Biomass burning aerosols in most climate models are too absorbing

Hunter Brown,Ben Johnson,Zheng Lu,Tommi Bergman,Rudra Pokhrel,Mıchael Schulz ,Gunnar Myhre,Xiaohong Liu,Johan P Beukes ,Pieter G Van Zyl ,Philip Stier,Shang Liu,Ville Vakkari,Ragnhild Bieltvedt Skeie ,Rawad Saleh,Duncan Watson-Parris ,Harri Kokkola,Tero Mielonen,Nicolas Bellouin,S M Murphy ,D Chand

doi:10.1038/s41467-020-20482-9

Abstract

Uncertainty in the representation of biomass burning (BB) aerosol composition and optical properties in climate models contributes to a range in modeled aerosol effects on incoming solar radiation. Depending on the model, the top-of-the-atmosphere BB aerosol effect can range from cooling to warming. By relating aerosol absorption relative to extinction and carbonaceous aerosol composition from 12 observational datasets to nine state-of-the-art Earth system models/chemical transport models, we identify varying degrees of overestimation in BB aerosol absorptivity by these models. Modifications to BB aerosol refractive index, size, and mixing state improve the Community Atmosphere Model version 5 (CAM5) agreement with observations, leading to a global change in BB direct radiative effect of −0.07 W m−2, and regional changes of −2 W m−2 (Africa) and −0.5 W m−2 (South America/Temperate). Our findings suggest that current modeled BB contributes less to warming than previously thought, largely due to treatments of aerosol mixing state.

Highlights

Uncertainty in the representation of biomass burning (BB) aerosol composition and optical properties in climate models contributes to a range in modeled aerosol effects on incoming solar radiation
While this analysis focused on Community Atmosphere Model version 5 (CAM5).4, all of the models in this study showed some degree of single scattering albedo (SSA) underestimation compared to observations
Mixed treatments produce up to twice the absorption enhancement when compared to the varying mixing states of particles in a composition resolving aerosol model[33], and core–shell mixing can overestimate absorption by ~30% when compared to non-spherical aerosols with a non-uniform composition[19]

Summary

Introduction

Uncertainty in the representation of biomass burning (BB) aerosol composition and optical properties in climate models contributes to a range in modeled aerosol effects on incoming solar radiation. Large uncertainties in the representation of biomass burning (BB) aerosols in Earth system models (ESM) and chemical transport models (CTM)[1,2] increase the range in their simulated climate impact[3] Reducing this range through improvements of aerosol emissions, atmospheric processes, and microphysical and optical properties can elucidate the effect of BB aerosols on climate[4,5,6], human health[7,8], as well as their role in the carbon cycle[9]. We approach the validation of these aerosol microphysical and optical properties through BB observations of single scattering albedo (SSA) versus BC-to-total-carbon (BC + OC) ratio (BC:TC)

Methods

Results

Conclusion