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. Display omitted •Uptakes of NO2 on urban grime increase non-linearly with the RH upon UV irradiation.•Surface adsorbed water plays an important role for uptakes of NO2 on urban grime.•Uptakes of NO2 are well correlated with the chemical composition of urban grime.•Unprecedented HONO surface flux is formed by two-month old grime.•N-containing organic compounds are released in the gas phase of the atmosphere.