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Abstract

Nitrous acid was measured in the lower, middle, and upper troposphere during 25 research flights of the HALO aircraft across two research missions, EMeRGe and CAFE-Africa in 2017 and 2018. These missions represent a variety of atmospheric conditions, from pristine low-NOx air over the Atlantic Ocean during the CAFE-Africa mission to polluted high-NOx air during the EMeRGe-EU and -Asia missions. Using the remote sensing mini-DOAS instrument, slant column densities retrieved from DOAS analysis are converted to volume mixing ratios with the O3/O4 scaling method. These observations are complemented by coincident measurements of formaldehyde and nitrogen dioxide from the mini-DOAS instrument. It is demonstrated that the O3/O4 scaling method is robust with respect to choice of the spectral interval as well as scaling gas. These measurements are combined with observations of trace gas concentrations and atmospheric parameters from a host of other instruments onboard the HALO aircraft, as well as model simulations. While the HCHO and NO2 measured by the mini-DOAS instrument compare well with photochemical model predictions and with measurements of complementary in situ instruments, the retrieved HONO is often in excess of what would be expected from the known gas phase formation mechanisms of nitrous acid, or what is predicted by the atmospheric chemistry models EMAC and MECO(n). With the co-measured species, potential heterogeneous and gas phase sources of this excess nitrous acid are investigated for the probed layers of the atmosphere, specifically the lower and upper troposphere. In the low-NOx marine boundary layer, the observed HONO is corroborated by previous studies in the same region, and may be formed by the photolysis of particulate nitrate. In the polluted lower troposphere, the heterogeneous conversion of NO2 on the surface or in the bulk of atmospheric aerosol is the most likely source of the observed HONO, though a precise mechanism cannot be determined with the information provided by the measurements of the available instrumentation. In the cold upper troposphere, a novel gas phase source of HONO, the oxidation of peroxynitrous acid is suggested and investigated. Overall, our measurements indicate a larger role of HONO than hitherto considered for the formation of OH, which is the major oxidant in the Earth's atmosphere.