NO x Lifetime and NO y 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 (29hr) than nighttime lifetime (6.3hr). 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 O-x budget suggests that approximately 15% of O-3 is lost overnight through N2O5 production and subsequent reaction with aerosol to form HNO3. Plain Language Summary The atmospheric lifetime (how long something persists in the atmosphere) and fate of nitrogen oxides in urban areas during the summer has been studied extensively, but relatively few studies have looked at the lifetime of nitrogen oxides in the atmosphere during winter. We use aircraft data from the East Coast of the United States during February-March 2015 to characterize the wintertime lifetime of nitrogen oxides when days are shorter, sunlight is reduced, and temperatures are colder. We are able to measure the wintertime lifetime of nitrogen oxides and assess the relative roles of mixing, deposition, and chemistry on their fate. We determine that nitrogen oxide loss during winter is dominated by nighttime rather than daytime chemistry and that this nighttime chemistry effectively removes ozone from the atmosphere.