Release of Nitrous Acid and Nitrogen Dioxide from Nitrate Photolysis in Acidic Aqueous Solutions

Nitrate (NO₃¯) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO₃¯ leads to NO₂ and HONO, both of which play important roles in tropospheric ozone and OH production. Field and laboratory studies suggest that NO₃¯ photochemistry is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO₂ and HONO emitted during photodegradation of aqueous NO₃¯ under acidic conditions. Nitrous acid is formed in higher quantities at pH 2–4 than expected based on consideration of primary photochemical channels alone. Both experimental and modeled results indicate that the additional HONO is not due to enhanced NO₃¯ absorption cross sections or effective quantum yields, but rather to secondary reactions of NO₂ in solution. We find that NO₂ is more efficiently hydrolyzed in solution when it is generated in situ during NO₃¯ photolysis than for the heterogeneous system where mass transfer of gaseous NO₂ into bulk solution is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO production 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO₃¯ photochemistry