Source attribution of cloud condensation nuclei and their impact on stratocumulus clouds and radiation in the south-eastern Atlantic
Authors: Che Haochi , Stier Philip , Watson-Parris Duncan , Gordon Hamish , Deaconu Lucia 
Published: August 2022

Abstract. The semi-permanent stratocumulus clouds over the south-eastern
Atlantic Ocean (SEA) can act as an “air conditioner” to the regional and
global climate system. The interaction of aerosols and clouds becomes
important in this region and can lead to negative radiative effects,
partially offsetting the positive radiative forcing of greenhouse gases. A
key pathway by which aerosols affect cloud properties is by acting as cloud
condensation nuclei (CCN). In this paper, we use the United Kingdom Earth
System Model (UKESM1) to investigate the sources of CCN (from emissions and
atmospheric processes) in the SEA as well as the response of the cloud droplet number
concentration (CDNC), the cloud liquid water path (LWP), and radiative forcing
to these sources during 2016 and 2017. Overall, free and upper troposphere
nucleated aerosols are the dominant source of the boundary layer CCN concentration at 0.2 % supersaturation (CCN0.2 %),
contributing an annual average of ∼ 41 % as they subside
and entrain into the marine boundary layer, which is consistent with
observations highlighting the important role of nucleation in the boundary
layer CCN concentration. In terms of emission sources, anthropogenic emissions (from
energy, industry, agriculture, etc.) contribute the most to the annual
average CCN0.2 % in the marine boundary layer (∼ 26 %), followed by biomass burning (BB, ∼ 17 %). In the
cloud layer, BB contributes about 34 % of the annual CCN0.2 %, midway
between the contributions from aerosol nucleation (36 %) and anthropogenic
sources (31 %). The contribution of aerosols from different sources to the
CDNC is consistent with their contribution to CCN0.2 % within the
marine boundary layer, with free and upper troposphere aerosol nucleation
being the most important source of the CDNC overall. In terms of emission
sources, anthropogenic sources are also the largest contributors to the
annual average CDNC, closely followed by BB. However, during the BB
season, BB and free and upper troposphere aerosol nucleation are equally the
most important sources of the CDNC. The contribution of BB to the CDNC is more
significant than its increase to CCN0.2 %, mainly because BB aerosols
are mostly located directly above the inversion layer in the model; thus, they can
increase the in-cloud CDNC by enhancing the supersaturation through the
dynamical feedback due to short-wave absorption. An aerosol source that
shows an increase in the CDNC also shows an increase in the LWP resulting from a
reduction in autoconversion. Due to the absorption effect, BB aerosol can
enhance existing temperature inversions and reduce the entrainment of
sub-saturated air, leading to a further increase in the LWP. As a result, the
contribution of BB to the LWP is second only to aerosol nucleation on annual
averages. These findings demonstrate that BB is not the dominant source of
CCN within the marine boundary layer from an emission source perspective.
However, as most BB aerosols are located directly above the inversion layer,
their effect on clouds increases due to their absorption effect (about the
same as anthropogenic sources for the CDNC and more than anthropogenic sources
for the LWP), highlighting the crucial role of their radiative effect on clouds.
The results on the radiative effects of aerosols show that BB aerosol
exhibits an overall positive RFari (radiative forcing associated with
aerosol–radiation interactions), but its net effective radiative forcing
remains negative due to its effect on clouds (mainly due to its absorbing effect).
By quantifying aerosol and cloud properties affected by different sources,
this paper provides a framework for understanding the effects of aerosol
sources on marine stratocumulus clouds and radiation in the SEA.