Atmospheric aerosol distributions and their climatic effects over South Africa using remote sensing observations and regional climate model

Abstract

Atmospheric aerosols are small solid and liquid particles suspended in the Earth's atmosphere which originate from anthropogenic and natural activities. Unlike greenhouse gases, aerosol particles are relatively short-lived in the atmosphere and exhibit multidimensional heterogeneity with respect to their composition, size, sources, mixing state and the spatio-temporal distributions. The concentration and climatic influences of atmospheric aerosols are much higher closer to their source regions. Therefore, to have a better understanding of the role of aerosols, their distribution and climatic impacts must be understood and quantified on a regional scale rather than on a global‐average basis. There are multiple sources of aerosols/precursor gases in South Africa (SA) which build a complex mixture of atmospheric particulates. This contribution presents a detailed study of aerosol climatology over SA as well as examines the direct radiative and semi-direct climatic effects of individual/total aerosol particles based on their sources. The climatological study has shown that, in terms of aerosol load spatial variation, SA can be classified into three parts: the upper, central, and lower part; which corresponds to high, medium and low aerosol loads. The seasonal variation of aerosol optical signatures shows that the prevailing sources of aerosols are different in each part of SA. The lower part is dominated by particles that are induced from the air mass transport from the surrounding marine environment and other SA/neighbouring regions. The central and upper parts of SA are primarily loaded by windblown mineral dust particles and aerosols that result from anthropogenic/biomass burning activities. Following the aerosol climatological study, using the 12 year (1997 - 2008) runs of the Regional Climate Model (RegCM4), the mass distribution, radiative influences and semi-direct climatic effects of wind-eroded desert dust particles, different species of aerosols that are induced from anthropogenic and biomass burning activities over SA are examined. Investigating the influence of aerosols, based on their sources, is essential to improve the scientific understanding about the two-way interaction and feedback among various species of aerosols, radiation and different climatic variables. This is also important to distinguish the climatic signals of anthropogenic aerosols from that of natural aerosols as well as to devise climate change mitigation strategies. Before employing RegCM4 for these purposes, the model’s performance in reproducing the major observational features of aerosol optical fields over SA was evaluated. Among various semi-direct climatic influences of aerosols, this study examined their effects on: surface temperature, surface sensible heat flux, net atmospheric radiative heating rate, hydrological variables (in terms of cloud cover and cloud liquid water path), boundary layer, surface pressure and surface wind fields. The study also assessed the dependency of aerosols’ semi-direct effects on seasonal variation of meteorological parameters as well as its reliance on atmospheric aerosol distributions and properties. Overall, the semi-direct effect assessments delivered not only an important contribution towards the understanding of the interaction and feedback between different types of aerosols-radiation-climate (at a regional level), but also offered insightful information about the mutual interrelationships among different climatic feedbacks. Among different aerosol species in SA, this study critically underscores that the wind-eroded desert dust particles have a dominant climatic signal in SA. Therefore, wind-eroded desert dust particles are of high importance and need to be incorporated in climate change studies over South Africa. Additionally, the dominance of dust particle climatic signals perhaps requires some attention from governmental or non-governmental environmental organizations which are working in and around South Africa: at least in terms of making some strategic plans on how to reduce the dust production and dispersion.