Atmospheric aerosol optical properties and radiative forcing over two
metros in South Africa
Abstract
Aerosol Robotic Network (AERONET) measurement data is used in the
validation of two prominent satellite aerosol property retrieval,
Multiangle Imaging Spectroradiometer (MISR) and Moderate Resolution
Imaging Spectroradiometer (MODIS), then applied to examine the
properties of aerosols and the direct influence on radiative forcing
(RF) over two metropolitan cities, Cape Town (CPT) and Pretoria (PRT) in
South Africa. The synoptic characteristics of aerosols over CPT for
2015-2019 indicate a general low aerosol optical depth (AOD) of an
average of 0.08 ± 0.014 and are prevalently sea salt (SS) aerosols. In
contrast, a high AOD value with an average of 0.23 ± 0.050 was observed
over PRT between 2011-2019 and predominated by sulphate/nitrate
aerosols. These two dominant aerosol types are found to be the primary
motivator of the net cooling effect of RF due to aerosol in each
location. While the average RF over CPT is -16.79 ± 5.61 during the
study period, the value over PRT is estimated to be more than two times
(-36.55 ± 10.54) of the former. The validation of MISR and MODIS
satellite aerosol properties retrieval for the region demonstrated
better accuracy over the land than in the maritime environment.
Meanwhile, MODIS underestimated AOD by ≈ 32% but generally reported
better precision across the board than the MISR instrument. Further
investigation into the seasonal variation of aerosols over the two
locations identified seasonality changes in the characteristics of
aerosols mainly influenced by the transport of high-absorbing
biomass-burning aerosols.