NOx Lifetime and NOy Partitioning During WINTER

Although urban NOx lifetimes have been examined extensively during summertime conditions, wintertime NOx chemistry has been comparatively less studied. We use measurements of NOx 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 NOx 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 NOx is long lived during the winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr). We demonstrate that wintertime NOx emissions have an overall lifetime controlled by the nighttime conversion of NOx to nitric acid (HNO3) via N2O5 heterogeneous chemistry, and we discuss constraints on the rates of NOx conversion to HNO3. Additionally, analysis of the nighttime Ox budget suggests that approximately 15% of O3 is lost overnight through N2O5 production and subsequent reaction with aerosol to form HNO3.