Spatial and temporal assessment of atmospheric OC and BC concentrations at South African debits sites

Abstract

The baseline of uncertainty in aerosol radiative forcing is large and depends on aerosol characteristics (e.g. size and composition), which can vary significantly on a regional scale. Sources (natural, anthropogenic and secondary) can be directly linked to the aerosol characteristics of a region, making monitoring campaigns to determine background aerosol composition in different regions very important. Up to 2005, Deposition of Biogeochemically Important Trace Species (DEBITS)-International Global Atmospheric Chemistry (IGAC) DEBITS in Africa (IDAF) project activities in southern Africa did not include aerosol measurements. Campaigns to initiate aerosol monitoring were launched during the 2005 to 2007 period. Additionally OC/BC measurements for PM2.5 and PM10 fractions were started in 2009. OC and BC measured in SA were explored in terms of spatial and temporal patterns, mass fractions of the total aerosol mass, as well as possible sources. PM10 and PM2.5 samples were collected at five sites in SA operated within the DEBITS-IDAF network. OC were higher than BC concentrations at all sites in both size fractions. OC/BC ratios reflected the location of the different sites, as well as possible sources impacting these sites. The OC and BC mass fraction percentages of the total aerosol mass varied up to 24% and 12%, respectively. A relatively well define seasonal pattern was observed, with higher OC and BC measured from May to October, which coincide with the dry season in the interior of SA. An inverse seasonal pattern was observed for the fractional mass contributions of OC and BC to the total aerosol mass, which indicates substantially higher aerosol load during this time of the year. Biomass burning fires contribute significantly to regional OC and BC during the burning season, while large point sources did not contribute that significantly to regional OC and BC.