Present-Day PM_2.5 over Asia: Simulation and Uncertainty in CMIP6 ESMs

This study assesses the ability of 10 Earth System Models (ESMs) that participated in the Coupled Model Intercomparison Project Phase 6 (CMIP6) to reproduce the present-day inhalable particles with diameters less than 2.5 micrometers (PM_2.5) over Asia and discusses the uncertainty. PM_2.5 accounts for more than 30% of the surface total aerosol (fine and coarse) concentration over Asia, except for central Asia. The simulated spatial distributions of PM_2.5 and its components, averaged from 2005 to 2020, are consistent with the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2) reanalysis. They are characterized by the high PM_2.5 concentrations in eastern China and northern India where anthropogenic components such as sulfate and organic aerosol dominate, and in northwestern China where the mineral dust in PM_2.5 fine particles (PM_2.5DU) dominates. The present-day multi-model mean (MME) PM_2.5 concentrations slightly underestimate ground-based observations in the same period of 2014–2019, although observations are affected by the limited coverage of observation sites and the urban areas. Those model biases partly come from other aerosols (such as nitrate and ammonium) not involved in our analyses, and also are contributed by large uncertainty in PM_2.5 simulations on local scale among ESMs. The model uncertainties over East Asia are mainly attributed to sulfate and PM_2.5DU; over South Asia, they are attributed to sulfate, organic aerosol, and PM_2.5DU; over Southeast Asia, they are attributed to sea salt in PM_2.5 fine particles (PM_2.5SS); and over central Asia, they are attributed to PM_2.5DU. They are mainly caused by the different representations of aerosols within individual ESMs including the representation of aerosol size distributions, dynamic transport, and physi-cal and chemistry mechanisms.