AIRBORNE STUDIES OF EMISSIONS FROM SAVANNA FIRES IN SOUTHERN AFRICA .1. AEROSOL EMISSIONS MEASURED WITH A LASER OPTICAL-PARTICLE COUNTER

During the SAFARI-92 experiment (Southern Africa Fire Atmosphere Resea rch Initiative, September-October 1992), we flew an instrumented DC-3 aircraft through plumes from fires in various southern African savanna ecosystems. Some fires had been managed purposely for scientific stud y (e.g., those in Kruger National Park, South Africa), while the other s were ''fires of opportunity'' which are abundant during the burning season in southern Africa. We obtained the aerosol (0.1-3.0 mu m diame ter) number and mass emission ratios relative to carbon monoxide and c arbon dioxide from 21 individual fires. The average particle number em ission ratio Delta N/Delta CO (Delta: concentrations in plume minus ba ckground concentrations) varied between 14+/-2 cm(-3) ppb(-1) for gras slands and 23+/-7 cm(-3) ppb(-1) for savannas. An exceptionally high v alue of 43+/-4 cm(-3) ppb(-1) was measured for a sugarcane fire. Simil arly, the mass emission ratio Delta M/Delta CO varied from 36+/-6 ng m (-3) ppb(-1) to 83+/-45 ng m(-3) ppb(-1), respectively, with again an exceptionally high value of 124+/-14 ng m(-3) ppb(-1) for the sugarcan e fire. The number and mass emission ratios relative to CO depended st rongly upon the fire intensity. Whereas the emission ratios varied gre atly from one fire to the other, the aerosol number and volume distrib utions as a function of particle size were very consistent, The averag e background aerosol size distribution was characterized by three mass modes (0.2-0.4 mu m, approximate to 1.0 mu m, and approximate to 2.0 mu m diameter). On the other hand, the aerosol size distribution in th e smoke plumes showed only two mass modes, one centered in the interva l 0.2-0.3 mu m and the other above 2 mu m diameter. From our mean emis sion factor (4+/-1 g kg(-1) dm) we estimate that savanna fires release some 11-18 Tg aerosol particles in the size range 0.1-3.0 mu m annual ly, a somewhat lower amount than emitted from tropical forest fires. W orldwide, savanna fires emit some 3-8x10(27) particles (in the same si ze range) annually, which is expected to make a substantial contributi on to the cloud condensation nuclei population in the tropics.