Showing papers by "Robert J. Delmas published in 1992"

1000 years of explosive volcanism recorded at the South Pole

Abstract: Cataclysmic volcanic eruptions generally disturb the global atmosphere markedly for around 2 years. During that time, long-life volcanic products (mainly H 2 SO 4 ), stored in the stratosphere gradually return to the troposphere. Antarctic snow may be subsequently contaminated and acid signals recorded. The recovery of these signals along Antarctic ice cores provides a history of past volcanic events, most often of global, but sometimes of just regional, significance. Several physical and chemical techniques have been used to analyse a 1000-year ice core drilled near Amundsen-Scott Base. Acid and ultrafine ash deposits of volcanic origin have been carefully investigated. 23 major volcanic eruptions have been detected, dated and tentatively identified. The results have been compared with similar Antarctic and Greenland records. The amount of volcanic sulfate deposited in Antarctic snow, calculated and averaged over the last millennium, is, however, minor (13% of the total sulfate) in comparison with that of the marine biogenic source. The 19th century was the period of the millennium most seriously disturbed by global explosive volcanic activity. The chlorine cycle, as recorded in Antarctic ice, seems to be little affected even by such a large event as Tambora, 1815. DOI: 10.1034/j.1600-0889.1992.00011.x

Precipitation chemistry in the Mayombé forest of equatorial Africa

Abstract: An automatic wet-only precipitation collector was operated in the coastal forest of equatorial Congo for a complete seasonal cycle (November 1986 to September 1987) Inorganic (Na+, K+, NH4+, Ca++, NO3−, Cl−, SO4=) and organic (HCOO−, CH3COO−) ions were determined in 169 samples which represent 51 rain events An average precipitation pH of 474 based on the volume weighted of H+ was obtained Rain from stratiform clouds showed higher acidity (pH = 462) than convective rainfall (pH = 481) This acidity results from a mixture of mineral acids (64%, of which about 42% is HNO3) and organic acids (36%) Most of the HNO3 component can be attributed to the emission of nitrogen oxides from vegetation burning To study the influence of variation in rainwater ion concentrations resulting from the differences in atmospheric liquid water content, rainfall events were stratified based on rainfall amount into convective and stratiform events The seasonal variation in the chemical composition of these types of rain events allowed us to compare the relative seasonal importance of the different sources (terrestrial biogenic, marine, soils, and biomass burning) Comparison between precipitation chemistry in Congo and in Amazonia shows that the African equatorial forest is influenced by local fires and savanna fires in the southern hemisphere during the dry season and by fires in the northern hemisphere during the wet season In Amazonia, on the other hand, the influence of biomass burning on rainwater chemistry appears to occur predominantly in the dry season Since the precipitation collector subdivides rainfall events into 10 sequential samples, we examined the evolution in chemical composition and deposition during four large convective events The results demonstrate the washout of ions at the onset of precipitation producing higher rainwater concentrations and their dilution as the rainfall intensity increases

Chemical composition of falling snow at Dumont d'Urville, Antarctica

Abstract: Fourteen samples of fresh falling snow were collected at Antarctic coastal base Dumont d'Urville in 1984. The samples have been analysed for major ions (including MSA) by ion chromatography and acid titration. The results are relevant to the chemical composition of background precipitation in polar marine conditions. The seasalt aerosol contribution is dominant. All samples are found to be acidic in the range 3–16 μeq/l. The calculated non-seasalt sulfate (nssSO42-) concentration is significantly negative for 3 of the 14 samples. NssSO42- is found to be relatively high in summer and fall. MSA also exhibits the same pattern probably linked to local marine biogenic activity and/or atmospheric photochemical processes. The MSA to nssSO42- ratio is in good agreement with values reported for coastal Antarctic ice cores and subantarctic acrosol. The background mean value for nitrate concentration is 1.1 μeq/l but two very strong spikes (up to 16 μeq/l) are observed. The first seems to be linked with long range transport of continental air masses while the second (in winter) is clearly due to a sudden input of nitric acid, possibly from the stratosphere.

Vertical profiles of ozone between 0 and 400 meters in and above the African equatorial forest

Abstract: Ozone concentrations have been measured in the northern Congo, near Impfondo (1°37 N, 18°04 E) during the DECAFE experiment in February 1988. This period corresponds to the dry season in the northern part of the equatorial region. The measurements were carried out simultaneously (1) at ground level in a large clearing, (2) inside the forest between 0 and 30 m, and (3) above the forest with a captive balloon flying up to 400 m. The results presented here are compared with the data obtained in the Mayombe forest in the southern part of Congo, near Dimonika in June 1988, during the dry season. For both northern and southern forested areas the ozone concentrations measured at ground level in a large clearing exhibit daily variations with maxima in the afternoon range between 10 and 30 parts per billion by volume (ppbv) and minima at the end of the night between 4 and 15 ppbv. The characterics of each surface ozone cycle are analyzed. Inside the forest, ozone concentrations are found very low near the ground and rarely exceed 15 ppbv above the canopy. At night the ozone concentrations are always close to zero at any level from the ground to the top of the canopy. Ozone removal processes are mainly located in the canopy. The ozone concentration above the forest is seen to increase in the early morning. Likely, this ozone production has a photochemical origin, the precursors coming from soils and biomass burning. The relationships among the vertical profiles of ozone, temperature, and water vapor are discussed.