Weaker cooling by aerosols due to dust–pollution interactions

Klaus Klingmüller,Vlassis A Karydis,Georgiy L Stenchikov,Jos Lelieveld,Sara Bacer

doi:10.5194/acp-20-15285-2020

Abstract

Abstract. The interactions between aeolian dust and anthropogenic air pollution, notably chemical ageing of mineral dust and coagulation of dust and pollution particles, modify the atmospheric aerosol composition and burden. Since the aerosol particles can act as cloud condensation nuclei, this affects the radiative transfer not only directly via aerosol–radiation interactions, but also indirectly through cloud adjustments. We study both radiative effects using the global ECHAM/MESSy atmospheric chemistry-climate model (EMAC) which combines the Modular Earth Submodel System (MESSy) with the European Centre/Hamburg (ECHAM) climate model. Our simulations show that dust–pollution–cloud interactions reduce the condensed water path and hence the reflection of solar radiation. The associated climate warming outweighs the cooling that the dust–pollution interactions exert through the direct radiative effect. In total, this results in a net warming by dust–pollution interactions which moderates the negative global anthropogenic aerosol forcing at the top of the atmosphere by (0.2 ± 0.1) W m−2.

Highlights

A prime objective of current atmospheric and climate science is the deeper understanding of ambient aerosols and their interactions with clouds
We study both radiative effects using the global ECHAM/MESSy atmospheric chemistry-climate model (EMAC) which combines the Modular Earth Submodel System (MESSy) with the European Centre/Hamburg (ECHAM) climate model
To quantify the effects of the different emission set-ups and the dust–pollution interactions on radiation, we consider the effective radiative forcing (ERF), which is defined as the change in net TOA downward radiative flux after allowing for atmospheric temperatures, water vapour and clouds to adjust but with sea surface temperature (SST) and sea ice cover fixed at climatological values (IPCC, 2014)

Summary

Introduction

A prime objective of current atmospheric and climate science is the deeper understanding of ambient aerosols and their interactions with clouds. The planetary albedo can be increased both directly by interactions of the anthropogenic aerosol particles with solar radiation and indirectly by enhanced cloudiness or cloud brightness caused by aerosol particles acting as cloud condensation nuclei. These direct and indirect effects are estimated to contribute a negative effective radiative forcing (ERF) of −0.45 W m−2 each, adding up to about −0.9 W m−2 (IPCC, 2014). We use the global ECHAM/MESSy atmospheric chemistry-climate model (EMAC) which combines the Modular Earth Submodel System (MESSy) with the European Centre/Hamburg (ECHAM) climate model It includes implementations of an extensive set of relevant physical and chemical processes, including detailed parametrisations of mineral dust ageing, cloud droplet activation and ice crystal formation in cirrus and mixed-phase clouds.

Model description

Methodology

Effects on the cloud condensate

Radiative effects

Findings

Conclusions