PNNL

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 (HNO3) and nitrate, using long-term ground-based observations and measurements from multiple aircraft campaigns. We find that both E3SMv2 and CESM2 overestimate the annual mean PM2.5 (particulate matter with diameter less than 2.5 ?m) nitrate surface concentration averaged over all sites, whereas the AeroCom models underestimate it. The observed nitrate PM2.5/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 PM2.5/PM10 ratio of 0.7. Most models underestimate the annual mean PM2.5/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. The ground-based observations give an annual mean surface (NO_3^-)/((NO_3^-+HNO_3)) molar ratio of 0.52. Most models underestimate the surface molar ratio 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.