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The volatility of the compounds comprising organic aerosol (OA) determines their distribution between the gas and particle phases. However, there is a disconnect between volatility distributions as typically derived from secondary OA (SOA) growth experiments and the effective particle volatility as probed in evaporation experiments. Specifically, the evaporation experiments indicate an overall much less volatile SOA. This raises questions regarding the use of traditional volatility distributions in the simulation and prediction of atmospheric SOA concentrations. Here, we present results from measurements of thermally induced evaporation of SOA for nine different SOA types (i.e., distinct volatile organic compound and oxidant pairs) encompassing both anthropogenic and biogenic compounds and O3 and OH to examine the extent to which the low effective volatility of SOA is a general phenomenon or specific to a subset of SOA types. The observed extents of evaporation with temperature were similar for all the SOA types and indicative of a low effective volatility. Furthermore, minimal variations in the composition of all the SOA types upon heating-induced evaporation were observed. These results suggest that oligomer decomposition likely plays a major role in controlling SOA evaporation, and since the SOA formation time scale in these measurements was less than a minute, the oligomer-forming reactions must be similarly rapid. Overall, these results emphasize the importance of accounting for the role of condensed phase reactions in altering the composition of SOA when assessing particle volatility.

Multi-wavelength aerosol extinction, absorption and scattering measurements made at two ground sites during the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) are analyzed using a spectral deconvolution method that allows extraction of particle size-related information, including the fraction of extinction produced by the fine mode particles and the effective radius of the fine mode. The spectral deconvolution method is typically applied to analysis of remote sensing measurements. Application to in situ measurements allows for comparison with more direct measurement methods and validation of the retrieval approach. Here, the retrieved fine mode fraction and effective radius generally compare well with other in situ measurements, including size distribution measurements and scattering and absorption measurements made separately for PM1 and PM10, but some limitations are also identified. These results indicate that for campaigns where size, composition, and multi-wavelength optical property measurements are made, comparison of the results can result in closure or can identify unusual circumstances. The comparison here also demonstrates that in situ multi-wavelength optical property measurements can be used to determine information about particle size distributions in situations where direct size distribution measurements are not available [ABSTRACT FROM AUTHOR]

Loss of sea ice is opening the Arctic to increasing development involving oil and gas extraction and shipping. Given the significant impacts of absorbing aerosol and secondary aerosol precursors emitted within the rapidly warming Arctic region, there is a need to characterize local anthropogenic aerosol sources and compare to natural conditions. From August-September 2015 in Utqiaġvik, AK, the chemical composition of individual atmospheric particles was measured by computer-controlled scanning electron microscopy with energy dispersive X-ray spectroscopy (0.13-4 μm projected area diameter) and real-time single particle mass spectrometry (0.2-1.5μm aerodynamic diameter). During Arctic Ocean influenced periods (70% of the study), our results show that fresh sea spray aerosol contributed ~20%, by number, of particles between 0.13-0.4μm, 40-70% between 0.4-1μm, and 80-100% of 1-4μm particles. In contrast, for periods influenced by emissions from Prudhoe Bay (10% of the study), the third largest oilfield in North America, there was a strong influence from submicron (0.13-1μm) combustion derived particles (20-50% OC, by number, 5-10% soot by number). While sea spray aerosol still comprised a large fraction of particles (90% by number from 1-4μm) detected under Prudhoe Bay influence, these particles were internally mixed with sulfate and nitrate indicative of aging processes during transport. In addition, the overall mode of the particle size number distribution shifted from 76nm during Arctic Ocean influence to 27nm during Prudhoe Bay influence with particle concentrations increasing from 130cm-3 to 920cm-3 due to transported particle emissions from the oil fields. The increased contributions of carbonaceous combustion products and partially aged SSA should be taken into consideration for future Arctic atmospheric composition and climate simulations. [ABSTRACT FROM AUTHOR]

Measurements of the optical properties (absorption, scattering and extinction) of PM1, PM2.5 and PM10 made at two sites around Sacramento, CA, during the June 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) are reported. These observations are used to establish relationships between various intensive optical properties and to derive information about the dependence of the optical properties on photochemical aging and sources. Supermicron particles contributed substantially to the total light scattering at both sites, about 50% on average. A strong, linear relationship is observed between the scattering Ångström exponent for PM10 and the fraction of the scattering that is contributed by submicron particles (fsca;PM1) at both sites and with similar slopes and intercepts (for a given pair of wavelengths), suggesting that the derived relationship may be generally applicable for understanding variations in particle size distributions from remote sensing measurements. At the more urban T0 site, the fsca,PM1 increased with photochemical age, whereas at the downwind, more rural T1 site the fsca,PM1 decreased slightly with photochemical age. This difference in behavior reflects differences in transport, local production and local emission of supermicron particles between the sites. Light absorption is dominated by submicron particles, but there is some absorption by supermicron particles (~15% of the total). The supermicron absorption derives from a combination of black carbon that has penetrated into the supermicron mode and from dust, and there is a clear increase in the mass absorption coefficient of just the supermicron particles with increasing average particle size. The mass scattering coefficient (MSC) for the supermicron particles was directly observed to vary inversely with the average particle size, demonstrating that MSC cannot always be treated as a constant in estimating mass concentrations from scattering measurements, or vice versa. The total particle backscatter fraction exhibited some dependence upon the relative abundance of sub- versus supermicron particles; however this was modulated by variations in the median size of particles within a given size range; variations in the submicron size distribution had a particularly large influence on the observed backscatter efficiency and an approximate method to account for this variability is introduced. The relationship between the absorption and scattering Ångström exponents is examined and used to update a previously suggested particle classification scheme. Differences in composition led to differences in the sensitivity of PM2.5 to heating in a thermodenuder to the average particle size, with more extensive evaporation (observed as a larger decrease in the PM2.5 extinction coefficient) corresponding to smaller particles; i.e., submicron particles were generally more susceptible to heating than the supermicron particles. The influence of heating on the particle hygroscopicity varied with the effective particle size, with larger changes observed when the PM2.5 distribution was dominated by smaller particles. [ABSTRACT FROM AUTHOR]