摘要:

The objective of the Dynamique et Chimie de l'Atmosphère en Forêt Equatoriale (DECAFE) program is to determine the influence of African tropics on atmospheric chemistry. Savannas and forests constitute the main ecosystems in this area. Because of the occurrence of biomass burning during the dry season, savannas of the northern and southern hemispheres are active sources of many constituents which are of interest to atmospheric chemistry. The rain forest, with its important biomass activity is also a source or sink for gases and aerosols. Man's impact on this ecosystem is important since the rate of deforestation is about 0.5% per year. Furthermore, a general air motion from savanna regions toward the tropical rain forest is observable in most parts of Africa, leading to important interactions between the respective emissions of these two ecosystems. The DECAFE experiments presented in this issue were located in the Mayombe forest (southern Congo) and in the forest of the northen Congo. A study of rain chemistry was undertaken in 1986 in the Mayombe forest, then continued in the northern forest. In February 1988 a coordinated experiment was held in the tropical rain forest of the north, near Impfondo. The paper presents an overview of the general meteorological and climatological situation in the African tropics, a study of the climatological features of the sites, and a short description of the Impfondo experiment, which is the subject of most of the contributions to this special issue. The local meteorological situation during the time of the experiment is analyze

ISSN:0148-0227    

DOI:10.1029/91JD02426

年代:1992

数据来源: WILEY

摘要:

We determined the distribution of ozone (O3) and Aitken condensation nuclei (CN) over the rain forest in equatorial Africa during February 12–25, 1988. A pronounced O3maximum with levels up to 70 ppbv was present in a layer between 1 and 3 km altitude throughout the period. It coincided with a CO maximum and with high levels of CO2and gaseous organic acids. In general, the vertical distribution of CN was similar to that of O3, with number densities ranging up to approximately 3000 cm−3. Dense haze was visible within this layer. O3and CN decreased sharply above the haze layer to values typical of the remote troposphere. Survey flights showed little change in levels of O3and CN or in their vertical distribution over distances of hundreds of kilometers. Meteorological observations suggest that this ozone and particulate enriched layer is formed from air masses which originate in northern Africa and subsequently advect over dry tropical regions where biomass burning emits large amounts of aerosols, CO, NO, and hydrocarbons. These air masses then become trapped in the equatorial region between the near‐surface monsoon flow from the southeast and the permanent easterly flow above 3–4 km. Differences in the vertical distribution of O3and CN result from the removal of O3by surface uptake and reactions with NO and hydrocarbons, leading to surface O3concentrations near zero and a steep O3gradient through the subcloud layer at night. During the day this gradient is reduced by convective mixing. CN concentrations showed no pronounced gradients in the subcloud layer, consistent with the absence of a strong sink for CN at the ground. CN gradients near the surface suggest emission of particles from the forest vegetation or from biomass

ISSN:0148-0227    

DOI:10.1029/91JD00961

年代:1992

数据来源: WILEY

摘要:

During Dynamique et Chimie de l'Atmosphère en Forêt Equatoriale (DECAFE) experiments, ozone mixing ratios higher than 65 ppb have been detected in the northern equatorial continental layer above the monsoon layer. In order to understand these data we have carried out simulation experiments with a numerical model which had been developed earlier in our laboratory. These studies show that the large amounts of ozone detected cannot be only due to the chemistry of biogenic emissions in forested areas. A remote origin related to the presence of neighboring savanna areas must be considered. The amounts of remote emissions in NO and light hydrocarbons (HC) necessary to account for the observed ozone ratios should be of the order of 400 μg N/m2h and 1800 μg C/m2h, respectively. These values can be interpreted in terms of biogenic biomass combustion. On the basis of the hypothesis it is shown that biogenic emissions coming from the forest account only for 20% of the total amount of ozone measu

ISSN:0148-0227    

DOI:10.1029/91JD00511

年代:1992

数据来源: WILEY

摘要:

In this report we give results of methane flux measurement in the flooded forest zone of the Congo River basin in central Africa. Methane fluxes are measured by the static chamber method above three types of soils characterized by various water contents. High methane emission is recorded on flooded soils (4.59 × 1012molecules/cm2/s) while methane uptake occurs in dry soil (−8.38 × 1010molecules/cm2/s). Methane flux is also derived from variations of surface concentrations of methane related to the variations of air stability and from vertical profiles in the lower troposphere. The four methods used to determine the average methane emission from this wetland type yield compatible values. The average yearly flux is estimated at 2 to 4 × 1012molecules/cm2/s or 45 to 90 mg/m2/d The total wetland area being about 105km2, global methane emission from the flooded forest zone of the Congo River basin would represent 1.6 to 3.2 Tg (CH4) per

ISSN:0148-0227    

DOI:10.1029/90JD02555

年代:1992

数据来源: WILEY

摘要:

Sources and sinks of methane were studied in the Mayombe forest, a tropical evergreen forest located in a mountainous region in central Africa. Important methane emissions, reaching 6×1013molecules/cm2/s, were measured in flooded lowlands where soil characteristics: pH and redox potential, favor the growth of methanogenic bacteria. However, basically, soils of this region constitute a sink of atmospheric methane with uptake rates ranging from 1010to 1011molecules/cm2/s. Methane emission from termite nests was also studied; it appeared to be a minor component of the methane budget. CH4concentrations were measured inside the forest and in the surrounding atmosphere, CO2being used as a qualitative tracer of air exchanges. In spite of intense but scattered and size‐limited sources this environment seems to be a net sink of atmospheric metha

ISSN:0148-0227    

DOI:10.1029/90JD02575

年代:1992

数据来源: WILEY

摘要:

Fifty measurements of nonmethane hydrocarbons (NMHC) and several other trace gases were made over an equatorial rain forest in February 1988 as part of the DECAFE experiment. The measurements were made independently by two different laboratories. Each laboratory used its own sample containers, gas chromatographic measurement procedure, and calibration. Also, the altitudinal distribution of the samples differed. Apart from propene andi‐pentane for which a substantial difference in the absolute calibration existed between the two laboratories, the average results were very similar, and the vertical profiles were identical within the scatter of the data. For NMHC with longer atmospheric residence times (e.g., ethane and acetylene) the average results agreed within a few percent. In the boundary layer, only small gradients could be found. In all cases where a significant vertical gradient existed, there was an increase of the mixing ratios with increasing altitude. This can be explained by the different origin of the air masses at different altitudes. Above the boundary the trace gas mixing ratios decrease. The observed NMHC pattern can primarily be described as photochemically aged emissions from biomass burning. The observed depletion of the photochemically reactive NMHC also agrees with the occurrence of enhanced ozone levels in the boundary laye

ISSN:0148-0227    

DOI:10.1029/91JD00289

年代:1992

数据来源: WILEY

摘要:

Gaseous short chain organic acids were measured during the dry season (February) in and above the rain forest of the northern Congo. Samples were taken at ground level and during several flights up to 4 km altitude. The organic acids were concentrated from the atmosphere by using “mist scrubbers,” which expose a mist of deionized water to the air to be probed. The organic acids absorbed in the water were subsequently analyzed by ion chromatography. Formic, acetic, and pyruvic acids were identified in the samples. At ground level, average mixing ratios of gaseous formic and acetic acid of 0.5±0.6 and 0.6±0.7 parts per billion by volume (ppbv) (1 s), respectively, were found. Boundary layer mixing ratios, however, were significantly higher (3.7±1.0 and 2.7±0.9 ppbv). This indicates a downward net flux of these atmospheric trace components from the boundary layer to the surface. Free tropospheric samples taken above the cloud convection layer show lower mixing ratios again (0.9±0.3 and 0.7±0.1 ppbv). On the basis of this vertical distribution, direct emission by vegetation is not considered to be the dominant source. Biomass burning and photochemical oxidation of biogenic precursors are the major processes contributing to the enhancement of organic acids observed in the boundary layer. The organic acids parallel the profiles of ozone and CO, which suggests that their generation processes are closely related. Pyruvic acid is not correlated with formic acid, indicating that the oxidation of isoprene is not of major importance. In emissions from biomass fires, CO correlates well with formic and acetic acid, and thus some of the enhancement of organic acids in the boundary layer can be explained due to burning. However, an additional gas phase source for organic acids must exist to explain the observed ratio of formic to acetic acid. This is most likely the ozonolysis of olefins which were released as pyrolysis products from biomas

ISSN:0148-0227    

DOI:10.1029/91JD01438

年代:1992

数据来源: WILEY

摘要:

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 precipitationpH of 4.74 based on the volume weighted of H+was obtained. Rain from stratiform clouds showed higher acidity (pH = 4.62) than convective rainfall (pH = 4.81). This acidity results from a mixture of mineral acids (64%, of which about 42% is HNO3) and organic acids (36%). Most of the HNO3component 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

ISSN:0148-0227    

DOI:10.1029/91JD00928

年代:1992

数据来源: WILEY

摘要:

We determined the distribution of gaseous and particulate sulfur compounds in the canopy of the tropical rain forest of northern Congo and the overlying atmosphere during February 12–25, 1988. Hydrogen sulfide and dimethylsulfide decayed exponentially with altitude from approximately 30–40 ppt at ground level to 3–5 ppt at around 3 km altitude. Emission fluxes from the forest to the atmosphere were estimated by fitting a one‐dimensional time‐dependent numerical model of chemistry and transport of the sulfur compounds to their observed vertical profiles. Emission fluxes of 0.6–1.0 nmol H2S m−2min−1and 0.3–0.7 nmol DMS m−2min−1were consistent with the observed vertical profiles of H2S and DMS. These fluxes compare well with fluxes reported previously for the Amazon rain forest during the dry season and support the view of a subordinate role of land biota in the global cycling of sulfur. The particulate sulfur concentration of 248 ppt was found below the forest canopy. Biomass burning is considered to be an important contributor to this particulate sulfur. Carbonyl sulfide was found to be enhanced above the 500 ppt tropospheric background throughout the mixing layer of 2–3 km depth, likely

ISSN:0148-0227    

DOI:10.1029/91JD01112

年代:1992

数据来源: WILEY