence on SOA formation was calculated offline based on measured and modeled VOC:NOx ratios. Finally, the SOA formation from all organic precursors (including SVOCs and IVOCs) was modeled based on recently derived parameterizations that accounted for vapor wall loss artifacts in chamber experiments. The updated model was used to simulate a 2-week summer episode over southern California at a model resolution of 8 km. When combustion-related POA was treated as semi-volatile, modeled POA mass concentrations were reduced by 15 %–40 % in the urban areas in southern California but were still too high when compared against “hydrocarbon-like organic aerosol” factor measurements made at Riverside, CA, during the Study of Organic Aerosols at Riverside (SOAR-1) campaign of 2005. Treating all POA (except that from marine sources) to be semi-volatile, similar to diesel exhaust POA, resulted in a larger reduction in POA mass concentrations and allowed for a better model–measurement comparison at Riverside, but this scenario is unlikely to be realistic since this assumes that POA from sources such as road and construction dust are semi-volatile too. Model predictions suggested that both SVOCs (evaporated POA vapors) and IVOCs did not contribute as much as other anthropogenic precursors (e.g., alkanes, aromatics) to SOA mass concentrations in the urban areas (< 5 % and < 15 % of the total SOA respectively) as the timescales for SOA production appeared to be shorter than the timescales for transport out of the urban airshed. Comparisons of modeled IVOC concentrations with measurements of anthropogenic SOA precursors in southern California seemed to imply that IVOC emissions were underpredicted in our updated model by a factor of 2. Correcting for the vapor wall loss artifact in chamber experiments enhanced SOA mass concentrations although the enhancement was precursor-dependent as well as NOx-dependent. Accounting for the influence of NOx using the VOC:NOx ratios resulted in better predictions of OA mass concentrations in rural/remote environments but still underpredicted OA mass concentrations in urban environments. The updated model's performance against measurements combined with the results from the sensitivity simulations suggests that the OA mass concentrations in southern California are constrained within a factor of 2. Finally, simulations performed for the year 2035 showed that, despite reductions in VOC and NOx emissions in the future, SOA mass concentrations may be higher than in the year 2005, primarily from increased hydroxyl radical (OH) concentrations due to lower ambient NO2 concentrations.

Physical properties of secondary photochemical aerosol from OH oxidation of a cyclic siloxane.

Cyclic volatile methyl siloxanes (cVMS) are high-production chemicals present in many personal care products. They are volatile, hydrophobic, and relatively long-lived due to slow oxidation kinetics. Evidence from chamber and ambient studies indicates that oxidation products may be found in the condensed aerosol phase. In this work, we use an oxidation flow reactor to produce ~ 100 μgm-3 of organosilicon aerosol from OH oxidation of decamethyl-cyclopentasiloxane (D5) with aerosol mass fractions (i.e., yields) of 0.2-0.5. The aerosols were assessed for concentration, size distribution, morphology, sensitivity to seed aerosol, hygroscopicity, volatility and chemical composition through a combination of aerosol size distribution measurement, tandem differential mobility analysis, and electron microscopy. Similar aerosols were produced when vapor from solid antiperspirant was used as the reaction precursor. Aerosol yield was sensitive to chamber OH and to seed aerosol, suggesting sensitivity of lower-volatility species and recovered yields to oxidation conditions and chamber operation. The D5 oxidation aerosol products were relatively non-hygroscopic, with an average hygroscopicity kappa of ~ 0.01, and nearly non-volatile up to 190 °C temperature. Parameters for exploratory treatment as a semi-volatile organic aerosol in atmospheric models are provided.

Characterization of submicron aerosol chemical composition and sources in the coastal area of Central Chile

Chemical characteristics and the sources of submicron particles (<1 μm in diameter) were investigated in Valle Alegre, the coastal area of Central Chile. The chemical composition of particles was studied by using a Soot particle Aerosol Mass Spectrometer and Multi-Angle Absorption Photometer. Submicron particles were dominated by organics (42% of mass) and sulfate (39% of mass) while the mass fractions of ammonium, nitrate and black carbon were much smaller (13, 2 and 4% of mass, respectively). Additionally, several metals (V, Zn, Fe, Cd, Cu, K, Na and Mg) were detected in submicron particles and also some of their inorganic salts (e.g. NaCl+, MgCl2+, CaCl2+, KCl+ and KNO3+). The sources of particles were examined by using Positive Matrix Factorization (PMF). Organic aerosol (OA) was divided into five factors by using PMF; hydrocarbon-like OA (HOA), biomass burning OA (BBOA), low-volatility oxygenated OA (LV-OOA), semi-volatile OA (SV-OOA) and marine oxygenated OOA (MOOA). Oxygenated factors (LV-OOA; SV-OOA and MOOA) comprised 75% of total OA with LV-OOA being the dominant factor (38% of OA). Sulfate had two major sources in Valle Alegre; ∼70% of sulfate was related to anthropogenic sources through the oxidation of gas phase SO2 whereas ∼24% of sulfate was associated with biogenic origin related to the oxidation of dimethyl sulfide in the marine environment. Regarding total submicron particle mass (campaign-average 9.5 μg m−3), the contribution of anthropogenic sources was at least as large as that of biogenic origin.

Can semi-volatile organic aerosols lead to fewer cloud particles?

Abstract. The impact of condensing organic aerosols on activated cloud number concentration is examined in a new aerosol microphysics box model, MATRIX-VBS. The model includes the volatility basis set (VBS) framework coupled with the aerosol microphysical scheme MATRIX (Multiconfiguration Aerosol TRacker of mIXing state) that resolves aerosol mass and number concentrations and aerosol mixing state. By including the condensation of organic aerosols, the new model produces fewer activated particles compared to the original model, which treats organic aerosols as nonvolatile. Parameters such as aerosol chemical composition, mass and number concentrations, and particle sizes that affect activated cloud number concentration are thoroughly tested via a suite of Monte Carlo simulations. Results show that by considering semi-volatile organics in MATRIX-VBS, there is a lower activated particle number concentration, except in cases with low cloud updrafts, in clean environments at above-freezing temperatures, and in polluted environments at high temperatures (310 K) and extremely low-humidity conditions.

Origin and variability in volatile organic compounds observed at an Eastern Mediterranean background site (Cyprus)

Abstract. More than 7000 atmospheric measurements of over 60 C2 − C16 volatile organic compounds (VOCs) were conducted at a background site in Cyprus during a 1-month intensive field campaign held in March 2015. This exhaustive dataset consisted of primary anthropogenic and biogenic VOCs, including a wide range of source-specific tracers, and oxygenated VOCs (with various origins) that were measured online by flame ionization detection–gas chromatography and proton transfer mass spectrometry. Online submicron aerosol chemical composition was performed in parallel using an aerosol mass spectrometer. This study presents the high temporal variability in VOCs and their associated sources. A preliminary analysis of their time series was performed on the basis of independent tracers (NO, CO, black carbon), meteorological data and the clustering of air mass trajectories. Biogenic compounds were mainly attributed to a local origin and showed compound-specific diurnal cycles such as a daily maximum for isoprene and a nighttime maximum for monoterpenes. Anthropogenic VOCs as well as oxygenated VOCs displayed higher mixing ratios under the influence of continental air masses (i.e., western Asia), indicating that long-range transport significantly contributed to the VOC levels in the area. Source apportionment was then conducted on a database of 20 VOCs (or grouped VOCs) using a source receptor model. The positive matrix factorization and concentration field analyses were hence conducted to identify and characterize covariation factors of VOCs that were representative of primary emissions as well as chemical transformation processes. A six-factor PMF solution was selected, namely two primary biogenic factors (relative contribution of 43 % to the total mass of VOCs) for different types of emitting vegetation; three anthropogenic factors (short-lived combustion source, evaporative sources, industrial and evaporative sources; 21 % all together), identified as being either of local origin or from more distant emission zones (i.e., the south coast of Turkey); and a last factor (36 %) associated with regional background pollution (air masses transported both from the Western and Eastern Mediterranean regions). One of the two biogenic and the regional background factors were found to be the largest contributors to the VOC concentrations observed at our sampling site. Finally, a combined analysis of VOC PMF factors with source-apportioned organic aerosols (OAs) helped to better distinguish between anthropogenic and biogenic influences on the aerosol and gas phase compositions. The highest OA concentrations were observed when the site was influenced by air masses rich in semi-volatile OA (less oxidized aerosols) originating from the southwest of Asia, in contrast with OA factor contributions associated with the remaining source regions. A reinforcement of secondary OA formation also occurred due to the intense oxidation of biogenic precursors.

MATRIX-VBS (v1.0): implementing an evolving organic aerosol volatility in an aerosol microphysics model

Abstract. The gas-particle partitioning and chemical aging of semi-volatile organic aerosol are presented in a newly developed box model scheme, where its effect on the growth, composition, and mixing state of particles is examined. The volatility-basis set (VBS) framework is implemented into the aerosol microphysical scheme MATRIX (Multiconfiguration Aerosol TRacker of mIXing state), which resolves mass and number aerosol concentrations and in multiple mixing-state classes. The new scheme, MATRIX-VBS, has the potential to significantly advance the representation of organic aerosols in Earth system models by improving upon the conventional representation as non-volatile particulate organic matter, often also with an assumed fixed size distribution. We present results from idealized cases representing Beijing, Mexico City, a Finnish forest, and a southeastern US forest, and investigate the evolution of mass concentrations and volatility distributions for organic species across the gas and particle phases, as well as assessing their mixing state among aerosol populations. Emitted semi-volatile primary organic aerosols evaporate almost completely in the intermediate-volatility range, while they remain in the particle phase in the low-volatility range. Their volatility distribution at any point in time depends on the applied emission factors, oxidation by OH radicals, and temperature. We also compare against parallel simulations with the original scheme, which represented only the particulate and non-volatile component of the organic aerosol, examining how differently the condensed-phase organic matter is distributed across the mixing states in the model. The results demonstrate the importance of representing organic aerosol as a semi-volatile aerosol, and explicitly calculating the partitioning of organic species between the gas and particulate phases.

Water uptake by organic aerosol and its influence on gas/particle partitioning of secondary organic aerosol in the United States

Abstract Organic aerosol (OA) is at least partly hygroscopic, i.e., water partitions into the organic phase to a degree determined by the relative humidity (RH), the organic chemical composition, and the particle size. This organic-phase water increases the aerosol mass and provides a larger absorbing matrix while decreasing its mean molecular weight, which can encourage additional condensation of semi-volatile organic compounds. Most regional and global atmospheric models account for water uptake by inorganic salts but do not explicitly account for organic-phase water and its subsequent impact on gas/particle partitioning of semi-volatile OA. In this work, we incorporated the organic-phase water model described by Pankow et al. (2015) into the UCD/CIT air quality model to simulate water uptake by OA and assessed its influence on total OA mass concentrations. The model was run for one summer month over two distinct regions: South Coast Air Basin (SoCAB) surrounding Los Angeles, California and the eastern United States (US). In SoCAB where the OA was dominated by non-hygroscopic primary OA (POA), there was very little organic-phase water uptake (0.1–0.2 μg m−3) and consequently very little enhancement (or growth) in total OA concentrations (OA + organic-phase water): a 3% increase in total OA mass was predicted for a 0.1 increase in relative humidity. In contrast, in the eastern US where secondary OA (SOA) from biogenic sources dominated the OA, substantial organic-phase water uptake and enhancement in total OA concentrations was predicted, even in urban locations. On average, the model predicted a 20% growth in total OA mass for a 0.1 increase in relative humidity; the growth was equivalent to a 250 nm particle with a hygroscopicity parameter (κ) of 0.15. Further, for the same relative humidity, the exact extent of organic-phase water uptake and total OA enhancement was found to be dependent on the particle mixing state. When the source-oriented mixing state of aerosols was considered, generally, less organic-phase water uptake was predicted than when simple internal mixing approximations were made. Overall, the results indicated that organic-phase water can significantly influence predicted total OA concentrations under certain conditions. Regional models applied in areas with high humidity and significant SOA formation should include calculations for organic-phase water in order to capture this effect.

Uptake of Semivolatile Secondary Organic Aerosol Formed from α-Pinene into Nonvolatile Polyethylene Glycol Probe Particles.

Semivolatile organic compounds (SVOCs) play an essential role in secondary organic aerosol (SOA) formation, chemical aging, and mixing of organic aerosol (OA) from different sources. Polyethylene glycol (PEG400) particles are liquid, polar, and nearly nonvolatile; they provide a new vehicle to study the interaction between SVOCs with OA. With a unique fragment ion C4H9O2(+) (m/z 89), PEG400 can be easily separated from α-pinene SOA in aerosol mass spectra. By injecting separately prepared PEG probe particles into a chamber containing SOA coated on ammonium sulfate seeds, we show that a substantial pool of SVOCs exists in equilibrium with the original SOA particles. Quantitative findings are based on bulk mass spectra, size-dependent composition, and the evolution of individual particle mass spectra, which we use to separate the two particle populations. We observed a larger fraction of SVOC vapors with increased amounts of reacted α-pinene. For the same amount of reacted α-pinene, the SOA formed from α-pinene oxidized by OH radicals had a higher fraction of SOA vapors than SOA formed by α-pinene ozonolysis. Compared to the PEG400 probe particles, we observed a lower mass fraction of SVOCs in poly(ethylene glycol) dimethyl ether (MePEG500) probe particles under otherwise identical conditions; this may be due to the lower polarity of the MePEG500 or caused by esterification reactions between the PEG400 and organic acids in the SOA.

Fundamental Time Scales Governing Organic Aerosol Multiphase Partitioning and Oxidative Aging.

Traditional descriptions of gas-particle partitioning of organic aerosols (OA) rely solely on thermodynamic properties (e.g., volatility). Under realistic conditions where phase partitioning is dynamic rather than static, the transformation of OA involves the interplay of multiphase partitioning with oxidative aging. A key challenge remains in quantifying the fundamental time scales for evaporation and oxidation of semivolatile OA. In this paper, we use isomer-resolved product measurements of a series of normal-alkanes (C18, C20, C22, and C24) to distinguish between gas-phase and heterogeneous oxidation products formed by reaction with hydroxyl radicals (OH). The product isomer distributions when combined with kinetics measurements of evaporation and oxidation enable a quantitative description of the multiphase time scales to be simulated using a single-particle kinetic model. Multiphase partitioning and oxidative transformation of semivolatile normal-alkanes under laboratory conditions is largely controlled by the particle phase state, since the time scales of heterogeneous oxidation and evaporation are found to occur on competing time scales (on the order of 10(-1) h). This is in contrast to atmospheric conditions where heterogeneous oxidation time scales are expected to be much longer (on the order of 10(2) h), with gas-phase oxidation being the dominant process regardless of the evaporation kinetics. Our results demonstrate the dynamic nature of OA multiphase partitioning and oxidative aging and reveal that the fundamental time scales of these processes are crucial for reliably extending laboratory measurements of OA phase partitioning and aging to the atmosphere.

Particulate emissions from residential wood combustion in Europe – revised estimates and an evaluation

Abstract. Currently residential wood combustion (RWC) is increasing in Europe because of rising fossil fuel prices but also due to climate change mitigation policies. However, especially in small-scale applications, RWC may cause high emissions of particulate matter (PM). Recently we have developed a new high-resolution (7 × 7 km) anthropogenic carbonaceous aerosol emission inventory for Europe. The inventory indicated that about half of the total PM2.5 emission in Europe is carbonaceous aerosol and identified RWC as the largest organic aerosol source in Europe. The inventory was partly based on national reported PM emissions. Use of this organic aerosol inventory as input for two chemical transport models (CTMs), PMCAMx and EMEP MSC-W, revealed major underestimations of organic aerosol in winter time, especially for regions dominated by RWC. Interestingly, this was not universal but appeared to differ by country. In the present study we constructed a revised bottom-up emission inventory for RWC accounting for the semivolatile components of the emissions. The revised RWC emissions are higher than those in the previous inventory by a factor of 2–3 but with substantial inter-country variation. The new emission inventory served as input for the CTMs and a substantially improved agreement between measured and predicted organic aerosol was found. The revised RWC inventory improves the model-calculated organic aerosol significantly. Comparisons to Scandinavian source apportionment studies also indicate substantial improvements in the modelled wood-burning component of organic aerosol. This suggests that primary organic aerosol emission inventories need to be revised to include the semivolatile organic aerosol that is formed almost instantaneously due to dilution and cooling of the flue gas or exhaust. Since RWC is a key source of fine PM in Europe, a major revision of the emission estimates as proposed here is likely to influence source–receptor matrices and modelled source apportionment. Since usage of biofuels in small combustion units is a globally significant source, the findings presented here are also relevant for regions outside of Europe.

Chemical composition, main sources and temporal variability of PM&amp;lt;sub&amp;gt;1&amp;lt;/sub&amp;gt; aerosols in southern African grassland

Abstract. Southern Africa is a significant source region of atmospheric pollution, yet long-term data on pollutant concentrations and properties from this region are rather limited. A recently established atmospheric measurement station in South Africa, Welgegund, is strategically situated to capture regional background concentrations, as well as emissions from the major source regions in the interior of South Africa. We measured non-refractive submicron aerosols (NR-PM1) and black carbon over a one year period in Welgegund, and investigated the seasonal and diurnal patterns of aerosol concentration levels, chemical composition, acidity and oxidation level. Based on air mass back trajectories, four distinct source regions were determined for NR-PM1. Supporting data utilised in our analysis included particle number size distributions, aerosol absorption, trace gas concentrations, meteorological variables and the flux of carbon dioxide. The dominant submicron aerosol constituent during the dry season was organic aerosol, reflecting high contribution from savannah fires and other combustion sources. Organic aerosol concentrations were lower during the wet season, presumably due to wet deposition as well as reduced emissions from combustion sources. Sulfate concentrations were usually high and exceeded organic aerosol concentrations when air-masses were transported over regions containing major point sources. Sulfate and nitrate concentrations peaked when air masses passed over the industrial Highveld (iHV) area. In contrast, concentrations were much lower when air masses passed over the cleaner background (BG) areas. Air masses associated with the anti-cyclonic recirculation (ACBIC) source region contained largely aged OA. Positive Matrix Factorization (PMF) analysis of aerosol mass spectra was used to characterise the organic aerosol (OA) properties. The factors identified were oxidized organic aerosols (OOA) and biomass burning organic aerosols (BBOA) in the dry season and low-volatile (LV-OOA) and semi-volatile (SV-OOA) organic aerosols in the wet season. The results highlight the importance of primary BBOA in the dry season, which represented 33% of the total OA. Aerosol acidity and its potential impact on the evolution of OOA are also discussed.

Physical and chemical characterization of ambient aerosol by HR‐ToF‐AMS at a suburban site in Hong Kong during springtime 2011

An Aerodyne high‐resolution time‐of‐flight aerosol mass spectrometer (HR‐ToF‐AMS) has been employed in a field sampling campaign at a suburban coastal site in Hong Kong in springtime 2011 to provide insights into the size‐resolved chemical composition of nonrefractory submicron aerosol species. This is the first time that such detailed real‐time measurements have been made in Hong Kong. The total nonrefractory PM1 was dominated by sulfate (51.0%) and organics (28.2%) with considerable acidity (average in situ pH = 0.95) and a characteristic bimodal particle size distribution with peaks at 200 and 570 nm of vacuum aerodynamic diameter (Dva). Source apportionment of organic aerosol yielded three characteristic aerosol fractions (hydrocarbon‐like organic aerosol, semivolatile organic aerosol and low‐volatile organic aerosol) with distinct temporal patterns and distributions in different particle size regions. The influence of air mass origin on species concentrations, particle size distributions and elemental ratios was investigated using backtrajectory analysis. Larger particle diameters, greater fractions of oxygenated organic aerosol and higher organic‐to‐carbon ratios were observed during coastal and continental air mass influence. Three major pollution events with elevated nonrefractory PM1 concentrations were observed in the sampling period, which were related to distinct meteorological and circulatory conditions. Accumulation and redistribution of local and regional pollutants were notable in a period of strong land‐sea breeze over the Pearl River Delta region, with considerable photochemical activity and particle aging. Increased fractions of oxygenated organic aerosol were apparent in foggy conditions, illustrating the importance of aqueous phase oxidation processes in a cooler and more humid time period.

Time Scales for Gas-Particle Partitioning Equilibration of Secondary Organic Aerosol Formed from Alpha-Pinene Ozonolysis

Most chemical transport models assume instantaneous equilibrium to represent gas-particle partitioning of semivolatile organic aerosol. This approach has been challenged by recent studies suggesting that secondary organic aerosol (SOA) cannot reach equilibrium within atmospheric time scales. The emergent hypothesis is that gas-particle partitioning rates are limited by diffusion within the condensed phase, which is thought to be "glassy." Here, we investigate the equilibration time scales of SOA formed from α-pinene ozonolysis by measuring the dynamic response to a modest step-change in temperature. Upon heating, equilibrium is disturbed, and the particles evaporate to restore equilibrium at the new temperature, which is attained when evaporation ceases. The SOA was formed at 10 °C and then heated to near room temperature (30 °C) so that the phase state (viscosity) of the condensed-phase after heating is similar to how it would be in the atmosphere. Experiments were performed in both a thermodenuder, with SOA loading of 350 μg/m(3), and in a smog chamber, with SOA loading of 2-12 μg/m(3). Both experiments show, contrary to previous findings, that the SOA achieves equilibrium with dynamic responses consistent with a mass accommodation coefficient of order 0.1. For typical atmospheric conditions, this translates into equilibration time scales on the order of minutes to tens of minutes, supporting the use of equilibrium partitioning in chemical transport models.

Characteristics, sources and water-solubility of ambient submicron organic aerosol in springtime in Helsinki, Finland

In this study the characteristics, sources and water-solubility of submicron organic aerosol (OA) were investigated in Helsinki, Finland. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was used to determine the submicron non-refractory aerosol components nitrate, sulfate, ammonium, chloride and organics between April 9 and May 8, 2009. The concentrations of the major water-soluble ions and water-soluble organic carbon (WSOC) were measured by a particle-into-liquid sampler (PILS) combined with a total organic carbon (TOC) analyzer and two ion chromatographs (IC) between April 25 and May 28, 2009. Parallel measurements of the submicron particulate matter (PM1), organic carbon (OC), black carbon (BC), meteorological quantities and trace gases were used to complement and validate the AMS and PILS-TOC-IC data.Sources or atmospheric processes affecting the organic aerosol were investigated by applying the Positive Matrix Factorization (PMF) analysis to the high-resolution mass spectra of the HR-ToF-AMS organics. All together seven factors were needed to describe the variation in the obtained dataset. The factors consisted of two different types of low-volatility oxygenated OA (LV-OOA), local and long-range-transported (LRT) biomass burning OA (BBOA), semi-volatile OA (SV-OOA), hydrocarbon-like OA (HOA), and one local source (coffee roastery). These factors were interpretable and could be connected to specific sources or chemical characteristics (biomass burning, traffic, biogenic emissions, oxidized long-range-transported aerosols, marine-processed aerosols and nearby industrial activity) of ambient aerosols. In order to study the organic fraction and PMF factors further, the elemental ratios OM:OC, O:C, H:C and N:C were calculated. The value of the OM:OC ratio varied between 1.4 and 2.1. A high OM:OC ratio (1.5–2.1) was observed for the highly-oxidized and water-soluble fraction, whereas this ratio was clearly lower (1.2–1.4) for local and fresh sources such as traffic. Two different factors representing local and long-range-transported biomass burning were observed. Local biomass burning emissions had a lower OM:OC ratio, indicating that this factor was less aged and had a different source area compared with the LRT BBOA. The water-solubilities of the OA factors were studied by investigating the correlation between these factors and WSOC and by reconstructing the concentration of water-soluble particulate organic matter (WSPOM) from the OA factors. The reconstructed WSPOM had a good correlation with the measured concentration of WSPOM.

Biogenic and anthropogenic organic matter in aerosol over continental Europe: source characterization in the east Baltic region

An Aerodyne quadruple aerosol mass spectrometer (Q-AMS) has been used to provide on-line measurements of size dependent chemical composition of fine aerosol particles (PM1) at the Air Pollution Research Station in Preila, Lithuania, representing the east Baltic region. The size dependent chemical composition measurements by AMS have revealed that in marine air masses 118 nm mode organics-containing particles were fresher compared to sulfate-containing particles (295 nm), likely originated as secondary aerosol from forest emissions or produced by primary sea spray over the Baltic Sea. In polluted continental air masses sulfate and organics were highly internally mixed and aged. The mass spectral results indicated that the major components of organic compounds were oxygenated organic species with strong signals at m/z 18, 43, 44 with several specific features. Positive matrix factorization (PMF) of AMS organic mass spectral data has identified three factors: aged oxygenated low-volatility organic aerosol (LV-OOA), less oxygenated semi-volatile organic aerosol (SV-OOA), and biogenic organic aerosol (BGOA) of either terrestrial or marine origin. The measurements were compared with a real-time particulate matter Beta Absorption Monitor (Thermo ESM Andersen) and Micro Orifice Uniform Deposit Impactor (MOUDI) data. The intercomparison showed a good correlation and a stable ratio between PM1 and PM2.5 concentrations. A comparison of the on-line Q-AMS data and the off-line MOUDI fine particle (<1 μm) data yielded a reasonable agreement in size distributions but not the absolute mass concentrations due to sampling conditions, evaporation of acidic species from sampling substrates and bounce of the particles in the MOUDI.