The impacts of aerosol mixing state on heterogeneous N₂O₅ hydrolysis

Dinitrogen pentoxide (N₂O₅) is an important nighttime reservoir for nitrogen oxides (NO_x). Heterogeneous hydrolysis on aerosol particles is a major loss pathway for N₂O₅, removing NO_x from the atmosphere. Current models use the bulk composition of the particle population to calculate the N₂O₅ reaction probability ((Formula presented.)). While this is appropriate when the aerosol is internally mixed, it remains an open question of how large the error is when the aerosol has a more complex mixing state, which is common in the real atmosphere. To better understand the role of the mixing state in calculating (Formula presented.) we used the stochastic particle-resolved model PartMC-MOSAIC to generate thousands of populations covering a wide range of mixing states. Our scenario library focused on submicron aerosols in urban environments, specifically examining mixtures of carbonaceous primary aerosols, secondary organic aerosols, and the inorganic species sulfate, nitrate, and ammonium. We excluded primary emissions of mineral dust or sea salt aerosols and focused on the processes in the residual layer during nighttime. To evaluate (Formula presented.) we compared three commonly used parameterizations: Riemer et al. (R09), Davis et al. (D08), and Bertram and Thornton (BT09). The results were qualitatively similar and highlighted that the internal-mixing assumption could lead to either over- or underestimation of (Formula presented.) The largest range of relative errors was obtained for the BT09 parameterization, ranging from −100% to 57%. We present a detailed process analysis that explains the reasons for the observed differences. Errors of this magnitude in reaction probability result in errors in the prediction of N₂O₅ in the range of (Formula presented.) 10% and of total nitrate in the range of (Formula presented.) 3%. The errors in ozone concentration were negligible, at less than 1%.