Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical AgingAllen L. Robinson, Neil M. Donahue, Manish K. Shrivastava, Emily A. Weitkamp, Amy M. Sage, Andrew P. Grieshop, Timothy E. Lane, Jeffrey R. Pierce, Spyros N. Pandis
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.