Photoenhanced Uptake of NO2 and HONO Formation on Real Urban Grime

Abstract

Nitrous acid (HONO) is one of the most important photochemical precursors of the hydroxyl radical in the sunlit urban atmosphere. The sources of HONO, however, are still poorly characterized, yet there is a disagreement between the field observations and the model results. Here, we show that light-induced NO2 heterogeneous chemistry on authentic urban grime can make an important contribution to the total HONO levels in the urban atmosphere. The obtained results indicate that the effective uptake coefficients of NO2 on urban grime in the presence of ultraviolet light [2.6 × 1015 photons cm–2 s–1 (300 nm < λ < 400 nm)] increased markedly from (1.1 ± 0.2) × 10–6 at 0% relative humidity (RH) to (5.8 ± 0.7) × 10–6 at 90% RH, exhibiting the following linear correlation with RH: γ(NO2) = (7.4 ± 3.3) × 10–7 + (5.5 ± 0.6) × 10–8 × RH%. The flux densities of HONO mediated by light-induced heterogeneous conversion of NO2 (46 ppb) on urban grime were enhanced by ∼1 order of magnitude from (2.3 ± 0.2) × 109 molecules cm–2 s–1 at 0% RH to (1.5 ± 0.01) × 1010 molecules cm–2 s–1 at 90% RH. This study promotes light-induced NO2 chemistry on urban grime being an important source of HONO and suggests that further experiments be performed in the future.