The Effect of Absorptive Aerosol Layers on Atmospheric Dynamics

Black Carbon (BC) is estimated to be the second largest anthropogenic driver of climate warming, after carbon dioxide. While past research has examined the radiative effects in the atmosphere due to BC, few studies have explored climate system response due to heating at different heights in the atmosphere from these absorptive aerosols. This research examines the dynamical and climatic impacts associated with vertical location of absorptive aerosols in the atmosphere.

The Community Earth System Model (CESM) with a slab ocean model was implemented with aerosol absorption optical depth (AAOD) prescribed in the midlatitudes to represent the presence of absorptive aerosols. Three scenarios — one with the prescribed AAOD located in the mid-troposphere, one with absorption located in the low troposphere, and a control with radiative forcings caused by absorptive aerosols zeroed — were executed out to 30 years. Comparison of these simulations (utilizing the last 10 years of the scenarios) enables analysis of the climate state after it has reached equilibrium due to aerosol absorption in different levels in the atmosphere.

While many significant direct radiative forcing effects were present in each scenario, there were major differences between the two prescribed scenarios. Principal findings included statistically significant alterations of low and high cloud fractions for the mid-tropospheric scenario as well as significant convective cloud fraction differences for both scenarios. Sea ice fraction fluctuations were also prevalent, especially in the low-tropospheric scenario. An increase in rainfall, totaling one to two feet per year, occurred in Southeast Asia for both scenarios, revealing implications on the region's monsoon season. Furthermore, stratospheric and southern hemispheric responses were observed to offset those in the northern hemisphere by cooling and/or dynamical weakening. Additionally, the global average surface temperature increased nearly half a degree Celsius more for the low-tropospheric scenario than the mid-tropospheric scenario, despite smaller top-of-atmosphere forcing. These findings, and many others, help in understanding the influence of absorptive aerosols, as they are prevalent constituents in our atmosphere.