How Does the Representation of Convection, Clouds, and Precipitation Influence Future Projections of PM2.5 in 21st Century Earth System Model Simulations?

Abstract:

As the planet warms in response to increases in greenhouse gas emissions, changes in the hydrological cycle (e.g., the characteristics of atmospheric humidity, clouds, and precipitation) can impact aerosol formation and removal processes in ways that may affect future air quality, even if aerosol emissions were to remain at present-day values. Aerosol (and precursor gas) emissions from biomass and fossil fuel burning impact air quality by injecting (and forming) fine particulate matter (PM2.5) into the atmosphere, which is hazardous to human health. These particles are subject to removal by dry and wet deposition, the latter of which is linked to precipitation frequency and amount. By fixing aerosol and precursor gas emissions at present-day levels in the Community Earth System Model (CESM), but increasing greenhouse gases through the 21st century, an overall increase in PM2.5 (particularly sulfate) is found to be associated largely with precipitation changes. A decrease in wet day frequency (5% globally) contributes to increases in the concentrations of black carbon, primary organic matter, and sulfate at the surface. The spatial patterns of these changes are strongly dependent on aerosol species, and sulfate is further influenced by the changing patterns of aqueous and gas-phase production. Thus, the representation of convection, clouds, and precipitation in CESM can have a large influence on these effects. Using CESM with conventional and superparameterization (i.e., convective parameterizations are replaced by high-resolution cloud resolving models), we investigate how the representation of convection impacts the changes in the hydrological cycle, and therefore air quality under a 21^st century scenario of continued reliance on fossil fuels (i.e., CMIP6 SSP5-8.5). We identify regions of poor air quality, which are more vulnerable to climate change and show increase in hazardous air quality days (>25 μg/m³ for a 24-hour period).