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 degrees S, 18 degrees E), a smoke plume originating from a controlled fire of 30 year 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 O-3 amounted to 370.3 ppm, 491 ppb, 1730 ppb and 47 ppb. respectively, whilst the mixing ratios of several halocarbons (F-ll, 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 5 ppb between air intakes at 4 and 30 m above the station (located at 230 m 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 13.0 mW cm (-2) and from 2.3 to 1.3 mW cm (-2), respectively, 5 h 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 (Delta NMHC/Delta CH4) have been obtained for the C-2-C-3 hydrocarbons (e.g., ethane at 229.3 ppt ppb (-1)). Whilst the C-4-C-7 hydrocarbons were characterised by the lowest ERs (e.g. n-hexane at 1.0 and rt-pentane at 0.8 ppt ppb (-1)).