Evaluating airborne fluxes of reactive nitrogen compounds over the marine boundary layer: from eddy covariance to wavelet transform

Abstract

Reactive nitrogen compounds (Nr) have different fates in the atmosphere due to differences in the governing process of chemical transformation, physical transport and deposition. Quantifying dry deposition of critical Nr including N2O5, HNO3, and ClNO2 is important for assessing their atmospheric lifetime and potential impacts on NOx cycling and ecosystems. However, little is known quantitatively and qualitatively about surface-atmosphere exchange of these species, especially under dark wintertime conditions. In this study, we use a comprehensive dataset from the Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) campaign to characterize the chemistry, sources, and removal rates of Nr. Employing both eddy covariance and wavelet analysis methods, we quantified exchange velocities and evaluate their validity using turbulence conditions, spectral patterns, and Nr budget constraints. We focus analysis on the wintertime marine boundary layer due to a greater prevalence of active vertical mixing, though evidence for surface layer decoupling is also present in the flux observations, as well as less stringent requirements on measurement frequency. We find that under conditions of strong vertical mixing, exchange velocities for N2O5, HNO3, and ClNO2 were -0.78 cm·s-1, -0.68 cm·s-1, -0.73 cm·s-1 on average indicating net deposition, with a range from -4 cm·s-1 to 3 cm·s-1 and an measurement uncertainty of 30%. A box model of nighttime reactive nitrogen chemistry is able reproduce the observations by invoking the observationally derived deposition velocities. To our knowledge, this is the first full simultaneous measurement and evaluation of N2O5, HNO3, and ClNO2 fluxes from an aircraft using eddy covariance, with which we provided an observation-based framework for assessing the impact of air-surface exchange on the fate of key nitrogen species.