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.

Organic aerosols (OA) are a highly dynamic system dominated by both variable gas particle partitioning and chemical evolution; however, these phenomena are poorly represented in current air quality models. The chemical transport model Comprehensive Air‐Quality Model with extensions dealing with particulate matter (PMCAMx) was extended to investigate the effects of partitioning and photochemical aging of primary emissions on OA concentrations in the eastern United States during July 2001 and January 2002. In both the summer and the winter, much of the traditionally defined primary OA (POA) emissions evaporate, creating a large pool of low‐volatility organic vapors. During the summertime, photochemical aging of these vapors creates substantial oxygenated OA that is regionally distributed. Little production of oxygenated OA is predicted in the winter because oxidant levels are low. OA formed from the oxidation of low‐volatility vapors is most important in and around urban areas located in the northeast and midwest. In rural locations and throughout the southeast, traditional secondary OA (SOA) formed from biogenic precursors is predicted to be the dominant class of oxidized OA. PMCAMx can only reproduce the large fractional contributions of oxidized OA observed in the atmosphere if some of the POA in the model evaporates. Sensitivity analysis illustrates that the volatility distribution of the existing POA emissions and the amount of intermediate volatility compounds not accounted for in current inventories are key uncertainties. At an upper bound, better accounting for emissions of low‐volatility organics has the potential to increase summertime OA concentrations in northeastern and midwestern cities by as much as 50%.

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Abstract. Existing parameterizations tend to underpredict the α-pinene aerosol mass fraction (AMF) or yield by a factor of 2-5 at low organic aerosol concentrations (<5 µg m −3 ). A wide range of smog chamber results obtained at various conditions (low/high NO x , presence/absence of UV radiation, dry/humid conditions, and temperatures ranging from 15-40 • C) collected by various research teams during the last decade are used to derive new parameterizations of the SOA formation from α-pinene ozonolysis. Parameterizations are developed by fitting experimental data to a basis set of saturation concentrations (from 10 −2 to 10 4 µg m −3 ) using an absorptive equilibrium partitioning model. Separate parameterizations for α-pinene SOA mass fractions are developed for: 1) Low NO x , dark, and dry conditions, 2) Low NO x , UV, and dry conditions, 3) Low NO x , dark, and high RH conditions, 4) High NO x , dark, and dry conditions, 5) High NO x , UV, and dry conditions. According to the proposed parameterizations the α-pinene SOA mass fractions in an atmosphere with 5 µg m −3 of organic aerosol range from 0.032 to 0.1 for reacted α-pinene concentrations in the 1 ppt to 5 ppb range.