Trace gas variations at Cape Point, South Africa, during May 1997 following a regional biomass burning episode

Abstract

During the continuous monitoring of atmospheric parameters at the station Cape Point (34°S, 18°E), a smoke plume originating from a controlled fire of 30-yr-old fynbos was observed on 6 May 1997. For this episode, which was associated with a nocturnal inversion and offshore airflow, atmospheric parameters (solar radiation and meteorological data) were considered and the levels of various trace gases compared with those measured at Cape Point in maritime air. Concentration maxima in the morning of 6 May for CO2, CO, CH4 and O3 amounted to 370.3ppm, 491ppb, 1730ppb and 47ppb, respectively, whilst the mixing ratios of several halocarbons (F-11, F-12, F-113, CCl4 and CH3CCl3) remained at background levels. In the case of CO, the maritime background level for this period was exceeded by a factor of 9.8. Differences in ozone levels of up to 5ppb between air intakes at 4 and 30m above the station (located at 230m above sea level) indicated stratification of the air advected to Cape Point during the plume event. Aerosols within the smoke plume caused the signal of global solar radiation and UV-A to be attenuated from 52.4 to 13.0mWcm-2 and from 2.3 to 1.3mWcm-2, respectively, 5h after the trace gases had reached their maxima. Emission ratios (ERs) calculated for CO and CH4 relative to CO2 mixing ratios amounted to 0.042 and 0.0040, respectively, representing one of the first results for fires involving fynbos. The CO ER is somewhat lower than those given in the literature for African savanna fires (average ER=0.048), whilst for CH4 the ER falls within the range of ERs reported for the flaming (0.0030) and smouldering phases (0.0055) of savanna fires. Non-methane hydrocarbon (NMHC) data obtained from a grab sample collected during the plume event were compared to background levels. The highest ERs (ΔNMHC/ΔCH4) have been obtained for the C2-C3 hydrocarbons (e.g. ethene at 229.3pptppb-1), whilst the C4-C7 hydrocarbons were characterised by the lowest ERs (e.g. n-hexane at 1.0 and n-pentane at 0.8pptppb-1). Copyright (C) 2001 Elsevier Science Ltd.