ALBERT

Description: We computed high-resolution (1º latitude x  1º longitude) seasonal and annual nitrous oxide (N2O) concentration fields for the Arabian Sea surface layer using a database containing more than 2400 values measured between December 1977 and July 1997. N2O concentrations are highest during the southwest (SW) monsoon along the southern Indian continental shelf. Annual emissions range from 0.33 to 0.70 Tg N2O and are dominated by fluxes from coastal regions during the SW and northeast monsoons. Our revised estimate for the annual N2O flux from the Arabian Sea is much more tightly constrained than the previous consensus derived using averaged in-situ data from a smaller number of studies. However, the tendency to focus on measurements in locally restricted features in combination with insufficient seasonal data coverage leads to considerable uncertainties of the concentration fields and thus in the flux estimates, especially in the coastal zones of the northern and eastern Arabian Sea. The overall mean relative error of the annual N2O emissions from the Arabian Sea was estimated to be at least 65%.

Description: The heterogeneous reaction SO2 + H2O2   H2SO4 on ice at 228 K has been studied in a low temperature coated-wall flow tube. With H2O2 in excess of SO2, the loss of SO2 on an ice surface is time dependent with the reaction most efficient on a freshly exposed surface. The deactivation of the surface arises because the protons formed in the reaction inhibit the dissociation of adsorbed SO2. This lowers the surface concentrations of HSO3-, a participant in the rate-determining step of the oxidation mechanism. For a fixed SO2 partial pressure of 1.4 x 10-4 Pa, the reaction probabilities for SO2 loss on a freshly exposed surface scale linearly with H2O2 partial pressures between 2.7 x 10-3 and 2.7 x 10-2 Pa because the H2O2 surface coverage is unsaturated in this regime. Conversely, the reaction probabilities decrease as the partial pressure of SO2 is raised from 2.7 x 10-5 to 1.3 x 10-3 Pa, for a fixed H2O2 partial pressure of 8.7 x 10-3 Pa. This is expected if the rate determining step for the mechanism involves HSO3- rather than SO2. It may also arise to some degree if there is competition between gas phase SO2 and H2O2 for adsorption sites. The reaction is sufficiently fast that the lifetime of SO2 within ice clouds could be controlled by this heterogeneous reaction and not by the gas-phase reaction with OH.

Description: A two-dimensional dynamic cloud model with detailed microphysics and a spectral treatment of gas scavenging was used to simulate trace gas vertical redistribution in precipitating continental and maritime clouds. A general picture of gas transport in such clouds has been developed by examining the sensitivity to a range of parameters, including cloud dynamic and microphysical structure, gas solubility, and the method of calculating gas uptake by droplets. Gases with effective Henry's law constants (H*) ranging from zero to greater than 109 mol dm-3 atm-1 were simulated. The abundance of highly soluble gases in the uppermost parts (top 1 km or so) of continental precipitating clouds was found to be as much as 20-50% of that of the insoluble tracer under conditions where the mixing ratio of the tracer was approximately 5% of its boundary layer value. The abundance of highly soluble gases was approximately 6 times higher in the uppermost parts of the continental cloud than in the maritime cloud, due to differences in wet removal efficiency in the two cloud types. A fully kinetic calculation of gas uptake, as opposed to assuming Henry's law equilibrium, was found to have a significant effect on gas transport, with the abundance of highly soluble gases in the uppermost parts of the cloud being a factor of 5 lower in the equilibrium simulations. The temperature dependence of the Henry's law constant was also found to be an important parameter in determining the abundance of soluble gases at cloud top, with the abundance of moderately soluble gases being as much as 70% lower when the temperature dependence of H* was included. This reduction in abundance was found to be equivalent to increasing the temperature-independent solubility by a factor of 7. The vertical transport of soluble gases could be parameterized in large-scale models by normalizing against the transport of tracers. However, our results suggest that there is no straightforward scaling factor, particularly if small concentrations of highly soluble gases in the upper troposphere need to be defined.

Description: A novel method to characterize the organic composition of aerosol particles has been developed. The method is based on organic vapor interaction with aerosol particles and it has been named an Organic Tandem Differential Mobility Analyzer (OTDMA). The OTDMA method has been tested for inorganic (sodium chloride and ammonium sulfate) and organic (citric acid and adipic acid) particles. Growth curves of the particles have been measured in ethanol vapor and as a comparison in water vapor as a function of saturation ratio. Measurements in water vapor show that sodium chloride and ammonium sulfate as well as citric acid particles grow at water saturation ratios (S) of 0.8 and above, whereas adipic acid particles do not grow at S

Description: The global mean OH concentration ([OH]GM) has been used as an indicator of the atmospheric oxidizing efficiency and its changes over time. It is also used for evaluating the performance of atmospheric chemistry models by comparing with other models or with observationally-based reference [OH]GM levels. We contend that the treatment of this quantity in the recent literature renders it problematic for either of these purposes. Several different methods have historically been used to compute [OH]GM: weighting by atmospheric mass or volume, or by the reaction with CH4 or CH3CCl3. In addition, these have been applied over different domains to represent the troposphere. While it is clear that this can lead to inconsistent [OH]GM values, to date there has been no careful assessment of the differences expected when [OH]GM is computed using various weightings and domains. Here these differences are considered using four different 3D OH distributions, along with the weightings mentioned above applied over various atmospheric domains. We find that the [OH]GM values computed based on a given distribution but using different domains for the troposphere can result in differences of 10% or more, while different weightings can lead to differences of up to 30%, comparable to the uncertainty which is commonly stated for [OH]GM or its trend. Thus, at present comparing [OH]GM values from different studies does not provide clearly interpretable information about whether the OH amounts are actually similar or not, except in the few cases where the same weighting and domain have been used in both studies. We define the atmospheric oxidizing efficiency of OH with respect to a given gas as the inverse of the lifetime of that gas, and show that this is directly proportional to the [OH]GM value weighted by the reaction with that gas, where the proportionality constant depends on the temperature distribution and the domain. We find that the airmass-weighted and volume-weighted [OH]GM values, in contrast, are generally poor indicators of the global atmospheric oxidizing efficiency with respect to gases such as CH4 and CH3CCl3 with a strong temperature dependence in their reaction with OH. We recommend that future studies provide both the airmass-weighted and the CH4-reaction-weighted [OH]GM values, over the domain from the surface to a climatological tropopause. The combination of these values helps to reduce the chance of coincidental agreement between very different OH distributions. Serious evaluations of modeled OH concentrations would best be done with airmass-weighted [OH]GM broken down into atmospheric sub-compartments, especially focusing on the tropics, where the atmospheric oxidizing efficiency is the greatest for most gases.

Description: The kinetics of the title reactions were investigated using the laser photolysis - resonance fluorescence method, employing the sequential two-photon dissociation of NO2 in the presence of H2  as the OH source. The 298 K rate constant for OH + C3H8 was found to be (1.15 ± 0.1) × 10-12 cm3 s-1, in excellent agreement with the literature recommendation, and with a separate determination using HNO3  photolysis at 248 nm as the OH source. The 298 K rate constants for OH + n - C3H7I and  i - C3H7I  were measured for the first time and found to be (1.47 ± 0.08) and (1.22 ± 0.06) × 10-12 cm3 s-1, respectively. The errors include an assessment of systematic error due to concentration measurement, which, for the propyl-iodides was minimised by on-line UV-absorption spectroscopy. These results show that reaction with OH is an important sink for  n - C3H7I and  i - C3H7I, which has implications for the reactive iodine budget of the marine boundary layer.

Description: At Mace Head, Eire, in the coastal East Atlantic, diiodomethane has been identified as an important precursor of iodine oxide radicals. Peak concentrations of both CH2I2 and IO at low water indicate that the intertidal region is a strong source of organo-iodines. Atmospheric measurements of CH2I2 made in marine air are compared with the concentrations predicted by a 2-dimensional model incorporating horizontal and vertical dispersion of surface emissions. The model shows that micrometeorological variability, proximity of the site to emissions, and photolysis all play important roles in determining the CH2I2 concentrations at Mace Head. In addition to a tidal-height dependent intertidal flux, which was estimated from seaweed production data, a contribution from offshore (non-local) sources was required in order to reproduce the strong signature of photolysis in the CH2I2 observations. A combination of an offshore flux and an intertidal flux (of up to 1.4 × 109 molecules cm-2s-1 at low water) results in good agreement between the measured and modelled CH2I2 concentrations. Although this study does not necessarily infer emission of CH2I2 from the open ocean, it suggests that air-sea exchange of CH2I2 in coastal waters does occur.

Description: As part of the German Research Foundation (DFG) project “Land Use Concepts for Marginal Regions”, since 1997 we have made analyses of the seed bank of 22 cultivated allotments, as well as of 15 meadow/pasture and 16 fallow allotments on former arable land of the Lahn-Dill Highlands, a marginal cultivated landscape in Hesse, Germany. One aim of this study is to determine according to which dynamic laws the seed bank of arable-land weeds is depleted after cultivation is abandoned. Depending on the intensity of the arable land use, the seed banks of rankers and cambisols that are presently still cultivated contain up to 1 004 800 seeds of arable-land species capable of germination m−3. Soils that were last cultivated less than ten years ago have much lower seed densities, while the seed bank of arable-land species is largely exhausted after only ca. 20 yr. This points to an exponential depletion of the seed bank of arable-land species and their abundance over time. On the basis of the present results,

Description: Spartina anglica (C. E. Hubbard) is a relatively new invasive perennial grass species in Ireland. It is well adapted to the intertidal mudflat environment and forms mono-specific swards. There have been concerns about its potential to impact negatively the ecosystems of mud flats and salt marshes. This ongoing project investigates the ecological effects of S. anglica, and its control, on the mudflats and saltmarsh at Bull Island, and the implications for management of S. anglica. The diversity and density of the macro-invertebrate infauna and some physical factors of the sediment were compared in: a) clumps of S. anglica, b) areas vegetated by Salicornia spp., c) an adjacent area of bare mud, and d) an unvegetated area. Presence of S. anglica had a significant effect on the density and diversity of macro-invertebrate infauna species. The results suggest that clumps of S. anglica can provide a habitat that supports a macro-invertebrate infauna as abundant and species rich as areas vegetated by Salicornia spp.

Description: The edge effect, generated by the interaction of patch and matrix, is preferably described by the interior-to-edge ratio of the patch. This ratio quantifies the extent of microclimatic changes at the boundary, and influences nature reserve design. As shown for elliptical and rectangular shapes, large and isodiametric patch designs are characterized by high interior-to-edge ratios. Different patch geometries can however lead to similar values of the ratio. A reference value, based upon the patch size, is therefore proposed to normalize the ratio to its maximum value, as observed for a perfectly isodiametric patch. The effect of patch geometry on the normalized ratio is discussed, as well as patch ranking based upon both the simple ratio and the normalized interior-to-edge ratio. An example is included using forest patches in the Belgian Campine region to illustrate the usefulness of the proposed index.