Investigation of the heterogeneous NO2 conversion on perchloric acid surfaces

Abstract

The heterogeneous conversion of NO₂ on perchloric acid surfaces into HONO, HNO₃ and N₂O was investigated in a bubbler system under simulated atmospheric conditions. NO₂, HONO and N₂O were monitored time resolved by long-path tunable infrared diode laser absorption spectrometry using single vibrational rotational lines. The experimental results show that NO₂ is heterogeneously converted on acid surfaces into HONO and consecutively into N₂O. HONO formation in the reaction system occurs by the heterogeneous reaction of NO₂ with water. The solubility of HONO in perchloric acid was determined at 298 K as a function of the acid concentration. The effective Henry's law constant (K_Heff) is well described by:

The experimental findings indicate that the nitrosyl ion (NO⁺) plays an important role in converting HONO into N₂O and HNO₃. A reaction scheme is proposed which is in excellent agreement with the experimental observations. For the global source strength of N₂O by heterogeneous conversion processes on acidic aerosols an upper limit of ca. 300 kt N₂O year^-1 was estimated which indicates that these reactions are probably of minor importance for the global N₂O budget.