NOx Lifetime and NOy Partitioning During WINTER.

Abstract: Although urban NO_x lifetimes have been examined extensively during summertime conditions, wintertime NO_x chemistry has been comparatively less studied. We use measurements of NO_x and its oxidation products from the aircraft‐based WINTER (Wintertime INvestigation of Transport, Emissions, and Reactivity) experiment over the northeastern United States during February–March 2015 to describe the NO_x lifetime during conditions when days are shorter, actinic flux is reduced, and temperatures are colder. By analyzing regional outflow from the East Coast, we show that NO_x is long lived during the winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr). We demonstrate that wintertime NO_x emissions have an overall lifetime controlled by the nighttime conversion of NO_x to nitric acid (HNO₃) via N₂O₅ heterogeneous chemistry, and we discuss constraints on the rates of NO_x conversion to HNO₃. Additionally, analysis of the nighttime O_x budget suggests that approximately 15% of O₃ is lost overnight through N₂O₅ production and subsequent reaction with aerosol to form HNO₃. [ABSTRACT FROM AUTHOR]