Impact of gas dry deposition parameterization on secondary particle formation in an urban canyon

This study investigates the impact of the parameterization of dry deposition on the local gas-particle partitioning between ammonium nitrate NH₄NO₃ and its precursor gases (ammonia, NH₃ and nitric acid, HNO₃) through chemistry-coupled large-eddy simulations on the dispersion of reactive gaseous and particulate pollutant in an idealized street canyon. A key factor in the parameterization of dry deposition is the effective Henry's Law constant H^*. Three distinct characterizations of H^* are compared: two drawn from existing literature (referred to as Model 1 and Model 2) and one typical of low pH conditions (referred to as Model 3), which could be more representative of urban areas. Model 3 shows contrasting gas-particle partitioning results between NH₃, HNO₃, and NH₄NO₃ with Model 1 and Model 2. In detail, the NH₄NO₃ concentrations in the street of Model 1 and Model 2 are smaller than the background NH₄NO₃ concentration. However, Model 3 shows higher NH₄NO₃ concentration in the street than the background NH₄NO₃ concentration. This is because the dry deposition fluxes of NH₃ and HNO₃ are higher in Model 1 and Model 2 than in Model 3, resulting in less available NH₃ and HNO₃ for NH₄NO₃ formation. These findings highlight the importance of selecting an appropriate H^* characterization that is tailored to the specific environmental conditions under investigation.