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The available database concerning rate constants for gas-phase reactions of the hydroxyl (OH) radical with alkanes through early 2003 is presented over the entire temperature range for which measurements have been made (~180-2000 K). Measurements made using relative rate methods are re-evaluated using recent rate data for the reference compound (generally recommendations from this review). In general, whenever more than one study has been carried out over an overlapping temperature range, recommended rate constants or temperature-dependent rate expressions are presented. The recommended 298 K rate constants, temperature-dependent parameters, and temperature ranges over which these recommendations are applicable are listed in Table 1.
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The Alps represent the largest barrier to meridional air flow in Europe, strongly influencing the weather and hence the distribution of atmospheric trace components. Here for the first time, chemical records from two ice cores retrieved from glaciers located in the northern and southern Swiss Alps were compared in conjunction with an analysis of "weather type", in order to assess geographical and seasonal trends in the deposition of trace species and to identify source regions and transport patterns.<br> <br> Using a correlation analysis, investigated trace species (NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup>−</sup>, SO<sub>4</sub><sup>2−</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Na<sup>+</sup>, K<sup>+</sup>, and Cl<sup>−</sup> were grouped into classes of different origin (anthropogenic, sea salt, or Saharan dust). Over the last 60 years, precipitation chemistry at both sites was dominated by NH<sub>4</sub><sup>+</sup>, NO<sub>4</sub><sup>−</sup>, and SO<sub>4</sub><sup>2−</sup>, all of anthropogenic origin and deposited mainly in summer by way of convective precipitation. The similarity of the SO<sub>4</sub><sup>2−</sup> profiles with historical records of SO<sub>4</sub> emissions from France and Italy indicated these two countries as key source areas for the anthropogenic species.<br> <br> In contrast, sea salt and Saharan dust showed major differences in transport pattern and deposition across the Alps. Currently, the sea-salt constituents Na<sup>+</sup>, K<sup>+</sup>, and Cl<sup>−</sup> are transported to the northern site during advective westerly-wind situations, independent of Saharan dust events. At the southern site, sea salt and Saharan dust are deposited simultaneously, indicating a coupled transport active mainly in summer during south-westerly wind situations.
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Laboratory experimental results of iodine oxide nucleation are presented. Nucleation was induced following UV photolysis of CF<sub>3</sub>I or CH<sub>2</sub>I<sub>2</sub> in the presence of excess ozone. Measurements were performed in a 70 L Teflon reactor with new particles detected using an Ultrafine Condensation Particle Counter, UCPC. The experimental results are interpreted using a coupled chemical - aerosol model to derive model parameters assuming single component homogeneous nucleation of OIO. The aerosol model results have been applied in an atmospheric box-model to interpret the possible implications of iodine oxide nucleation in the marine boundary layer. The model calculations demonstrate that IO and OIO concentrations reported in recent field measurements using long path absorption (Allan et al., 2000, 2001) are not sufficient to account for significant aerosol production either in the coastal or open ocean marine boundary layer using the mechanism presented. We demonstrate that inhomogeneous sources of iodine oxides, i.e. "hot" spots with elevated iodine species emissions, could account for the aerosol production bursts observed in the coastal region near Mace Head, Ireland.
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A sensitivity study of the treatment of isoprene and related parameters in 3D atmospheric models was conducted using the global model of tropospheric chemistry MATCH-MPIC. A total of twelve sensitivity scenarios which can be grouped into four thematic categories were performed. These four categories consist of simulations with different chemical mechanisms, different assumptions concerning the deposition characteristics of intermediate products, assumptions concerning the nitrates from the oxidation of isoprene and variations of the source strengths. The largest differences in ozone compared to the reference simulation occured when a different isoprene oxidation scheme was used (up to 30-60% or about 10 nmol/mol). The largest differences in the abundance of peroxyacetylnitrate (PAN) were found when the isoprene emission strength was reduced by 50% and in tests with increased or decreased efficiency of the deposition of intermediates. The deposition assumptions were also found to have a significant effect on the upper tropospheric HO<sub>x</sub> production. Different implicit assumptions about the loss of intermediate products were identified as a major reason for the deviations among the tested isoprene oxidation schemes. The total tropospheric burden of O<sub>3</sub> calculated in the sensitivity runs is increased compared to the background methane chemistry by 26±9 Tg( O<sub>3</sub>) from 273 to an average from the sensitivity runs of 299 Tg(O<sub>3</sub>). % revised Thus, there is a spread of ± 35% of the overall effect of isoprene in the model among the tested scenarios. This range of uncertainty and the much larger local deviations found in the test runs suggest that the treatment of isoprene in global models can only be seen as a first order estimate at present, and points towards specific processes in need of focused future work.
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Ambient continental-rural fine aerosol (K-puszta, Hungary, PM<sub>1.5</sub>) was sampled on quartz fibre filters in winter and summer 2001. Water-soluble matter (WSM) was extracted in MilliQ-water, and, in a second step, solid phase extraction was used to isolate the less hydrophilic fraction (ISOM) of the water-soluble organic matter (WSOM) from remaining inorganic salts and "most" hydrophilic organic matter (MHOM). This approach allowed ISOM, which constitutes the major fraction of WSOM, to be isolated from ambient aerosols and investigated in pure form. Hygroscopic properties of both WSM and ISOM extracts as well as of aquatic reference fulvic and humic acids were investigated using a Hygroscopicity Tandem Differential Mobility Analyser (H-TDMA). ISOM deliquesced between 30% and 60% relative humidity (RH), and hygroscopic growth factors at 90% RH ranged from 1.08 to 1.17. The hygroscopicity of ISOM is comparable to secondary organic aerosols obtained in smog chamber experiments, but lower than the hygroscopicity of highly soluble organic acids. The hygroscopic behaviour of investigated fulvic and humic acids had similarities to ISOM, but hygroscopic growth factors were slightly smaller and deliquescence was observed at higher RH (75-85% and 85-95% RH for fulvic acid and humic acid, respectively). These differences probably originate from larger average molecular mass and lower solubility of fulvic and humic acids.<br> <br> Inorganic composition data, measured ISOM hygroscopicity, and a presumed value for the hygroscopicity of the small remaining MHOM fraction were used to predict hygroscopic growth of WSM extracts. Good agreement between model prediction and measured water uptake was observed with differences (by volume) ranging from +1% to -18%. While deliquescence properties of WSM extracts were mainly determined by the inorganic salts (42-53 wt % of WSM), the WSOM accounted for a significant fraction of particulate water. At 90% RH, according to model predictions and measurements, about 80-62% of particulate water in the samples are associated with inorganic salts and about 20-38% with WSOM. The relative contributions of both distinguished WSOM fractions, ISOM and MHOM, remains uncertain since MHOM was not available in isolated form, but the results suggest that the less abundant MHOM is also important due to its presumably larger hygroscopicity.
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We have compared satellite and balloon observations of methane (CH<sub>4</sub>) and hydrogen fluoride (HF) during the Arctic winter 1999/2000 with results from the MA-ECHAM4 middle atmospheric general circulation model (GCM). For this purpose, the meteorology in the model was nudged towards ECMWF analyses. This nudging technique is shown to work well for this middle atmospheric model, and offers good opportunities for the simulation of chemistry and transport processes. However, caution must be used inside the polar vortex, particularly late in the winter. The current study focuses on transport of HF and CH<sub>4</sub>, initialized with satellite measurements from the HALOE instrument aboard the UARS satellite. We have compared the model results with HALOE data and balloon measurements throughout the winter, and analyzed the uncertainties associated with tracer initialization, boundary conditions and the passive tracer assumption. This comparison shows that the model represents some aspects of the Arctic vortex well, including relatively small-scale features. However, while profiles outside the vortex match observations well, the model underestimates HF and overestimates CH<sub>4</sub> concentrations inside the vortex, particularly in the middle stratosphere. This problem is also evident in a comparison of vortex descent rates based upon vortex average tracer profiles from MA-ECHAM4, and various observations. This could be due to an underestimate of diabatic subsidence in the model, or due to too much mixing between vortex and non-vortex air.
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Mass spectrometric analysis of volatile and semi-volatile (=non-refractory) aerosol particles have been performed during a field study in the summer Eastern Mediterranean. A size-resolved, quantitative mass spectrometric technique (the Aerodyne Aerosol Mass Spectrometer, AMS) has been used, and the results are compared to filter sampling methods and particle sizing techniques. The different techniques agree with the finding that the fine particle mode (<i>D</i><1.2 <font face="Symbol">m</font>m) consisted mostly of ammonium sulfate and of organic material. The aerosol sulfate ranged between 2 and 12 <font face="Symbol">m</font>g/m<sup>3</sup>. On most days, ammonium was closely correlated with sulfate, suggesting ammonium sulfate as the major aerosol component, but on days with high sulfate mass concentrations, the sulfate was not fully neutralized by ammonium. Trajectories indicate that the aerosol and/or its precursors originate from South-Eastern Europe. The source of the ammonium sulfate aerosol is most likely fossil fuel burning, whereas the organic aerosol may also originate from biomass burning. Ion series analysis of the organics fraction in the mass spectrometer indicated that the major component of the organics were oxygenated organics which are a marker for aged, photochemically processed aerosol or biomass burning aerosol. The non-refractory aerosol compounds, measured with the Aerosol Mass Spectrometer, contributed between 37 and 50% to the total aerosol mass in the fine mode. A second mass spectrometer for single particle analysis by laser ablation has been used for the first time in the field during this study and yielded results, which agree with filter samples of the coarse particle mode. This mode consisted of sea salt particles and dust aerosol.
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The possibility to calculate linear-source receptor relationships for the transport of atmospheric trace substances with a Lagrangian particle dispersion model (LPDM) running in backward mode is shown and presented with many tests and examples. This mode requires only minor modifications of the forward LPDM. The derivation includes the action of sources and of any first-order processes (transformation with prescribed rates, dry and wet deposition, radioactive decay, etc.). The backward mode is computationally advantageous if the number of receptors is less than the number of sources considered. The combination of an LPDM with the backward (adjoint) methodology is especially attractive for the application to point measurements, which can be handled without artificial numerical diffusion. Practical hints are provided for source-receptor calculations with different settings, both in forward and backward mode. The equivalence of forward and backward calculations is shown in simple tests for release and sampling of particles, pure wet deposition, pure convective redistribution and realistic transport over a short distance. Furthermore, an application example explaining measurements of Cs-137 in Stockholm as transport from areas contaminated heavily in the Chernobyl disaster is included.
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A new modelling study of the role of transition metal ions on cloud chemistry has been performed. Developments of the Model of Multiphase Cloud Chemistry (M2C2; Leriche et al., 2001) are described, including the transition metal ions reactivity emission/deposition processes and variable photolysis in the aqueous phase. The model is then applied to three summertime scenarios under urban, remote and marine conditions, described by Ervens et al. (2003).<br> <br> Chemical regimes in clouds are analyzed to understand the role of transition metal ions on cloud chemistry and especially, on H<sub>x</sub>O<sub>y</sub> chemistry, which consequently influences the sulphur and the VOCs chemistry in droplets. The ratio of Fe(II)/Fe(III) exhibits a diurnal variation with values in agreement with the available measurements of Fe speciation. In the urban case, sensitivity tests with and without TMI chemistry, show an enhancement of OH concentration in the aqueous phase when TMI chemistry is considered.
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Downwelling longwave fluxes, DLFs, have been derived for each month over a ten year period (1984-1993), on a global scale with a spatial resolution of 2.5x2.5 degrees and a monthly temporal resolution. The fluxes were computed using a deterministic model for atmospheric radiation transfer, along with satellite and reanalysis data for the key atmospheric input parameters, i.e. cloud properties, and specific humidity and temperature profiles. The cloud climatologies were taken from the latest released and improved International Satellite Climatology Project D2 series. Specific humidity and temperature vertical profiles were taken from three different reanalysis datasets; NCEP/NCAR, GEOS, and ECMWF (acronyms explained in main text). DLFs were computed for each reanalysis dataset, with differences reaching values as high as 30 Wm<sup>-2</sup> in specific regions, particularly over high altitude areas and deserts. However, globally, the agreement is good, with the rms of the difference between the DLFs derived from the different reanalysis datasets ranging from 5 to 7 Wm<sup>-2</sup>. The results are presented as geographical distributions and as time series of hemispheric and global averages. The DLF time series based on the different reanalysis datasets show similar seasonal and inter-annual variations, and similar anomalies related to the 86/87 El Niño and 89/90 La Niña events. The global ten-year average of the DLF was found to be between 342.2 Wm<sup>-2</sup> and 344.3 Wm<sup>-2</sup>, depending on the dataset. We also conducted a detailed sensitivity analysis of the calculated DLFs to the key input data. Plots are given that can be used to obtain a quick assessment of the sensitivity of the DLF to each of the three key climatic quantities, for specific climatic conditions corresponding to different regions of the globe. Our model downwelling fluxes are validated against available data from ground-based stations distributed over the globe, as given by the Baseline Surface Radiation Network. There is a negative bias of the model fluxes when compared against BSRN fluxes, ranging from -7 to -9 Wm<sup>-2</sup>, mostly caused by low cloud amount differences between the station and satellite measurements, particularly in cold climates. Finally, we compare our model results with those of other deterministic models and general circulation models.
