Nitrogen oxides (NOx = NO + NO2) influence both the gas and aerosol phases of tropospheric chemistry, with impacts on air quality, climate, and nutrient cycling in ecosystems. The lifetime of NOx in the atmosphere is controlled by conversion to its permanent oxidative sinks: organic nitrates (RONO2) and nitric acid (HNO3). Much of the previous observational work to understand the lifetime and fate of NOx has focused on summer conditions when the daytime NOx lifetime is relatively short (2-4 hr) and NOx plays a key role in tropospheric O3 production. Here I use observations from four aircraft experiments to provide new constraints on the lifetime and fate of NOx and its oxidation products in urban areas by exploring the importance of condensed phase and dark reactions. First, I examine the lifetime and fate of NOx during wintertime conditions in the Northeast US and show that NOx loss is dominated by nocturnal, condensed phase reactions that produce HNO3. Second, I examine the importance of nocturnal production of RONO2 as a loss pathway for NOx in threechemically distinct locations in the US and Korea. Finally, I examine the fate of RONO2, an important oxidative sink of NOx, by assessing its partitioning into the condensed phase and its role in secondary organic aerosol (SOA) formation using observations from Korea as an example of urban chemistry.

Although urban NOx lifetimes have been examined extensively during summertime conditions, wintertime NOx chemistry has been comparatively less studied. I use measurements of NOx and its oxidation products from the aircraft-based WINTER (Wintertime INvestigation of Transport, Emissions, and Reactivity) experiment over the northeastern UnitedStates during February - March 2015 to describe the urban NOx lifetime during conditions when days are shorter, actinic flux is reduced, and temperatures are colder. By analyzing regional outflow from the East Coast, I show that NOx is long lived during the winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr). Moreover, I demonstrate that wintertime urban NOx emissions have an overall lifetime controlled by the nighttime conversion of NOx to HNO3 via heterogeneous chemistry.

In warm, rural environments dominated by biogenic emissions, nocturnal NO3-initiated production of RONO2 is known to be competitive with daytime OH-initiated RONO2 production. However, in urban areas, OH-initiated production of RONO2 has been assumed dominant and NO3-initiated production considered negligible. I show evidence for nighttime RONO2 production similar in magnitude to daytime production during the three aircraft campaigns in chemically distinct summertime environments: Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS, 2013) in the rural Southeastern United States, Front Range Air Pollution and Photochemistry Experiment (FRAPPE, 2014) in the Colorado Front Range, and Korea-United States Air Quality Study (KORUS-AQ, 2016) around the megacity of Seoul, South Korea. During each campaign, morning observations show RONO2 enhancements at constant, near-background Ox (= O3 + NO2) concentrations, indicating that the RONO2 are from a non-photochemical source, whereas afternoon observations show a strong correlation between RONO2 and Ox resulting from photochemical production. Furthermore, I show that there are sufficient precursors for nocturnal RONO2 formation during all three campaigns.

Finally, I examine the fate of RONO2 using observations from KORUS-AQ during May -June 2016. I use measurements of particle-phase RONO2 and total (gas + particle) RONO2 to explore the phase partitioning of RONO2 and the contribution of organic nitrates to SOA production. These measurements show that about 1/4 of RONO2 is in the condensed phase, and from our observations, I estimate that 15% of the organic aerosol (OA) mass can be attributed to RONO2. I observe that the fraction of RONO2 in the condensed phase increases with total OA concentration, evidence that equilibrium absorptive partitioning controls the phase distribution of RONO2. I use our observations in conjunction with the Community Multiscale Air Quality (CMAQ) Modeling System to show that our current understanding of RONO2 chemistry can only account for one third of the observed RONO2; there is a large missing source of semi-volatile, anthropogenically-derived RONO2 around Seoul.