Spatial and temporal assessment of atmospheric organic carbon and black carbon concentrations at South African DEBITS sites
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 and anthropogenic) can be directly linked to the aerosol characteristics of a region, making monitoring campaigns to determine aerosol composition in different regions very important. Limited data currently exists for atmospheric aerosol black carbon (BC) and organic carbon (OC) in South Africa. In this study, BC and OC concentrations were explored in terms of spatial and temporal patterns, mass fractions of BC and OC of the overall aerosol mass, as well as possible sources. Primary pollutants, of which BC is an example, are emitted directly from the source. Certain primary pollutants can react with other pollutants to form secondary pollutants. OC can either be a primary or secondary pollutant, e.g. formed by gas-to-particle conversion of volatile organic compounds (VOCs) in the atmosphere (nucleation and condensation of gaseous precursors). Greenhouse gases (GHG) and BC absorb terrestrial long wave radiation causing an increase of atmospheric temperature. In contrast, OC generally reflects incoming radiation, cooling the atmosphere. GHGs have a long residence time in the atmosphere (10 to 100 years), while the residence time of aerosols is usually only a week or more. The climatic effects of aerosols are therefore particularly important from a regional perspective. Aerosols are also important from an air quality perspective, especially since ultrafine particles (diameter smaller than 100nm) are small enough to go through the membranes of the respiratory tract and into the blood stream. They can then be transported to the brain. Up to 2005, DEBITS (Deposition of Biogeochemical Import Trace Species) activities in South Africa did not include aerosol measurements. In order to initiate aerosol monitoring, campaigns were launched during the 2005 to 2007 period. Additionally, OC and BC measurements for the PM10 and PM2.5 (particulate matter smaller than or equal to 10 and 2.5 μm, respectively) fractions were started in 2009. PM10 and PM2.5 samples were collected at five sampling sites in South Africa operated within the DEBITS network, i.e. Louis Trichardt, Skukuza, Vaal Triangle, Amersfoort and Botsalano, with MiniVol samplers. The selected sites are mostly located in rural areas, but with the surrounding atmosphere influenced by industries, transportation, biomass burning, etc. Winters are characterised by an increase in biomass burning (fires) and combustion for domestic use (cooking and space heating). Samples were analysed with a Thermal/Optical Carbon analyser (Desert Research Institute). OC and BC results showed that the total carbonaceous content decreased during the summer due to less biomass burning (fires). BC was the highest at the industrially influenced sites, while OC was highest at regional background sites. OC was higher than BC concentrations at all sites in both size fractions. Most OC and BC occurred in the PM2.5 fraction. OC/BC ratios reflected the setting of the different DEBITS sites, with sites in or close to anthropogenic source regions having the lowest OC/BC ratios, while background sites had the highest OC/BC ratios. The OC mass fraction percentage of the total aerosol weight varied up to 24% and the BC up to 12%. The highest OC mass fraction was found at Skukuza, which was attributed to both natural (lies within the savannah biome) and anthropogenic (dominant path of air mass movement from the anthropogenic industrial hub of South Africa) reasons. The highest mass fraction of BC was found in the Vaal Triangle, since it is situated within a well-known anthropogenic source region. Household combustion for space heating and cooking also seemed to make a significant contribution to BC at this site in the cold winter months. A relatively well-defined seasonal pattern was observed, with higher OC and BC concentrations measured from May to October, which coincides with the dry season in the interior of South Africa. Positive correlations between OC and BC concentrations with the distance back trajectories passed fires were observed, indicating that fires contribute significantly to both atmospheric OC and BC during the burning season.