Winter ozone formation and VOC incremental reactivities in the Upper Green River Basin of Wyoming

Abstract

The Upper Green River Basin (UGRB) in Wyoming experiences ozone episodes in the winter when the air is relatively stagnant and the ground is covered by snow. A modeling study was carried out to assess relative contributionsofoxidesofnitrogen(NOx)andindividualvolatileorganiccompounds(VOCs),andnitrousacid (HONO) in winter ozone formation episodes in this region. The conditions of two ozone episodes, one in February 2008 and one in March 2011, were represented using a simplified box model with all pollutants present initially, but with the detailed SAPRC-07 chemical mechanism adapted for the temperature and radiation conditions arising from the high surface albedo of the snow that was present. Sensitivity calculations were conducted to assess effects of varying HONO inputs, ambient VOC speciation, and changing treatmentsoftemperatureandlightingconditions.Thelocationsmodeledwerefoundtobequitedifferentin VOCspeciationandsensitivitiestoVOCandNOxemissions,withonesitemodeledforthe2008episodebeing highly NOx-sensitive and insensitive to VOCs and HONO, and the other 2008 site and both 2011 sites being verysensitivetochangesinVOCandHONOinputs.IncrementalreactivityscalescalculatedforVOC-sensitive conditions in the UGRB predict far lower relative contributions of alkanes to ozone formation than in the traditional urban-based MIR scale and that the major contributors to ozone formation were the alkenes and the aromatics, despite their relatively small mass contributions. The reactivity scales are affected by the variable ambient VOC speciation and uncertainties in ambient HONO levels. These box model calculations areusefulforindicatinggeneralsensitivitiesandreactivitycharacteristicsofthesewinterUGRBepisodes,but fully three-dimensional models will be required to assess ozone abatement strategies in the UGRB.