Ozone and alkyl nitrate formation from the Deepwater Horizon oil spill atmospheric emissions

Abstract

Ozone (O 3), alkyl nitrates (RONO 2), and other photochemical products were formed in the atmosphere downwind from the Deepwater Horizon (DWH) oil spill by photochemical reactions of evaporating hydrocarbons with NOx (=NO+NO 2) emissions from spill response activities. Reactive nitrogen species and volatile organic compounds (VOCs) were measured from an instrumented aircraft during daytime flights in the marine boundary layer downwind from the area of surfacing oil. A unique VOC mixture, where alkanes dominated the hydroxyl radical (OH) loss rate, was emitted into a clean marine environment, enabling a focused examination of O 3 and RONO 2 formation processes. In the atmospheric plume from DWH, the OH loss rate, an indicator of potential O 3 formation, was large and dominated by alkanes with between 5 and 10 carbons per molecule (C 5 -C 10). Observations showed that NOx was oxidized very rapidly with a 0.8h lifetime, producing primarily C 6 -C 10 RONO 2 that accounted for 78% of the reactive nitrogen enhancements in the atmospheric plume 2.5h downwind from DWH. Both observations and calculations of RONO 2 and O 3 production rates show that alkane oxidation dominated O 3 formation chemistry in the plume. Rapid and nearly complete oxidation of NOx to RONO 2 effectively terminated O 3 production, with O 3 formation yields of 6.0±0.5 ppbv O 3 per ppbv of NOx oxidized. VOC mixing ratios were in large excess of NOx, and additional NOx would have formed additional O 3 in this plume. Analysis of measurements of VOCs, O 3, and reactive nitrogen species and calculations of O 3 and RONO 2 production rates demonstrate that NOx-VOC chemistry in the DWH plume is explained by known mechanisms. Copyright 2012 by the American Geophysical Union.