Insights into NOx and HONO in the subtropical marine boundary layer during MarParCloud campaign at Cape Verde

Chemical processing of reactive nitrogen species, especially NO_x(=NO+NO₂) and nitrous acid (HONO), determines/alters critically the photochemical ozone production in the troposphere, affecting the climate change, biological cycle and human healthy. However, the characteristics and sources of nitrous acid (HONO) and NO_x in the remote marine atmosphere are still poorly understood. Herein, based on the data sets of HONO-related species as well as other parameters measured during MarParCloud campaign at Cape Verde in October 2017, the multiphase chemistry model SPACCIM equipped with the state-of-the-art multiphase chemistry mechanism CAPRAM was adopted with input of current literature parametrizations for various HONO sources in the tropospheric boundary layer (gas reaction of NO and OH, ocean-surface-mediated conversion of NO₂ to HONO, NO₂ reacted with organics on mineral dust, NH₃ oxidation process, and dust-surface-photocatalytic conversions of reactive nitrogen species to HONO) to reveal the relative importance of each source for HONO in the remote boundary layer at Cape Verde. Each simulation was performed for 72 hours in different clusters obtained from the backward trajectories model analysis with HYSPLIT. The simulations well reproduced the observed HONO level and its diurnal pattern, and significantly improved the model performance for NO_x and O₃ in every cluster after 72 hours of operation, when considering the mechanisms of dust-surface-photocatalytic conversions of reactive nitrogen species. Furthermore, photolysis of the absorbed HNO₃ on the dust is modelled to be the prevailing contributor for the daytime HONO at Cape Verde, which accounted for about 56%, following by the photo-enhanced of NO₂ absorbed on the dust (41%). In contrast, the ocean-surface-mediated conversion of NO₂ to HONO and other pathways were found unimportant for HONO formation at Cape Verde. For OH sources, HONO photolysis only accounted for a small proportion source (~3%) of the ambient OH level in remote marine boundary layer due to the low HONO concentration at Cape Verde. In summary, this study highlights the key role of dust aerosols in the formation of HONO and NO_x at Cape Verde.