.[1] Nitric acid (HNO 3 ) is the dominant end product of NO x (= NO + NO 2 ) oxidation in the troposphere, and its dry deposition is considered to be a major removal pathway for the atmospheric reactive nitrogen. Here we present both field and laboratory results to demonstrate that HNO 3 deposited on ground and vegetation surfaces may undergo effective photolysis to form HONO and NO x , 1 -2 orders of magnitude faster than in the gas phase and aqueous phase. With this enhanced rate, HNO 3 photolysis on surfaces may significantly impact the chemistry of the overlying atmospheric boundary layer in remote low-NO x regions via the emission of HONO as a radical precursor and the recycling of HNO 3 deposited on ground surfaces back to NO x .

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