A. Khedim

Field study of the emissions of methyl chloride and other halocarbons from biomass burning in Western Africa

A field study of trace gas emissions from biomass burning in Equatorial Africa gave methyl chloride emission ratios of 4.3×10−5±0.8×10−5 mol CH3Cl/mol CO2. Based on the global emission rates for CO2 from biomass burning we estimate a range of 226−904×109 g/y as global emission rate with a best estimate of 515×109 g/y. This is somewhat lower than a previous estimate which has been based on laboratory studies. Nevertheless, our emission rate estimates correspond to 10–40% of the global turnover of methyl chloride and thus support the importance of biomass burning as methyl chloride source. The emission ratios for other halocarbons (CH2Cl2, CHCl3, CCl4, CH3CCl3, C2HCl3, C2Cl4, F-113) are lower. In general there seems to be a substantial decrease with increasing complexity of the compounds and number of halogen atoms. For dichloromethane biomass burning still contributes significantly to the total global budget and in the Southern Hemisphere biomass burning is probably the most important source for atmospheric dichloromethane. For the global budgets of other halocarbons biomass burning is of very limited relevance.

Comparison of measured OH concentrations with model calculations

The influence of chemical precursors and sunlight on the atmospheric OH abundance is investigated by a comparison of locally measured tropospheric OH with model calculations. The latter are based on the gas phase reaction mechanism of the regional acid deposition model (RADM2) which incorporates an explicit inorganic and a comprehensive organic chemistry. The experimental data were obtained in the planetary boundary layer during two sets of campaigns. In Deuselbach (1983) and Schauinsland (1984), rural conditions were encountered with NOx concentrations on the average of 2.2 and 0.9 ppb, respectively. This data set was already compared with model calculations based upon an older and less detailed chemical reaction scheme (Perner et al., 1987). Since then the experimental data were reanalyzed leading to modified measured OH concentrations and also to modified precursor concentrations. For a consistent comparison with the more recent campaigns in Julich (1987 and 1988) we have redone the calculations. The modeled and measured OH concentrations of the campaigns in 1983 and 1984 correlate well with a coefficient of correlation of r = 0.73. The model overpredicts OH by about 20%. Under more polluted conditions in Julich with average NOx concentrations of 4 ppb the correlation coefficient between experimental and modeled data are significantly smaller (r = 0.61). Possible reasons are the influence of not measured precursors, for example isoprene, and the inapplicability of a quasi-steady state model under the spatially inhomogeneous conditions in Julich. Again the model overpredicts the OH concentration by about 15%, which is somewhat smaller than for the rural case. The precision of the comparison is limited by the uncertainties of the chemical reaction rate constants.

Problems Connected with the Analysis of Halocarbons and Hydrocarbons in the Non-Urban Atmosphere

The problems connected with the measurement of hydrocarbons outside urban areas are considerable: The atmospheric mixing ratios of most of the hydrocarbons are very low--from a few ppb down to some ppt; the mixture of hydrocarbons is extremely complex, ranging from light n-alkanes to alkyl benzenes and terpenes; for measurements in remote areas the logistic conditions often restrict the instrumentation which can be used for sample collection or in situ measurements (such as lack of electric power supply, weight restrictions etc.). Nevertheless, sensitive and sufficiently reliable measurements of hydrocarbons in the non-urban atmosphere are important. Hydrocarbons are important factors in the tropospheric photochemistry (e.g. ozone formation) and can be used as valuable tracers for man-made atmospheric pollutants etc. Other useful tracers for anthropogenic emission are halocarbons such as dichlormethane, tri- and tetrachloroethen etc. The impact of man-made hydrocarbons on the chemistry of the troposphere can only be understood if the extent of natural (biogenic) contributions is known. From measurements of a large variety of hydrocarbons and halocarbons it is often possible to obtain information about the sources of the most important atmospheric hydrocarbon species, even for trace gases with both significant anthropogenic and biogenic sources. In this presentation some of the problems and their solutions connected with such measurements of atmospheric hydrocarbons and halocarbons are presented and discussed. Some of the results obtained by several series of measurements are described, indicating that man-made as well as biogenic hydrocarbons can be important factors for the chemistry of the atmosphere.

The Use of Automated “On Line” Gaschromatrography for the Monitoring of Organic Trace Gases in the Atmosphere at Low Levels

Abstract The usual procedure for measurements of hydrocarbons, halocarbons, and similar trace gases in the atmosphere is sample collection in the field and subsequent analysis in the laboratory. However this procedure is not adequate if intensive field investigtions with a high measuring frequency are desired. In this case the use of in-situ measurement techniques is necessary. For the measurement of volatile organic trace substances in the atmosphere the by far best technique is gaschromatography. However even if very sensitive ionization detection is employed, a preconcentration step is necessary if the technique is to be used at low pollution levels. In this paper the basic design and some applications are described for an automated gaschromatograph suitable for unattended operation. The instrument allows automated prconcentration of the ambient air sample, injection etc. including calibration with an external standard. The lower limits of detection are in the ppt range, the reproducibility of the meas...

Emissions of Light Nonmethane Hydrocarbons from the Atlantic into the Atmosphere

During two Atlantic cruises of the German research vessel Polarstern, 1988 and 1989, the concentrations of light nonmethane hydrocarbons (NMHC) in seawater were measured. On the basis of a simple budget analysis, the oceanic mixed layer represents a NMHC reservoir with an internal production and a major loss by emission into the atmosphere. As a consequence, the concentrations of NMHC depend on the rates of ocean-atmosphere exchange: high exchange rates reduce the concentrations and vice versa. With the prevailing transfer velocities the emission rates were calculated according to ocean-atmosphere exchange models. The regional averages of the alkene emission rates vary by 1 order of magnitude. For ethene the maximum value was 5×108 molecules cm−2 s−1. The emissions of the various alkanes were generally below 1×108 molecules cm−2s−1. The total C2-C4 hydrocarbon emissions during both cruises average 6×108 molecules cm−2 s−1, 70 % of which are alkene emissions, with ethene alone contributing 42 % to the total. No indications for enhanced emissions of NMHC at high phytoplankton concentrations or in the proximity to coastlines were observed. Thus we regard the emissions as representative for the mid-Atlantic and the season of the investigations, August to October. The calculated emission rates of the shortlived alkenes are validated by comparison with atmospheric measurements of NMHC. The emission rates are substantially lower than the majority of reported oceanic emission estimates by up to about 2 orders of magnitude.

Determination of C2–C5 hydrocarbons in the atmosphere at low parts per 109 to high parts per 1012 levels

Abstract By far the most abundant hydrocarbon in unpolluted air is methane (mixing ratio ca. 1.6 ppm). The mixing ratios of other hydrocarbons are typically in the low parts per 109 (ppb) and parts per 1012 (ppt) ranges. Although methane is several orders of magnitude more abundant in clean air, it is conceivable that other hydrocarbons are still of considerable importance to clean air photchemistry, because their reaction with hydroxyl radicals proceeds much faster than that of methane. Owing to this high reactivity of many of the light non-methane hydrocarbons (NMHC), mixing ratios of NMHC as low as a few ppb or several ppt can have a considerable influence on the photochemistry of unpolluted air. For this reason a gas chromatographic method has been developed that permits the determination of several C2–C5 hydrocarbons with detection limits of a few ppt from grab samples of 0.5– dm3 (STP). The samples are collected in evacuated 2-1 stainless-steel containers with metal bellows-sealed stainless-steel valves. These sample collection and storage cans are specially pre-treated and cleaned to avoid changes in sample composition during transport of the samples to the laboratory. In the laboratory the samples are analysed by enrichment of the hydrocarbons on a packed pre-column at sub-ambient temperatures (ca. - 35°C) and subsequent separation on a 7 m x 0.8 mm I.D. packed column (Spherosil XOB075). A flame-ionization detector is used. This method allowed survey measurements on a global scale of C2–C5 hydrocarbons, which gave an estimate of the contributions of light hydrocarbons to atmospheric photochemical reactions.

Light hydrocarbons in the tropospheric boundary layer over tropical Africa

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 and i-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 layer.

The influence of ozone on light nonmethane hydrocarbons during cryogenic preconcentration

A number of recent measurement series of nonmethane hydrocarbons (NMHCs) based on in situ analysis report very low alkene concentrations in the remote troposphere. It was speculated that during preconcentration or thermal desorption of the sample, atmospheric ozone may react with the reactive hydrocarbons, e.g., alkenes. Therefore the behavior of ozone in different inlet systems at different conditions was investigated, in order to indicate where O3 interferences may arise. The results for the inlet and preconcentration system used for our measurements show that up to 50% of the ambient ozone is lost during passage of a heated stainless steel inlet line. The remaining ozone is preconcentrated together with the hydrocarbons. During the process of thermal desorption the remaining ozone is lost within minutes leading to a loss of reactive hydrocarbons of the order of 2–10% which is usually less than the error of measurement. These results were confirmed when different amounts of ozone were added to samples of pressurized air with moderate and low NMHC concentrations. For ozone mixing ratios of up to 100 ppb no significant change in the concentration of light alkenes was observed. The results show that our system used for cryogenic preconcentration of NMHC with subsequent thermal desorption is suitable for quantitative measurements even of reactive light alkenes in the atmosphere without an additional ozone trap.

Hydrocarbons in the Non-Urban Atmosphere: Analysis, Ambient Concentrations and Impact on the Chemistry of the Atmosphere

Abstract The role of hydrocarbons for the chemistry of the atmosphere outside of heavily polluted urban or industrial areas is not yet fully understood. One of the reasons is the rather limited data base for hydrocarbon mixing ratios in rural and semi-rural areas. This is-at least partly-due to analytical problems. The mixing ratios of hydrocarbons-except methane-in remote and semi-remote areas are very low, a few ppb or even less. In addition, the hydrocarbon pattern is-especially in semi-rural areas-extremely complex, a potpourri of engine exhaust, refinary emissions, natural gas leakage, biogenic emissions from plants, decaying leaves etc. In the past few years we have developed gas chromatographic technics for the identification and quantification of low and medium molecular weight hydrocarbons outside polluted areas. The results of these measurements show that these non-methane hydrocarbons (NMHC) are present in the non-urban atmosphere at very low mixing ratios-generally fractions of a ppb, rarely m...

Vertical profiles of acetylene in the troposphere and stratosphere

Stratospheric measurements of acetylene up to altitudes of 30 km are presented. The air samples were collected during three different balloon flights, two of them at 44°N, one at 32°N using balloon borne, liquid neon-cooled, cryosamplers. Their acetylene concentration was measured in the laboratory by flame ionisation gaschromatography. The different profiles at 32°N and 44°N are discussed with respect to possible vertical exchange processes and compared with published model calculations.