Observationally Constrained Analysis on the Distribution of Fine and Coarse Mode Nitrate in Global Climate Models

Abstract. Nitrate plays an important role in the Earth’s climate and air quality. A key challenge in simulating the lifecycle of nitrate aerosol in global climate models is to accurately represent mass size distribution of nitrate aerosol. In this study, we evaluate the performance of the Energy Exascale Earth System Model version 2 (E3SMv2) and the Community Earth System Model version 2 (CESM2), along with Aerosol Comparisons between Observations and Models (AeroCom) phase III models, in simulating spatial distribution of fine-mode nitrate, mass size distribution of fine- and coarse-mode nitrate, and the gas-aerosol partitioning between nitric acid gas and nitrate, using long-term ground-based observations and measurements from multiple aircraft campaigns. We find that most models underestimate the annual mean PM_2.5 (particulate matter with diameter less than 2.5 μm) nitrate surface concentration averaged over all sites. The observed nitrate PM_2.5/PM₁₀ and PM₁/PM₄ ratios are influenced by the relative contribution of fine sulfate/organic particles and coarse dust/sea salt particles. Overall, the ground-based observations give an annual mean surface nitrate PM_2.5/PM₁₀ ratio of 0.7. Most models underestimate the annual mean PM_2.5/PM₁₀ ratio in all regions. There are large spreads in the modeled nitrate PM₁/PM₄ ratios, which span the full range from 0 to 1. Most models underestimate the surface molar ratio of nitrate to total inorganic nitrate averaged across all sites. Our study indicates the importance of gas-aerosol partition parameterization and simulation of dust and sea salt in correctly simulating mass size distribution of nitrate.