Mass balance of the atmospheric aerosol in a South African subtropical savanna (Nylsvley, May 1997)

Aerosol samples were collected at Nylsvley Natural Reserve in northeastern South Africa at the beginning of the dry season, when large-scale fires were absent. Aerosol was collected on quartz fiber filters with open face stack filters in an inlet with an upper cutoff mass median diameter of 30 mu m. The samples were analyzed for soluble ions (Cl-, NO3-, SO42-, oxalate, NH4+, Na+, K+, Ca2+, Mg2+), trace metals and crustal elements (Fe, Mn, Ca, Mg, Pb, Cu, Ag, Cd), "black carbon" (BC), total carbon (TC); and in selected samples for aerosol humidity, for mono- and dicarboxylic acids and other polar organic components, and for cellulose as a tracer for plant debris. Aerosol mass balances for the individual samples were constructed from the sum of following groups: Humidity (determined as weight loss at 100 degrees C with a microthermobalance), ammonium sulfate, Cl- and NO3-, organic material, BC, and soil dust. On the average, the fit between the sum of the determined groups of components and the gravimetrically determined mass was within 6%, whereas deviations ranged for individual samples from -12 to +27% relative to the gravimetrically determined mass. The main component in aerosol was soil dust (36%), followed by organic material (28%), and ammonium sulfate (27%). Humidity was 6%, BC was 2%, and Cl- lard NO3- were 1% of aerosol mass. The low BC/TC ratio of 0.09 indicated little influence from combustion sources. The diurnal trend of the BC/TC ratio of 0.06 during day and 0.14 during night indicated a daytime source for organic components with no BC associated. As there were no industrial or other anthropogenic sources evident, the daytime source of organic components is assumed to be gas-particle conversion from biogenic emissions. Plant debris and organic acids were the major analytically accessible groups; however, together they form only 7.2% of the carbonaceous material. With 0.4% other polar organics and 5.7% BC in this group, 86.7% of the carbonaceous material remains unidentified.