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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.