On the pH dependence of photo‐induced volatilization of nitrogen oxides from frozen solutions containing nitrate

[1] Photochemically induced emissions from the surfaces of snow and ice crystals are known to alter the composition of the troposphere. In particular, photolysis of the nitrate anion in snow and ice has been shown to yield NOx (= NO + NO2) and nitrous acid (HONO) in the gas-phase; photolysis of HONO and NOx alters the oxidative balance of the atmosphere and increases the rate of formation of the greenhouse gas ozone. We report here that the photochemical release of NO2 from irradiated frozen solutions containing nitrate anion is promoted by acidic conditions similar to those found in moderately polluted environments. In unbuffered ice or under basic conditions (pH 9.5), the main gas-phase nitrogen oxide observed was NO2 (>95% relative to odd nitrogen, or NOy). The yield of gas-phase NO2 increases with decreasing pH (measured prior to freezing) in the range from 9.5 to 4.5. At pH 4.5 and lower, acidic nitrogen oxides, mainly HONO and nitric acid (HONO2), are observed in the gas-phase, as well as trace amounts of peroxynitric acid (HO2NO2) and peroxynitrous acid (HO2NO). We propose that the acid-promoted increase in the rate of NO2 release involves formation of HOONO as a volatile intermediate. This acid-promoted pathway for conversion of condensed-phase NO3− to gas-phase NO2 is currently not considered in atmospheric chemistry and aerosol models but may be a significant and overlooked aerosol nitrate volatilization pathway.