Spatiotemporal analysis of absorbing aerosols and radiative forcing over environmentally distinct stations in East Africa during 2001-2018

Geoffrey W Khamala; John W Makokha; Richard Boiyo; Kanike Raghavendra Kumar

doi:10.1016/j.scitotenv.2022.161041

Spatiotemporal analysis of absorbing aerosols and radiative forcing over environmentally distinct stations in East Africa during 2001-2018

Sci Total Environ. 2023 Mar 15:864:161041. doi: 10.1016/j.scitotenv.2022.161041. Epub 2022 Dec 21.

Authors

Geoffrey W Khamala¹, John W Makokha², Richard Boiyo³, Kanike Raghavendra Kumar⁴

Affiliations

¹ Department of Science Technology and Engineering, Kibabii University, P.O. Box 1699-50200, Bungoma, Kenya. Electronic address: khamalawanjala@gmail.com.
² Department of Science Technology and Engineering, Kibabii University, P.O. Box 1699-50200, Bungoma, Kenya.
³ Department of Physical Sciences, Meru University of Science and Technology, P.O. Box 972-60200, Meru, Kenya; Department of Environment, Water, Energy and Natural Resources, County Government of Vihiga, Maragoli, Kenya.
⁴ Department of Engineering Physics, College of Engineering, Koneru Lakshmaiah Education Foundation, Vaddeswaram, 522 302 Guntur, Andhra Pradesh, India.

PMID: 36563754
DOI: 10.1016/j.scitotenv.2022.161041

Abstract

East Africa (EA) suffers from the inadequate characterization of atmospheric aerosols, with far-reaching consequences of its inability to quantify precisely the impacts of these particles on regional climate. The current study aimed at characterizing absorption and radiative properties of aerosols using the long-term (2001-2018) AErosol RObotic NETwork (AERONET) and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) data over three environmentally specific sites in EA. The annual mean absorption aerosol optical depth (AAOD_{440 nm}), absorption Angstrom Exponent (AAE_{440-870 nm}), total effective radius (R_Eff), and total volume concentration (μm³/μm²) revealed significant spatial heterogeneity over the domain. The study domain exhibited a significant contribution of fine-mode aerosols compared to the coarse-mode particles. The monthly variation in SSA_{440 nm} over EA explains the strength in absorption aerosols that range from moderate to strong absorbing aerosols. The aerosols exhibited significant variability over the study domain, with the dominance of absorbing fine-mode aerosols over Mbita accounting for ∼40 to ∼50 %, while weakly absorbing coarse-mode particles accounted for ∼8.2 % over Malindi. The study conclusively determined that Mbita was dominated by AAOD mainly from biomass burning in most of the months, whereas Malindi was coated with black carbon. The direct aerosol radiative forcing (DARF) retrieved from both the AERONET and MERRA-2 models showed strong cooling at the top of the atmosphere (TOA; -6 to -27 Wm^-2) and the bottom of the atmosphere (BOA, -7 to -66 Wm^-2). However, significant warming was noticed within the atmosphere (ATM; +14 to +76 Wm^-2), an indication of the role of aerosols in regional climate change. The study contributed to understanding aerosol absorption and radiative characteristics over EA and can form the basis of other related studies over the domain and beyond.

Keywords: AERONET; Absorption aerosols; East Africa; Radiative forcing; Single scattering albedo; Volume size distribution.