Aerosol optical depth over a remote semi-arid region of South Africa from spectral measurements of the daytime solar extinction and the nighttime stellar extinction

Spectral daytime aerosol optical depths have been measured at Sutherland, South Africa (32°22' S, 20°48' E), from January 1998 to November 1999. Sutherland is located in the semi-arid Karoo desert, approximately 400-km northeast from Cape Town. The site, remote from major sources of aerosols, hosts the South African Astronomical Observatory (SAAO), where nighttime stellar extinction is being measured. The comparison of daytime and nighttime measurements for the years 1998-1999 makes it possible to validate the astronomical dataset of aerosol optical depth (τ_a) dating back to 1991. The 1998 and 1999 annually averaged daytime τ_a at 500 nm are 0.04 ± 0.04 and 0.06 ± 0.06, respectively. Half-day averages vary between 0.03 and 0.44, with peak values in August-September. This pronounced seasonality is linked to the biomass-burning season in the Southern Hemisphere. Smoke haze layers transported to Sutherland originated primarily on the African landmass at latitudes between 10° and 20°S and passed over Namibia and Angola. On one occasion, aerosols from fires in Brazil transported across the Atlantic Ocean were likely detected. The haze layers reaching Sutherland are therefore at least 2-3 days old. The spectral dependence of the aerosol optical depth for the smoke layers supports the bimodality of the volume size distribution for biomass burning aerosols. The accumulation mode has a volume modal diameter of 0.32 μm, consistent with the hypothesis of aged haze. The stellar measurements (1991-2001) show that, due to the eruption of Mt. Pinatubo, the atmospheric extinction depth at 550 nm in the years 1991-1993, increased by 33% with respect to the average value (0.14 ± 0.03) for the period 1994-2001. Outside the Pinatubo event, extinction is largest in the period 1997-1999.