Source attribution of cloud condensation nuclei and their impact on stratocumulus clouds and radiation in the south-eastern Atlantic

Abstract

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.

Cloud condensation nuclei concentrations derived from the CAMS reanalysis

“…This provides another reason to exclude dust from CCN in this study. Furthermore, in a modelling study by Che et al (2022), where wetting and coating of insoluble dust with soluble material is enabled by internal mixing, they conclude that dust does not have a notable impact on CCN. Therefore, we assume for now that DU has minor contributions to overall CCN due to its large size (and thus high mass but low number concentrations) and its low hygroscopicity.…”

Aerosol‐Correlated Cloud Activation for Clean Conditions in the Tropical Atlantic Boundary Layer During LASIC

“…The average N ss,Dcrit / N CCN at <0.3% supersaturation in November‐May and June–October was 6 ± 3%. This result supports similar observations of minor sea‐spray contributions to CCN in marine boundary layers (Modini et al., 2015; Quinn et al., 2017, 2019; Russell et al., 2023; Sanchez et al., 2021; Wex et al., 2016; Zheng et al., 2018), in agreement with the modeled contribution of sea salt to tropical Atlantic N CCN (Che, Stier, et al., 2022). N ss,Dcrit / N CCN was largest during clean conditions of June–October with an average value of 10 ± 5% and maximum of 50% at 0.1% supersaturation.…”

Impacts of an aerosol layer on a midlatitude continental system of cumulus clouds: how do these impacts depend on the vertical location of the aerosol layer?

“…This interest is motivated by aerosol layers that originate from biomass burning sites in southern Africa (Mari et al, 2008;Menut et al, 2018;Haslett et al, 2019;Denjean et al, 2020). These layers are lifted and transported to the southeastern Atlantic (SEA) region and S. S. Lee et al: Impacts of an aerosol layer on a midlatitude continental system of cumulus clouds located above or around the top of a large layer or deck of warm cumulus and stratocumulus clouds (Roberts et al, 2005;van der Werf et al, 2010;Che et al, 2022). Note that aerosols in the transported aerosol layers contain organic and black carbon, and these aerosols act as radiation absorbers as well as CCN (Wilcox, 2010;Deaconu et al, 2019;Chaboureau et al, 2022).…”