Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces

Abstract

During the last few decades, nitrous acid (HONO) has attracted significant attention as a major source of the OH radical, the detergent of the atmosphere. However, the different daytime sources identified in the laboratory are still the subject of controversial discussion. In the present study, one of these postulated HONO sources, the heterogeneous photolysis of nitric acid (HNO₃), was studied on quartz glass surfaces in a photo flow-reactor under atmospherically relevant conditions. In contrast to other investigations, a very low HNO₃ photolysis frequency for HONO formation of J(HNO₃ → HONO) = 2.4 × 10⁻⁷ s⁻¹ (0° SZA, 50% r.h.) was determined. If these results can be translated to atmospheric surfaces, HNO₃ photolysis cannot explain the significant HONO levels in the daytime atmosphere. In addition, it is demonstrated that even the small measured yields of HONO did not result from the direct photolysis of HNO₃ but rather from the consecutive heterogeneous conversion of the primary photolysis product NO₂ on the humid surfaces. The secondary NO₂ conversion was not photo-enhanced on pure quartz glass surfaces in good agreement with former studies. A photolysis frequency for the primary reaction product NO₂ of J(HNO₃ → NO₂) = 1.1 × 10⁻⁶ s⁻¹ has been calculated (0° SZA, 50% r.h.), which indicates that renoxification by photolysis of adsorbed HNO₃ on non-reactive surfaces is also a minor process in the atmosphere.