Heterogeneous N₂O₅ Uptake During Winter: Aircraft Measurements During the 2015 WINTER Campaign and Critical Evaluation of Current Parameterizations

Nocturnal dinitrogen pentoxide (N₂O₅) heterogeneous chemistry impacts regional air quality and the distribution and lifetime of tropospheric oxidants. Formed from the oxidation of nitrogen oxides, N₂O₅ is heterogeneously lost to aerosol with a highly variable reaction probability, γ(N₂O₅), dependent on aerosol composition and ambient conditions. Reaction products include soluble nitrate (HNO₃ or NO₃ ⁻) and nitryl chloride (ClNO₂). We report the first-ever derivations of γ(N₂O₅) from ambient wintertime aircraft measurements in the critically important nocturnal residual boundary layer. Box modeling of the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the eastern United States derived 2,876 individual γ(N₂O₅) values with a median value of 0.0143 and range of 2 × 10⁻⁵ to 0.1751. WINTER γ(N₂O₅) values exhibited the strongest correlation with aerosol water content, but weak correlations with other variables, such as aerosol nitrate and organics, suggesting a complex, nonlinear dependence on multiple factors, or an additional dependence on a nonobserved factor. This factor may be related to aerosol phase, morphology (i.e., core shell), or mixing state, none of which are commonly measured during aircraft field studies. Despite general agreement with previous laboratory observations, comparison of WINTER data with 14 literature parameterizations (used to predict γ(N₂O₅) in chemical transport models) confirms that none of the current methods reproduce the full range of γ(N₂O₅) values. Nine reproduce the WINTER median within a factor of 2. Presented here is the first field-based, empirical parameterization of γ(N₂O₅), fit to WINTER data, based on the functional form of previous parameterizations.