Measurement of heterogeneous uptake of NO on inorganic particles, sea water and urban grime.

Heterogeneous reactions of NO on different surfaces play an important role in atmospheric NO removal and HONO formation, having profound impacts on photochemistry in polluted urban areas. Previous studies have suggested that the NO uptake on the ground or aerosol surfaces could be a dominant source for elevated HONO during the daytime. However, the uptake behavior of NO varies with different surfaces, and different uptake coefficients were used or derived in different studies. To obtain a more holistic picture of heterogeneous NO uptake on different surfaces, a series of laboratory experiments using different flow tube reactors was conducted, and the NO uptake coefficients (γ) were determined on inorganic particles, sea water and urban grime. The results showed that heterogeneous reactions on those surfaces were generally weak in dark conditions, with the measured γ varied from <10 to 3.2 × 10 under different humidity. A photo-enhanced uptake of NO on urban grime was observed, with the obvious formation of HONO and NO from the heterogeneous reaction. The photo-enhanced γ was measured to be 1.9 × 10 at 5% relative humidity (RH) and 5.8 × 10 at 70% RH on urban grime, showing a positive RH dependence for both NO uptake and HONO formation. The results demonstrate an important role of urban grime in the daytime NO-to-HONO conversion, and could be helpful to explain the unknown daytime HONO source in the polluted urban area.