Nitryl Chloride in the Urban Winter: Results from Recent Aircraft Campaigns (Invited Presentation)

Field observations over the last decade have demonstrated that nitryl chloride, ClNO₂, is a major product of the nighttime heterogeneous uptake of N₂O₅in the lower atmosphere. Morning photolysis of ClNO₂following its overnight production represents a large source of reactive chlorine. This mechanism is important in regionally polluted environments with elevated levels of nitrogen oxides (NO_x= NO + NO₂) that react to produce N₂O₅at night, and it is prevalent during the winter season when N₂O₅represents the dominant pathway for NO_xoxidation. Recent aircraft campaigns have investigated ClNO₂and N₂O₅in the urban winter atmosphere. The WINTER (Wintertime Investigation of Transport Emissions and Reactivity) campaign surveyed urban regions and outflow over the Eastern U.S. during the winter of 2015 using the NSF C130 aircraft. The UWFPS (Utah Winter Fine Particulate Study) campaign was an intensive study of regional air pollution in the urban mountain basins of Northern Utah, including Salt Lake City, using the NOAA Twin Otter aircraft. Both campaigns included measurements of nitrogen oxides, ozone, N₂O₅, ClNO₂and aerosol composition that allow for detailed analysis of the factors governing N₂O₅uptake and ClNO₂production. Yields of ClNO₂from both campaigns show that it differs considerably from current parameterizations used in models, suggesting more complexity in ClNO₂formation chemistry than has been represented from laboratory studies to date. This presentation will survey results from these campaigns, implications for the role of halogens in urban winter environments, and needs for further research to define wintertime heterogeneous chemistry of nitrogen oxides and halogens.