Formation and growth of organic aerosols downwind of the Deepwater Horizon oil spill

[1] Aerosol size distributions were measured in June 2010 downwind of the surface oil slick produced by the Deepwater Horizon oil spill in the Gulf of Mexico. Previous work has shown that rapid condensation of partially oxidized hydrocarbons was responsible for formation of a plume of secondary organic aerosol downwind of the spill region. Here we examine new particle formation and growth kinetics. New particles were formed upwind of the freshest oil but downwind of oil that surfaced less than about 100 hours previously. Four nm particles formed at a rate of ∼3 cm−3 s−1 and subsequently grew by condensation at a rate of ∼20 nm hr−1; preexisting accumulation mode particles grew by ∼10 nm hr−1. The gas-phase concentration of a condensing species necessary to support irreversible growth with unit accommodation coefficient is estimated to be ∼0.04–0.09 μg m−3 (∼3–8 pptv). Gas-phase concentrations may have been higher if condensation were limited by volatility. The ratio of growth rates for newly formed particles to accumulation mode particles was consistent within error limits with irreversible condensation. The absence of new particle formation away from the <100 hr-old oil slick indicates that the oxidation products of gas-phase hydrocarbon species were directly involved in the formation and growth of new particles.