Insights Into NO_<i>x</i> and HONO Chemistry in the Tropical Marine Boundary Layer at Cape Verde During the MarParCloud Campaign

Abstract

Abstract Chemical processing of reactive nitrogen species, especially of NO x (= NO + NO 2 ) and nitrous acid (HONO), determines the photochemical ozone production and oxidation capacity in the troposphere. However, sources of HONO and NO x in the remote marine atmosphere are still poorly understood. In this work, the multiphase chemistry mechanism CAPRAM in the model framework SPACCIM was used to study HONO formation at Cape Verde (CVAO) in October 2017, adopted with the input of current parameterizations for various HONO sources. Three simulations were performed that adequately reproduced ambient HONO levels and its diurnal pattern. The model performance for NO x and O 3 improves significantly when considering dust‐surface‐photocatalytic conversions of reactive nitrogen compounds with high correlation coefficients up to 0.93, 0.56, and 0.89 for NO, NO 2 , and O 3 , respectively. Photocatalytic conversion of the adsorbed HNO 3 on dust is modeled to be the predominant contributor for daytime HONO at CVAO, that is, accounting for about 62% of the chemical formation rate at noontime. In contrast, the ocean‐surface‐mediated conversion of NO 2 to HONO and other discussed pathways are less important. The average OH levels at midday (9:00–16:00) modeled for cluster trajectory 1, 2, and 3 are 5.2, 5.1, and 5.2 × 10 6 molecules cm −3 , respectively. Main OH formation is driven by O 3 photolysis with a contribution of 74.6% to the total source rate, while HONO photolysis is negligible (∼1.8%). In summary, this study highlights the key role of dust aerosols for HONO formation and NO x cycling at CVAO and possibly in other dust‐affected regions, urgently calling for further investigations using field and model studies.