Constraints on Aerosol Nitrate Photolysis as a Potential Source of HONO and NO x .

The concentration of nitrogen oxides (NO _x ) plays a central role in controlling air quality. On a global scale, the primary sink of NO _x is oxidation to form HNO ₃ . Gas-phase HNO ₃ 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 ₃ undergoes photolysis 10-300 times more rapidly than gas-phase HNO ₃ . We present here constraints on the rate of particle-phase HNO ₃ photolysis based on observations of NO _x and HNO ₃ collected over the Yellow Sea during the KORUS-AQ study in summer 2016. The fastest proposed photolysis rates are inconsistent with the observed NO _x to HNO ₃ ratios. Negligible to moderate enhancements of the HNO ₃ 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 ₃ photolysis would not significantly affect the HNO ₃ budget but could help explain observations of HONO and NO _x in highly aged air.