Global Radiative Impacts of Black Carbon Acting as Ice Nucleating Particles.

Zachary Mcgraw,Camilla Weum Stjern,Bjørn Hallvard Samset,Trude Storelvmo

doi:10.1029/2020gl089056

Zachary Mcgraw, Camilla Weum Stjern + Show 2 more

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https://doi.org/10.1029/2020gl089056

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Abstract

Black carbon (BC) aerosols from incomplete combustion generally warm the climate, but the magnitudes of their various interactions with climate are still uncertain. A key knowledge gap is their role as ice nucleating particles (INPs), enabling ice formation in clouds. Here we assess the global radiative impacts of BC acting as INPs, using simulations with the Community Earth System Model 2 climate model updated to include new laboratory‐based ice nucleation parameterizations. Overall, we find a moderate cooling through changes to stratiform cirrus clouds, counteracting the well‐known net warming from BC's direct scattering and absorption of radiation. Our best estimates indicate that BC INPs generally thin cirrus by indirectly inhibiting the freezing of solution aerosol, with a global net radiative impact of −0.13 ± 0.07 W/m2. Sensitivity tests of BC amounts and ice nucleating efficiencies, and uncertainties in the environment where ice crystals form, show a potential range of impacts from −0.30 to +0.02 W/m2.

Highlights

Black carbon (BC) influences Earth's climate through a variety of mechanisms (Bond et al, 2013; Peng et al, 2016), many of which are still poorly constrained
Our core result is an estimate of the current effective radiative forcings (ERFs) due to BC particles acting as ice nucleating particles (INPs)
Impacts of BC INPs in mixed‐phase clouds and deep convective cores were not found to be significant, and we focus on stratiform cirrus for the remainder of this letter

Summary

Introduction

Black carbon (BC) influences Earth's climate through a variety of mechanisms (Bond et al, 2013; Peng et al, 2016), many of which are still poorly constrained. There are still considerable model differences, linked partly to the low confidence in assessments of BC's diverse roles in clouds (Bond et al, 2013; Stjern et al, 2017). One such mechanism is BC's ability to act as surfaces where ice crystals form at relatively low saturations (ice nucleating particles [INPs]). The impact of BC INPs depends on the number of BC particles at ice cloud altitudes, their ice nucleating efficiency, and their influence on processes that compete for water vapor. INP changes may either strengthen or weaken ice cloud warming effects depending on the balance between LW and SW impacts

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