Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high-time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

Organic aerosol (OA) data acquired by the Aerosol Mass Spectrometer (AMS) in 37 field campaigns were deconvolved into hydrocarbon‐like OA (HOA) and several types of oxygenated OA (OOA) components. HOA has been linked to primary combustion emissions (mainly from fossil fuel) and other primary sources such as meat cooking. OOA is ubiquitous in various atmospheric environments, on average accounting for 64%, 83% and 95% of the total OA in urban, urban downwind, and rural/remote sites, respectively. A case study analysis of a rural site shows that the OOA concentration is much greater than the advected HOA, indicating that HOA oxidation is not an important source of OOA, and that OOA increases are mainly due to SOA. Most global models lack an explicit representation of SOA which may lead to significant biases in the magnitude, spatial and temporal distributions of OA, and in aerosol hygroscopic properties.

Measurements of cloud condensation nuclei (CCN), aerosol size distribution and chemical composition were obtained at the UNH‐AIRMAP Thompson Farms site, during the ICARTT 2004 campaign. This work focuses on the analysis of a week of measurements, during which semiurban and continental air were sampled. Predictions of CCN concentrations were carried out using “simple” Köhler theory; the predictions are subsequently compared with CCN measurements at 0.2%, 0.3%, 0.37%, 0.5% and 0.6% supersaturation. Using size‐averaged chemical composition, CCN are substantially overpredicted (by 35.8 ± 28.5%). Introducing size‐dependent chemical composition substantially improved closure (average error 17.4 ± 27.0%). CCN closure is worse during periods of changing wind direction, suggesting that the introduction of aerosol mixing state into CCN predictions may sometimes be required. Finally, knowledge of the soluble salt fraction is sufficient for description of CCN activity.

Abstract.A long-term, high time-resolution volatile organic compound (VOC) data set from a ground site that experiences urban, rural, and marine influences in the Northeastern United States is presented. A proton-transfer-reaction mass spectrometer (PTR-MS) was used to quantify 15 VOCs: a marine tracer dimethyl sulfide (DMS), a biomass burning tracer acetonitrile, biogenic compounds (monoterpenes, isoprene), oxygenated VOCs (OVOCs: methyl vinyl ketone (MVK) plus methacrolein (MACR), methanol, acetone, methyl ethyl ketone (MEK), acetaldehyde, and acetic acid), and aromatic compounds (benzene, toluene, C 8 and C 9 aromatics). Time series, overall and seasonal medians, with 10th and 90th percentiles, seasonal mean diurnal profiles, and inter-annual comparisons of mean summer and winter diurnal profiles are shown. Methanol and acetone exhibit the highest overall median mixing ratios 1.44 and 1.02 ppbv, respectively. Comparing the mean diurnal profiles of less well understood compounds (e.g., MEK) with better known compounds (e.g., isoprene, monoterpenes, and MVK + MACR) that undergo various controls on their atmospheric mixing ratios provides insight into possible sources of the lesser known compounds. The constant diurnal value of ∼0.7 for the toluene:benzene ratio in winter, may possibly indicate the influence of wood-based heating systems in this region. Methanol exhibits an initial early morning release in summer unlike any other OVOC (or isoprene) and a dramatic late afternoon mixing ratio increase in spring. Although several of the OVOCs appear to have biogenic sources, differences in features observed between isoprene, methanol, acetone, acetaldehyde, and MEK suggest they are produced or emitted in unique ways.

[1] The composition and size of aerosols were measured using an Aerodyne quadrupole aerosol mass spectrometer at Thompson Farm in Durham, NH, during the International Consortium for Atmospheric Research on Transport and Transformation campaign during summer 2004. Submicron, non-refractory aerosol was dominated by organic matter and sulfate (averages of 5.7 mg m À3 and 3.6 mg m À3 , respectively), with smaller contributions from nitrate and ammonium (averages of 0.3 mg m À3 and 1.02 mg m À3 , respectively). Organic aerosol (OA) mass correlates well with anthropogenic tracers such as carbon monoxide (CO, R 2 = 0.58) and black carbon (R 2 = 0.59), but multiple analyses indicate possible contributions from primary, secondary, anthropogenic, and biogenic OA. Multivariate statistical analysis of the OA mass spectra indicates the presence of two types of oxygenated OA (OOA) and a hydrocarbon-like OA (HOA) component that also contains contributions from biomass burning OA (BBOA). On average, the HOA/BBOA component accounts for 21% of the total OA mass while the two OOA components account for 24% and 55%, respectively, of the OA burden. Observed nitrate correlates well with OA (R 2 = 0.67), suggesting interference, the presence of organic nitrates, processing/ uptake of nitric acid by OA, or other temporally coincident processes because of the ammonia-poor environment with respect to sulfate. The relative increase of OA with respect to background compared to that of CO (average of 72.7 mg m À3 ppmv À1) indicates values that are higher than those based on previous measurements in New England.