Secondary organic aerosol (SOA) accounts for a significant fraction of ambient tropospheric aerosol and a detailed knowledge of the formation, properties and transformation of SOA is therefore required to evaluate its impact on atmospheric processes, climate and human health. The chemical and physical processes associated with SOA formation are complex and varied, and, despite considerable progress in recent years, a quantitative and predictive understanding of SOA formation does not exist and therefore represents a major research challenge in atmospheric science. This review begins with an update on the current state of knowledge on the global SOA budget and is followed by an overview of the atmospheric degradation mechanisms for SOA precursors, gas-particle partitioning theory and the analytical techniques used to determine the chemical composition of SOA. A survey of recent laboratory, field and modeling studies is also presented. The following topical and emerging issues are highlighted and discussed in detail: molecular characterization of biogenic SOA constituents, condensed phase reactions and oligomerization, the interaction of atmospheric organic components with sulfuric acid, the chemical and photochemical processing of organics in the atmospheric aqueous phase, aerosol formation from real plant emissions, interaction of atmospheric organic components with water, thermodynamics and mixtures in atmospheric models. Finally, the major challenges ahead in laboratory, field and modeling studies of SOA are discussed and recommendations for future research directions are proposed.

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The formation, properties and impact of secondary organic aerosol: current and emerging issues

The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

/pdf/organic-aerosol-and-global-climate-modelling-a-review-4ojfzx6x50.pdf

Organic aerosol and global climate modelling: a review

Most primary organic-particulate emissions are semivolatile; thus, they partially evaporate with atmospheric dilution, creating substantial amounts of low-volatility gas-phase material. Laboratory experiments show that photo-oxidation of diesel emissions rapidly generates organic aerosol, greatly exceeding the contribution from known secondary organic-aerosol precursors. We attribute this unexplained secondary organic-aerosol production to the oxidation of low-volatility gas-phase species. Accounting for partitioning and photochemical processing of primary emissions creates a more regionally distributed aerosol and brings model predictions into better agreement with observations. Controlling organic particulate-matter concentrations will require substantial changes in the approaches that are currently used to measure and regulate emissions.

http://science.sciencemag.org/content/sci/315/5816/1259.full.pdf

Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging

http://krollgroup.mit.edu/papers/Kroll_2008.pdf

Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere

The amount of primary organic aerosol (POA) depends on the gas-particle partitioning of 1000s of individual organic compounds. This partitioning can be expressed in terms of the volatility distribution of the emissions. A volatility distribution can be constructed from speciated emissions data by lumping species with similar saturation vapor pressures. In practice, this is not possible because less than 10% of the condensed and semivolatile mass has been identified on a compound-by-compound basis (

Supporting Online Material for Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging

An inorganic aerosol equilibrium model is used to investigate the response of inorganic particulate matter (PM) concentrations with respect to the precursor concentrations of sulfuric acid, ammonia, and nitric acid over a range of temperatures and relative humidities. Diagrams showing regions of PM response to precursor concentrations are generated, thus allowing the qualification of assumptions concerning the response of PM to sulfate and overall sensitivity to ammonia and nitric acid availability. The PM concentration level responds nonlinearly to sulfate and shows overall sensitivity to ammonia and nitric acid availability for specific atmospheric conditions and precursor concentrations. The generated diagrams are applied as a means of approximating the PM response to precursor concentrations for two urban polluted areas. In both cases, reductions in ammonia emissions have the most significant impact on the total PM level. However, such a reduction will result in significant increases in atmospheric acidity.

Response of Inorganic PM to Precursor Concentrations

https://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.files/fileID/12839

Prediction of multicomponent inorganic atmospheric aerosol behavior

Reductions in airborne sulfate concentration may cause inorganic fine particulate matter (PM25) to respond nonlinearly, as nitric acid gas may transfer to the aerosol phase. Where this occurs, reductions in sulfur dioxide (SO2) emissions will be much less effective than expected at reducing PM2.5. As a measure of the efficacy of reductions in sulfate concentration on PM , we define marginal PM2.5 as the local change in PM2.5 resulting from a small change in sulfate concentration. Using seasonal-average conditions and assuming thermodynamic equilibrium, we find that the conditions for PM2.5 to respond nonlinearly to sulfate reductions are common in the eastern United States in winter, occurring at half of the sites considered, and uncommon in summer, due primarily to the influence of temperature. Accounting for diurnal and intraseasonal variability, we find that seasonal-average conditions provide a reasonable indicator of the time-averaged PM2.5 response. These results indicate that reductions in...

https://www.tandfonline.com/doi/pdf/10.1080/10473289.1999.10463973

Marginal PM25: Nonlinear Aerosol Mass Response to Sulfate Reductions in the Eastern United States

The hygroscopic nature of atmospheric aerosol has generally been associated with its inorganic fraction. In this study, a group contribution method is used to predict the water absorption of secondary organic aerosol (SOA). Compared against growth measurements of mixed inorganic−organic particles, this method appears to provide a first-order approximation in predicting SOA water absorption. The growth of common SOA species is predicted to be significantly less than common atmospheric inorganic salts such as (NH4)2SO4 and NaCl. Using this group contribution method as a tool in predicting SOA water absorption, an integrated modeling approach is developed combining available SOA and inorganic aerosol models to predict overall aerosol behavior. The effect of SOA on water absorption and nitrate partitioning between the gas and aerosol phases is determined. On average, it appears that SOA accounts for approximately 7% of total aerosol water and increases aerosol nitrate concentrations by approximately 10%. At h...

Asif S. Ansari

Papers

Response of Inorganic PM to Precursor Concentrations

Prediction of multicomponent inorganic atmospheric aerosol behavior

Marginal PM25: Nonlinear Aerosol Mass Response to Sulfate Reductions in the Eastern United States

Water Absorption by Secondary Organic Aerosol and Its Effect on Inorganic Aerosol Behavior

Integrated approaches to modeling the organic and inorganic atmospheric aerosol components