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Sesquiterpenes (C15H24) are a class of biogenic volatile organic compounds that are significant secondary organic aerosol (SOA) precursors due to their high reactivity with oxidants and their high SOA yields. Previous studies have focused almost exclusively on β-caryophyllene, and there is relatively little known about the other sesquiterpenes. In this study we focus on another major sesquiterpene, α-humulene, which has three endo-cyclic double bonds. A series of experiments quantified the SOA production during the ozonolysis of α-humulene in the Carnegie Mellon atmospheric simulation chamber. The experiments resulted in high SOA yields ranging from 30 to 70% for SOA concentration in the range 10 to 100 μg m−3. Most of the SOA had effective volatility equal to or less than 1 μg m−3at 298 K and the average SOA effective vaporization enthalpy was 115 ± 23 kJ mol−1. The SOA aerosol mass spectrometer (AMS) spectrum was slowly evolving during experiments, which suggested modest differences, from the AMS point of view, between the SOA compounds produced initially and the SOA compounds produced towards the end of the experiment. The α-humulene SOA mass spectrum resembled that of β-caryophyllene SOA but it was less similar to α-pinene SOA.

Pellet combustion in residential heating stoves has increased globally during the last decade. Despite their high combustion efficiency, the widespread use of pellet stoves is expected to adversely impact air quality. The atmospheric aging of pellet emissions has received even less attention, focusing mainly on daytime conditions, while the degree to which pellet emissions undergo night-time aging as well as the role of relative humidity remain poorly understood. In this study, environmental simulation chamber experiments were performed to characterize the fresh and aged organic aerosol (OA) emitted by a pellet stove. The fresh pellet stove PM1(particulate matter with an aerodynamic diameter less than 1 μm) emissions consisted mainly of OA (93 ± 4%, mean ± standard deviation) and black carbon (5 ± 3%). The primary OA (POA) oxygen-to-carbon ratio (O : C) was 0.58 ± 0.04, higher than that of fresh logwood emissions. The fresh OA at a concentration of 70 μg m−3(after dilution and equilibration in the chamber) consisted of semi-volatile (68%), low and extremely low volatility (16%) and intermediate-volatility (16%) compounds. The oxidation of pellet emissions under dark conditions was investigated by injecting nitrogen dioxide (NO2) and ozone (O3) into the chamber, at different (10–80%) relative humidity (RH) levels. In all dark aging experiments secondary organic aerosol (SOA) formation was observed, increasing the OA levels after a few hours of exposure to NO3radicals. The change in the aerosol composition and the extent of oxidation depended on RH. For low RH, the SOA mass formed was up to 30% of the initial OA, accompanied by a moderate change in both O : C levels (7–8% increase) and the OA spectrum. Aging under higher RH conditions (60–80%) led to a more oxygenated aerosol (increase in O : C of 11–18%), but only a minor (1–10%) increase in OA mass. The increase in O : C at high RH indicates the importance of heterogeneous aqueous reactions in this system, that oxidize the original OA with a relatively small net change in the OA mass. These results show that the OA in pellet emissions can chemically evolve under low photochemical activity (e.g.the wintertime period) with important enhancement in SOA mass under certain conditions.

A method for the measurement of the water solubility distribution of atmospheric organic aerosols is presented. This method is based on the extraction of organic aerosols collected on filters, using different amounts of water and measurement of the corresponding water-soluble organic carbon concentration. The solubility distribution is then estimated using the solubility basis set. The method was applied on both ambient and source-specific aerosols. Approximately 60% of the atmospheric urban organic aerosol analyzed had water solubility higher than 0.6 g L–1. Around 10% of the fresh cooking organic aerosol had water solubility higher than 10 g L–1, while 80% of the total fresh cooking organic aerosol had solubility lower than 0.1 g L–1. The ambient measurements suggested that the solubility distributions are roughly consistent with the positive matrix factorization analysis results determined during the analysis of the high-resolution time-of-flight aerosol mass spectrometry data. Most of the oxidized organic aerosol appears to have water solubility above 0.6 g L–1, while the hydrocarbon-like organic aerosol and cooking organic aerosol have water solubility less than 0.002 and 0.1 g L–1, respectively. The biomass burning organic aerosol seems to have mostly intermediate solubility in water, between 0.04 and 0.6 g L–1. The proposed approach can quantify the solubility distribution in the 0.002–15 g L–1range. Future extension of the method to higher solubility ranges would be useful for capturing the complete solubility range for atmospheric cloud condensation studies (0.1–100 g L–1).

Particulate matter from biomass burning emissions affects air quality, ecosystems and climate; however, quantifying these effects requires that the connection between primary emissions and secondary aerosol production is firmly established. We performed atmospheric simulation chamber experiments on the chemical oxidation of residential biomass burning emissions under dark conditions. Biomass burning organic aerosol was found to age under dark conditions, with its oxygen-to-carbon ratio increasing by 7–34% and producing 1–38 μg m−3of secondary organic aerosol (5–80% increase over the fresh organic aerosol) after 30 min of exposure to NO3radicals in the chamber (corresponding to 1–3 h of exposure to typical nighttime NO3radical concentrations in an urban environment). The average mass concentration of SOA formed under dark-oxidation conditions was comparable to the mass concentration formed after 3 h (equivalent to 7–10 h of ambient exposure) under ultraviolet lights (6 μg m−3or a 47% increase over the emitted organic aerosol concentration). The dark-aging experiments showed a substantial increase in secondary nitrate aerosol (0.12–3.8 μg m−3), 46–100% of which is in the form of organic nitrates. The biomass burning aerosol pH remained practically constant at 2.8 throughout the experiment. This value promotes inorganic nitrate partitioning to the particulate phase, potentially contributing to the buildup of nitrate aerosol in the boundary layer and enhancing long-range transport. These results suggest that oxidation through reactions with the NO3radical is an additional secondary aerosol formation pathway in biomass burning emission plumes that should be accounted for in atmospheric chemical-transport models.

Acidity profoundly affects almost every aspect that shapes the composition of ambient particles and their environmental impact. Thermodynamic analysis of gas-particle composition datasets offers robust estimates of acidity, but they are not available for long periods of time. Fog composition datasets, however, are available for many decades; we develop a thermodynamic analysis to estimate the ammonia in equilibrium with fog water and to infer the pre-fog aerosol pH starting from fog chemical composition and pH. The acidity values from the new method agree with the results of thermodynamic analysis of the available gas-particle composition data. Applying the new method to historical (25 years) fog water composition at the rural station of San Pietro Capofiume (SPC) in the Po Valley (Italy) suggests that the aerosol has been mildly acidic, with its pH decreasing by 0.5–1.5 pH units over the last decades. The observed pH of the fog water also increased 1 unit over the same period. Analysis of the simulated aerosol pH reveals that the aerosol acidity trend is driven by a decrease in aerosol precursor concentrations, and changes in temperature and relative humidity. Currently, NOxcontrols would be most effective for PM2.5reduction in the Po valley both during summer and winter. In the future, however, seasonal transitions to the NH3-sensitive region may occur, meaning that the NH3reduction policy may become increasingly necessary.

A dual smog chamber system was used to quantify the formation rates of secondary organic and inorganic aerosol in an urban environment (Pittsburgh, US). Ambient air was introduced in both chambers, and HONO photolysis was used to produce hydroxyl radicals (OH) in the perturbed chamber. The second chamber was used as a reference. The production rate of secondary organic aerosol (SOA) under typical noon-time OH concentrations ranged from 0.2 to 0.8 μg m–3h–1. The production rate of sulfate was approximately five times less than that of the SOA. Nucleation and growth of new particles were observed in the perturbation chamber. The produced SOA had a similar composition with the preexisting oxygenated ambient OA. The reacted amounts of the measured VOCs were able to explain 5–50% of the formed SOA in the perturbed chamber. Intermediate volatility organic compounds could be responsible for the rest. The oxygen to carbon ratio (O:C) in the perturbed chamber remained approximately the same during SOA production, while an increase was observed in the control chamber. A possible explanation could be the loss of less oxidized species to the chamber walls. After 2 h, the OA increased by 70% on average and the sulfate by 40%.

The chemical transport model PMCAMx is used to examine the effect of climate change on fine (under 2.5 µms) particulate matter (PM2.5) during the summer in the eastern United States. Meteorology from 10 years in the 1990s (present) and 10 years in the 2050s (future) based on the Intergovernmental Panel on Climate Change A2 scenario is used. Anthropogenic pollutant emissions are assumed to remain constant, while biogenic emissions are climate sensitive and, depending on species, increase between 15 and 27% on average. The predicted changes of PM2.5are modest (increases of less than 10% on average across the domain) and quite variable in space, ranging from +13% in the Plains to −7% in the Northeast. Variability is driven concurrently by changes in temperature, wind speed, rainfall, and relative humidity, with no single dominant meteorological factor. Sulfate and organic aerosol are responsible for most of the PM2.5change. The improved treatment of organic aerosol using the volatility basis set does not increase significantly its sensitivity to climate change compared to traditional treatments that neglect the volatility of primary particles and do not simulate the chemical aging processes. Future organic aerosol is predicted to be more oxidized due to increases of its secondary biogenic and anthropogenic components. These results suggest that the effects of planned and expected emission anthropogenic emission controls will be more important than those of climate change for PM2.5concentrations in 2050. Maximum daily 8 h average ozone increases by 5% on average are predicted, with a marked increase in the Northeast, Southeast, and Midwest. Updated GRE‐CAPS model used for pollutant prediction in eastern U.S.Future (2050s) pollutant concentrations driven by many competing factorsPM2.5increases on average but varies in magnitude and sign

The purpose of this study was to estimate cardiopulmonary mortality associations for long-term exposure to PM2.5species and sources (i.e., components) within the U.S. National Health Interview Survey cohort. Exposures were estimated through a chemical transport model for six species (i.e., elemental carbon (EC), primary organic aerosols (POA), secondary organic aerosols (SOA), sulfate (SO4), ammonium (NH4), nitrate (NO3)) and five sources of PM2.5(i.e., vehicles, electricity-generating units (EGU), non-EGU industrial sources, biogenic sources (bio), “other” sources). In single-pollutant models, we found positive, significant (p< 0.05) mortality associations for all components, except POA. After adjusting for remaining PM2.5(total PM2.5minus component), we found significant mortality associations for EC (hazard ratio (HR) = 1.36; 95% CI [1.12, 1.64]), SOA (HR = 1.11; 95% CI [1.05, 1.17]), and vehicle sources (HR = 1.06; 95% CI [1.03, 1.10]). HRs for EC, SOA, and vehicle sources were significantly larger in comparison to those for remaining PM2.5(per unit μg/m3). Our findings suggest that cardiopulmonary mortality associations vary by species and source, with evidence that EC, SOA, and vehicle sources are important contributors to the PM2.5mortality relationship. With further validation, these findings could facilitate targeted pollution regulations that more efficiently reduce air pollution mortality.

The organic fraction of ambient aerosols is a complex mixture of hundreds of organic compounds varying in chemical structure and physical properties. The cloud condensation nuclei (CCN) activities of single‐component organic particles were experimentally investigated. Activation diameters were determined using a Tandem Differential Mobility Analyzer and a thermal diffusion Cloud Condensation Nucleus Counter. Studies were performed at supersaturations of 0.3 and 1% with dry particle diameters ranging between 0.02 and 0.2 μm. The focus was on both hygroscopic secondary organics and hydrophobic primary organics. Laboratory experiments were performed with sodium chloride, ammonium sulfate, glutaric acid, adipic acid, pinonic acid, glutamic acid, leucine, cholesterol, pinic acid, norpinic acid, hexadecane, hexadecanol, myristic acid, palmitic acid, and stearic acid. The results were compared with the classical Köhler theory and a theory that accounts for the limited solubility of many organics. It was discovered that organic species with bulk solubilities in water less than 0.01 g cm−3can be a good source of CCN in the atmosphere if their contact angles with water are zero. Experiments confirmed that Köhler theory works well in predicting activation of soluble inorganic species and organics that are wettable by water, while an extension of the theory appears necessary to account for the low‐solubility organic species that are not wettable by water.