Constraints on Aerosol Nitrate Photolysis As a Potential Source of HONO and NO

The concentration of nitrogen oxides (NO ) plays a central role in controlling air quality. On a global scale, the primary sink of NO is oxidation to form HNO. Gas-phase HNO photolyses slowly with a lifetime in the troposphere of 10 days or more. However, several recent studies examining HONO chemistry have proposed that particle-phase HNO undergoes photolysis 10-300 times more rapidly than gas-phase HNO. We present here constraints on the rate of particle-phase HNO photolysis based on observations of NO and HNO collected over the Yellow Sea during the KORUS-AQ study in summer 2016. The fastest proposed photolysis rates are inconsistent with the observed NO to HNO ratios. Negligible to moderate enhancements of the HNO photolysis rate in particles, 1-30 times faster than in the gas phase, are most consistent with the observations. Small or moderate enhancement of particle-phase HNO photolysis would not significantly affect the HNO budget but could help explain observations of HONO and NO in highly aged air.