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

During the continuous monitoring of atmospheric parameters at the station C ape Point (34 degreesS, 18 degreesE), a smoke plume originating from a cont rolled fire of 30-yr-old fynbos was observed on 6 May 1997. For this episod e, which was associated with a nocturnal inversion and offshore airflow, at mospheric parameters (solar radiation and meteorological data) were conside red and the levels of various trace gases compared with those measured at C ape 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 strati fication 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. Emissi on ratios (ERs) calculated for CO and CH4 relative to CO2 mixing ratios amo unted to 0.042 and 0.0040, respectively, representing one of the first resu lts for fires involving fynbos. The CO ER is somewhat lower than those give n 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 (0.00 30) and smouldering phases (0.0055) of savanna fires. Non-methane hydrocarb on (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, ethene 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)). (C) 2001 Else vier Science Ltd. All rights reserved.