Air quality studies using long-term observations at Welgegund, South Africa

Quality-controlled long-term measurements of atmospheric composition are central in identifying the sources and processes that are most important for air quality in a particular environment. In South Africa, continuous measurements of various atmospheric constituents have been carried out at the Welgegund measurement station since May 2010 in close collaboration between North-West University, Finnish Meteorological Institute and University of Helsinki. Welgegund is a regionally representative continental site approx. 100 km west of Johannesburg with no local sources. Before Welgegund, measurements were operated at Marikana village, which is an urban location with large industrial point sources in the vicinity. At both sites measurements included particulate matter smaller than 10 µm in diameter (PM10), black carbon (BC) and trace gases (SO₂, NO, NO_x, O₃, CO) among others.

Numerous industrial point sources surrounding Johannesburg result in elevated NO_x and SO₂ levels in the region, but these concentrations remain below air quality standards. On the other hand, PM10 does exceed 50 µg m³ rather frequently and more often at Marikana than at Welgegund. However, strong correlation with CO suggests that the periods of elevated PM10 are related to incomplete combustion rather than the industrial emissions at both locations. At Marikana the major source appears to be domestic heating during winter time, while at Welgegund landscape fires seem more important. Also, O₃ exceeds air quality standards frequently at both sites and the highest O₃ cases appear to be linked with landscape fires. Furthermore, our observations suggest that O₃ formation is not NO_x-limited but rather VOC-limited.