Global Perspectives on Nitrate Aerosol Dynamics: A Comprehensive Sensitivity Analysis

Abstract. In recent years, nitrate aerosols have emerged as a dominant component of atmospheric composition, surpassing sulfate aerosols in both concentration and climatic impact. However, accurately simulating nitrate aerosols remains a significant challenge for global atmospheric models due to the complexity of their formation and regional variability. This study investigates key factors influencing nitrate aerosol formation to improve simulation accuracy in highly polluted regions. Using the advanced EMAC climate and chemistry model, we assess the effects of grid resolution, emission inventories, and thermodynamic, chemical, and aerosol scavenging processes. The ISORROPIA II thermodynamic model is employed to simulate the formation of inorganic aerosols. Model predictions are compared with surface observations of particulate nitrate in PM₁ and PM_2.5 size fractions, including PM_2.5 data from filter-based observational networks and PM₁ data from aerosol mass spectrometer field campaigns across Europe, North America, East Asia, and India. Results show that the model overestimates PM_2.5 nitrate concentrations, especially in East Asia, with biases up to a factor of three. Increasing grid resolution, adjusting N₂O₅ hydrolysis uptake coefficient, and utilizing an appropriate emission database (e.g., CMIP6) improve performance. However, these adjustments do not necessarily enhance PM₁ predictions, which remain underestimated, especially in urban downwind sites. Seasonal variations and diurnal trends reveal discrepancies in model performance, especially in Europe and urban downwind locations. In Europe, model bias is driven by an unrealistically sharp decrease in nitrate aerosol levels from morning maxima to evening minima. Sensitivity tests show relatively small impact on total tropospheric nitrate burden, with variations within 25 %.