Chamber studies to simulate secondary organic aerosol formation from the Deepwater Horizon oil spill

Because atmospheric organic species are generally emitted from a large number of sources, over wide spatial and temporal scales, it is generally challenging to ascribe ambient organic aerosol (OA) to the oxidation of specific secondary organic aerosol (SOA) precursors. However, the Deepwater Horizon (DWH) oil spill (April 20-July 15, 2010), provided the unique circumstance of a large, well-defined source of gas-phase organics introduced into a relatively clean atmosphere. Here we describe a laboratory simulation of SOA formation downwind of the DWH spill, via the oxidation of South Louisiana-light (SL) crude oil by OH radicals in an environmental chamber. Intermediate and semi-volatile fractions of the SL crude oil are vaporized and oxidized by gas-phase OH radicals (formed from the photolysis of HONO). The chemical composition is monitored as a function of OH exposure. When OH exposures are approximately matched, laboratory-generated SOA and OA measured downwind of the oil spill exhibit extremely similar aerosol mass spectra, in strong support of the hypothesis that the OA measured downwind of the DWH oil spill was secondary in nature. More generally, this agreement indicates that in cases when SOA precursors are well-constrained, chamber experiments can reasonably reproduce key properties of ambient OA. Results of chamber studies on sub-fractions of the SL crude oil, aimed at identifying the classes of oil components most responsible for SOA formation, will be discussed.