Photolysis of nitrous acid (HONO) is the most important primary source of hydroxyl radicals (OH) in the air of large urban agglomerates such as Guangzhou. Therefore, the accurate quantification of HONO is of paramount importance regarding the OH concentrations and thus the oxidation capacity of the urban atmosphere.
Recent observations pointed out that neither a photostationary state model (PSS) nor a more complete photochemical model (Master chemical mechanism (MCM)) containing currently known and postulated sources of HONO (relevant for urban environment) can reproduce the daytime HONO levels measured in dense urban area; hence, any missing HONO source in models can lead to an underestimation of the oxidative capacity of the atmosphere.
To shed new insights into the chemistry of this complex system, here for the first time we suggest that light-induced heterogeneous NO2 processing on authentic urban grime collected in downtown Guangzhou, can be an important additional source of HONO in large agglomerates where most of the population live. The obtained results indicate that the effective uptake coefficients (γ) of NO2 (46 ppb) on urban grime in presence of UV light ((2.6 · 1015 photons cm-2 s-1 (300nm<λ<400nm)) increased markedly from (1.1±0.2) · 10-6 at 0 % RH to (5.8±0.7) · 10-6 at 90 % RH, exhibiting the following linear correlation with the 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 about one 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. These results indicate that the level of HONO formed by this “new” source is one order of magnitude higher during “wet” season compared to that of “dry” season in Guangzhou.
This study promotes the light-induced NO2 chemistry on urban grime as an important source of HONO and suggests further experiments to be performed in the future.
The effective uptake coefficients from this study can be integrated in the photochemical models to account for the unknown daytime HONO sources in an urban area. For example, 1D framework based on regional and 3D chemical transport models (CTMs) or The Weather Research and Forecasting/Chemistry (WRF-Chem) model can consider the effective uptake coefficients from this study and account the additional HONO formation in Guangzhou.
Find the Just Accepted ES&T Letters paper at: https://pubs.acs.org/toc/estlcu/0/ja