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  1. Abstract

    Organic nitrates (RONO2) are an important NOxsink. 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 Ox(≡ O3+NO2) concentrations, indicating that the RONO2are from a non‐photochemical source, whereas afternoon observations show a strong correlation between RONO2and Oxresulting from photochemical production. We show that there are sufficient precursors for nighttime RONO2formation during all three campaigns. This evidence impacts our understanding of nighttime NOxchemistry.

  2. Abstract

    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 (ICIMS), and aerosol mass spectrometry. Species investigated include NO2, NO, total nitrogen oxides (NOy), N2O5, ClNO2, and HNO3. Particulate‐phase nitrate is also included for comparisons of HNO3and NOy. 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 NOymeasured by CRDS and CL the average was 1.01 ± 0.00; and for N2O5measured by CRDS and ICIMS the average was 0.89 ± 0.01. NOybudget closure to within 20% is demonstrated. We observe nonlinearity in NO2and NOycorrelations at concentrations above ~30 ppbv that maymore »be related to the NO discrepancy noted above. For ClNO2there were significant differences between ICIMS 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.

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  3. Abstract

    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 directmore »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.

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