Evidence of nitric acid uptake in warm cirrus anvil clouds during the NASA TC4 campaign

[1] Uptake of HNO3 onto cirrus ice may play an important role in tropospheric NOx cycling. Discrepancies between modeled and in situ measurements of gas‐phase HNO3 in the troposphere suggest that redistribution and removal mechanisms by cirrus ice have been poorly constrained. Limited in situ measurements have provided somewhat differing results and are not fully compatible with theory developed from laboratory studies. We present new airborne measurements of HNO3 in cirrus clouds from anvil outflow made during the Tropical Composition, Cloud, and Climate Coupling Experiment (TC4). Upper tropospheric (>9 km) measurements made during three flights while repeatedly traversing the same cloud region revealed depletions of gas‐phase HNO3 in regions characterized by higher ice water content and surface area. We hypothesize that adsorption of HNO3 onto cirrus ice surfaces could explain this. Using measurements of cirrus ice surface area density and some assumptions about background mixing ratios of gas‐phase HNO3, we estimate molecular coverages of HNO3 on cirrus ice surface in the tropical upper troposphere during the TC4 racetracks to be about 1 × 10 molecules cm. This likely reflects an upper limit because potential dilution by recently convected, scavenged air is ignored. Also presented is an observation of considerably enhanced gas‐phase HNO3 at the base of a cirrus anvil suggesting vertical redistribution of HNO3 by sedimenting cirrus particles and subsequent particle sublimation and HNO3 evaporation. The impact of released HNO3, however, appears to be restricted to a very thin layer just below the cloud.