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Item Open Access A Comparison of Measured OH Concentrations with Model Calculations(AGU, 1994) Poppe, D.; Zimmerman, J.; Bauer, R.; Brauers, T.; Brüning, D.; Callies, J.; Dorn, H.P; Hofzumahaus, A.; Johnen, F.J.; Khedim, A.; Hoch, H.; Koppmann, R.; London, H.; Müller, K.P; Neuroth, R.; Plass-Dülmer, C.H; Platt, U.; Rohrer, F.; Röth, E.P; Rudolph, J.; Schmidt, U.; Wallasch, M.; Ehhalt, D.HThe 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 NO x 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 Jülich (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 Jülich with average NO x 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 Jülich. 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.Item Open Access The distribution of light nonmethane hydrocarbons over the mid-Atlantic: Results of the Polarstern cruise ANT VII/1(Springer Netherlands, 1992) Koppmann, R.; Bauer, R.; Johnen, F.J.; Plass-Dulmer, C.; Rudolph, J.During the cruise ANT VII/1 (September/October 1988) of the German research vessel Polarstern the latitudinal distributions of several nonmethane hydrocarbons were measured over the Atlantic between 45°N and 30°S by in-situ gas chromatography. On the average, the highest mixing ratios of ethane, propane, i- and n-butane, ethene and acetylene were observed in the Northern Hemisphere around 40° N and just north of the intertropical convergence zone, respectively. South of the equator, a bulge in the mixing ratios of ethane and acetylene was observed indicating aged biomass burning emissions. This observation coincided with enhanced tropospheric ozone found in this region at this season. On the average ethane and acetylene mixing ratios were around 500 and 100 ppt, respectively, whereas the levels of the other NMHC were in the range of some ppt up to 100 ppt. compared with the results of the cruise ANT V/5 (March/April, 1987), the ethane mixing ratios in September/October proved to be a factor of 3 lower in the Northern Hemisphere and a factor of 2 higher in the Southern Hemisphere, probably due to seasonal effects. Possible causes are the higher OH radical concentrations in summer, which result in a faster removal of ethane or stronger emission from biomass burning which also peaks in the dry season. The relative pattern of the hydrocarbons just north of the ITCZ was very similar for both measurement series. In this region, the NMHC were advected by long-range transport from the continent, whereas generally the ocean itself acts as a major NMHC source. This is supported by the results of a balance calculation between oceanic emissions and atmospheric removal rates.Item Open Access The Distribution of Methyl Chloride, Dichloromethane, Trichloroethene, and Tetrachloroethene over the Atlantic(AGU, 1993) Koppmann, R.; Johnen, F.J.; Plass-Dülmer, C.; Rudolph, J.During the cruise ANT VIII/1 of the German R/V Polarstern in August/September 1989 the latitudinal distributions of the atmospheric concentrations of methylchloride, dichloromethane, trichloroethene, and tetrachloroethene were measured over the Atlantic between 45°N and 30°S by in situ gas chromatography. With the exception of trichloroethene they showed mixing ratios well above the lower limit of detection. The methylchloride distribution was uniform with average mixing ratios of 532 ± 8 and 550 ± 12 ppt in the northern and southern Hemispheres, respectively. Dichloromethane increased linearly between the Intertropical Convergence Zone and 45°N with average mixing ratios of 36 ± 6 ppt and was almost constant in the southern hemisphere with an average of 18 ± 1 ppt. Tetrachloroethene mixing ratios were between less than 1 and 10 ppt in the northern hemisphere and always below 3 ppt in the southern hemisphere. Similar to dichloromethane, tetrachloroethene was nearly constant in the southern hemisphere and increased linearly toward northern latitudes. This is compatible with the predominantly industrial origin of these compounds. Trichloroethene varied between 0.3 ppt and about 15 ppt in the northern hemisphere with an average of 3 ± 1 ppt and was generally lower than 1 ppt in the southern hemisphere with mixing ratios often near or below the detection limit of 0.1 ppt. For CH3Cl we estimate a global turnover of 3.5 × 1012 g/yr which is compatible with previous results. Using a simple model calculation our measurements imply a global turnover for CH2Cl2 and C2Cl4 of 0.9 × 1012 g/yr and 0.6 × 1012 g/yr, respectively.Item Open Access Emissions of Light Nonmethane Hydrocarbons from the Atlantic into the Atmosphere(AGU, 1993) Plass-Dülmer, C.; Koppmann, R.; Johnen, F.J.; Rudolph, J.; Kuosa, H.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.Item Open Access The Influence of Ozone on Light Hydrocarbons During Cryogenic Preconcentration(AGU, 1995) Koppmann, R.; Johnen, F.J.; Khedim, A.; Rudolph, J.; Wedel, A.; Wiards, B.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.Item Open Access Measurements of light atmospheric hydrocarbons over the Atlantic in regions of low biological activity(AGU, 1990) Rudolph, J.; Johnen, F.J.More than 200 in situ measurements of several selected nonmethane hydrocarbons (NMHCs) were made in the remote marine atmosphere over the Atlantic between 40°S and 50°N. Ethane, the by far longest lived of the NMHCs, showed southern hemispheric mixing ratios around 280 ppt, comparable in magnitude to most other previous measurements. The mixing ratios of ethene, propene, propane, and i‐ and n‐butane in the southern hemisphere were in the range of 10–30 ppt. For i‐pentane and n‐pentane the atmospheric mixing ratios in the southern hemisphere were below the detection limit of 15–20 ppt. These values are rather low compared to other published measurements in the marine atmosphere. This is due to the remoteness of the measuring locations and the very low biological activity in the surrounding ocean areas. For all alkanes the latitudinal profiles exhibit a considerable decrease from north to south. The relative hydrocarbon patterns show that the alkanes in the northern hemisphere are primarily due to long‐range transport from continental or coastal areas. In general the transport times exceed several days. Consequently, the observed northern hemispheric ethene and propene mixing ratios must be, with few exceptions, primarily the result of oceanic emissions. The average difference of these compounds by a factor of 2 between the southern and the northern hemisphere can be explained by change of the phytoplankton concentration in ocean water. There is little or no indication for the existence of significant diurnal cycles for these two alkenes.Item Open Access An optimized method for airborne peroxyacetyl nitrate (PAN) measureme(Springer Netherlands, 1995) Schrimpf, W.; Muller, K.P.; Johnen, F.J.; Lienaerts, K; Rudolph, J.In this paper we describe a gas-chromatographic method for PAN measurements in the background atmosphere, which has been adapted to the special requirements of aircraft based campaigns. The instrument is installed in a 1.21 m high, 19 inch rack which has a total weight of 70 kg and a power consumption of 750 VA. The gas chromatograph is equipped with a commercial liquid injector and a valve system for injection of gaseous samples. The gas-inlet system allows automatic injection of samples with defined and constant mass, independent from ambient pressure variations. Two different methods are used for calibration: Liquid PAN calibration samples and a diffusion source for gas-phase calibrations. Both methods have reproducibilities better than 90% and agree with each other to better than 85%. An optimum selectivity of the gas-chromatographic separation is obtained by a combination of two short megabore capillary columns of different polarity. The flow rates are 15 cm3/min, the column temperature is 26°C. For detection an electron-capture detector, operated at 30°C, is used. To allow a reliable control of these relatively low temperatures the instrument is equipped with peltier cooling. To avoid baseline or signal drifts caused by pressure variations in the aircraft cabin an electronic control of the system pressure is integrated into the instrument. The lower limit of detection is better than 15 ppt (3 ), the time needed for one measurement is less than 4 min. Preliminary results from a flight campaign conducted in June 1994 demonstrate the suitability of the instrument for airborne PAN measurements.Item Open Access The Use of Automated "On Line" Gaschromatrography for the Monitoring of Organic Trace Gases in the Atmosphere at Low Levels(Taylor & Francis, 1990) Pilwat, G.; Khedim, A.; Johnen, F.J.; Rudolph, J.