Simulation of nitrous acid formation taking into account heterogeneous pathways: application to the Milan metropolitan area

Nitrous acid (HONO) is of particular importance in atmospheric chemistry; its photolysis significantly enhances photooxidation processes early in the morning or during wintertime, through the rapid production of OH radicals in the presence of long wavelength radiation, and in urban areas with low O3 levels. Previous studies reveal that neither direct HONO emissions from combustion sources nor homogeneous gas-phase reactions are sufficient to explain the observed HONO concentrations. Therefore, the role of heterogeneous production pathways is analysed in this work by means of the multilayer photochemical model MUSE, where a parameterization of heterogeneous NOy chemistry has been implemented. MUSE was then applied to simulate pollutant dispersion and transformation over the Milan metropolitan area on 9 March 1994, a day in the 1994 Milan field measurement campaign characterized by high pollutant concentrations. Simulation results for the concentrations of O3 and NOx are in good agreement with the observations. During most of the time, the predicted HONO concentration levels are distinctly lower than the measured levels. The results are discussed and perspectives for further work are provided.