Coupling between the Tropospheric Photochemistry of Nitrous Acid (HONO) and Nitric Acid (HNO₃)

Environmental Context. Nitrous acid (HONO) is formed in the troposphere in urban, rural and remote environments via several uncertain heterogeneous and photochemical processes that involve nitric acid (HNO₃). A recently recognised process is initiated by the deposition and migration of HNO₃ within snow-pack surfaces to form nitrate anions (NO₃⁻). Photo-reduction of NO₃⁻ followed by acidification of the nitrite (NO₂⁻) photo-product leads to emissions of gas-phase HONO. Seasonal observations at Halley, Antarctica are consistent with the formation of HONO via this process, which is potentially of global significance because much of the Earth’s land (and sea) surface is covered with snow and is sunlit for much of the year. Both HONO and HNO₃ significantly influence the production of ozone (O₃), which acts as a greenhouse gas in the troposphere, via their respective roles as a source of hydroxyl radicals (OH^•) and as a sink for OH^• and nitrogen dioxide (NO₂).

Abstract. The tropospheric photochemistry of nitrous acid (HONO) and its coupling with that of nitric acid (HNO₃) in urban, rural and remote atmospheres are highlighted in terms of established and uncertain homogeneous and heterogeneous sources and sinks, together with known and potential effects and impacts. Observations made at Halley, Antarctica, via optical detection of an azo dye derivative of HONO are consistent with snow-pack photochemical production of HONO, which has potential significance for the production of hydroxyl radicals (OH^•) and ozone (O₃) on regional and global scales. Recent developments in measurement methods for HONO and HNO₃ are also highlighted. It is now timely to conduct a formal intercomparison of the methods in order to evaluate and enhance their capabilities, and to validate the growing body of HONO and HNO₃ data obtained in urban, rural and remote locations.