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Organic nitrates (RONO₂) are an important NO_xsink. In warm, rural environments dominated by biogenic emissions, nocturnal NO₃‐initiated production of RONO₂is competitive with daytime OH‐initiated RONO₂production. However, in urban areas, OH‐initiated production of RONO₂has been assumed dominant and NO₃‐initiated production considered negligible. We show evidence for nighttime RONO₂production similar in magnitude to daytime production during three aircraft campaigns in chemically distinct summertime environments: Studies of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC⁴RS) in the rural Southeastern United States, Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) in the Colorado Front Range, and Korea‐United States Air Quality Study (KORUS‐AQ) around the megacity of Seoul. During each campaign, morning observations show RONO₂enhancements at constant, near‐background O_x(≡ O₃+NO₂) concentrations, indicating that the RONO₂are from a non‐photochemical source, whereas afternoon observations show a strong correlation between RONO₂and O_xresulting from photochemical production. We show that there are sufficient precursors for nighttime RONO₂formation during all three campaigns. This evidence impacts our understanding of nighttime NO_xchemistry.

We present a comparison of instruments measuring nitrogen oxide species from an aircraft during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the northeast United States. Instrument techniques compared here include chemiluminescence (CL), thermal dissociation laser‐induced fluorescence (TD‐LIF), cavity ring‐down spectroscopy (CRDS), high‐resolution time of flight, iodide‐adduct chemical ionization mass spectrometry (I^‐CIMS), and aerosol mass spectrometry. Species investigated include NO₂, NO, total nitrogen oxides (NO_y), N₂O₅, ClNO₂, and HNO₃. Particulate‐phase nitrate is also included for comparisons of HNO₃and NO_y. Instruments generally agreed within reported uncertainties, with individual flights sometimes showing much better agreement than the data set taken as a whole, due to flight‐to‐flight slope changes. NO measured by CRDS and CL showed an average relative slope of 1.16 ± 0.01 across all flights, which is outside of combined uncertainties. The source of the error was not identified. For NO₂measured by CRDS and TD‐LIF the average was 1.02 ± 0.00; for NO_ymeasured by CRDS and CL the average was 1.01 ± 0.00; and for N₂O₅measured by CRDS and I^‐CIMS the average was 0.89 ± 0.01. NO_ybudget closure to within 20% is demonstrated. We observe nonlinearity in NO₂and NO_ycorrelations at concentrations above ~30 ppbv that may be related to the NO discrepancy noted above. For ClNO₂there were significant differences between I^‐CIMS and TD‐LIF, potentially due in part to the temperature used for thermal dissociation. Although the fraction of particulate nitrate measured by the TD‐LIF is not well characterized, it improves comparisons to include particulate measurements.

Nitryl chloride (ClNO₂) plays an important role in the budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species. ClNO₂is formed from the heterogeneous uptake and reaction of dinitrogen pentoxide (N₂O₅) on chloride‐containing aerosol, with a production yield,ϕ(ClNO₂), defined as the moles of ClNO₂produced relative to N₂O₅lost. Theϕ(ClNO₂) has been increasingly incorporated into 3‐D chemical models where it is parameterized based on laboratory‐derived kinetics and currently accepted aqueous‐phase formation mechanism. This parameterization modelsϕ(ClNO₂) as a function of the aerosol chloride to water molar ratio. Box model simulations of night flights during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign derived 3,425 individualϕ(ClNO₂) values with a median of 0.138 and range of 0.003 to 1. Comparison of the box model median to those predicted by two other field‐basedϕ(ClNO₂) derivation methods agreed within a factor of 1.3, within the uncertainties of each method. In contrast, the box model median was 75–84% lower than predictions from the laboratory‐based parameterization (i.e., [parameterization − box model]/parameterization). An evaluation of factors influencing this difference reveals a positive dependence ofϕ(ClNO₂) on aerosol water, opposite to the currently parameterized trend. Additional factors may include aqueous‐phase competition reactions for the nitronium ion intermediate and/or direct ClNO₂loss mechanisms. Further laboratory studies of ClNO₂formation and the impacts of aerosol water, sulfate, organics, and ClNO₂aqueous‐phase reactions are required to elucidate and quantify these processes on ambient aerosol, critical for the development of a robustϕ(ClNO₂) parameterization.