Abstract:
Biomass burning has an impact on atmospheric composition as well as human health and
wellbeing. In South Africa, the biomass burning season extends from July to October and affects
the aerosol loading and tropospheric ozone concentrations which in turn impact solar ultraviolet
radiation (UVR) levels at the surface. Using ground-based observations of aerosols, tropospheric
ozone and solar UVR (as well as modelled solar UVR) we investigated the impact of aerosols
and tropospheric ozone on solar UVR in August, September, and October over Pretoria. Aerosol
optical depth (AOD) and tropospheric ozone reached a peak between September and October each
year. On clear-sky days, the average relative difference between the modelled and observed solar
Ultraviolet Index (UVI) levels (a standard indicator of surface UVR) at solar noon was 7%. Using
modelled UVR—which included and excluded the effects of aerosols and tropospheric ozone from
biomass burning—aerosols had a larger radiative effect compared to tropospheric ozone on UVI levels
during the biomass burning season. Excluding only aerosols resulted in a 10% difference between
the modelled and observed UVI, while excluding only tropospheric ozone resulted in a difference of
−2%. Further understanding of the radiative effect of aerosols and trace gases, particularly in regions
that are affected by emissions from biomass burning, is considered important for future research.
