NO_<b>x</b> Lifetime and NO_<b>y</b> Partitioning During WINTER

Abstract

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 (HNO 3 ) via N 2 O 5 heterogeneous chemistry, and we discuss constraints on the rates of NO x conversion to HNO 3 . Additionally, analysis of the nighttime O x budget suggests that approximately 15% of O 3 is lost overnight through N 2 O 5 production and subsequent reaction with aerosol to form HNO 3 .