Strong wintertime ozone events in the Upper Green River basin, Wyoming

摘要

During recent years, elevated ozone (O) values have been observedrepeatedly in the Upper Green River basin (UGRB), Wyoming, during wintertime.This paper presents an analysis of high ozone days in late winter 2011(1 h average up to 166 ppbv – parts per billion by volume). Intensive operational periods (IOPs) ofambient monitoring were performed, which included comprehensive surface andboundary layer measurements. On IOP days, maximum O values arerestricted to a very shallow surface layer. Low wind speeds in combinationwith low mixing layer heights (~ 50 m above ground levelaround noontime) are essential for accumulation of pollutants within theUGRB. Air masses contain substantial amounts of reactive nitrogen (NO)and non-methane hydrocarbons (NMHC) emitted from fossil fuel explorationactivities in the Pinedale Anticline. On IOP days particularly in the morning hours, reactive nitrogen (up to 69%), aromatics and alkanes(~ 10–15%; mostly ethane and propane) are majorcontributors to the hydroxyl (OH) reactivity. Measurements at the Bouldermonitoring site during these time periods under SW wind flow conditions showthe lowest NMHC / NO ratios (~ 50), reflecting arelatively low reactive NMHC mixture, and a change from a NO-limitedregime towards a NMHC-limited regime as indicated by photochemicalindicators, e.g., O /NO, O /NO, and O / HNOand the EOR (extent of reaction). OH production on IOP days is mainly due tonitrous acid (HONO). On a 24 h basis and as determined for a measurementheight of 1.80 m above the surface HONO photolysis on IOP days cancontribute ~ 83% to OH production on average, followed byalkene ozonolysis (~ 9%). Photolysis by ozone and HCHOphotolysis contribute about 4% each to hydroxyl formation. High HONOlevels (maximum hourly median on IOP days: 1096 pptv – parts per trillion by volume) are favored by acombination of shallow boundary layer conditions and enhanced photolysisrates due to the high albedo of the snow surface. HONO is most likely formedthrough (i) abundant nitric acid (HNO) produced in atmosphericoxidation of NO, deposited onto the snow surface and undergoingphoto-enhanced heterogeneous conversion to HONO (estimated HONO production:10.2 ± 40% ppbv h) and (ii) combustion-related emission of HONO(estimated HONO production: ~ 0.1 ± 30% ppbv h). HONOproduction is confined to the lowermost 10 m of the boundary layer. HONO,serves as the most important precursor for OH, strongly enhanced due to thehigh albedo of the snow cover (HONO photolysis rate 10.7 ± 30%ppbv h). OH radicals will oxidize NMHCs, mostly aromatics (toluene,xylenes) and alkanes (ethane, propane), eventually leading to an increase inozone.