We examine the distribution and fate of nitrogen oxides (NO x ) in the lower troposphere over the Northeast United States (NE US) using aircraft observations from the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign in February-March 2015, as well as the GEOS-Chem chemical transport model and concurrent ground-based observations. We find that the National Emission Inventory from the U.S. Environmental Protection Agency is consistent with WINTER observations of total reactive nitrogen ( T NO y ) to within 10% on average, in contrast to the significant overestimate reported in past studies under warmer conditions. Updates to the dry deposition scheme and dinitrogen pentoxide (N 2 O 5 ) reactive uptake probability, ɣ(N 2 O 5 ), result in an improved simulation of gas-phase nitric acid (HNO 3 ) and submicron particulate nitrate (pNO 3 À ), reducing the longstanding factor of 2-3 overestimate in wintertime HNO 3 + pNO 3 À to a 50% positive bias. We find a NO x lifetime against chemical loss and deposition of 22 hr in the lower troposphere over the NE US. Chemical loss of NO x is dominated by N 2 O 5 hydrolysis (58% of loss) and reaction with OH (33%), while 7% of NO x leads to the production of organic nitrates. Wet and dry deposition account for 55% and 45% of T NO y deposition over land, respectively. We estimate that 42% of the NO x emitted is exported from the NE US boundary layer during winter, mostly in the form of HNO 3 + pNO 3 À (40%) and NO x (38%).Plain Language Summary Nitrogen oxides are a key family of pollutants emitted by cars, electric utilities, and industry. The fate of nitrogen oxides remains poorly understood especially during the winter season, when low sunlight leads to their persistence in the atmosphere. We analyze comprehensive aircraft observations of nitrogen oxides and their atmospheric products over the Northeast United States during winter 2015. This detailed chemical information allows to resolve a long-standing overestimate of the oxidation products of nitrogen oxides and places new constraints on their deposition to land ecosystems and export to the global atmosphere. Key Points: • Existing anthropogenic NO x inventory is consistent with aircraft and ground-based observations over Northeast United States during winter • NO x has a 22 hr lifetime, with half of NO y present as NO x , 37% as HNO 3 and pNO 3 À , and remaining 13% mostly as PAN • Model reproduces NO y partitioning and predicts a 42% NO x export efficiency in winter, with a 55-45% split between wet and dry deposition Supporting Information: • Supporting Information S1

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.

Nitrogen dioxide (NO2) is a ubiquitous air pollutant with high concentrations particularly close to main roads. The focus of this study was on possible differences in NO2 concentrations between adult and child heights as a function of different distances from heavily trafficked roads in urban areas. Passive diffusion tubes were used to measure NO2 concentrations at heights of 0.8 m (approximate inhalation height of children and closer to vehicle exhaust height) and 2.0 m (approximate inhalation height of adults) above the ground at a number of locations and over several weeks in the city of Edinburgh, UK. Evidence for significant differences in NO2 between heights was observed up to at least 1.2 m from kerbside of busy roads, with tubes at 0.8 m measuring concentrations 5–15 % (a few μg m−3) greater than at 2.0 m. The vertical NO2 concentration difference was not observable at distances 2.5 m or greater from the kerbside. Fitting of horizontal transects of NO2 concentrations away from main roads demonstrated the strong influence of wind speed in yielding faster fall-off in NO2 concentration from the roadside, and in near-ground vertical gradient in NO2, and lower background NO2 concentrations. These observations have potential public health implications for differential NO2 exposures between children walking, or in buggies, close to heavily trafficked urban roads compared with adults.

Abstract. The atmospheric multiphase reaction of dinitrogen pentoxide (N2O5) with chloride-containing aerosol particles produces nitryl chloride (ClNO2), which has been observed across the globe. The photolysis of ClNO2 produces chlorine radicals and nitrogen dioxide (NO2), which alter pollutant fates and air quality. However, the effects of local meteorology on near-surface ClNO2 production are not yet well understood, as most observational and modeling studies focus on periods of clear conditions. During a field campaign in Kalamazoo, Michigan, from January–February 2018, N2O5 and ClNO2 were measured using chemical ionization mass spectrometry, with simultaneous measurements of atmospheric particulate matter and meteorological parameters. We examine the impacts of atmospheric turbulence, precipitation (snow, rain) and fog, and ground cover (snow-covered and bare ground) on the abundances of ClNO2 and N2O5. N2O5 mole ratios were lowest during periods of lower turbulence and were not statistically significantly different between snow-covered and bare ground. In contrast, ClNO2 mole ratios were highest, on average, over snow-covered ground, due to saline snowpack ClNO2 production. Both N2O5 and ClNO2 mole ratios were lowest, on average, during rainfall and fog because of scavenging, with N2O5 scavenging by fog droplets likely contributing to observed increased particulate nitrate concentrations. These observations, specifically those during active precipitation and with snow-covered ground, highlight important processes, including N2O5 and ClNO2 wet scavenging, fog nitrate production, and snowpack ClNO2 production, that govern the variability in observed atmospheric chlorine and nitrogen chemistry and are missed when considering only clear conditions.

The role of anthropogenic NOx emissions in secondary organic aerosol (SOA) production is not fully understood but is important for understanding the contribution of emissions to air quality. Here, we examine the role of organic nitrates (RONO2) in SOA formation over the Korean Peninsula during the Korea–United States Air Quality field study in Spring 2016 as a model for RONO2 aerosol in cities worldwide. We use aircraft-based measurements of the particle phase and total (gas + particle) RONO2 to explore RONO2 phase partitioning. These measurements show that, on average, one-fourth of RONO2 are in the condensed phase, and we estimate that ≈15% of the organic aerosol (OA) mass can be attributed to RONO2. Furthermore, we observe that the fraction of RONO2 in the condensed phase increases with OA concentration, evidencing that equilibrium absorptive partitioning controls the RONO2 phase distribution. Lastly, we model RONO2 chemistry and phase partitioning in the Community Multiscale Air Quality modeling system. We find that known chemistry can account for one-third of the observed RONO2, but there is a large missing source of semivolatile, anthropogenically derived RONO2. We propose that this missing source may result from the oxidation of semi- and intermediate-volatility organic compounds and/or from anthropogenic molecules that undergo autoxidation or multiple generations of OH-initiated oxidation.