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Analysis of Radiative Properties and Direct Radiative Forcing Estimates of Dominant Aerosol Clusters over an Urban-desert Region in West Africa

Category: Optical/Radiative Properties and Remote Sensing

Article In Press
DOI: 10.4209/aaqr.2017.12.0600
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Olusegun G. Fawole 1,2, Xiaoming Cai2, Rachel T. Pinker4, A. Robert MacKenzie2,3

  • 1 Department of Physics and Engineering Physics, Obafemi Awolowo University, Ile-Ife 220005, Nigeria
  • 2 School of Geography, Earth and Environmental Sciences, University of Birmingham, B15 2TT, UK
  • 3 Birmingham Institute of Forest Research (BIFoR), University of Birmingham, B15 2TT, UK
  • 4 Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20742, USA

Highlights

In-depth analysis of radiative properties of aerosols in an urban-desert region.
Estimates of the radiative forcing abilities of dominant aerosol classes.
Comparison of the radiative forcing abilities of the dominant aerosol classes.
Estimate of the DRF and forcing ability of aerosols of gas flaring origin.


Abstract

The strategic location of the AERONET site in Ilorin, Nigeria, makes it possible to obtain information on several aerosol types and their radiative effects. The strong reversal of wind direction occasioned by the movement of the ITCZ during the West Africa Monsoon (WAM) plays a major role in the variability of aerosol nature at this site, which is confirmed by aerosol optical depth (AOD) (675 nm) and Ångström exponent (AE) (440–870 nm) values with 1st and 99th percentile values of 0.08 and 2.16, and 0.11 and 1.47, respectively. The direct radiative forcing (DRF) and radiative forcing efficiency (RFE) of aerosol, as retrieved from the AERONET sun-photometer measurements, are estimated using radiative transfer calculations for the periods of 2005–2009 and 2011–2015. The DRF and RFE of the dominant aerosol classes—desert dust (DD), biomass burning (BB), urban (UB) and gas flaring (GF)—have been estimated. The median (± standard deviation) values of the DRF at the top of the atmosphere (TOA) for the DD, BB, UB and GF aerosol classes are –27.5 ± 13.2 Wm–2, –27.1 ± 8.3 Wm–2, –11.5 ± 13.2 Wm–2 and –9.6 ± 8.0 Wm–2, respectively, while those of the RFE are –26.2 ± 4.1 Wm–2 δ–1, –35.2 ± 4.6 Wm–2 δ–1, –31.0 ± 8.4 Wm–2 δ–1 and –37.0 ± 10.3 Wm–2 δ–1, respectively. Arguably due to its high SSA and assymetric values, the DD aerosol class shows the largest DRF but the smallest RFE. Its smallest AOD notwithstanding, the GF class can cause greater perturbation of the earth-atmosphere system in the sub-region both directly and indirectly, possibly due to the presence of black carbon and other co-emitted aerosol and the ageing of the GF aerosols. This study presents the first estimate of DRF for aerosols of gas flaring origin and shows that its radiative potential can be similar in magnitude to that of biomass burning and urban aerosol in West Africa.

Keywords

Gas flaring, West African Monsoon Direct radiative forcing Radiative forcing efficiency Assymetric parameter


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