Physical and chemical characterization of urban grime: An impact on the NO2 uptake coefficients and N-containing product compounds

Abstract

Urban grime represents an important environmental surface for heterogeneous reactions in urban environment. Here, we assess the physical and chemical properties of urban grime collected during six consecutive months in downtown of Guangzhou, China. There is a significant variation of the uptake coefficients of NO₂ on the urban grime as a function of the relative humidity (RH). In absence of water molecules (0% RH), the light-induced uptake coefficients of NO₂ on urban grime samples collected during six months are very similar in order of ≈10^-6. At 80% RH, depending on the sampling month the light-induced uptake coefficient of NO₂ can reach one order of magnitude higher values (1.5 × 10^-5, at 80% RH) compared to those uptakes at 0% RH. In presence of 80% RH, there are strong correlations between the measured NO₂ uptakes and the concentrations of the water soluble carbon, soluble anions, polycyclic aromatic hydrocarbons and n-alkanes depicted in the urban grime. These correlations, demonstrate that surface adsorbed water on urban grime play an important role for the uptakes of NO₂. The heterogeneous conversion of NO₂ on two-month old urban grime under sunlight irradiation (68 W m^-2, 300 nm < λ < 400 nm) at 60% RH leads to the formation of unprecedented HONO surface flux of 4.7 × 10¹⁰ molecules cm^-2 s^-1 which is higher than all previously observed HONO fluxes, thereby affecting the oxidation capacity of the urban atmosphere. During the heterogeneous chemistry of NO₂ with urban grime, the unsaturated and N-containing organic compounds are released in the gas phase which can affect the air quality in the urban environment.

Keywords: HONO; Heterogeneous reactions; Nitrogen oxides; Oxidation; Photochemistry; Urban grime.