15 years of measurements at Welgegund atmospheric measurement site in South Africa

Long-term measurements of aerosol particles and trace gases are available from several well-equipped research stations in Europe and North America, but global coverage of such measurements is far from complete. Notably, Welgegund measurement station in South Africa is the only site in continental Africa where e.g. submicron aerosol size distributions are continuously monitored. Welgegund is located in a grazed grassland savanna environment approx. 100 km west of Johannesburg (26.57°S, 26.94°E) at 1480 m above sea level. The station has been operational since May 2010 and is equipped with continuous measurements of aerosol properties such as submicron size distributions, PM10, aerosol particle scattering, absorption-based black carbon (BC) with a MAAP, basic trace gases (SO₂, O₃, NO, NO_x, CO) as well as ecosystem fluxes (CO₂, H₂O).

Seasonality at Welgegund is characterised with a wet season from October to April, and a dry season from May to September. During the dry season, BC is elevated due to both increased sources (e.g. landscape fires) and reduced wet removal. In the BC time series, a decrease is observed between May 2015 and May 2017. Until May 2015, the mean wet season BC is 0.33 µg m^-3, but after May 2017, the mean wet season BC is 0.24 µg m^-3. Simultaneously, the mean dry season BC drops from 0.98 µg m^-3 to 0.75 µg m^-3. For both seasons, the decrease is statistically significant with p-value ≤ 0.001 using Mann-Whitney U test. The decrease in BC affects single scattering albedo at 635 nm, which increases from 0.88 to 0.91 for the wet season and from 0.81 to 0.86 for the dry season, respectively.

Last year, an ACSM for on-line measurement of non-refractory submicron aerosol chemical composition and an on-line GC-MS system for volatile organic compound measurements were added to Welgegund instrumentation. These new measurements, together with the existing long time series, enable more detailed characterisation of aerosol particles and their precursors at the grassland savanna environment at Welgegund.