Untersuchung organischer Spurengase in der Troposphäre : globale Verteilung, jahreszeitliche Variationen und langfristige Trends

In this study the spatial and temporal variations of the mixing ratios of organic trace gases in the lower troposphere were investigated . Air samples were collected in stainless steel canisters and analyzed for their contents of carbon dioxide, carbon monoxide, methane, nonmethane hydrocarbons and halocarbons. Characterizations of the used gas chromatographic systems and the resuits of international intercomparison experiments showed that the analytical methods are suitable for the measurement of organic trace gases at Iow concentration levels. The latitudinal distribution of trace gases in the boundary layer over the Atlantic was measured during the cruise of the German research vessel `Polarstern' as part of the ALBATROSS campaign (October/November 1996) . The measurements covered a latitude range between 67°N and 45°S . In this work the distribution of some halogenated hydrocarbons in marine air was measured for the First time . Highest mixing ratios of hydrocarbons and several anthropogenic halocarbons (CH2C1-,, CHCL, 1,2-C 2H4C12 , C7HC13, C2CL und CH3Br) were observed in the Northern Hemisphere between 42°N to 67°N latitude. The mixing ratios of 1,1-dichloroethene, 1,1,1-trichloroethane, dibromomethane, tribromomethane and iodomethane showed a rather uniform distribution in both hemispheres . The observed mixing ratios of methylchloride and tetrachloromethane showed Iow maxima near the equator. The seasonal variations of the mixing ratios of organic trace gases were measured in whole air samples collected in Alert (Canadian Arctic) over a time period of 7 years between January 1989 and July 1996 . The resuits allowed to estimate lang term trends of mixing ratios for a number of compounds . The mixing ratios of most hydrocarbons decreased between 2.2 to 14 .4 % per year. Except tetrachloroethene, for other halocarbons (CH3CI, CH-C12 and C,HCL), no significant trends of the Fnixing ratios were found. Tetrachloroethene showed an annual decrease of 8 .3 % per year. The observed mixing ratios of the trace gases showed pronounced seasonal variations with maxima in winter and minima in sumrner. These variations are mainly caused by the meteorologieal conditions in the Arctic. The ratios of winter and summer concentrations were different for each compound. The plot of the winter/summer ratio as a function of the reaction rates with OH radicals showed a maximum for C4-C5 -alkanes. A global Chemical Tracer Model (CTM) allowed a qualitative simulation of the observed seasonal variations. For the quantitative description of the observed data with the CTM local sources of hydrocarbons are necessary. The estimated source strength ranged from less then 0 .09 up to 0 .9 ng .m 2 . s -' depending an the hydrocarbon . Due to the location of Alert the ocean is discussed as a local source for these hydrocarbon

Diffusion technique for the production of gas standards for atmospheric measurements

For the calibration of gas chromatographic measurements of volatile organic compounds in ambient air samples, standard gas mixtures at low concentrations are needed with high accuracy. For this purpose we developed a diffusion device combined with a dynamic dilution system. Pure liquid compounds are placed in glass vials. They diffuse through a capillary on top of each vial into a diffusion chamber flushed with synthetic air. In an additional dynamic dilution step the final concentration is adjusted with a flow of purified synthetic air to typical mixing ratios between several ppt (v/v) and ppb (v/v). The diffusion rates are determined from the mass loss of the vials. Extensive tests over 21 months showed that the diffusion rates varied little with time, between 1.4% and 3.1%, depending on the compound. The system proved to be suitable for compounds with a wide range of boiling points, from 305 K (1,1-dichlorethene) to 418 K (1,2-dimethylbenzene) The diffusion device was applied to a gas chromatographic system with a flame ionization detector and an electron-capture detector. The linearity of the diffusion device was checked with different standard mixtures with mixing ratios ranging from 0.32 ppt (v/v) (tribromomethane) to 1353 ppt (v/v) (n-pentane). The regression analysis of peak area versus concentration showed excellent agreement among the standards for each compound with correlation coefficients (r(2)) between 0.9826 and 0.9998. The temporal stability of the diffusion device was determined from more than 270 measurements of one standard mixture. The reproducibility of the peak areas ranged between 2.2% and 12.7% depending on the compound. (C) 1999 Elsevier Science B.V. All rights reserved

Meridional distribution of hydroperoxides and formaldehyde in the marine boundary layer of the Atlantic (48N - 35S) measured during the ALBATROSS campaign

AbstractGas phase H2O2, organic peroxides, and formaldehyde (HCHO) have been measured in situ during October/November 1996 on board RV Polarstern in surface air over the Atlantic from 48°N-35°S with different analytical methods. The results indicate that recombination and self-reactions of peroxy radicals largely dominate over scavenging by NO. The peroxy radical chemistry was governed by the photooxidation of CH4 and CO, as could be deduced from our failure to detect organic hydroperoxides other than CH3OOH (methyl hydroperoxide (MHP)). Hydroperoxide and formaldehyde mixing ratios were highest within the tropics with peak values of around 2000 parts per trillion by volume (pptv) (H2O2), 1500 pptv (MHP), and 1000 pptv (HCHO). In the case of H2O2 and MHP we observed diurnal variations of the mixing ratios in the tropical North Atlantic and derived deposition rates of around (1.8±0.6) 10-5 s-1 for H2O2 and (1.2±0.4) 10-5 s-1 for MHP. The measured MHP/(H2O2+MHP) and MHP/HCHO ratios corresponded to 0.32±0.12 and 0.87±0.4, respectively. HCHO mixing ratios observed during the expedition were significantly higher than predicted by current photochemical theory based on the photooxidation of CH4 and CO

Removal of SO2 from the marine boundary layer over the Atlantic Ocean : a case study on the kinetics of the S(IV)oxidation on marine aerosols

Measurements of SO2 and NSS-SO42- were made over the Atlantic Ocean on board the RV Polarstern from October 9 to November 2, 1996, as part of the ALBATROSS campaign. The measurements were performed between 66.7 degrees N and 37.8 degrees S with a mean longitude of approximately 30 degrees W. The most frequent background values for SO2 were found to be 13 parts per trillion by volume (pptv) (0.54 mnol m(-3) at standard ambient temperature and pressure (SATP)) in the Southern Hemisphere, and 15 pptv (0.62 nmol m(-3) SATP) in the Northern Hemisphere. The mean values for total NSS-SO42- in particles with a d > 0.2 mu m were (5.99 +/- 2.93) nmol m(-3) (SATP) in the Southern Hemisphere, and (8.93 +/- 5.29) nmol m(-3) (SATP) in the Northern Hemisphere. An analysis of the size-fractionated aerosol samples (d > 1 mu m and 0.2 mu m 1 mu m. The main fraction of this NSS-SO42-. is most likely produced by the oxidation of dissolved SO2 via heterogeneous reactions occurring in the aqueous phase of coarse mode marine aerosols. A case study on the kinetics of this oxidation pathway was conducted during ALBATROSS. October 12, 1996, the ship sailed in the plume of a volcano on Iceland during its eruption from September 30 to October 13, 1996, as indicated by trajectory analysis and by the measurements of NSS-SO42- SO2, CO, and Hg, An empirical physicochemical approach considering the atmosphere as a natural flow reactor is used for the presented case study. The determined pseudo-first-order reaction rate constant for;the oxidation of SO, on marine aerosols is 3.31 x 10(-4) s(-1) at 25 degrees C. Assuming that the occurrence of coarse mode marine aerosols is the rate-limiting variable of the reaction, the second-order reaction rate constant is found to be 1.32 x 10(-6) cm(3) s(-1) particle(-1) at 25 degrees C. These values are in good agreement with results of previous field experiments as well as with the results of model studies