PNNL

Abstract

Atmospheric carbonaceous aerosols play an important role in the climate system by influencing the Earth’s radiation budgets and modifying the cloud properties. Despite the importance, their representations in large-scale atmospheric models are still crude, which can influence model simulated burden, lifetime, physical, chemical and optical properties, and the climate forcing of carbonaceous aerosols. In this study, we improve the current 3-mode version of modal aerosol module (MAM3) in the Community Atmosphere Model version 5 (CAM5) by introducing a primary carbon mode to explicitly account for the aging process of primary carbonaceous aerosols in the atmosphere. Compared to MAM3, the 4-mode version of MAM (MAM4) significantly increases the column burdens of primary organic matter (POM) and black carbon (BC) by up to 40% in many remote regions away from the sources, where the in-cloud scavenging plays an important role in determining the aerosol concentrations. Changes of the column burdens for other types of aerosol (e.g., sulfate, secondary organic aerosols, mineral dust, sea salt) are less than 1%. We evaluated the MAM4 simulation against in situ surface and aircraft observations. MAM4 significantly improves the model simulation of seasonal variation of BC concentrations in the polar regions, by increasing the BC concentrations in all seasons and particularly in cold seasons. However, it exacerbates the overestimation of modeled BC concentrations in the upper troposphere in the Pacific regions. Future improvements of the representation of aerosol processes related to (1) in-cloud scavenging and vertical transport in convective clouds and (2) emissions of anthropogenic and biomass burning aerosols are needed to further reduce the model biases of POM and BC aerosols.