Organic process. (A) Oil from the DWH well spread across the ocean surface in a narrow plume, from which the most volatile organic carbon compounds (VOCs) quickly evaporated. Semivolatile and intermediate-volatility compounds (SVOCs and IVOCs) spread over a wider area because they took longer to evaporate. Oxidation of IVOCs and SVOCs in the atmosphere contributed heavily to the formation of organic aerosols. (B) The distribution of mass fraction of the hydrocarbons as a function of their volatility in the oil, represented by the saturation concentration C* (8). At the DWH spill, the size of the oil slick that best matched the downstream plume of organic particulate was consistent with precursor compounds to aerosol formation with C* ≈ 10^5 µg m−3, which equates to hydrocarbons composed of 14 to 16 carbon atoms. "CREDIT: ADAPTED BY P. HUEY/SCIENCE"