NOxLifetime and NOyPartitioning During WINTER

Although urban NOxlifetimes have been examined extensively during summertime conditions, wintertime NOxchemistry has been comparatively less studied. We use measurements of NOxand 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 NOxlifetime 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 NOxis long lived during the winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr). We demonstrate that wintertime NOxemissions have an overall lifetime controlled by the nighttimeconversion of NOxto nitric acid (HNO3) via N2O5heterogeneous chemistry, and we discuss constraints on the rates of NOxconversion to HNO3. Additionally, analysis of the nighttime Oxbudget suggests that approximately 15% of O3is lost overnight through N2O5production and subsequent reaction with aerosol to form HNO3. 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. NOxis long lived during winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr)Wintertime NOxlifetime is controlled by the nighttime conversion of NOxto HNO3via N2O5heterogeneous chemistryFifteen percent of O3is removed at night via the dark reactions of N2O5