Aircraft measurements in the Uintah Basin of Utah in winter 2013 during a high ozone event

Photochemical production of ozone under wintertime conditions at mid latitudes of a continental interior is considered unlikely given the the low levels of solar insolation and cold temperatures. During the winter of 2013 (January – early March) ozone concentrations exceeded the U.S. 8-hour standard of 75 ppb on ~40 days in the Uintah Basin of Utah. Persistent snow cover throughout the winter not only produced a strong temperature inversion layer but also resulted in a very high surface albedo that greatly enhanced available ultraviolet radiation to drive the ozone photochemistry. As part of the Uintah Basin Winter Ozone Study (UBWOS) aircraft flights were conducted throughout the basin with continuous measurements of ozone, methane, nitrogen dioxide, carbon monoxide, and carbon dioxide. During the course of seven flights from January 31 to February 7, 2013, ozone levels built up from near background conditions of ~55 ppb to ~140 ppb following a similar progression seen in surface and tethered ozonesonde measurements. Methane concentrations increased during the episode from background levels of ~2 ppm to over 10 ppm. In the gas fields, methane readings regularly exceeded 15 ppm suggesting this portion of the basin as the likely source region of volatile organic compounds (VOCs) that were highly correlated with methane based on flask samples obtained from the aircraft. Nitrogen dioxide levels were also very high in this part of the basin, likely because of extensive gas processing facilities. Vertical profiles obtained during the flights showed that the high ozone concentrations as well as other constituents were confined to a shallow layer between the lowest flight level at ~1450 meters above sea level and 1700 masl. During these afternoon flights, constituents were generally well-mixed through this layer. Ozone values increased across the basin as the high ozone episode developed over the course of the week. The presence of high ozone values across the basin suggests that ozone is being produced from precursors that spread through the basin since highest ozone amounts are not seen in close proximity to the the precursor sources. Also enhanced levels of methane and accompanying VOCs spread from the primary source in the gas fields through the basin indicating the continuing availability of ozone precursors.