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

Abstract

Abstract. During recent years, elevated ozone (O3) values have been observed repeatedly 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). Intensive Operational Periods (IOPs) of ambient monitoring were performed which included comprehensive surface and boundary layer measurements. On IOP days, maximum O3 values are restricted to a very shallow surface layer. Low wind speeds in combination with low mixing layer heights (~50 m a.g.l. around noontime) are essential for accumulation of pollutants within the UGRB. Air masses contain substantial amounts of reactive nitrogen (NOx) and non-methane hydrocarbons (NMHC) emitted from fossil fuel exploration activities in the Pinedale Anticline. On IOP days in the morning hours in particular, reactive nitrogen (up to 69%), aromatics and alkanes (~10–15%; mostly ethane and propane) are major contributors to the hydroxyl (OH) reactivity. Measurements at the Boulder monitoring site during these time periods under SW wind flow conditions show the lowest NMHC/NOx ratios (~50), reflecting a relatively low NMHC mixture, and a change from a NOx-limited regime towards a NMHC limited regime as indicated by photochemical indicators, e.g. O3/NOy, O3/NOz, and O3/HNO3 and the EOR (Extent of Reaction). OH production on IOP days is mainly due to nitrous acid (HONO). Until noon on IOP days, HONO photolysis contributes between 74–98% of the entire OH-production. Ozone photolysis (contributing 2–24%) is second to HONO photolysis. However, both reach about the same magnitude in the early afternoon (close to 50%). Photolysis of formaldehyde (HCHO) is not important (2–7%). High HONO levels (maximum hourly median on IOP days: 1096 pptv) are favored by a combination of shallow boundary layer conditions and enhanced photolysis rates due to the high albedo of the snow surface. HONO is most likely formed through (i) abundant nitric acid (HNO3) produced in atmospheric oxidation of NOx, deposited onto the snow surface and undergoing photo-enhanced heterogeneous conversion to HONO (estimated HONO production: 2250 pptv h−1) and (ii) combustion related emission of HONO (estimated HONO production: ~585 pptv h−1). HONO, serves as the most important precursor for OH, strongly enhanced due to the high albedo of the snow cover (HONO photolysis rate 2900 pptv h−1). OH radicals will oxidize NMHCs, mostly aromatics (toluene, xylenes) and alkanes (ethane, propane), eventually leading to an increase in ozone.