Long-term measurements of sulphur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone in southern Africa using passive samplers

Abstract

This paper reports the measurements of monthly mean gaseous sulphur dioxide, nitrogen dioxide, ammonia, nitric acid and ozone at four remote sites - Louis Trichardt (South Africa), Cape Point (South Africa), Amersfoort (South Africa) and Okaukuejo (Namibia) - in southern Africa, over a period of nine to 11 years, using passive samplers. The 10-year mean sulphur dioxide concentrations varied from 0.43 parts per billion (ppb) at Okaukuejo to 2.8 ppb at Amersfoort. Nitrogen dioxide varied from 0.34 ppb at Okaukuejo to 2.5 ppb at Amersfoort, nitric acid from 0.23 ppb at Louis Trichardt to 0.90 ppb at Amersfoort, and ammonia from 1.2 ppb at Amersfoort to 1.5 ppb at Cape Point. Ozone exhibited a fairly constant high mean value of about 27 ppb throughout the region except for the Louis Trichardt site, with a relatively high 10-year mean of 35 ppb. The annual concentrations of all chemical species measured decreased from 1995 to 2001, after which they increased slightly. The large amounts of biofuels that are combusted on the continent, mainly in winter, substantially influence the concentrations of all measured species. The seasonal concentrations of all the gases seem to peak in winter and spring at all sites, except for ammonia at Cape Point and Okaukuejo, and sulphur dioxide at Cape Point, which peaks during the summer. Strong inter-annual variations were observed that prove the scientific value of decision-making based on long-term observations.