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Atmospheric Environment (v.39, #23-24)

Editorial board (pp. i).

List for forthcoming papers (pp. i-ii).

Introduction to FEBUKO and MODMEP by Hartmut Herrmann Guest Editor, Coordinator of FEBUKO; Ralf Wolke Coordinator of MODMEP (pp. 4167-4167).

FEBUKO and MODMEP: Field measurements and modelling of aerosol and cloud multiphase processes by H. Herrmann; R. Wolke; K. M�ller; Bruggemann E. Br�ggemann; T. Gnauk; P. Barzaghi; S. Mertes; K. Lehmann; A. Massling; W. Birmili; A. Wiedensohler; W. Wieprecht; K. Acker; W. Jaeschke; H. Kramberger; B. Svrcina; Bachmann K. B�chmann; J.L. Collett Jr.; D. Galgon; K. Schwirn; A. Nowak; D. van Pinxteren; A. Plewka; R. Chemnitzer; Rud C. R�d; D. Hofmann; A. Tilgner; K. Diehl; B. Heinold; D. Hinneburg; O. Knoth; A.M. Sehili; M. Simmel; S. Wurzler; Z. Majdik; G. Mauersberger; Muller F. M�ller (pp. 4169-4183).

An overview of the two FEBUKO aerosol–cloud interaction field experiments in the Thüringer Wald (Germany) in October 2001 and 2002 and the corresponding modelling project MODMEP is given. Experimentally, a variety of measurement methods were deployed to probe the gas phase, particles and cloud droplets at three sites upwind, downwind and within an orographic cloud with special emphasis on the budgets and interconversions of organic gas and particle phase constituents. Out of a total of 14 sampling periods within 30 cloud events three events (EI, EII and EIII) are selected for detailed analysis. At various occasions an impact of the cloud process on particle chemical composition such as on the organic compounds content, sulphate and nitrate and also on particle size distributions and particle mass is observed. Moreover, direct phase transfer of polar organic compound from the gas phase is found to be very important for the understanding of cloudwater composition.For the modelling side, a main result of the MODMEP project is the development of a cloud model, which combines a complex multiphase chemistry with detailed microphysics. Both components are described in a fine-resolved particle/drop spectrum. New numerical methods are developed for an efficient solution of the entire complex model. A further development of the CAPRAM mechanism has lead to a more detailed description of tropospheric aqueous phase organic chemistry. In parallel, effective tools for the reduction of highly complex reaction schemes are provided. Techniques are provided and tested which allow the description of complex multiphase chemistry and of detailed microphysics in multidimensional chemistry-transport models.

Keywords: Hill cap cloud experiment; Aerosol; Cloud water; Trace gases; Physico-chemical analysis

Meteorological characterisation of the FEBUKO hill cap cloud experiments, Part I: Synoptic characterisation of measurement periods by A. Tilgner; B. Heinold; A. Nowak; H. Herrmann (pp. 4185-4194).

The synoptic and local meteorological conditions during the ground-based cloud passage experiment FEBUKO performed at the Schm�cke Mountain (Th�ringer Wald) during October 2001 and 2002 are reviewed and discussed. A general description of the weather types and a classification of air masses are presented. In the second part the meteorological situations are illustrated in detail for the different experimental cloud events. The main objective of this two-part study is to classify the cloud events with respect to the occurring weather conditions and consistency to the philosophy of cloud passage experiments. Therefore, particular emphasis is placed on the incident flow conditions and on the separation of orographic and non-orographic cloud types. In the case of the flow characterisation, weather charts and calculated backward trajectories are used to determine the horizontal wind pattern and the rawinsonde data for the vertical structure of wind vectors. Additionally, in order to describe the local flow conditions the observed wind speed and direction at the experimental site on the summit are applied for the total of 14 cloud episodes. For the examination of the orographic character and properties of clouds, satellite pictures of different spectral channels, vertical thermodynamic data of the rawinsonde as well as the measured liquid water content and the cloud base height are evaluated. The resulting event evaluation provides a basis for subsequent local analysis of the flow over and/or around the mountain range (Part II of the study). Generally, it is found that more suitable conditions were encountered in October 2001 than in October 2002. Especially for the anticyclonic southwest weather-type, stable incoming flow condition as well as orographically induced clouds could be clearly identified.

Keywords: FEBUKO; Orographic cloud; Air mass investigation; Meteorological event classification

Meteorological characterisation of the FEBUKO hill cap cloud experiments, Part II: Tracer experiments and flow characterisation with nested non-hydrostatic atmospheric models by B. Heinold; A. Tilgner; W. Jaeschke; W. Haunold; O. Knoth; R. Wolke; H. Herrmann (pp. 4195-4207).

The mesoscale and local flow conditions during the ground-based cloud passage experiment FEBUKO performed at the Schm�cke Mountain (Th�ringer Wald) during October 2001 and 2002 are investigated and discussed. Several methods are applied to characterise and classify the cloud episodes in terms of the flow conditions and their consistency to the philosophy of cloud passage experiments. For this the flow over the mountain range and a flow that connects the experimental sites are of crucial importance. The resulting selection of events is based on a synoptical evaluation (Part I of the work) and provides a recommendation of events, which are adequate for subsequent investigations. The mesoscale air flow over the complex terrain is characterised by means of non-dimensional flow parameters like Froude number and the non-hydrostatic meteorological model LM. An analysis of the locally measured natural tracer ozone is intended to assure that measurements were performed in identical air masses at the different locations during the 14 cloud events. It is found that the flow connecting the measurement sites is distinctly associated with the flow over and/or around the Th�ringer Wald, which in turn is determined by the synoptical flow and the thermal stratification. Furthermore, applications of tracer techniques using the inert SF₆ for studies of transport processes in the experimental site and verification of the location of measurement stations are presented. For the tracer experiments in October 2001 and 2002 an attempt is made to reproduce them with an anelastic non-hydrostatic model in conservation form in order to understand the tracer dispersion.

Keywords: Flow over complex terrain; FEBUKO; Flow parameters; Flow modelling; Tracer studies

Aerosol characterisation at the FEBUKO upwind station Goldlauter (I): Particle mass, main ionic components, OCEC, and mass closure by T. Gnauk; Bruggemann E. Br�ggemann; Muller K. M�ller; R. Chemnitzer; Rud C. R�d; D. Galgon; A. Wiedensohler; K. Acker; R. Auel; W. Wieprecht; Moller D. M�ller; W. Jaeschke; H. Herrmann (pp. 4209-4218).

This contribution presents characterisation efforts of the gas phase and particle phase main components during the FEBUKO orographic cloud passage experiments in autumn 2001 and 2002 in the Thüringer Wald (Germany). Three events out of a total of 14 were chosen as the best events considering all meteorological conditions. Gas phase and size-segregated particle phase data obtained from physical (dry size distribution) and chemical (particle mass, main ions, OCEC, and water-soluble metals) measurements are presented for the upwind site. The total particulate mass concentration (PM₁₀) was found to be between 8 and 17μgm⁻³. Particles with an aerodynamic diameter up to 1.2μm contribute about 80% of the mass concentration. About 90% of the total ion concentration consists of nitrate, sulphate and ammonium. The OC concentration in all three events amounts to about 1.0μgm⁻³, whereas EC concentrations were between 0.40 and 1.0μgm⁻³. The contribution of OC and EC to stage mass ranged from 5% to 35% and from 2% to 17%, respectively. The water content of particles was estimated to be 16–18%. Physical and chemical mass closure is discussed in detail and the results are in a reasonable agreement. The complex data set obtained for each event can be used in the initialisation of models for the multiphase processes during and after the cloud passage of the characterised air mass.

Keywords: Size-segregated particle characterisation; Major ions; OCEC; Physical and chemical mass closure

Aerosol characterisation at the FEBUKO upwind station Goldlauter (II): Detailed organic chemical characterisation by Muller K. M�ller; D. van Pinxteren; A. Plewka; B. Svrcina; H. Kramberger; D. Hofmann; Bachmann K. B�chmann; H. Herrmann (pp. 4219-4231).

An extensive set of gaseous and particulate organic compounds was quantified before an orographic cloud passage at the upwind site of the research region in Thüringer Wald. Samples were collected with two different time resolutions, 2h for gaseous species and spray absorber samples and the whole cloud event duration to determine the concentrations of ketones, aldehydes, monocarboxylic acids, dicarboxylic acids (DCA), hydrocarbons, biogenic sugars and alcohols in both the gas and particle phase.The measurement of different groups of organic compounds delivered size-segregated concentrations at the upwind site of a cloud experiment. The size distribution of DCA showed a peak in the mass-rich impactor stage 3 (0.42–1.2μm). The concentrations of DCA from the filters, the impactor foils as well as the spray absorber samples decreased with increasing C-number. The time resolved measurements revealed an increasing mixing ratio from night time to midday for carboxylic and DCA, and related carbonyl compounds.The biogenic compounds xylitol (up to 103ngm⁻³), levoglucosan (up to 62ngm⁻³) and pinonaldehyde (up to 34ngm⁻³) were the compounds found in highest concentrations in the particle phase beside the oxalate (up to 104ngm⁻³).The organic trace gases with the highest mixing ratios identified were formaldehyde (up to 1.47ppbv), acetaldehyde (up to 0.84ppbv) and acetone (up to 0.65ppbv), acetic acid (up to 0.43ppbv) and formic acid (up to 0.41ppbv).

Keywords: Filter; Impactor; Spray collector; Carbonyl compounds; Acids; Organic semivolatiles

Evolution of particle concentration and size distribution observed upwind, inside and downwind hill cap clouds at connected flow conditions during FEBUKO by S. Mertes; D. Galgon; K. Schwirn; A. Nowak; K. Lehmann; A. Massling; A. Wiedensohler; W. Wieprecht (pp. 4233-4245).

The concentration and size distribution of atmospheric particles were traced upwind, inside and downwind a hill cap cloud within the ground-based cloud passage experiment FEBUKO, which was carried out at the mountain ridge Thüringer Wald (Germany) during October 2001 and 2002. Three cloud events were examined in detail for which the connected flow between all three sites was meteorologically confirmed and the influence of entrainment and local sources on particle concentration and size distribution was only of minor importance. Modifications of the number size distributions between 60 and 300nm particle diameter that are attributed to cloud processing were observed comparing upwind and downwind dry particle size distributions. Despite a small interference of droplet deposition, a maximum in-cloud mass production of 0.38μgm⁻³ was found accounting for 5% of the upwind aerosol mass. The corresponding mass production rate was estimated to 1.16μgm⁻³h⁻¹. A formation of new ultra-fine particles was detected in the outflow of the orographic clouds during nighttime yielding number concentrations up to 300cm⁻³ at a pre-existing dry particle surface area of about 300μm²cm⁻³. At a more than twice as high particle surface area, i.e. when adsorption of condensable gases on existing particles became more important, no new particle formation was observed. Whereas the in-cloud mass production proceeded rather continuously throughout the cloud events, the particle production occurred during periods of 15min up to 2h within the cloud events. In the absence of actinic radiation, ternary nucleation of gaseous substances outgassing from the evaporating droplets at high relative humidity is hypothesized as the most likely particle formation mechanism but the reason for the temporary appearance of the particle production is not known.

Keywords: Particle number size distribution; Cloud processing; In-cloud mass production; New particle formation

Link between aerosol hygroscopic growth and droplet activation observed for hill-capped clouds at connected flow conditions during FEBUKO by S. Mertes; K. Lehmann; A. Nowak; A. Massling; A. Wiedensohler (pp. 4247-4256).

Within the ground-based cloud passage experiment FEBUKO, which was carried out at the mountain ridge Thüringer Wald (Germany) during October 2001 and 2002, the dry number size distribution and hygroscopic growth of aerosol particles upwind cloud and the dry number size distributions of interstitial particles and cloud droplet residuals inside cloud were measured at connected flow conditions. The connected flow between the upwind and in-cloud summit site was meteorologically predicted and experimentally confirmed for three selected cloud events. For these events, it could be verified that entrainment and droplet deposition had only a minor influence on the evolution of the particle size distribution between the two sites. Hence, the size resolved soluble volume fraction of the cloud input aerosol particles determined from the hygroscopic growth measurements could be related to the particle activation inferred from the particle size distributions observed inside cloud. The shape and steepness of the scavenging fraction as a function of particle diameter was found to correlate with the increase of soluble volume fraction with size, which had implications for the droplet activation diameter of the cloud condensation nuclei (CCN) that ranged between 110 and 180nm. The minimum soluble volume fractionεmin that was required to serve as CCN was determined for three different dry diameters from the relation of the particle volume fraction and scavenging fraction. From the comparison withεmin predictions from classical Köhler theory it is inferred that aerosol particles remained in the interstitial phase although they should have been activated. A discussion of different processes which have the general ability to explain this finding favoured the hypothesis of organic surface films retarding the uptake of water molecules.

Keywords: Particle soluble volume fraction; Cloud droplet residues; Interstitial particles; Droplet activation; Cloud condensation nuclei

Size-resolved soluble volume fractions of submicrometer particles in air masses of different character by K. Lehmann; A. Massling; A. Tilgner; S. Mertes; D. Galgon; A. Wiedensohler (pp. 4257-4266).

As a contribution to the joint research project FEBUKO, hygroscopic properties of atmospheric Aitken and accumulation mode particles were measured in the Thueringer Wald, Germany, using a hygroscopicity-tandem differential mobility analyser (H-TDMA).The hygroscopic growth of particles with initial dry diameters ofDp=50, 150, and 250nm at 90% relative humidity was used to calculate average distributions of the soluble volume fraction with respect to the hygroscopic growth of ammonium sulphate. The application of this parameterisation procedure was tested by analysing the dataset with respect to the dependence of the soluble volume fraction on particle size and air masses character. With increasing dry particle size, the fraction of particles containing high soluble volume fractions was found to increase. The number of accumulation mode particles in marine air masses passing the sampling site having a large soluble volume fraction was significantly higher than in air masses of more continental character. For particles withDp=50nm, no air mass dependence of the soluble volume fraction was found. In marine air masses, particles withDp=150 and 250nm are assumed to undergo similar evolution processes, whereas in continental air masses this seemed to be the case for particles withDp=50nm and 150nm.

Keywords: H-TDMA; Soluble volume fraction; Hygroscopicity; Air masses

Cloud physics and cloud water sampler comparison during FEBUKO by W. Wieprecht; K. Acker; S. Mertes; J. Collett Jr.; W. Jaeschke; Bruggemann E. Br�ggemann; Moller D. M�ller; H. Herrmann (pp. 4267-4277).

Optical methods for counting and sizing cloud droplets and a wide range of cloud water sampling methods were used to characterize the atmospheric liquid phase during the FEBUKO cloud experiments. Results near cloud base as well as more than 300m inside the hill cap clouds are presented, reflecting their inhomogeneous nature. The cloud droplet number varies from 50 to 1000cm⁻³ and drop sizes between 1 and 20μm diameter are most frequent. Variations in the liquid water content (LWC) and in the total ion content (TIC) are much smaller when the measurement position is deeper in the cloud. Near cloud base variability in updraft strength and, near cloud top, entrainment processes (droplet evaporation by mixing with drier air, aerosol and gas scavenging) disturb the adiabatic conditions and produce large variations in LWC and chemical composition. Six different active cloud water collectors and impactors were running side by side; they differ in the principle of sampling, in the throughput of cloudy air per unit time and in the calculated 50% cutoff diameter, which influence also their sampling efficiency. Two of them are designed to collect cloud water in two droplet size fractions. Three cloud events were selected by the FEBUKO team for detailed cloud physical and chemical analyses because they serve best the modelling demands concerning connected flow between the upwind, summit and downwind sites for process studies. Frequency distributions of the LWC and, also of the cloud base height are given as statistical parameters for both FEBUKO experiments.

Keywords: Cloud water sampling; Liquid water content; Cloud base height; Droplet size distribution; Cloud chemistry

H₂O₂ and organic peroxide measurements in an orographic cloud: The FEBUKO experiment by J. Valverde-Canossa; W. Wieprecht; K. Acker; G.K. Moortgat (pp. 4279-4290).

The H₂O₂ and organic peroxides are known to be important oxidants in cloud-water, influencing the oxidising capacity of the atmosphere. Measurements of H₂O₂ in cloud-water have shown a wide range of concentrations depending on the season and measuring site. Moreover, organic peroxide measurements are scarce in spite of their importance. Measurements of peroxides were carried out in the Thuringian Forest, Germany, during the FEBUKO research cluster in the Fall 2001. The measuring stations were located at three sites: upwind (gas phase), summit (cloud-water and gas phase) and downwind (gas phase). Analysis was achieved by high performance liquid chromatography (enzymatic method). From the different peroxides only H₂O₂ was detected in the gas phase at the upwind site with mixing ratios <130ppt. In the cloud-water, besides hydrogen peroxide (H₂O₂), hydroxymethylhydroperoxide (HMHP), 1-hydroxyethylhydroperoxide (1-HEHP) and methylhydroperoxide (MHP) were also detected with concentrations normalised with the liquid water content up to 1.30, 0.075, 0.065 and 0.015nmolm⁻³, respectively. Organic peroxides (HMHP+1-HEHP+MHP) constitute up to 80% of the total peroxides during nighttime while during daytime they accounted for about 14%. Consequently, organic peroxides might play an important role in nighttime cloud chemistry.

Keywords: Organic peroxides; Hydrogen peroxide; Ozonolysis; Sulphate production

Schm�cke hill cap cloud and valley stations aerosol characterisation during FEBUKO (I): Particle size distribution, mass, and main components by E. Br�ggemann; T. Gnauk; S. Mertes; K. Acker; R. Auel; W. Wieprecht; D. M�ller; J.L. Collett Jr.; H. Chang; D. Galgon; R. Chemnitzer; C. R�d; R. Junek; W. Wiedensohler; H. Herrmann (pp. 4291-4303).

Hill cap cloud field experiments were performed during autumn 2001 and 2002 in the Thüringer Wald (Germany). Gas phase trace compounds were determined at an upwind, summit, and downwind sites and major particulate components at an upwind and downwind site. Cloud water and total cloud components (drop residuals and interstitial particles) were determined at a summit site. Three events were fulfilling the criteria for the best conditions defined by during a connected flow upwind–summit–downwind sites and further detailed analysis was performed on these events. Cloud water components were compared with particle concentration at upwind and downwind site. The concentrations of non-volatile components in cloud water were found to be in good agreement with corresponding particle phase concentrations at the upwind site. Downwind site particulate component concentrations of non-volatile compounds were lower than in cloud water indicating loss processes during transport such as deposition. The concentrations of volatile components were found to be higher in cloud water than in the particle phase concentrations at up- and downwind site samples probably due to a loss from impactor sampling technique as well as a transport loss. Indications for changes of aerosol composition by cloud processes were found from a limited number of cases. Elevated sulphate and ammonium concentrations from upwind to downwind site in the smallest particle size range (PM_0.05–0.14) were found during event I (20% and 17%) and event III (70% and 150%), respectively. In the particle size range of PM_0.14–0.42 an increase of OC by about 20% for event I was observed. Considering the relative contributions of components to the single size range mass (avoiding physical sink processes), comparatively higher increases for sulphate, nitrate, ammonium, OC, and EC could be observed. Indications of an increase of aerosol mass can be derived in some cases from the aerosol number and volume size distributions. Results from a complex multiphase model (SPACCIM) are consistent showing an increase in concentrations of some compounds for some cases.

Keywords: Size-segregated particle characterisation; Cloud water; Major ions; OCEC; Metals

Schm�cke hill cap cloud and valley stations aerosol characterisation during FEBUKO (II): Organic compounds by D. van Pinxteren; A. Plewka; D. Hofmann; Muller K. M�ller; H. Kramberger; B. Svrcina; Bachmann K. B�chmann; W. Jaeschke; S. Mertes; J.L. Collett Jr.; H. Herrmann (pp. 4305-4320).

An extensive speciation of organic compounds was conducted during the FEBUKO cloud experiments in autumn 2001 and 2002. Three measurement sites were chosen at the Schmücke mountain in the Thüringer Wald region, Germany, which allowed to characterise air masses chemically before, during, and after their passage of a hill cap cloud. Concentrations of 33 organic carbonyl compounds, 5 monocarboxylic acids (MCAs), and 10 dicarboxylic acids (DCAs) are reported for different atmospheric phases at the three sites. Some of them were determined for the first time in cloud water. The concentration levels of the compounds were usually low, consistent with the rural sampling region. The identified fraction of dissolved organic carbon in the cloud water was 17.3%, 14.7%, and 10.1%, on average, for three independent cloud events. For the gas phase compounds the phase partitioning between liquid phase and interstitial gas phase inside the cloud was determined and compared to the theoretically expected values considering thermodynamic equilibrium conditions (Henry's law). For relatively polar organic carbonyl compounds (with a high Henry constant and a high effective water solubility), the ratio of measured to calculated liquid phase fractions was close to 1 (0.6–3.4). For the more hydrophobic compounds, however, a significant liquid phase supersaturation with respect to the gas phase concentrations was observed (ratios of 45–912). For the MCAs, only small deviations from Henry's law were determined, comparable to the ones of the polar carbonyl compounds. The scavenging efficiency of the particulate DCAs inside of the cloud was close to 100%. Concentrations of both particulate and gas phase organic compounds were usually lower at the downwind site than at the upwind site. This was most likely due to physical sink processes during the passage of the air parcel over the forested Schmücke mountain.

Keywords: Cloud water chemistry; Organic carbonyl compounds; Organic acids; Phase partitioning; Aerosol processing

Non-dissipative cloud transport in Eulerian grid models by the volume-of-fluid (VOF) method by Detlef Hinneburg; Oswald Knoth (pp. 4321-4330).

The formation of clouds is coupled to the vapour saturation condition. Cloud modelling is therefore dramatically disturbed by dilution processes, which are induced by recurrent interpolations on the fixed (Eulerian) grid. The numerical diffusion gives rise to degeneration and premature disappearance of the modelled clouds. The difficulties increase, if sectional mass representation in the drop microphysics and aerosol chemistry is considered. To tackle this problem, stringently defined and tracked phase boundaries are required.The numerical diffusion of clouds can be totally suppressed by the volume-of-fluid (VOF) method, which is applied here in connection with an atmospheric model. The cloud phase is distinguished by prognosing the partial cloud volume in all grid cells near the cloud boundary. Adopting elementary geometrical forms for the intracellular cloud volume and simple diagnostic rules of their alignment, the standard transport fluxes can be used in the new equation. Separate variables for the cloud and environmental phase complete the transport scheme.The VOF method and its realisation are described in detail. Advection, condensation, evaporation, and turbulent diffusion are considered within the VOF framework. The variation of the grid resolution and turbulence conditions for a rising thermal leads to striking arguments in favour of the VOF method, resulting in higher intensity, lifting, and lifetime as well as clear boundaries of the simulated clouds (even for low grid resolution).

Keywords: Cloud model; Cloud volume; Diffusion; Entrainment

A parcel model for the combined treatment of microphysical and multiphase chemical processes by Oswald Knoth (pp. 4331-4340).

The accurate and efficient description of aerosol microphysical and chemical processes is required for the assessment of radiative and chemical effects of natural and anthropogenic atmospheric aerosols. The combined modeling of microphysical and chemical processes in the gas and aqueous phase such as meteorological changes, transformation of chemical species in the gas and liquid phase and the transfer of species from one phase to the other is required. Since the aforementioned processes proceed on similar time scales the usual time splitting schemes which perform process by process in a sequential order are not appropriate. In contrast to other approaches where a microphysical and a cloud chemistry model are coupled, the new approach treats both processes in a unified way both from the modeling and numerical point of view. It is argued that this new model type is better suited for incorporation in multidimensional atmospheric and transport models. Essential parts of the model are outlined. The differential equations are discretized in mass space by a discontinuous Galerkin method and integrated after that in time by an implicit–explicit time integration scheme. Numerous simulations are performed to show the reliability of the new approach. The Eulerian fixed grid approach is compared with a 2000 bin moving simulation to demonstrate the merits and demerits of a fixed grid.

Keywords: Aerosols; Growth equation; Aqueous phase chemistry; Mathematical model

ISSA (iterative screening and structure analysis)—a new reduction method and its application to the tropospheric cloud chemical mechanism RACM/CAPRAM2.4 by G. Mauersberger (pp. 4341-4350).

An automated reduction method ISSA (iterative screening and structure analysis) has been developed. It is aimed at the analysis of complex atmospheric chemical multiphase mechanisms and produces reduced mechanisms for specifiable application purposes. Cyclic and non-cyclic reactions identified by a structure analysis are separately evaluated. The normalized valuation coefficients are calculated in a box model framework by using time-averaged reaction rates. Starting with a set of target species, important reactions and species are selected together in an iteration procedure. So, only one threshold value fixed for all box model scenarios is necessary. For every scenario a specific reduced mechanism is obtained. The sum of reactions and species included in the specific reduced mechanisms generates then the ISSA-reduced mechanism. All reactants in the reduced mechanism are included in the verification procedure where the concentrations simulated with the full and the reduced mechanism are compared. The maximum relative deviation of daily maxima was found to be a suitable deviation measure for atmospheric trace species concentrations.An application of the ISSA method to the large cloud chemical mechanism RACM/CAPRAM2.4 resulted in reduction rates of 55% for reactions (46% gas phase, 60% liquid phase), 23% for species, and 23% for phase transfers. The deviation between full and reduced mechanism averaged over all scenarios and reactants was 2.5%. The liquid-phase part of this application was compared with a condensed version of the CAPARAM2.4 mechanism developed simultaneously with the full version. It was found that these two reduced versions of CAPRAM2.4 differ significantly. Whereas the condensed version achieves good verification results only for the target species, the ISSA-reduced version reproduce very well the complete full mechanism results and should be useful for future large-scale models, which will include both detailed microphysics and complex (reduced) multiphase chemistry.

Keywords: Modelling; Automatic mechanism reduction; Multiphase photochemistry; Deviation measure; Cloud-chemical box model

Towards a more detailed description of tropospheric aqueous phase organic chemistry: CAPRAM 3.0 by H. Herrmann; A. Tilgner; P. Barzaghi; Z. Majdik; S. Gligorovski; L. Poulain; A. Monod (pp. 4351-4363).

CAPRAM 3.0 is the latest development of the chemical aqueous phase radical mechanism (CAPRAM) series which is incorporating CAPRAM 2.4 (Ervens et al., 2003a, Journal of Geophysical Research—Atmospheres 108) and a new extended reaction mechanism for atmospherically relevant hydrocarbons containing more than two and up to six carbon atoms. The chemistry of organics containing three and four carbon atoms is now described in detail. Almost 400 new reactions are now implemented considering the chemistry of organic compounds containing different functional groups, i.e. alcohols, carbonyl compounds, mono- and dicarboxylic acids, polyfunctional compounds as well as some esters and one heterocyclic compound.The aqueous chemistry has been coupled to the gas phase mechanism RACM (regional atmospheric chemistry modeling) (Stockwell et al., 1997, Journal of Geophysical Research—Atmpspheres 102, 25847–25879), and phase exchange is treated using the resistance model of Schwartz (1986. In: Jaeschke, W. (Ed.), Chemistry of Multiphase Atmospheric Systems, NATO ASI Series, Springer, Berlin, pp. 415–471). The CAPRAM remote scenario which was chosen as the standard scenario showed that the introduction of the higher organic chemistry has a relevant influence on the standard subsystems. The diurnal peak concentration of OH radical in the droplets decreases with about 40% and the reactions of OH with hydrocarbons containing 3 or 4 carbon atoms account for about 10% out of the total sinks of OH in the droplets. A slightly stronger acidification of the aqueous phase in comparison to CAPRAM 2.4 is observed.The simulations for the standard scenario showed that there is an increase of organic mass within the droplets where the organic compounds containing 4 carbon atoms represent the 67.5% of the total mass, whereas in the urban and in the marine scenario the contribution of two carbon atom compounds is dominating.The formation and accumulation of substituted mono- and dicarboxylic acids such as tartaric, mesoxalic and acetic acid in the aqueous phase are also observed.

Keywords: Modeling; Multiphase chemistry; VOC oxidation; Box model

Numerical simulation of the microphysics of an orographic cloud: Comparison with measurements and sensitivity studies by Martin Simmel; Karoline Diehl; Sabine Wurzler (pp. 4365-4373).

The formation and evolution of orographic clouds are modeled using a parcel model with sectional microphysics based on the Linear Discrete Method and a size-dependent representation of the soluble particle fraction. The model results are compared to observations from three periods of the field experimental campaigns FEBUKO 2001 and 2002 covering about 150 single cases. Processing of aerosol is sensitive to cloud droplet number and size. Therefore, droplet nucleation is emphasized. Sensitivity studies concerning the soluble particle fractionε, the water accommodation coefficientαC, and model dynamics were carried out. The size-dependent representation ofε turned out to be very important for a correct nucleation description whereas a shift of the soluble fraction by ±0.1 induces much smaller effects. DecreasingαC and increasing vertical velocity both lead to enhanced droplet formation due to higher supersaturations reached. This effect often occurred for the same parameter configuration. Entrainment was shown to be important to reach better agreement between the calculated and the observed data, reducing the liquid water contents below the respective adiabatic values and leading to a broadening of drop size distributions including an increase of small droplets.

Keywords: Spectral cloud model; Condensation; Nucleation; Cloud microphysics; Drop growth

SPACCIM: A parcel model with detailed microphysics and complex multiphase chemistry by R. Wolke; A.M. Sehili; M. Simmel; O. Knoth; A. Tilgner; H. Herrmann (pp. 4375-4388).

Multiphase processes, such as the uptake of gases by clouds or the production of gas phase halogens from particulate halides are of increasing importance for the understanding of the tropospheric system. Mass transfer and chemical reactions modify the concentrations of stable compounds and oxidants in either phase. The parcel model SPACCIM is presented which combines a complex multiphase chemical model with a detailed microphysical model. For this purpose, a new coupling scheme is implemented. The description of both components is given for a fine-resolved particle/drop spectrum. The SPACCIM approach allows the coupling of multiphase chemical models with microphysical codes of various types. An efficient numerical solution of such systems is only possible utilizing the special structure. An implicit time-integration scheme with an adapted sparse solver for the linear systems is applied. Its numerical efficiency and robustness is analyzed for two scenarios and versions of different complexity of the multiphase chemistry mechanism CAPRAM. The sensitivity of simulation results against variations in the particle/droplet size resolution, the coupling time step and numerical control parameters is discussed. Guidelines for an “optimal�? choice of control parameters are derived from this sensitivity study. The coupling scheme operation is always robust and reliable. Model simulations are compared with several measurements from the FEBUKO field campaign. Simulated and measured results show a reasonable agreement.

Keywords: Air pollution modeling; Multiphase chemistry; Chemical kinetics; Stiff ODE solution; Implicit integration schemes; Sparse linear solver

SPACCIM: Simulations of the multiphase chemistry occurring in the FEBUKO hill cap cloud experiments by A. Tilgner; Z. Majdik; A.M. Sehili; M. Simmel; R. Wolke; H. Herrmann (pp. 4389-4401).

The parcel model SPACCIM is applied to investigate the effect of multiphase cloud processing of tropospheric aerosol particles and trace gases resulting from a passage through an orographic cloud at Mt. Schmücke (Germany) during the joint research project FEBUKO. The applied model combines a complex microphysical and a detailed multiphase chemistry model with about 261 gas phase and 776 aqueous phase reactions. The chemical multiphase model incorporates a detailed description of the inorganic and organic multiphase chemistry based on time-dependent size-resolved aerosol/cloud spectra. The data measured at the upwind site provided the basis for the chemical and physical model initialisation under real environmental conditions. The simulation results were compared to experimental cloud water composition data at Schmücke summit site as well as gas and aerosol measurements at downwind site in order to interpret the experimental data and to evaluate the model results. To this end, a detailed analysis of the chemical multiphase system was performed including source and sinks studies with special emphasis on aqueous phase oxidants and S(IV) to S(VI) conversion. A central objective of the study has been to assess in-cloud oxidations of organic compounds and results for important C2 and C3 oxidation subsystems are presented. This modelling study shows that the observed multiphase chemistry is strongly affected by dynamic microphysical processes. Furthermore, a significant cloud condensation nuclei (CCN) modification with sizes up to about 400nm, mass productions up to about 0.7μgm⁻³ and acidification caused by cloud processing was identified in the model in agreement with the experimental findings. However, for organic compounds with low solubilities the cloud water measurements show considerably higher concentrations than expected from both (i) their Henry solubilities and (ii) the complex multiphase modelling as performed by the model.

Keywords: Spectral model; Microphysics; Multiphase chemistry; Aerosol cloud processing

Comparison of different model approaches for the simulation of multiphase processes by A.M. Sehili; R. Wolke; O. Knoth; M. Simmel; A. Tilgner; H. Herrmann (pp. 4403-4417).

Cloud-chemistry models are developed intensively with increasing complexity, leading to new knowledge and offering new possibilities to understand the physico-chemical processes taking place in the atmosphere. Intercomparing such detailed models is the way to test the robustness and reliability of their parameterizations and numerical schemes. The present study involves newly developed parcel models treating microphysics and chemistry with equal rigor. The description of both kinds of processes is given for a size-resolved particle/droplet spectrum. Three different types of models are compared. In thespaccim approach, one- and two-dimensional particle/drop microphysical schemes are used in a time-splitting setup between chemistry and microphysics. Thegalerkin model employs a one-dimensional scheme in a fully coupled setup. For each of the three types, “fixed bin�? and “moving bin�? approaches are implemented. A comparison between “fixed�? and “moving bin�? approaches makes sense only for scenarios without coagulation and breakup. The paper focuses on the effects of different microphysical and numerical approaches on the multiphase chemistry. The resulting changes in the particle/droplet composition feed back on cloud microphysics. Substantiated conclusions can only be derived if these effects are studied for a wide range of cases. Thus, the simulations are performed for three chemical reaction mechanisms of different complexity and four scenarios derived from field measurements. The interaction between numerical schemes, microphysics and multiphase chemistry is discussed. Mostly, the results of the participating models agree in an appreciable way. Observable differences are noticed between the “moving bin�? approach and models using fixed grids for the discretization of the particle/droplet spectrum. Furthermore, the initial aerosol composition influences the fate of chemical species as well as the behavior of the numerical solver in a substantial way.

Keywords: Air pollution modeling; Multiphase chemistry; Microphysics; Cloud processing; Time integration schemes; Model comparison

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Atmospheric Environment (v.39, #23-24)

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Atmospheric Environment (v.39, #23-24)