R. Borchers

Tropospheric trends for CF4 and C2F6 since 1982 derived from SF6 dated stratospheric air

The stratosphere can be used as an archive of past tropospheric conditions. Measurements of the long-lived trace gases CF4, C2F6, and SF6 have been carried out on stratospheric air samples obtained by balloon borne cryosampling from altitudes of up to 34 km between 1987 and 1995. SF6 observations were used to ‘date’ the air samples. Above 25 km height the SF6 concentrations are lagging the tropospheric ones by several years: 4.5 years for the tropics, 6 years for mid-latitudes and up to 10 years for the arctic winter vortex. The high ages of stratospheric air found inside the upper polar vortex may indicate that the lag of the peak halogen burden between the troposphere and the polar stratosphere is longer than previously assumed. The age information from SF6 allows to reconstruct tropospheric trends for CF4 and C2F6 since 1982. Accordingly, for the period 1982 to 1995, concentrations increased from 62 to 75 ppt for CF4 and from 1.5 to 2.6 ppt for C2F6. Mean increase rates of 1.00 ppt yr−1 for CF4 and 0.084 ppt yr−1 for C2F6 translate into annual injection rates of about 14,600 t yr−1 and 1,900 t yr−1 respectively. With aluminium production as the only major source, average emissions per ton of primary aluminium are thus calculated to be 0.77 kg for CF4 and 0.10 kg of C2F6 in 1994.

Monitoring the vertical structure of the Arctic Polar Vortex over northern Scandinavia during EASOE: Regular N2O profile observations

Different whole air samplers were flown on large balloons launched from Kiruna (67.9°N; 21.1°E) in the period from 30 November 1991 until 20 March 1992. Thirteen vertical profiles of the N2O mixing ratio were obtained from the analyses of the stratospheric air samples collected at altitudes between about 10 km and 31 km. The series of profile observations illustrates the temporal variation of the vertical structure of the Arctic polar vortex over Northern Scandinavia during EASOE. Already by mid December the N2O mixing ratios observed at the 600 K isentropic level (about 24–25 km) were as low as 20 ppbv, due to the effect of diabatic subsidence. In December subsidence rates of about 100–180 m/day were observed. Dynamic wave activity at the edge of the vortex led to significant sideways erosion at lower altitudes. However, inside the vortex, at altitudes above 600 K, N2O mixing ratios of less than 10 ppbv were observed until the end of March.

Isotopic enrichment of nitrous oxide (15N14NO, 14N15NO, 14N14N18O) in the stratosphere and in the laboratory

Nitrous oxide (N2O) extracted from stratospheric whole air samples has been analyzed for its 15N and 18O isotopic composition, and strong enrichments in the heavy isotopes are observed concomitant with decreasing N2O mixing ratio. Notably, the N enrichment is strongly different at the two nonequivalent positions in the molecule. Laboratory broadband photolysis experiments at wavelengths representative for the stratosphere confirm that photolysis is the prime cause for the observed fractionation in the stratosphere. However, the in situ stratospheric fractionation constants are significantly reduced compared to the laboratory data, reflecting the importance of dynamic processes. In addition, small but significant variations in the ratio of the two 15N fractionation constants indicate the influence of additional chemical processes like the oxidation of N2O by O(1 D).

Intercomparison of balloon borne cryogenic whole air samples during the MAP/GLOBUS '83 campaign

Abstract Two cryogenic whole air samplers were flown almost simultaneously, but on seperate gondolas as part of the MAP/GLOBUS campaign in September 1983. Two sets of air samples were collected between 34 and 10 km altitude. This paper describes the technical aspects of this intercomparison. The data obtained from gas-chromatographic analyses for CH4, N2O, CFCl3 and CF2Cl2 are discussed. An error analysis shows that the observational uncertainty of CH4, N2O and CF2Cl2 is comparable to the analytical error for these gases. In the case of CFCl3 it indicates the existence of unidentified experimental uncertainties.

The total hydrogen budget in the Arctic winter stratosphere during the European Arctic Stratospheric Ozone Experiment

Vertical profiles of H2O and CH4 were measured using in situ techniques in the Arctic winter stratosphere during the European Arctic Stratospheric Ozone Experiment (EASOE) 1991/1992 in the pressure range between 12 and 80 hPa. The measurements were grouped according to the meteorological situation, i.e., their position relative to the polar vortex. The mixing ratio of the quantity 2·CH4 + H2O was determined in the altitude region between the hygropause and 30 km to be quasi constant at a mean mixing ratio of 6.91 ± 0.41 ppmv. No significant difference of 2·CH4 + H2O was observed inside and outside the vortex. From the correlation between CH4 and H2O ΔH2O/ΔCH4, i.e., the water vapor yield of the methane oxidation, was calculated to be −1.82 ± 0.21. The globally averaged H2O mixing ratio of air entering the stratosphere was found to be 3.61 ± 0.28 ppmv. The data are compared with studies on the total hydrogen budget over the past 20 years. Despite the variety of instrumental techniques used in these studies, an increase of total hydrogen is apparent over this period.

Natural fluorinated organics in fluorite and rocks

Results of measurements of fluorinated compounds in gasses extracted from igneous and metamorphic rocks are reported. A new extraction method analogous to a pepper mill for geological samples is described. It permits extraction at low temperatures and ensures a rapid transfer of extracted gases from active surfaces to cryogenic pre-concentration loop. Values for CF4, CF3Cl, CF2Cl2, CFCl3, CHF3, SF6 and NF3 in fluorites, granites, basalts and other igneous and metamorphic rocks are reported. It is proposed that trifluoroacetic acid (TFA) that was recently discovered in various environmental archives could also origin from similar geogenic sources.

The variation of available chlorine, Cly, in the arctic polar vortex during EASOE

Whole air samplers were flown on large balloons that were launched from Kiruna (67.9°N, 21.1°E) in the period from 30 November 1991 until 20 March 1992 during EASOE. Thirteen vertical profiles of the CCl2F2 mixing ratio were obtained from the analyses of stratospheric air samples that were collected at altitudes between about 10 and 31 km. An empirical correlation function derived from simultaneous profile observations of CCl2F2 and the five other most abundant organic chlorine species was used to determine the vertical distribution of total organic chlorine (Σ Clorg) for each flight. These profiles allowed to estimate vertical profiles of the mixing ratio of total available chlorine, Cly, and the temporal variation of its column abundance.

Global stratospheric distribution of halocarbons

A 16 yr database of stratospheric halocarbon profiles obtained by cryogenic whole air sampling and subsequent trace gas analyses is presented.Twenty-eight balloon ascents of cryogenic samplers were carried out at 44°N, supplemented by 2 balloon flights at 17.5°N and 3 flights at 69°N, between 1977 and 1993. The resultant vertical profiles of CCl4, CCl3F, CCl2F2, CClF3, CF4, CCl2FCClF2, CClF2.CClF2, CClF2CF3, CF3CF3, CH3Cl, CHClF2, CH3CCl3, CBrClF2 and CBrF3 were averaged using relative values with respect to tropospheric mixing ratios, yielding reference profiles for middle, low and high northern latitudes. In addition, latitude-altitude cross-sections of CCl3F, CCl2F2, and CH3CCl3 derived from the balloon data and additional aircraft data are shown for the northern hemisphere.

The Vertical Distribution of Halocarbons in the Stratosphere

By means of cryogenic sampling and subsequent gas-chromatographic analysis vertical profiles of CCl4, CCl3F, CCl2F2, CClF3, CF4, C2Cl3F3, C2Cl2F4, C2ClF5, C2F6, CH3Cl and CH3CCl3 were derived for stratospheric heights up to 35 km. Vertical profiles of halocarbons computed by means of one-dimensional and two-dimensional models fall off less rapidly in the stratosphere than the measured profiles, this systematic discrepancy being due to deficiencies in the radiation and transport schemes of present models. It is shown that measured profiles of fully halogenated hydrocarbons provide a tool for systematically studying these deficiencies and thus improving the models. Sources and sinks of halocarbons are discussed, and an assessment of past and future sources of organically bound chlorine in the atmosphere is made.

Formation of volatile iodinated alkanes in soil: results from laboratory studies.

Volatile iodinated organic compounds play an important role in the tropospheric photochemical system, but the current knowledge of the known sources and sinks of these alkyl iodides is still incomplete. This paper describes a new source of alkyl iodides from the pedosphere. Different organic-rich soils and humic acid were investigated for their release of volatile organoiodides. Six volatile organoiodides, iodomethane, iodoethane, 1-iodopropane, 2-iodopropane, 1-iodobutane and 2-iodobutane were identified and their release rates were determined. We assume an abiotic reaction mechanism induced by the oxidation of organic matter by iron(III). The influence of iron(III), iodide and pH on the formation of alkyl iodides was investigated. Additionally, different organic substances regarded as monomeric constituents of humus were examined for the production of alkyl iodides. Two possible reaction pathways for the chemical formation of alkyl iodides are discussed. As humic acids and iron(III) are widespread in the terrestrial environment, and the concentration of iodide in soil is strongly enriched (compared to seawater), this soil source of naturally occurring organoiodides is suggested to contribute significantly to the input of iodine into the troposphere.