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Abstract The dark reaction of NO x and H 2 O vapor in 1 atm of air was studied for the purpose of elucidating the recently discussed unknown radical source in smog chambers. Nitrous acid and nitric oxide were found to be formed by the reaction of NO 2 and H 2 O in an evacuable and bakable smog chamber. No nitric acid was observed in the gas phase. The reaction is not stoichiometric and is thought to be a heterogeneous wall reaction. The reaction rate is first order with respect to NO 2 and H 2 O, and the concentrations of HONO and NO initially increase linearly with time. The same reaction proceeds with a different rate constant in a quartz cell, and the reaction of NO 2 and H 2 18 O gave H 18 ONO exclusively. Taking into consideration the heterogeneous reaction of NO 2 and H 2 O, the upper limit of the rate constant of the third‐order reaction NO + NO 2 + H 2 O → 2HONO was deduced to be (3.0 ± 1.4) × 10 −10 ppm −2 ‐min −1 , which is one order of magnitude smaller than the previously reported value. Nitrous acid formed by the heterogeneous dark reaction of NO 2 and H 2 O should contribute significantly to both an initially present HONO and a continuous supply of OH radicals by photolysis in smog chamber experiments.
Sakamaki et al. (Sat,) studied this question.