C. Plass-Dülmer

Strong daytime production of OH from HNO2 at a rural mountain site

Nitrous acid and OH were measured concurrently with a number of other atmospheric components and relevant photolysis frequencies during two campaigns at the Meteorological Observatory Hohenpeissenberg (980 m a.s.l.) in summer 2002 and 2004. On most of the 26 measurement days the HNO 2 concentration surprisingly showed a broad maximum around noon (on average 100 pptv) and much lower concentrations during the night (∼30 pptv). The results indicate a strong unknown daytime source of HNO 2 with a production rate on the order of 2-4 x 10 6 cm -3 s -1 . The data demonstrate an important contribution of HNO 2 to local HO x levels over the entire day, comparable with the photolysis of O 3 and HCHO. On average during the 2004 campaign, 42% of integrated photolytic HO x formation is attributable to HNO 2 photolysis.

Chemical ionization mass spectrometer for long-term measurements of atmospheric OH and H2SO4

Abstract An atmospheric pressure/chemical ionization mass spectrometer (AP/CIMS) has been developed for continuous long-term measurements of atmospheric OH and H2SO4. The corresponding methods both involve chemical ionization of H2SO4 by NO3− ions with OH being first titrated by excess SO2 to form equivalent concentrations of H2SO4 in the system. The chemical ionization mass spectrometry (CIMS) system has been operated since April 1998 at the Meteorological Observatory Hohenpeissenberg, a mountain research station of the German Weather Service in South Germany. A technical description of the apparatus is presented followed by a detailed estimate of uncertainties in calibration and ambient air measurements resulting from changes in instrumental and/or ambient parameters. Examples from both calibration runs and ambient air measurements are shown. For the present system and operating conditions accuracy, precision, and detection limit are estimated to be 39%, 30%, and 3 × 104 molecules cm−3 for H2SO4, and 54%, 48%, and 5 × 105 molecules cm−3 for OH measurements, respectively, based on 5 min signal integration.

Distribution of methylchloride, dichloromethane, trichloroethene and tetrachloroethene over the north and south Atlantic

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.

Evolution of newly formed aerosol particles in the continental boundary layer : A case study including OH and H2SO4 measurements

An event of new particle formation is presented, based on simultaneous measurements of aerosol number size distributions, relevant gaseous components including H2SO4 and OH, and meteorological parameters. Measurements were conducted at Hohenpeissenberg, a rural continental mountain site in southern Germany. The event was observed under intense solar radiation, with total particle number concentrations increasing from 6000 to 25000 cm−3 within one hour, and ultrafine particles (3–11 nm) accounting for more than 50% of total number. Observed OH and H2SO4 concentrations reached maximum levels around 107 cm−3. A lower limit of the particle nucleation rate was estimated to be 3 cm−3·s−1, consistent with present models of ternary nucleation involving the H2SO4-H2O-NH3 system. Roughly 80% of the subsequent drop in ultrafine mode particle number concentration could be explained by coagulation. The observed particle growth rate of 2.1±0.1 nm/h was largely attributed to the condensation of measured H2SO4, assuming neutralization by ammonia.

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.

Intercomparison of tropospheric OH radical measurements by multiple folded long‐path laser absorption and laser induced fluorescence

An intercomparison of in-situ OH measurements by differential optical absorption spectroscopy (DOAS) and laser-induced fluorescence spectroscopy (LIF) was carried out in August 1994 in a clean rural environment in North-East Germany. A large data set of temporally overlapping OH measurements with well defined measurement errors was obtained and compared. Both instruments encountered the same air masses, except when the wind came from NNW and caused a perturbation of the DOAS measurements. Excluding that wind sector, the weighted regression analysis of 137 data pairs (70% of all available data pairs) yields a linear relationship between the DOAS and LIF measurements with a correlation coefficientr = 0.90. The unity slope (1.01±0.04) and the non-significant intercept (0.28±0.15) × 106 cm−3 demonstrate that both OH instruments agreed excellently in their calibrations and accurately measured OH.

Production mechanism of C2‐C4 hydrocarbons in seawater: Field measurements and experiments

The production mechanism of light nonmethane hydrocarbons (NMHC) in seawater was investigated during the North Atlantic atmospheric chemistry program (NATAC) in April and May 1991 in the European coastal seas and the North Atlantic. A significant alkene production occurred in the presence of light only. Under conditions of negligible NMHC emissions (low wind velocity) increasing hydrocarbon concentrations were observed during daytime, whereas the concentrations remained constant during night. NMHC formation experiments were carried out with seawater filled in quartz glass bottles and showed the same dependence of light. Experiments with differently pretreated seawater samples indicated that the presence of dissolved organic material (DOM) is also necessary for alkene production. We suggest a two-step production mechanism for alkenes: first DOM is released, probably from algae, then part of this material is photochemically transformed into alkenes. The production rates in the quartz glass bottles were comparable to the production rates in the ocean surface. This indicates that the processes occurring in the experimental setups represent the processes occurring in the field. Since the production - and emission rates were in the same range it can be concluded that the budget of light alkenes in the remote marine environment is determined by the production in seawater as the dominant source and the flux into the atmosphere as the main loss process.

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.

POPCORN: A Field Study of Photochemistry in North-Eastern Germany

The intensive field study POPCORN (Photo-Oxidant Formation by Plant Emitted Compounds and OH Radicals in North-Eastern Germany) was carried out in a rural area of North-Eastern Germany during August 1994. An overview of the objectives, measurements and major results of this campaign is presented. Measurements of a set of relevant atmospheric trace compounds, including the hydroxyl radical, along with meteorological data were performed to increase the understanding of OH radical chemistry and photo-oxidant formation. Additionally, plant emissions and the exchange of trace gases between a maize field and the atmosphere were investigated. Budgets of selected trace gases were calculated to assess the relative importance of local sources, chemistry or transport. Intercomparisons between measurement techniques were a central issue of POPCORN and included measurements of OH, hydrocarbons, formaldehyde, photolysis frequencies and vertical fluxes. OH radical concentrations were measured simultaneously by LIF (Laser Induced Fluorescence) and DOAS (Differential Optical Absorption Spectroscopy). Both methods showed good agreement. Maximum OH concentrations were around 107 cm−3 and the diurnal cycles closely followed the rate of primary production via ozone photolysis. Generally, the trace gas composition during POPCORN was characterized by relatively low concentrations of most compounds, e.g. CO: 85–200 ppb, ethane: 0.6–2 ppb, and moderate NOx levels: 0.5–5 ppb (at noontime). Concentrations of individual biogenic volatile organic compounds (VOC) were mostly well below 100 ppt. However, formaldehyde and acetaldehyde which partly originate from biological sources were observed at mixing ratios of some ppb.

Sampling, storage, and analysis of C2-C7 non-methane hydrocarbons from the US National Oceanic and Atmospheric Administration Cooperative Air Sampling Network glass flasks

An analytical technique was developed to analyze light non-methane hydrocarbons (NMHC), including ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, n-hexane, isoprene, benzene and toluene from whole air samples collected in 2.5l-glass flasks used by the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division (NOAA ESRL GMD, Boulder, CO, USA) Cooperative Air Sampling Network. This method relies on utilizing the remaining air in these flasks (which is at below-ambient pressure at this stage) after the completion of all routine greenhouse gas measurements from these samples. NMHC in sample aliquots extracted from the flasks were preconcentrated with a custom-made, cryogen-free inlet system and analyzed by gas chromatography (GC) with flame ionization detection (FID). C2-C7 NMHC, depending on their ambient air mixing ratios, could be measured with accuracy and repeatability errors of generally < or =10-20%. Larger deviations were found for ethene and propene. Hexane was systematically overestimated due to a chromatographic co-elution problem. Saturated NMHC showed less than 5% changes in their mixing ratios in glass flask samples that were stored for up to 1 year. In the same experiment ethene and propene increased at approximately 30% yr(-1). A series of blank experiments showed negligible contamination from the sampling process and from storage (<10 pptv yr(-1)) of samples in these glass flasks. Results from flask NMHC analyses were compared to in-situ NMHC measurements at the Global Atmospheric Watch station in Hohenpeissenberg, Germany. This 9-months side-by-side comparison showed good agreement between both methods. More than 94% of all data comparisons for C2-C5 alkanes, isoprene, benzene and toluene fell within the combined accuracy and precision objectives of the World Meteorological Organization Global Atmosphere Watch (WMO-GAW) for NMHC measurements.