Volatility-Based Aging of Biomass Burning Organic Aerosol in the Unified Model

Organic aerosol (OA) often accounts for over half of ambient aerosol mass, especially in biomass burning aerosol (BBA), which affects climate through direct (scattering/absorption) and indirect (cloud interactions) pathways. Despite advances in OA modeling techniques over the last two decades, general circulation models (GCMs) still tend to oversimplify OA formation processes.

Model treatment of OA drastically affects the mass burden and lifetime of OA in the atmosphere, which in turn impact the radiative effects of OA. While chemical transport models (CTMs) typically follow the instantaneous equilibrium model with advanced OA chemistry and numerous species, GCMs typically include fewer OA species and represent fewer chemical processes.

We apply simple versions of the volatility basis set (VBS) method commonly used in air quality models to the Unified climate model (UM) and examine its effects on BBA against the 2018 WECAN campaign in the western US. We evaluate multiple volatility modeling approaches to determine the simplest configuration that still has good performance. One possibility is the technique discussed in Lou et al. 2020 for the E3SM model, which assumes kinetically-limited condensation but allows both primary and secondary OA to condense/evaporate based on saturation vapor pressure. We argue that relatively simple approaches can improve model representation of OA without introducing a myriad of unconstrained parameters.