Nitrous Acid (HONO) Formation from the Irradiation of Aqueous Nitrate Solutions in the Presence of Marine Chromophoric Dissolved Organic Matter: Comparison to Other Organic Photosensitizers
Stephanie L. Mora García, Shubhrangshu Pandit, Juan G. Navea, Vicki H. Grassian
Abstract
Nitrous acid (HONO), a highly reactive trace atmospheric gas, is often underestimated in global atmospheric models due to the poor understanding of its sources and sinks, especially in the marine boundary layer (MBL). Herein, we have investigated HONO formation from the irradiation of nitrate solutions in the presence of increasingly complex photosensitizers including marine dissolved organic matter (m-DOM), which contains chromophoric organic matter, collected from a large-scale mesocosm experiment. In particular, aqueous nitrate solutions in the presence of m-DOM, humic acid (HA), and 4-benzoylbenzoic acid (4-BBA) as well as ethylene glycol (EG) were irradiated with a solar simulator. Gas-phase HONO and NO2 produced during the irradiation of these samples were detected using incoherent broad band cavity enhanced absorption spectroscopy (IBBCEAS). The relative amounts of HONO and NO2 formation varied for the different samples. The addition of all of these different organic containing samples (m-DOM, HA, 4-BBA, and EG) to nitrate solutions caused an enhancement in HONO formation, with m-DOM showing the greatest total amount over a 6 h time period. Mechanisms for this enhancement are discussed as well as the strong pH dependence, with the greatest amount of HONO at a low pH. Overall, HONO formation from nitrate photolysis in the presence of m-DOM provides insights into the HONO formation pathway in the MBL and ultimately contributes to improving atmospheric models.