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The effect of ammonia on the partitioning of two dicarboxylic acids, oxalic (C2) and adipic (C6) is determined. Measurements by a tandem differential mobility analysis system and a thermodenuder (TD-TDMA) system are used to estimate the saturation vapor pressure and enthalpy of vaporization of ammonium oxalate and adipate. Ammonia dramatically lowered the vapor pressure of oxalic acid, by several orders of magnitude, with an estimated vapor pressure of 1.7 ± 0.8 × 10(–6) Pa at 298 K. The vapor pressure of ammonium adipate was 2.5 ± 0.8 × 10(–5) Pa at 298 K, similar to that of adipic acid. These results suggest that the dominance of oxalate in diacid concentrations measured in ambient aerosol could be attributed to the salt formation with ammonia.

The measurement of Water-Soluble Organic Carbon (WSOC) in atmospheric aerosol is usually carried out by sample collection on filters, extraction in ultrapure water, filtration, and measurement of the total organic carbon. This paper investigates the role of different conditions of sampling and extraction as well as the range of solubilities of the organic compounds that contribute to the WSOC. The sampling and extraction of WSOC can be described by a single parameter, P, expressing the ratio of water used per volume of air sampled on the analyzed filter. Two cases are examined in order to bound the range of interactions of the various organic aerosol components with each other. In the first we assume that the organic species form an ideal solution in the particle and in the second that the extraction of a single compound is independent of the presence of the other organics. The ideal organic solution model predicts that species with water solubility as low as 10(-4) g L(-1) contribute to the measured WSOC. In the other end, the independent compounds model predicts that low-solubility (as low as 10(-7) g L(-1)) compounds are part of the WSOC. Studies of the WSOC composition are consistent with the predictions of the ideal organic solution model. A value of P = 0.1 cm(3) m(-3) is proposed for the extraction of WSOC for typical organic aerosol concentrations (1-10 μg m(-3)). WSOC measurements under high concentration conditions often used during source sampling will tend to give low WSOC values unless higher P values are used.

We use a two-dimensional volatility basis set (2D-VBS) box model to simulate secondary organic aerosol (SOA) mass yields of linear oxygenated molecules: n-tridecanal, 2- and 7-tridecanone, 2- and 7-tridecanol, and n-pentadecane. A hybrid model with explicit, a priori treatment of the first-generation products for each precursor molecule, followed by a generic 2D-VBS mechanism for later-generation chemistry, results in excellent model-measurement agreement. This strongly confirms that the 2D-VBS mechanism is a predictive tool for SOA modeling but also suggests that certain important first-generation products for major primary SOA precursors should be treated explicitly for optimal SOA predictions.

According to the pseudo-ideal mixing assumption employed in practically all chemical transport models, organic aerosol components from different sources interact with each other in a single solution, independent of their composition. This critical assumption greatly affects modeled organic aerosol concentrations, but there is little direct experimental evidence to support it. A main experimental challenge is that organic aerosol components from different sources often look similar when analyzed with an aerosol mass spectrometer. We developed a new experimental method to overcome this challenge, using isotopically labeled compounds ((13)C or D) and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). We generated mixtures of secondary organic aerosol (SOA) from isotopically labeled toluene and from unlabeled α-pinene and used the HR-ToF-AMS data to separate these different SOA types. We evaluated their interaction by comparing the aerosol mass yields of toluene and α-pinene when the SOA was formed in these mixtures to their yields when the SOA was formed in isolation. At equilibrium, our results are consistent with pseudo-ideal mixing of anthropogenic and biogenic SOA components from these chemically dissimilar precursors.

Secondary organic aerosol (SOA) consists of compounds with a wide range of volatilities and its ambient concentration is sensitive to this volatility distribution. Recent field studies have shown that the typical mass spectrum of ambient oxygenated organic aerosol (OOA) as measured by the Aerodyne Aerosol Mass Spectrometer (AMS) is quite different from the SOA mass spectra reported in smog chamber experiments. Part of this discrepancy is due to the dependence of SOA composition on the organic aerosol concentration. High precursor concentrations lead to higher concentrations of the more volatile species in the produced SOA while at lower concentrations the less volatile compounds dominate the SOA composition. alpha-Pinene, beta-pinene, d-limonene, and beta-caryophyllene ozonolysis experiments were performed at moderate concentration levels. Using a thermodenuder the more volatile SOA species were removed achieving even lower SOA concentration. The less volatile fraction was then chemically characterized by an AMS. The signal fraction of m/z44, and thus the concentration of C02+, is significantly higher for the less volatile SOA. High NO(x) conditions result in less oxidized SOA than low NO(x) conditions, while increasing relative humidity levels results in more oxidized products for limonene but has little effect on alpha-and beta-pinene SOA. Combining a smog chamber with a thermodenuder model employing the volatility basis-set framework, the AMS SOA mass spectrum for each experiment and for each precursor is deconvoluted into low, medium, and high volatility component mass spectra. The spectrum of the surrogate component with the lower volatility is quite similar to that of ambient OOA.

Recent developments in our understanding of atmospheric organic particulate matter formation have been used to develop a state-of-the-art organic aerosol module for regional chemical transport models (CTMs). The module has been added to the regional CTM, PMCAMx, and has been evaluated against observations in the eastern U.S. The new module uses the volatility basis set framework to simulate primary organic aerosol (POA) partitioning between the gas and particulate phases and the gas-phase oxidation of the corresponding vapors. The formation and chemical aging of secondary organic aerosol (SOA) are modeled using the same volatility distribution approach. The module uses recent results from smog chamber studies for the formation of SOA from anthropogenic and biogenic hydrocarbons. Hourly organic aerosol predictions are evaluated using data from the Pittsburgh Air Quality Study (PAQS), and daily averaged predictions for July 2001 are compared to ambient measurements from the EPA Speciated Trends Network (STN) and the Interagency Monitoring of Protected Visual Environments (IMPROVE). The model reproduces both the absolute organic aerosol concentrations in urban and rural locations as well as the high degree of oxidation of these compounds. The chemical aging of anthropogenic SOA is consistent with the ambient organic aerosol concentration field and has a significant impact on the absolute ground-level concentrations of these compounds.

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.

The SOA formation from beta-pinene ozonolysis at modest precursor concentrations (2-40 ppb) was investigated in the temperature range of 0-40 degrees C. The presence of inert seeds and high ozone concentrations is necessary to minimize losses of semivolatile vapors to the walls of the smog chamber. beta-pinene secondary organic aerosol production increases significantly with decreasing temperature. An increase by a factor of 2-3, depending on the reacted beta-pinene concentration, was observed as the temperature decreased from 40 to 0 degrees C. This increase appearsto be due mainly to the shifting of partitioning of the semivolatile SOA componentstoward the particulate phase and not to a change of the beta-pinene product distribution with temperature. The measurements are used to develop a new temperature-dependent parametrization for the four-component basis-set. The parametrization predicts much higher SOA production for beta-pinene ozonolysis for typical atmospheric conditions than the values that have been suggested by previous studies.

The temperature-dependence of secondary organic aerosol (SOA) concentrations is measured using a temperature-controlled smog chamber. Aerosols are generated from reaction of alpha-pinene (14-150 ppb) and ozone at a constant temperature of 22 +/- 2 degrees C in the presence of the OH-scavenger 2-butanol. After the reactions are completed the chamber is heated or cooled in a range from 20 to 40 degrees C. SOA volume concentrations increase at temperatures below the initial formation temperature and decrease at elevated temperatures. The response to the temperature change as measured by percent mass change per degree ranged from -0.4 to -3.6% K(-1), for a total mass reduction of 5-60% upon heating from 22 to 35 degrees C. The reported range is due to two factors: (1) experimental uncertainty, arising mainly from uncertainty in evaporation and condensation behavior of particles lost to the chamber wall; (2) differences in the temperature response from experiment to experiment. Aerosol temperature sensitivity was also measured by tandem differential mobility analysis (TDMA) where similarly generated SOA were heated from 20 to 25 degrees C to 30-40 degrees C with residence times of 0.5-1.5 min, resulting in particle volume reductions of up to 20%. The TDMA experiments indicate that evaporation of the SOA particles in this system occurs with a potentially significant mass transfer limitation (e.g., accommodation coefficient <0.1).

Ammonium sulfate particles were generated by atomization and introduced into a smog chamber where they were coated with secondary organic aerosol from ozonolysis of limonene or alpha-pinene. These mixed particles were then sampled with a humidified Tandem-DMA system where a monodisperse aerosol population was selected, humidified, and dried to observe the relative humidity (RH) at which the particles returned to the original dry diameter. The volume fraction of secondary organic aerosol (SOA) in the mixed particles ranged from 0.59 to 0.94 for limonene SOA and 0.54 to 0.72 for alpha-pinene SOA. Efflorescence RHs for our mixed aerosols were in the range of 28-34%, similar to our observation of 32% ERH for pure ammonium sulfate nanoparticles. These findings indicate that the effect of SOA on the ERH of inorganic salts in the atmosphere may be negligible.

A substantial fraction of the total ultrafine particulate mass is comprised of organic compounds. Of this fraction, a significant subfraction is secondary organic aerosol (SOA), meaning that the compounds are a by-product of chemistry in the atmosphere. However, our understanding of the kinetics and mechanisms leading to and following SOA formation is in its infancy. We lack a clear description of critical phenomena; we often don't know the key, rate limiting steps in SOA formation mechanisms. We know almost nothing about aerosol yields past the first generation of oxidation products. Most importantly, we know very little about the derivatives in these mechanisms; we do not understand how changing conditions, be they precursor levels, oxidant concentrations, co-reagent concentrations (i.e., the VOC/NOx ratio) or temperature will influence the yields of SOA. In this paper we explore the connections between fundamental details of physical chemistry and the multitude of steps associated with SOA formation, including the initial gas-phase reaction mechanisms leading to condensible products, the phase partitioning itself, and the continued oxidation of the condensed-phase organic products. We show that SOA yields in the alpha-pinene + ozone are highly sensitive to NOx, and that SOA yields from beta-caryophylene + ozone appear to increase with continued ozone exposure, even as aerosol hygroscopicity increases as well. We suggest that SOA yields are likely to increase substantially through several generations of oxidative processing of the semi-volatile products.

New particle formation and growth events have been observed in several urban areas and are of concern due to their potential negative effects on human health. The main purpose of this study was to investigate the chemistry of ultrafine particles during the growth phase of the frequently observed nucleation events in Pittsburgh (approximately 100 events per year) and therefore infer the mechanisms of new particle growth in the urban troposphere. An Aerodyne aerosol mass spectrometer (AMS) and two SMPS systems were deployed at the U.S. EPA Pittsburgh Supersite during September 2002. Significant nucleation events were observed in 3 out of the 16 days of this deployment, including one of the 10 strongest nucleation events observed in Pittsburgh over a period of 15 months. These events appear to be representative of the climatology of new particle formation and growth in the Pittsburgh region. Distinctive growth of sulfate, ammonium, organics, and nitrate in the ultrafine mode (33-60 nm in a vacuum aerodynamic diameter or approximately 18-33 nm in physical diameter) was observed during each of these three events, with sulfate always being the first (and the fastest) species to increase. Ultrafine ammonium usually increased 10-40 min later than sulfate, causing the ultrafine mode particles to be more acidic during the initial stages of the nucleation events. Significant increase of ultrafine organics often happened after 11:00 a.m., when photochemistry is more intense. This observation coupled with a parallel increase of ultrafine m/z 44, a mass fragment generally representative of oxygenated organic compounds, indicates that secondary organic species contribute significantly to the growth of particles at a relatively later time of the event. Among all these four species, nitrate was always a minor component of the ultrafine particles and contributed the least to the new particle growth.

The formation of secondary organic aerosols (SOA) is simulated for the Nashville/western Tennessee domain using three recent SOA modules incorporated into the three-dimensional air quality model, CMAQ. The Odum/Griffin et al. and CMU/STI modules represent SOA absorptive partitioning into a mixture of primary and secondary particulate organic compounds (OC), with some differences in the formulation of the absorption process and the selection of SOA species and their precursors. Empirical representations based on measured laboratory SOA yields are used for condensable organic products in both these modules. The AEC module simulates SOA absorption into organic and aqueous particulate phases, and a representation based on an explicit gas-phase mechanism is used in the AEC module. Predicted SOA concentrations can vary by a factor of 10 or more. In general, the gas-phase mechanistic approach predicts a higher yield of SOA than those based on laboratory yields. There exist some differences in the two empirical modules despite their similar basis on experimental data. All three modules predict a dominance of SOA of biogenic origin as compared to SOA of anthropogenic origin. The causes for differences among the three SOA modules include the representation of terpenes, the mechanistic versus empirical representation of SOA-forming reactions, the identities of SOA, and the parameters used in the gas/particle partitioning calculations. Two sensitivity studies show that formation of water-soluble SOA and temperature dependence may be areas of key uncertainties affecting current models.

Measuring the mass concentration (PM0.1) of ultrafine particles (UFPs; diameter less than 100 nm) continuously, automatically, and accurately poses a considerable challenge because of the typically low PM0.1 concentration levels and susceptibility to interference from larger particles during the measurement processes. Previous work has shown that PM0.1 has a strong and stable correlation with the number concentration of particles with diameter above 50 nm (N50). This suggests that the indirect measurement of PM0.1 might be possible by measuring N50. In this work, the feasibility of using a Condensation Particle Counter (CPC) that measures PM0.1 by measuring N50 is examined. A water-based CPC was modified to measure N50 (CPC50). The CPC50's ability to measure PM0.1 was then evaluated by comparing its measurements with the PM0.1 measurements of a Scanning Mobility Particle Sizer (SMPS) in a suburban area. PM0.1 measured by the CPC50 was in good agreement with PM0.1 measured by the SMPS with a fractional bias (FBIAS) of 4% and a fractional error (FERROR) of 21% for hourly measurements. These findings indicate that a CPC50 could serve as a cheaper alternative to the SMPS, allowing for continuous and real-time measurement of PM0.1. Copyright © 2024 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The Cloud-AerosoL InteractionS in the Helmos background TropOsphere (CALISHTO) campaign took place in autumn 2021 at the NCSR Demokritos background high-altitude Helmos Hellenic Atmospheric Aerosol & Climate Change station (HAC)2 to study the interactions between aerosols and clouds. The current study presents the chemical characterization of the Non-Refractory (NR) PM1 aerosol fraction using a Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM). A comparative offline aerosol filter analysis by a High-Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) showed consistent results regarding the species determined. Source apportionment applied on both datasets (ACSM-ToF and offline AMS analysis on filter extracts) yielded the same factors for the organic aerosol (one primary and two secondary factors). Additionally, the Positive Matrix Factorization (PMF) model was applied on the total PM1 fraction by the ToF-ACSM (including both organic and inorganic ions). Five different types were identified, including a primary organic factor, ammonium nitrate, ammonium sulphate, and two secondary organic aerosols; one more and one less oxidized. The prevailing atmospheric conditions at the station, i.e. cloud presence, influence from emissions from the Planetary Boundary Layer (PBL) and air mass origin were also incorporated in the study. The segregation between PBL and Free Troposphere (FT) conditions was made by combining data from remote sensing and in-situ measurement techniques. The types of air masses arriving at the site were grouped as continental, marine, dust and marine-dust based on back trajectories data. Significant temporal variability in the aerosol characteristics was observed throughout the campaign; in September, air masses from within the PBL were sampled most of the time, resulting in much higher mass concentrations compared to October and November when concentrations were reduced by a factor of 5. Both in-cloud and FT measurement periods resulted in much lower concentration levels, while similar composition was observed in PBL and FT conditions. We take advantage of using a recently developed "virtual filtering" technique to separate interstitial from activated aerosol sampled from a PM10 inlet during cloudy periods. This allows the determination of the chemical composition of the interstitial aerosol during in-cloud periods. Ammonium sulphate, the dominant PMF factor in all conditions, contributed more when air masses were arriving at (HAC)2 during Dust events, while higher secondary organic aerosol contribution was observed when air masses arrived from continental Europe. [ABSTRACT FROM AUTHOR]

Inconsistencies have often been observed between the measurements obtained from the Aerosol Mass Spectrometer (AMS) and the Scanning Mobility Particle Sizer (SMPS) during laboratory experiments focused on secondary organic aerosol (SOA) formation. We show that these discrepancies are due to the use of a size-independent collection efficiency (CE) especially in experiments in which new particle formation takes place. A new methodology for the estimation of a composition and size-dependent CE is proposed. This approach significantly improves the agreement between the measurements from the two instruments with the average deviation of the two types of measurements remaining below 10% for both total aerosol and SOA concentration. We show that in these experiments, ammonium sulfate particles both pure and thinly coated with SOA have a lower CE than the pure SOA particles or particles thickly coated with SOA. Copyright © 2024 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

This study explores the differences in performance and results by various versions of the ISORROPIA thermodynamic module implemented within the ECHAM/MESSy Atmospheric Chemistry (EMAC) model. Three different versions of the module were used, ISORROPIA II v1, ISORROPIA II v2.3, and ISORROPIA-lite. First, ISORROPIA II v2.3 replaced ISORROPIA II v1 in EMAC to improve pH predictions close to neutral conditions. The newly developed ISORROPIA-lite has been added to EMAC alongside ISORROPIA II v2.3. ISORROPIA-lite is more computationally efficient and assumes that atmospheric aerosols exist always as supersaturated aqueous (metastable) solutions, while ISORROPIA II includes the option to allow for the formation of solid salts at low RH conditions (stable state). The predictions of EMAC by employing all three aerosol thermodynamic models were compared to each other and evaluated against surface measurements from three regional observational networks in the polluted Northern Hemisphere (Interagency Monitoring of Protected Visual Environments (IMPROVE), European Monitoring and Evaluation Programme (EMEP), and Acid Deposition Monitoring Network of East Asia (EANET)). The differences between ISORROPIA II v2.3 and ISORROPIA-lite were minimal in all comparisons with the normalized mean absolute difference for the concentrations of all major aerosol components being less than 11 % even when different phase state assumptions were used. The most notable differences were lower aerosol concentrations predicted by ISORROPIA-lite in regions with relative humidity in the range of 20 % to 60 % compared to the predictions of ISORROPIA II v2.3 in stable mode. The comparison against observations yielded satisfactory agreement especially over the USA and Europe but higher deviations over East Asia, where the overprediction of EMAC for nitrate was as high as 4 µ g m -3 (∼20%). The mean annual aerosol pH predicted by ISORROPIA-lite was on average less than a unit lower than ISORROPIA II v2.3 in stable mode, mainly for coarse-mode aerosols over the Middle East. The use of ISORROPIA-lite accelerated EMAC by nearly 5 % compared to the use of ISORROPIA II v2.3 even if the aerosol thermodynamic calculations consume a relatively small fraction of the EMAC computational time. ISORROPIA-lite can therefore be a reliable and computationally efficient alternative to the previous thermodynamic module in EMAC. [ABSTRACT FROM AUTHOR]

Atmospheric organic compounds with an effective saturation concentration (C∗) at 298 K between 10 3 and 10 6 µgm-3 are called intermediate-volatility organic compounds (IVOCs), and they have been identified as important secondary organic aerosol (SOA) precursors. In this work, we simulate IVOCs emitted from on-road diesel and gasoline vehicles over Europe with a chemical transport model (CTM), utilizing a new approach in which IVOCs are treated as lumped species that preserve their chemical characteristics. This approach allows us to assess both the overall contribution of IVOCs to SOA formation and the role of specific compounds. For the simulated early-summer period, the highest concentrations of SOA formed from the oxidation of on-road IVOCs (SOA-iv) are predicted for major European cities, like Paris, Athens, and Madrid. In these urban environments, on-road SOA-iv can account for up to a quarter of the predicted total SOA. Over Europe, unspeciated cyclic alkanes in the IVOC range are estimated to account for up to 72 % of the total on-road SOA-iv mass, with compounds with 15 to 20 carbons being the most prominent precursors. The sensitivity of the predicted SOA-iv concentrations to the selected parameters of the new lumping scheme is also investigated. Active multigenerational aging of the secondary aerosol products has the most significant effect as it increases the predicted SOA-iv concentrations by 67 %. [ABSTRACT FROM AUTHOR]

Wildfires are a significant source of organic aerosol during summer, with major impacts on air quality and climate. However, studies in Europe suggest a surprisingly low (less than 10%) contribution of biomass burning organic aerosol to average summertime fine particulate matter levels. In this study we combine field measurements and atmospheric chemical transport modeling, to demonstrate that the contribution of wildfires to fine particle levels in Europe during summer is seriously underestimated. Our work suggests that the corresponding contribution has been underestimated by a factor of 4–7 and that wildfires were responsible for approximately half of the total OA in Europe during July 2022. This discrepancy with previous work is due to the rapid physicochemical transformation of these emissions to secondary oxidized organic aerosol with an accompanying loss of its organic chemical fingerprints. These atmospheric reactions lead to a regionally distributed background organic aerosol that is responsible for a significant fraction of the health-related impacts caused by fine particles in Europe and probably in other continents. These adverse health effects can occur hundreds or even thousands of kilometers away from the fires. We estimate that wildfire emissions are responsible for 15–22% of the deaths in Europe due to exposure to fine particulate matter during summer. [ABSTRACT FROM AUTHOR]

Water is a key component of atmospheric aerosol, affecting many aerosol processes including gas-to-particle partitioning of semi-volatile compounds. Water related to secondary organic aerosol (SOAW) is often neglected in atmospheric chemical transport models and is not considered in gas-to-particle partitioning calculations for inorganic species. We use a new inorganic aerosol thermodynamics model, ISORROPIA-lite, which considers the effects of SOAW, to perform chemical transport model simulations for 1 year over the continental United States to quantify its effects on aerosol mass concentration and composition. SOAW can increase average fine aerosol water levels by up to a factor of 2 when secondary organic aerosol (SOA) is a major PM 1 component. This is often the case in the south-eastern US, where SOA concentrations are higher. Although the annual average impact of this added water on total dry PM 1 concentrations due to increased partitioning of nitrate and ammonium is small (up to 0.1 µ g m -3), total dry PM 1 increases of up to 2 µ g m -3 (with nitrate levels increases of up to 200 %) can occur when RH levels and PM 1 concentrations are high. [ABSTRACT FROM AUTHOR]

Atmospheric organic compounds with an effective saturation concentration (C*) at 298 K between 103 and 106 μg m-3 are called intermediate volatility organic compounds (IVOCs) and they have been identified as important secondary organic aerosol (SOA) precursors. In this work, we simulate IVOCs emitted from on-road diesel and gasoline vehicles over Europe with a chemical transport model (CTM), utilizing a new approach, where IVOCs are treated as lumped species that preserve their chemical characteristics. This approach allows us to assess both the overall contribution of IVOCs to SOA formation and the role of specific compounds. For the simulated early summer period, the highest concentrations of SOA formed from the oxidation of on-road IVOCs (SOA-iv) are predicted for major European cities, like Paris, Athens, and Madrid. In these urban environments, on-road SOA-iv can account for up to a quarter of the predicted total SOA. Over Europe, unspeciated cyclic alkanes in the IVOC range are estimated to account for up to 72 % of the total on-road SOA-iv mass, with compounds with 15 to 20 number of carbons being the most prominent precursors. The sensitivity of the predicted SOA-iv concentrations to the selected parameters of the new lumping scheme is also investigated. Active multigenerational aging of the secondary aerosol products has the most significant effect as it increases the predicted SOA-iv concentrations by 67 %. [ABSTRACT FROM AUTHOR]

This study explores the differences in performance and results by various versions of the ISORROPIA thermodynamic module implemented within the global atmospheric chemistry model EMAC. Three different versions of the module were used, ISORROPIA II v1, ISORROPIA II v2.3, and ISORROPIA-lite. First, ISORROPIA II v2.3 replaced ISORROPIA II v1 in EMAC to improve pH predictions close to neutral conditions. The newly developed ISORROPIA-lite has been added to EMAC alongside ISORROPIA II v2.3. ISORROPIA-lite is more computationally efficient and assumes that atmospheric aerosols exist always as supersaturated aqueous (metastable) solutions while ISORROPIA II includes the option to allow the formation of solid salts at low RH conditions (stable state). The predictions of EMAC by employing all three aerosol thermodynamic models were compared to each other and evaluated against surface measurements from three regional observational networks (IMPROVE, EMEP, EANET) in the polluted Northern Hemisphere. The differences between ISORROPIA II v2.3 and ISORROPIA-lite were minimal in all comparisons with the normalized mean absolute difference for the concentrations of all major aerosol components being less than 10 % even when different phase state assumptions were used. The most notable differences were lower aerosol concentrations predicted by ISORROPIA-lite in regions with relative humidity in the range of 20% to 60% compared to the predictions of ISORROPIA II v2.3 in stable mode. The comparison against observations yielded satisfactory agreement especially over the US and Europe, but higher deviations over East Asia, where the overprediction of EMAC for nitrate was as high as 4 μg m-3 (~ 20%). The mean annual aerosol pH predicted by ISORROPIAlite was on average less than a unit lower than ISORROPIA II v2.3 in stable mode, mainly for coarse mode aerosols over Middle East. The use of ISORROPIA-lite accelerated EMAC by 5 % compared to the use of ISORROPIA II v2.3 even if the aerosol thermodynamic calculations consume a relatively small fraction of the EMAC computational time. ISORROPIA-lite can therefore be a reliable and computationally effective replacement of the previous thermodynamic module in EMAC. [ABSTRACT FROM AUTHOR]

Primary emissions from wood and pellet stoves were aged in an atmospheric simulation chamber under daytime and nighttime conditions. The aerosol was analyzed with online aerosol mass spectrometry and offline Fourier transform infrared spectroscopy (FTIR). Measurements using the two techniques agreed reasonably well in terms of the organic aerosol (OA) mass concentration, OA:OC trends, and concentrations of biomass burning markers – lignin-like compounds and anhydrosugars. Based on aerosol mass spectrometry, around 15 % of the primary organic aerosol (POA) mass underwent some form of transformation during daytime oxidation conditions after 6–10 h of atmospheric exposure. A lesser extent of transformation was observed during the nighttime oxidation. The decay of certain semi-volatile (e.g., levoglucosan) and less volatile (e.g., lignin-like) POA components was substantial during aging, highlighting the role of heterogeneous reactions and gas–particle partitioning. Lignin-like compounds were observed to degrade under both daytime and nighttime conditions, whereas anhydrosugars degraded only under daytime conditions. Among the marker mass fragments of primary biomass burning OA (bbPOA), heavy ones (higher m/z) were relatively more stable during aging. The biomass burning secondary OA (bbSOA) became more oxidized with continued aging and resembled that of aged atmospheric organic aerosols. The bbSOA formed during daytime oxidation was dominated by acids. Organonitrates were an important product of nighttime reactions in both humid and dry conditions. Our results underline the importance of changes to both the primary and secondary biomass burning aerosols during their atmospheric aging. Heavier fragments from aerosol mass spectrometry seldom used in atmospheric chemistry can be used as more stable tracers of bbPOA and, in combination with the established levoglucosan marker, can provide an indication of the extent of bbPOA aging. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) is a major fraction of the total organic aerosol (OA) in the atmosphere. SOA is formed by the partitioning onto pre-existent particles of low-vapor-pressure products of the oxidation of volatile, intermediate-volatility, and semivolatile organic compounds. Oxidation of the precursor molecules results in a myriad of organic products, making the detailed analysis of smog chamber experiments difficult and the incorporation of the corresponding results into chemical transport models (CTMs) challenging. The volatility basis set (VBS) is a framework that has been designed to help bridge the gap between laboratory measurements and CTMs. The parametrization of SOA formation for the VBS has been traditionally based on fitting yield measurements of smog chamber experiments. To reduce the uncertainty in this approach, we developed an algorithm to estimate the SOA product volatility distribution, effective vaporization enthalpy, and effective accommodation coefficient combining SOA yield measurements with thermograms (from thermodenuders) and areograms (from isothermal dilution chambers) from different experiments and laboratories. The algorithm is evaluated with "pseudo-data" produced from the simulation of the corresponding processes, assuming SOA with known properties and introducing experimental error. One of the novel features of our approach is that the proposed algorithm estimates the uncertainty in the predicted yields for different atmospheric conditions (temperature, SOA concentration levels, etc.). The uncertainty in these predicted yields is significantly smaller than that of the estimated volatility distributions for all conditions tested. [ABSTRACT FROM AUTHOR]

The offline aerosol mass spectrometry technique is a useful tool for the source apportionment of organic aerosol (OA) in areas and periods during which an aerosol mass spectrometer (AMS) is not available. However, the technique is based on the extraction of aerosol samples in water, while several atmospheric OA components are partially or fully insoluble in water. In this work an improved offline technique was developed and evaluated in an effort to capture most of the partially soluble and insoluble organic aerosol material, reducing significantly the uncertainty of the corresponding source apportionment. A major advantage of the proposed approach is that no corrections are needed for the offline analysis to account for the limited water solubility of some OA components. The improved offline AMS analysis was tested in three campaigns: two during winter and one during summer. Collocated online AMS measurements were performed for the evaluation of the offline method. Source apportionment analysis was performed separately for the online and the offline measurements using positive matrix factorization (PMF). The PMF results showed that the fractional contribution of each factor to the total OA differed between the online and the offline PMF results by less than 15 %. The differences in the AMS spectra of the factors of the two approaches could be significant, suggesting that the use of factor profiles from the literature in the offline analysis may lead to complications. Part of the good agreement between the online and the offline PMF results is due to the ability of the improved offline AMS technique to capture a bigger part of the OA, including insoluble organic material. This was evident by the significant fraction of submicrometer suspended insoluble particles present in the water extract and by the reduced insoluble material on the filters after the extraction process. More than half of the elemental carbon (EC) was on average missing from the filters after the water extraction. Significant EC concentrations were measured in the produced aerosol that was used as input to the AMS during the offline analysis. [ABSTRACT FROM AUTHOR]

Various parameterizations of organic aerosol (OA) formation and its subsequent evolution in the two-dimensional Volatility Basis Set (2D-VBS) framework are evaluated using ground measurements collected in the 2013 PEGASOS field campaign in the boreal forest station of Hyytiälä in southern Finland. A number of chemical aging schemes that performed well in the polluted environment of the Po Valley in Italy during the PEGASOS 2012 campaign are examined, taking into account various functionalization and fragmentation pathways for biogenic and anthropogenic OA components. All seven aging schemes considered have satisfactory results, consistent with the ground measurements. Despite their differences, these schemes predict similar contributions of the various OA sources and formation pathways for the periods examined. The highest contribution comes from biogenic secondary OA (bSOA), as expected, contributing 40–63% depending on the modeling scheme. Anthropogenic secondary OA (aSOA) is predicted to contribute 11–18% of the total OA, while SOA from intermediate-volatility compounds (SOA-iv) oxidation contributes another 18–27%. The fresh primary OA (POA) contributes 4%, while the SOA resulting from the oxidation of the evaporated semivolatile POA (SOA-sv) varies between 4 and 6%. Finally, 5–6% is predicted to be due to long-range transport from outside the modeling domain. [ABSTRACT FROM AUTHOR]

The off-line Aerosol Mass Spectrometry (AMS) technique is a useful tool for the source apportionment of organic aerosol (OA) in areas and periods during which an AMS is not available. However, the technique is based on the extraction of aerosol samples in water, while several atmospheric OA components are partially or fully insoluble in water. In this work an improved off-line technique was developed and evaluated in an effort to capture most of the partially soluble and insoluble organic aerosol material, reducing significantly the uncertainty of the corresponding source apportionment. A major advantage of the proposed approach is that no corrections are needed for the off-line analysis to account for the limited water solubility of some OA components. The improved off-line AMS analysis was tested in three campaigns: two during winter and one during summer. Collocated on-line AMS measurements were performed for the evaluation of the off-line method. Source apportionment analysis was performed separately for the on-line and the off-line measurements using Positive Matrix Factorization (PMF). The PMF results showed that the fractional contribution of each factor to the total OA differed between the on-line and the off-line PMF results by less than 15%. The differences in the AMS spectra of the factors of the two approaches could be significant suggesting that the use of factor profiles from the literature in the off-line analysis may lead to complications. Part of the good agreement between the on-line and the off-line PMF results is due to the ability of the improved off-line AMS technique to capture a bigger part of the OA, including insoluble organic material. This was evident by the significant fraction of submicrometer suspended insoluble particles present in the water extract, and by the reduced insoluble material on the filters after the extraction process. More than half of the elemental carbon (EC) was on average missing from the filters after the water extraction. Significant EC concentrations were measured in the produced aerosol that was used as input to the AMS during the off-line analysis. [ABSTRACT FROM AUTHOR]

Norpinic acid is a major semi-volatile oxidation product of α-pinene and β-pinene, two of the most important biogenic atmospheric volatile organic compounds. In this study we characterized the physicochemical properties of norpinic acid aerosol using a variety of techniques, and we investigated its reaction with OH radicals. The Aerosol Mass Spectrometer (AMS) spectrum of norpinic acid was characterized by a pronounced peak at m/z 82 (C5H6O+), which can be used as its chemical signature. The measured density of norpinic acid particles was 1.3 g cm−3. Its saturation concentration at 298 K was estimated to be equal to 8.9 μg m−3 using thermodenuder measurements and 12.8 μg m−3 using isothermal dilution. Its vaporization enthalpy was equal to 71 kJ mol−1. After reaction with OH radicals for an equivalent atmospheric period of 0.6–5 days under UV radiation and low RH, there were no noticeable changes in the AMS spectrum of the particles, while the wall-loss corrected mass concentration slightly decreased. This suggests that the atmospheric aging products of norpinic acid particles are quite similar to the parent molecule when measured by the AMS, and the aging reactions lead to a small change in particle mass concentration. [ABSTRACT FROM AUTHOR]

PMCAMx-UF, a three-dimensional chemical transport model focusing on the simulation of the ultrafine particle size distribution and composition has been extended with the addition of reactions of chemical aging of semivolatile anthropogenic organic vapors, the emissions and chemical aging of intermediate-volatility organic compounds (IVOCs), and the production of extremely low-volatility organic compounds (ELVOCs) by monoterpenes. The model is applied in Europe to quantify the effect of these processes on particle number concentrations. The model predictions are evaluated against both ground measurements collected during the PEGASOS 2012 summer campaign across many stations in Europe and airborne observations by a zeppelin measuring above Po Valley, Italy. PMCAMx-UF reproduces the ground level daily average concentrations of particles with a diameter larger than 100 nm (N100) with normalized mean error (NME) of 45 % and normalized mean bias (NMB) close to 10 %. For the same simulation, PMCAMx-UF tends to overestimate the concentration of particles with a diameter larger than 10 nm (N10) with a daily NMB of 23 % and a daily NME of 63 %. The model was able to reproduce more than 75 % of the N10 and N100 airborne observations (zeppelin) within a factor of 2. According to the PMCAMx-UF predictions, the ELVOC production by monoterpenes leads to surprisingly small changes of the average number concentrations over Europe. The total number concentration decreased due to the ELVOC formation by 0.2 %, N10 decreased by 1.1 %, N50 (particles with a diameter larger than 50 nm) increased by 3 %, and N100 increased by 4 % due to this new secondary organic aerosol (SOA) source. This small change is due to the nonlinearity of the system, with increases predicted in some areas and decreases in others, but also the cancelation of the effects of the various processes like accelerated growth and accelerated coagulation. Locally, the effects can be significant. For example, an increase in N100 by 20 %–50 % is predicted over Scandinavia and significant increases (10 %–20 %) are predicted over some parts of central Europe. The ELVOCs contributed on average around 0.5 µgm-3 and accounted for 10 %–15 % of the PM2.5 OA. The addition of IVOC emissions and their aging reactions led to a surprising reduction of the total number of particles (Ntot) and N10 by 10 %–15 % and 5 %–10 %, respectively, and to an increase in the concentration of N100 by 5 %–10 %. These were due to the accelerated coagulation and reduced nucleation rates. [ABSTRACT FROM AUTHOR]

Sub-50 nm particles originating from traffic emissions pose risks to human health due to their high lung deposition efficiency and potentially harmful chemical composition. We present a modeling study using an updated European Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) number emission inventory, incorporating a more realistic, empirically justified particle size distribution (PSD) for sub-50 nm particles from road traffic as compared with the previous version. We present experimental PSDs and CO2 concentrations, measured in a highly trafficked street canyon in Helsinki, Finland, as an emission factor particle size distribution (EFPSD), which was then used in updating the EUCAARI inventory. We applied the updated inventory in a simulation using the regional chemical transport model PMCAMx-UF over Europe for May 2008. This was done to test the effect of updated emissions at regional and local scales, particularly in comparison with atmospheric new particle formation (NPF). Updating the inventory increased the simulated average total particle number concentrations by only 1 % , although the total particle number emissions were increased to a 3-fold level. The concentrations increased up to 11 % when only 1.3–3 nm sized particles (nanocluster aerosol, NCA) were considered. These values indicate that the effect of updating overall is insignificant at a regional scale during this photochemically active period. During this period, the fraction of the total particle number originating from atmospheric NPF processes was 91 % ; thus, these simulations give a lower limit for the contribution of traffic to the aerosol levels. Nevertheless, the situation is different when examining the effect of the update closer spatially or temporally or when focusing on the chemical composition or the origin of the particles. For example, the daily average NCA concentrations increased by a factor of several hundred or thousand in some locations on certain days. Overall, the most significant effects – reaching several orders of magnitude – from updating the inventory are observed when examining specific particle sizes (especially 7–20 nm), particle components, and specific urban areas. While the model still has a tendency to predict more sub-50 nm particles compared to the observations, the most notable underestimations in the concentrations of sub-10 nm particles are now overcome. Additionally, the simulated distributions now agree better with the data observed at locations with high traffic densities. The findings of this study highlight the need to consider emissions, PSDs, and composition of sub-50 nm particles from road traffic in studies focusing on urban air quality. Updating this emission source brings the simulated aerosol levels, particularly in urban locations, closer to observations, which highlights its importance for calculations of human exposure to nanoparticles. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) formation from mixtures of volatile precursors may be influenced by the molecular interactions of the components of the mixture. Here, we report measurements of the volatility distribution of SOA formed from the photo-oxidation of o -cresol, α -pinene, and their mixtures, representative anthropogenic and biogenic precursors, in an atmospheric simulation chamber. The combination of two independent thermal techniques (thermal denuder, TD, and the Filter Inlet for Gases and Aerosols coupled to a high-resolution time-of-flight chemical ionization mass spectrometer, FIGAERO-CIMS) to measure the particle volatility, along with detailed gas- and particle-phase composition measurements, provides links between the chemical composition of the mixture and the resultant SOA particle volatility. The SOA particle volatility obtained by the two independent techniques showed substantial discrepancies. The particle volatility obtained by the TD was wider, spanning across the LVOC and SVOC range, while the respective FIGAERO-CIMS derived using two different methods (i.e. calibrated Tmax and partitioning calculations) was substantially higher (mainly in the SVOC and IVOC, respectively) and narrow. Although the quantification of the SOA particle volatility was challenging, both techniques and methods showed similar trends, with the volatility of the SOA formed from the photo-oxidation of α -pinene being higher than that measured in the o -cresol system, while the volatility of the SOA particles of the mixture was between those measured at the single-precursor systems. This behaviour could be explained by two opposite effects, the scavenging of the larger molecules with lower volatility produced in the single-precursor experiments that led to an increase in the average volatility and the formation of unique-to-the-mixture products that had higher O:C , MW, OSc‾ and, consequently, lower volatility compared to those derived from the individual precursors. We further discuss the potential limitations of FIGAERO-CIMS to report quantitative volatilities and their implications for the reported results, and we show that the particle volatility changes can be qualitatively assessed, while caution should be taken when linking the chemical composition to the particle volatility. These results present the first detailed observations of SOA particle volatility and composition in mixed anthropogenic and biogenic systems and provide an analytical context that can be used to explore particle volatility in chamber experiments. [ABSTRACT FROM AUTHOR]

Ecosystem productivity is strongly modulated by the atmospheric deposition of inorganic reactive nitrogen (the sum of ammonium and nitrate). The individual contributions of ammonium and nitrate vary considerably over space and time, giving rise to complex patterns of nitrogen deposition. In the absence of rain, much of this complexity is driven by the large difference between the dry deposition velocity of nitrogen-containing molecules in the gas or condensed phase. Here we quantify how aerosol liquid water and acidity, through their impact on gas–particle partitioning, modulate the deposition velocity of total NH 3 and total HNO 3 individually while simultaneously affecting the dry deposition of inorganic reactive nitrogen. Four regimes of deposition velocity emerge: (i) HNO 3 – fast, NH 3 – slow, (ii) HNO 3 – slow, NH 3 – fast, (iii) HNO 3 – fast, NH 3 – fast, and (iv) HNO 3 – slow, NH 3 – slow. Conditions that favor partitioning of species to the aerosol phase strongly reduce the local deposition of reactive nitrogen species and promote their accumulation in the boundary layer and potential for long-range transport. Application of this framework to select locations around the world reveals fundamentally important insights: the dry deposition of total ammonia displays little sensitivity to pH and liquid water variations, except under conditions of extreme acidity and/or low aerosol liquid water content. The dry deposition of total nitric acid, on the other hand, is quite variable, with maximum deposition velocities (close to gas deposition rates) found in the eastern United States and minimum velocities in northern Europe and China. In the latter case, the low deposition velocity leads to up to 10-fold increases in PM 2.5 nitrate aerosol, thus contributing to the high PM 2.5 levels observed during haze episodes. In this light, aerosol pH and associated liquid water content can be considered to be control parameters that drive dry deposition flux and can accelerate the accumulation of aerosol contributing to intense haze events throughout the globe. [ABSTRACT FROM AUTHOR]

A source-resolved three-dimensional chemical transport model, PMCAMx-SR (Particulate Matter Comprehensive Air-quality Model with extensions – Source Resolved), was applied in the continental US to investigate the contribution of the various components (primary and secondary) of biomass burning organic aerosol (bbOA) to organic aerosol levels. Two different schemes based on the volatility basis set were used for the simulation of the bbOA during different seasons. The first is the default scheme of PMCAMx-SR, and the second is a recently developed scheme based on laboratory experiments of the bbOA evolution. The simulations with the alternative bbOA scheme predict much higher total bbOA concentrations when compared with the base case ones. This is mainly due to the high emissions of intermediate-volatility organic compounds (IVOCs) assumed in the alternative scheme. The oxidation of these compounds is predicted to be a significant source of secondary organic aerosol. The impact of the other parameters that differ in the two schemes is low to negligible. The monthly average maximum predicted concentrations of the alternative bbOA scheme were approximately an order of magnitude higher than those of the default scheme during all seasons. The performance of the two schemes was evaluated against observed total organic aerosol concentrations from several measurement sites across the US. The results were different for the different seasons examined. The default scheme performed better during July and September, while the alternative scheme performed a little better during April. These results illustrate the uncertainty of the corresponding predictions and the need to quantify the emissions and reactions of IVOCs from specific biomass sources and to better constrain the total (primary and secondary) bbOA levels. [ABSTRACT FROM AUTHOR]

Volatility and viscosity have substantial impacts on gas–particle partitioning, formation and evolution of aerosol and hence the predictions of aerosol-related air quality and climate effects. Here aerosol volatility and viscosity at a rural site (Gucheng) and an urban site (Beijing) in the North China Plain (NCP) in summer and winter were investigated by using a thermodenuder coupled with a high-resolution aerosol mass spectrometer. The effective saturation concentration (C*) of organic aerosol (OA) in summer was smaller than that in winter (0.55 µ g m -3 vs. 0.71–0.75 µ g m -3), indicating that OA in winter in the NCP is more volatile due to enhanced primary emissions from coal combustion and biomass burning. The volatility distributions varied and were largely different among different OA factors. In particular, we found that hydrocarbon-like OA (HOA) contained more nonvolatile compounds compared to coal-combustion-related OA. The more oxidized oxygenated OA (MO-OOA) showed overall lower volatility than less oxidized OOA (LO-OOA) in both summer and winter, yet the volatility of MO-OOA was found to be relative humidity (RH) dependent showing more volatile properties at higher RH. Our results demonstrated the different composition and chemical formation pathways of MO-OOA under different RH levels. The glass transition temperature (Tg) and viscosity of OA in summer and winter are estimated using the recently developed parameterization formula. Our results showed that the Tg of OA in summer in Beijing (291.5 K) was higher than that in winter (289.7–290.0 K), while it varied greatly among different OA factors. The viscosity suggested that OA existed mainly as solid in winter in Beijing (RH = 29 ± 17 %), but as semisolids in Beijing in summer (RH = 48 ± 25 %) and Gucheng in winter (RH = 68 ± 24 %). These results have the important implication that kinetically limited gas–particle partitioning may need to be considered when simulating secondary OA formation in the NCP. [ABSTRACT FROM AUTHOR]

The dependence of aerosol acidity on particle size, location, and altitude over Europe during a summertime period is investigated using the hybrid version of aerosol dynamics in the chemical transport model PMCAMx. The pH changes more with particle size in northern and southern Europe owing to the enhanced presence of non-volatile cations (Na, Ca, K, Mg) in the larger particles. Differences of up to 1–4 pH units are predicted between sub- and supermicron particles, while the average pH of PM1-2.5 can be as much as 1 unit higher than that of PM1. Most aerosol water over continental Europe is associated with PM1 , while coarse particles dominate the water content in the marine and coastal areas due to the relatively higher levels of hygroscopic sea salt. Particles of all sizes become increasingly acidic with altitude (0.5–2.5 units pH decrease over 2.5 km) primarily because of the decrease in aerosol liquid water content (driven by humidity changes) with height. Inorganic nitrate is strongly affected by aerosol pH with the highest average nitrate levels predicted for the PM1-5 range and over locations where the pH exceeds 3. Dust tends to increase aerosol pH for all particle sizes and nitrate concentrations for supermicron range particles. This effect of dust is quite sensitive to its calcium content. The size-dependent pH differences carry important implications for pH-sensitive processes in the aerosol. [ABSTRACT FROM AUTHOR]

A source-resolved three-dimensional chemical transport model, PMCAMx-SR, was applied in the continental U.S. to investigate the contribution of the various components (primary and secondary) of biomass burning organic aerosol (bbOA) to organic aerosol levels. Two different schemes based on the volatility basis set were used for the simulation of the bbOA during different seasons. The first is the default scheme of PMCAMx-SR and the second is a recently developed scheme based on laboratory experiments of the bbOA evolution. The simulations with the alternative bbOA scheme predict much higher total bbOA concentrations when compared with the base case ones. This is mainly due to the high emissions of intermediate volatility organic compounds (IVOCs) assumed in the alternative scheme. The oxidation of these compounds is predicted to be a significant source of secondary organic aerosol. The impact of the other parameters that differ in the two schemes is low to negligible. The monthly average maximum predicted concentrations of the alternative bbOA scheme were approximately an order of magnitude higher than those of the default scheme during all seasons. The performance of the two schemes was evaluated against observed total organic aerosol concentrations from several measurement sites across the US. The results were mixed. The default scheme performed better during July and September while the alternative scheme performed a little better during April. These results illustrate the uncertainty of the corresponding predictions, the need to quantify the emissions and reactions of IVOCs from specific biomass sources, and to better constrain the total (primary and secondary) bbOA levels. [ABSTRACT FROM AUTHOR]

A month-long set of summertime measurements in a remote area in the Mediterranean is used to quantify aerosol absorption and the role of black and brown carbon. The suite of instruments included a high-resolution aerosol mass spectrometer (HR-ToF-AMS) and a scanning mobility particle sizer (SMPS), both coupled to a thermodenuder and an Aethalometer, a photoacoustic extinctiometer (PAX 405), and a single particle soot photometer (SP2). The average refractory black carbon (rBC) concentration during the campaign was 0.14 µ g m -3 , representing 3 % of the fine aerosol mass. The measured light absorption was two or more times higher than that of fresh black carbon (BC). Mie theory indicated that the absorption enhancement due to the coating of BC cores by nonrefractory material could explain only part of this absorption enhancement. The role of brown carbon (BrC) and other non-BC light-absorbing material was then investigated. A good correlation (R2=0.76) between the unexplained absorption and the concentration of extremely low volatility organic compounds (ELVOCs) mass was found. [ABSTRACT FROM AUTHOR]

Volatility is one of the most important physical properties of organic aerosol (OA), as it determines the partitioning of its components between the vapor and particulate phases. Despite their atmospheric importance, multicomponent OA volatility estimates remain quite uncertain. This study combined thermodenuder (TD) and isothermal dilution measurements to characterize secondary OA (SOA) generated from the ozonolysis of α-pinene and cyclohexene. The SOA from both precursors evaporated similarly in the TD, but behaved quite differently when isothermally diluted by similar amounts. The α-pinene ozonolysis SOA evaporated by only 20% after 2 h of dilution by a factor of around 20, while 65% of the cyclohexene ozonolysis SOA evaporated at the same conditions. The volatility distributions were first estimated by fitting only the evaporation in the TD. This approach resulted in similar volatility distributions for the two systems. Then, the model was used to fit both the evaporation in the TD and the dilution chamber. This technique estimated drastically different volatility distributions with the α-pinene ozonolysis SOA consisting of mostly low-volatility compounds and the cyclohexene ozonolysis SOA consisting of mostly semi-volatile compounds. In the next stage of analysis, the model was updated to account for vapor-phase wall-losses occurring in the dilution chamber. This approach resulted in slightly less volatile SOA and provided some information about the losses of vapors to the walls, but the results were fairly uncertain. These results show the necessity of combining thermal measurements with other techniques to accurately estimate OA volatility. Copyright © 2020 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NOx) and ammonia (NH3) from anthropogenic and biogenic emissions are central contributors to particulate matter (PM) concentrations worldwide. The response of PM to changes in the emissions of both compounds is typically studied on a case-by-case basis, owing in part to the complex thermodynamic interactions of these aerosol precursors with other PM constituents. Here we present a simple but thermodynamically consistent approach that expresses the chemical domains of sensitivity of aerosol particulate matter to NH3 and HNO3 availability in terms of aerosol pH and liquid water content. From our analysis, four policy-relevant regimes emerge in terms of sensitivity: (i) NH3 sensitive, (ii) HNO3 sensitive, (iii) NH3 and HNO3 sensitive, and (iv) insensitive to NH3 or HNO3. For all regimes, the PM remains sensitive to nonvolatile precursors, such as nonvolatile cations and sulfate. When this framework is applied to ambient measurements or predictions of PM and gaseous precursors, the "chemical regime" of PM sensitivity to NH3 and HNO3 availability is directly determined. The use of these regimes allows for novel insights, and this framework is an important tool to evaluate chemical transport models. With this extended understanding, aerosol pH and associated liquid water content naturally emerge as previously ignored state parameters that drive PM formation. [ABSTRACT FROM AUTHOR]

Volatility plays a key role in affecting mass concentrations and the lifetime of aerosol particles in the atmosphere, yet our knowledge of aerosol volatility in relatively polluted environment, e.g., north China, remains poor. Here aerosol volatility in Beijing in summer 2017 and 2018 was measured using a thermodenuder (TD) coupled with an Aerodyne high-resolution aerosol mass spectrometer (AMS) and a soot particle AMS. Our results showed overall similar thermograms for most non-refractory aerosol species compared with those reported in previous studies. However, high mass fraction remaining and NO+/NO2+ ratio for chloride and nitrate, each above 200 ∘ C, indicated the presence of considerable metallic salts and organic nitrates in Beijing. The volatility distributions of organic aerosol (OA) and four OA factors that were resolved from positive matrix factorization were estimated using a mass transfer model. The ambient OA comprised mainly semi-volatile organic compounds (SVOCs; 63 %) with an average effective saturation concentration (C*) of 0.55 µ g m -3 , suggesting overall more volatile properties than OA in megacities of Europe and the US. Further analysis showed that the freshly oxidized secondary OA was the most volatile OA factor (SVOC = 70 %) followed by hydrocarbon-like OA (HOA). In contrast, the volatility of more oxidized oxygenated OA (MO-OOA) was comparable to that of cooking OA with SVOC on average accounting for 60.2 %. We also compared the volatility of ambient and black-carbon-containing OA. Our results showed that the BC-containing primary OA (POA) was much more volatile than ambient POA (C*=0.69 µ g m -3 vs. 0.37 µ g m -3), while the BC-containing SOA was much less volatile, highlighting the very different composition and properties between BC-containing and ambient aerosol particles. [ABSTRACT FROM AUTHOR]

We explore the morphologies of multicomponent nanoparticles through atomistic molecular dynamics simulations under atmospherically relevant conditions. The particles investigated consist of both organic (cis-pinonic acid – CPA, 3-methyl-1,2,3-butanetricarboxylic acid – MBTCA, n - C20H42 , n - C24H50 , n - C30H62 or mixtures thereof) and inorganic (sulfate, ammonium and water) compounds. The effects of relative humidity, organic mass content and type of organic compound present in the nanoparticle are investigated. Phase separation is predicted for almost all simulated nanoparticles either between organics and inorganics or between hydrophobic and hydrophilic constituents. For oxygenated organics, our simulations predict an enrichment of the nanoparticle surface in organics, often in the form of islands depending on the level of humidity and organic mass fraction, giving rise to core–shell structures. In several cases the organics separate from the inorganics, especially from the ions. For particles containing water-insoluble linear alkanes, separate hydrophobic and hydrophilic domains are predicted to develop. The surface partitioning of organics is enhanced as the humidity increases. The presence of organics in the interior of the nanoparticle increases as their overall mass fraction in the nanoparticle increases, but this also depends on the humidity conditions. Apart from the organics–inorganics and hydrophobics–hydrophilics separation, our simulations predict a third type of separation (layering) between CPA and MBTCA molecules under certain conditions. [ABSTRACT FROM AUTHOR]

The chemical transport model PMCAMx was extended to investigate the effects of partitioning and photochemical aging of biomass burning emissions on organic aerosol (OA) concentrations. A source-resolved version of the model, PMCAMx-SR, was developed in which biomass burning emissions and their oxidation products are represented separately from the other OA components. The volatility distribution and chemical aging of biomass burning OA (BBOA) were simulated based on recent laboratory measurements. PMCAMx-SR was applied to Europe during an early summer period (1–29 May 2008) and a winter period (25 February–22 March 2009). During the early summer, the contribution of biomass burning (both primary and secondary species) to total OA levels over continental Europe was estimated to be approximately 16 %. During winter the contribution was nearly 47 %, due to both extensive residential wood combustion but also wildfires in Portugal and Spain. The intermediate volatility compounds (IVOCs) with effective saturation concentration values of 10 5 and 10 6 µ g m -3 are predicted to contribute around one third of the BBOA during the summer and 15 % during the winter by forming secondary OA (SOA). The uncertain emissions of these compounds and their SOA formation potential require additional attention. Evaluation of PMCAMx-SR predictions against aerosol mass spectrometer measurements in several sites around Europe suggests reasonably good performance for OA (fractional bias less than 35 % and fractional error less than 50 %). The performance was weaker during the winter suggesting uncertainties in residential heating emissions and the simulation of the resulting BBOA in this season. [ABSTRACT FROM AUTHOR]

Black carbon (BC)-containing particles are the most strongly light absorbing aerosols in the atmosphere. Measurements of BC are challenging because of its semi-empirical definition based on physical properties and not chemical structure, the complex and continuously changing morphology of the corresponding particles, and the effects of other particulate components on its absorption. In this study, we compare six available commercial continuous instruments measuring BC using biomass burning aerosol. The comparison involves a Soot Particle Aerosol Mass Spectrometer (SP-AMS), a Single-Particle Soot Photometer (SP2), an aethalometer, a Multiangle Absorption Photometer (MAAP), and a blue and a green photoacoustic extinctiometer (PAX). An SP-AMS collection efficiency equal to 0.35 was measured for this aerosol system. The corrected SP-AMS BC mass measurements agreed within 6% with the SP2 refractory BC mass values. Two regimes of behavior were identified for the optical instruments corresponding to high and low organic/BC ratio. The mass absorption cross-sections (MAC) measured varied from 26% to two times the instrument default values depending on the instrument and the regime. The presence of high organic aerosol concentration in this system can lead to overestimation of the BC mass by the optical instruments by as much as a factor of 2.7. In general, the discrepancy among the BC measurements increased as the organic carbon content of the BC-containing particles increased. © 2018 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Hygroscopicity, oxidation level, and volatility are three crucial properties of organic pollutants. This study assesses the feasibility of a novel measurement and analysis technique to determine these properties and establish their relationship. The proposed experimental setup utilizes a cloud condensation nuclei (CCN) counter to quantify hygroscopic activity, an aerosol mass spectrometer to measure the oxidation level, and a thermodenuder to evaluate the volatility. The setup was first tested with secondary organic aerosol (SOA) formed from the ozonolysis of α-pinene. The results of the first experiments indicated that, for this system, the less volatile SOA contained species that had on average lower O:C ratios and hygroscopicities. In this SOA system, both low- and high-volatility components can have comparable oxidation levels and hygroscopicities. The method developed here can be used to provide valuable insights about the relationships among organic aerosol hygroscopicity, oxidation level, and volatility. [ABSTRACT FROM AUTHOR]

Organic compounds from combustion sources such as biomass burning and fossil fuel use are major contributors to the global atmospheric load of aerosols. We analyzed the sensitivity of model-predicted global-scale organic aerosols (OA) to parameters that control primary emissions, photochemical aging, and the scavenging efficiency of organic vapors. We used a computationally efficient module for the description of OA composition and evolution in the atmosphere (ORACLE) of the global chemistry-climate model EMAC (ECHAM/MESSy Atmospheric Chemistry). A global dataset of aerosol mass spectrometer (AMS) measurements was used to evaluate simulated primary (POA) and secondary (SOA) OA concentrations. Model results are sensitive to the emission rates of intermediate-volatility organic compounds (IVOCs) and POA. Assuming enhanced reactivity of semi-volatile organic compounds (SVOCs) and IVOCs with OH substantially improved the model performance for SOA. The use of a hybrid approach for the parameterization of the aging of IVOCs had a small effect on predicted SOA levels. The model performance improved by assuming that freshly emitted organic compounds are relatively hydrophobic and become increasingly hygroscopic due to oxidation. [ABSTRACT FROM AUTHOR]

The authors reflect on the knowledge of chemical engineers regarding the atmospheric nanoparticles and climate change. They are critical about the effect of formation of atmospheric particles, such as gases and nanoparticles, on the climate system. They relate on the potential of these nanoparticles to grow that affects cloud condensation. They add that chemical engineers have advanced on addressing atmospheric chemistry issues through new measurement techniques.

Abstract: Changes in summertime organic aerosol (OA) concentrations in the Eastern U.S. are investigated for different temperature change scenarios using the chemical transport model PMCAMx-2008. OA is simulated using the volatility basis set approach, assuming that the primary emissions are semi-volatile and that the intermediate volatile and semi-volatile organic compounds are oxidized in the gas phase, resulting in products with lower volatility. For the basic temperature change scenario where biogenic emissions are kept constant, ground-level OA decreases by −0.3% K−1 on average. Increases in the north (+0.1% K−1) and decreases in the south (−0.5% K−1) are predicted. The effect of the uncertain temperature dependence of the aging rate constant is modest, changing the OA by only 0.1% K−1 over the temperature-independent case. For the more realistic scenario in which biogenic OA precursor emissions are allowed to increase with temperature (up to 10% K−1), however, average OA increases by 4.1% K−1, with even higher increases in southern regions. These results suggest that as temperature increases, complicated changes in production, partitioning and chemical aging will take place. Nevertheless, the change in biogenic emissions and subsequent production of biogenic OA is more than an order of magnitude more important than the changes in the rates of chemical and physical atmospheric processes. [Copyright &y& Elsevier]