Cloud Chemistry Model Research Articles

Influence of strong iron-binding ligands on cloud water oxidant capacity

Iron (Fe) plays a dual role in atmospheric chemistry: it is involved in chemical and photochemical reactivity and serves as a micronutrient for microorganisms that have recently been shown to produce strong organic ligands. These ligands control the reactivity, mobility, solubility and speciation of Fe, which have a potential impact on Fe bioavailability and cloud water oxidant capacity.In this work, the concentrations of Fe-binding ligands and the conditional stability constants were experimentally measured for the first time by Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) technique in cloud water samples collected at puy de Dôme (France). The conditional stability constants, which indicate the strength of the Fe-ligand complexes, are higher than those considered until now in cloud chemistry (mainly Fe-oxalate). To understand the effect of Fe complexation on cloud water reactivity, we used the CLEPS cloud chemistry model. According to the model results, we found that Fe complexation impacts the hydroxyl radical formation rate: contrary to our expectations, Fe complexation by natural organic ligands led to an increase in hydroxyl radical production. These findings have important impacts on cloud chemistry and the global iron cycle.

Convective uplift of pollution from the Sichuan Basin into the Asian monsoon anticyclone during the StratoClim aircraft campaign

Abstract. The StratoClim airborne campaign took place in Nepal from 27 July to 10 August 2017 to document the physical and chemical properties of the South Asian upper troposphere–lower stratosphere (UTLS) during the Asian summer monsoon (ASM). In the present paper, simulations with the Meso-NH cloud-chemistry model at a horizontal resolution of 15 km are performed over the Asian region to characterize the impact of monsoon deep convection on the composition of Asian monsoon anticyclone (AMA) and on the formation of the Asian tropopause aerosol layer (ATAL) during the StratoClim campaign. StratoClim took place during a break phase of the monsoon with intense convective activity over South China and Sichuan. Comparisons between brightness temperatures (BTs) at 10.8 µm observed by satellite sensors and simulated by Meso-NH highlight the ability of the model to correctly reproduce the life cycle of deep convective clouds. A comparison between CO and O3 concentrations from Meso-NH and airborne observations (StratoClim and IAGOS (In-service Aircraft for a Global Observing System)) demonstrates that the model captures most of the observed variabilities. Nevertheless, for both gases, the model tends to overestimate the concentrations and misses some thin CO plumes related to local convective events probably because the resolution is too coarse, but the convective uplift of pollution is very well captured by the model. We have therefore focused on the impact of Sichuan convection on the AMA composition. A dedicated sensitivity simulation showed that the 7 August convective event brought large amounts of CO deep into the AMA and even across the 380 K isentropic level located at 17.8 km. This Sichuan contribution enhanced the CO concentration by ∼15 % to reach more than 180 ppbv over a large area around 15 km height. It is noteworthy that Meso-NH captures the impact of the diluted Sichuan plume on the CO concentration during a StratoClim flight south of Kathmandu, highlighting its ability to reproduce the transport pathway of Sichuan pollution. According to the model, primary organic aerosol and black carbon particles originating from Sichuan are transported following the same pathway as CO. The large particles are heavily scavenged within the precipitating part of the convective clouds but remain the most important contributor to the particle mass in the AMA. Over the whole AMA region, the 7 August convective event resulted in a 0.5 % increase in CO concentration over the 10–20 km range that lasted about 2 d. The impact of pollution uplift from three regions (India, China, and Sichuan) averaged over the first 10 d of August has also been evaluated with sensitivity simulations. Even during this monsoon break phase, the results confirm the predominant role of India relative to China with respective contributions of 11 % and 7 % to CO concentration in the 10–15 km layer. Moreover, during this period a large part (35 %) of the Chinese contribution comes from the Sichuan Basin alone.

Anthropogenic and biogenic hydrophobic VOCs detected in clouds at the puy de Dôme station using Stir Bar Sorptive Extraction: Deviation from the Henry's law prediction

Abstract Cloud droplets are able to trap many gaseous compounds and represent a very reactive media in the atmosphere. However, phase-partitioning of Volatile Organic Compounds (VOCs) between gaseous phase (interstitial air) and atmospheric liquid phase (cloud droplets) is still poorly characterized. In the present work, air samples and cloud water were collected simultaneously and hydrophobic organic compounds from anthropogenic and biogenic origins were quantified in order to evaluate their air/water partitioning. For this, cloud events were sampled at the puy de Dome station (PUY). A new analytical approach was optimized to explore dissolved VOCs in the aqueous phase by Stir Bar Sorptive Extraction (SBSE) and gaseous samples were collected using sorbent tubes. All samples were analyzed by Thermal Desorption (TD) Gas Chromatography (GC) coupled with Mass Spectrometry (MS). Extraction efficiencies by SBSE vary between 22% and 97% and the detection limit of VOCs in aqueous cloud samples lies between 1.0 and 8.7 ng L−1. Experimental partitioning is evaluated towards the theoretical one described by the Henry's Law constants. Our results show that observed concentrations of hydrophobic VOCs are supersaturated by a factor of 10–103 in comparison with Henry's law equilibrium. Calculations have been performed to evaluate the potential role of adsorption at the air/water interface and of sorption with non-dissolved organic carbon. However, the supersaturation in the cloud droplets requires more laboratory experiments to understand processes at the air/water interface and constrain cloud chemistry models.

First evaluation of the effect of microorganisms on steady state hydroxyl radical concentrations in atmospheric waters

Clouds are complex multiphasic media where efficient chemical reactions take place and where microorganisms have been found to be metabolically active. Hydroxyl radical is the main oxidant in cloud water, and more generally in the atmosphere, during the day and drives the cloud oxidative capacity. However, only one measurement of the steady state hydroxyl radical concentrations in cloud water has been reported so far. Cloud chemistry models are used to estimate the hydroxyl radical concentrations with values ranging from 10−12 to 10−15 M that are surely overestimated due to a lack of knowledge about the speciation of the organic matter acting as a sink for hydroxyl radicals. The aim of this work is to quantify the concentration of hydroxyl radicals at steady state in rain and cloud waters and to measure the impact of native microflora on this concentration. First, the non-toxicity of terephthalic acid as probe is controlled before the analysis in real atmospheric water samples. Higher concentrations of hydroxyl radicals are found in cloud waters than in rain waters, with a mean value “1.6 ± 1.5” × 10−16 M and “7.2 ± 5.0” × 10−16 M for rain and cloud waters respectively and no real impact of microorganisms was observed. This method allows the measurement of steady state hydroxyl radical levels at very low concentrations (down to 10−17 M) and it is biocompatible, fast and easy to handle. It is a useful tool, complementary to other methods, to give a better overview of atmospheric water oxidant capacity.

Siderophores in Cloud Waters and Potential Impact on Atmospheric Chemistry: Photoreactivity of Iron Complexes under Sun-Simulated Conditions.

In the present work, the photoreactivity of a mixture of iron(III)–pyoverdin (Fe(III)–Pyo) complexes was investigated under simulated cloud conditions. Pyoverdins are expected to complex ferric ions naturally present in cloudwater, thus modifying their availability and photoreactivity. The spectroscopic properties and photoreactivity of Fe(III)-Pyo were investigated, with particular attention to their fate under solar irradiation, also studied through simulations. The photolysis of the Fe(III)–Pyo complex leads to the generation of Fe(II), with rates of formation (RFe(II)f) of 6.98 and 3.96 × 10–9 M s–1 at pH 4.0 and 6.0, respectively. Interestingly, acetate formation was observed during the iron-complex photolysis, suggesting that fragmentation can occur after the ligand-to-metal charge transfer (LMCT) via a complex reaction mechanism. Moreover, photogenerated Fe(II) represent an important source of hydroxyl radical via the Fenton reaction in cloudwater. This reactivity might be relevant for the estimation of the rates of formation and steady-state concentrations of the hydroxyl radical by cloud chemistry models and for organic matter speciation in the cloud aqueous phase. In fact, the conventional models, which describe the iron photoreactivity in terms of iron–aqua and oxalate complexes, are not in accordance with our results.

Convective transport and scavenging of peroxides by thunderstorms observed over the central U.S. during DC3

AbstractOne of the objectives of the Deep Convective Clouds and Chemistry (DC3) field experiment was to determine the scavenging of soluble trace gases by thunderstorms. We present an analysis of scavenging of hydrogen peroxide (H2O2) and methyl hydrogen peroxide (CH3OOH) from six DC3 cases that occurred in Oklahoma and northeast Colorado. Estimates of H2O2 scavenging efficiencies are comparable to previous studies ranging from 79 to 97% with relative uncertainties of 5–25%. CH3OOH scavenging efficiencies ranged from 12 to 84% with relative uncertainties of 18–558%. The wide range of CH3OOH scavenging efficiencies is surprising, as previous studies suggested that CH3OOH scavenging efficiencies would be <10%. Cloud chemistry model simulations of one DC3 storm produced CH3OOH scavenging efficiencies of 26–61% depending on the ice retention factor of CH3OOH during cloud drop freezing, suggesting ice physics impacts CH3OOH scavenging. The highest CH3OOH scavenging efficiencies occurred in two severe thunderstorms, but there is no obvious correlation between the CH3OOH scavenging efficiency and the storm thermodynamic environment. We found a moderate correlation between the estimated entrainment rates and CH3OOH scavenging efficiencies. Changes in gas‐phase chemistry due to lightning production of nitric oxide and aqueous‐phase chemistry have little effect on CH3OOH scavenging efficiencies. To determine why CH3OOH can be substantially removed from storms, future studies should examine effects of entrainment rate, retention of CH3OOH in frozen cloud particles during drop freezing, and lightning‐NOx production.

A better understanding of hydroxyl radical photochemical sources in cloud waters collected at the puy de Dôme station – experimental versus modelled formation rates

Abstract. The oxidative capacity of the cloud aqueous phase is investigated during three field campaigns from 2013 to 2014 at the top of the puy de Dôme station (PUY) in France. A total of 41 cloud samples are collected and the corresponding air masses are classified as highly marine, marine and continental. Hydroxyl radical (HO•) formation rates (RHO•f) are determined using a photochemical setup (xenon lamp that can reproduce the solar spectrum) and a chemical probe coupled with spectroscopic analysis that can trap all of the generated radicals for each sample. Using this method, the obtained values correspond to the total formation of HO• without its chemical sinks. These formation rates are correlated with the concentrations of the naturally occurring sources of HO•, including hydrogen peroxide, nitrite, nitrate and iron. The total hydroxyl radical formation rates are measured as ranging from approximately 2 × 10−11 to 4 × 10−10 M s−1, and the hydroxyl radical quantum yield formation (ΦHO•) is estimated between 10−4 and 10−2. Experimental values are compared with modelled formation rates calculated by the model of multiphase cloud chemistry (M2C2), considering only the chemical sources of the hydroxyl radicals. The comparison between the experimental and the modelled results suggests that the photoreactivity of the iron species as a source of HO• is overestimated by the model, and H2O2 photolysis represents the most important source of this radical (between 70 and 99 %) for the cloud water sampled at the PUY station (primarily marine and continental).

Evaluation of modeled cloud chemistry mechanism against laboratory irradiation experiments: The HxOy/iron/carboxylic acid chemical system

Currently, cloud chemistry models are including more detailed and explicit multiphase mechanisms based on laboratory experiments that determine such values as kinetic constants, stability constants of complexes and hydration constants. However, these models are still subject to many uncertainties related to the aqueous chemical mechanism they used. Particularly, the role of oxidants such as iron and hydrogen peroxide in the oxidative capacity of the cloud aqueous phase has typically never been validated against laboratory experimental data. To fill this gap, we adapted the M2C2 model (Model of Multiphase Cloud Chemistry) to simulate irradiation experiments on synthetic aqueous solutions under controlled conditions (e.g., pH, temperature, light intensity) and for actual cloud water samples. Various chemical compounds that purportedly contribute to the oxidative budget in cloud water (i.e., iron, oxidants, such as hydrogen peroxide: H2O2) were considered. Organic compounds (oxalic, formic and acetic acids) were taken into account as target species because they have the potential to form iron complexes and are good indicators of the oxidative capacity of the cloud aqueous phase via their oxidation in this medium. The range of concentrations for all of the chemical compounds evaluated was representative of in situ measurements. Numerical outputs were compared with experimental data that consisted of a time evolution of the concentrations of the target species.The chemical mechanism in the model describing the “oxidative engine” of the HxOy/iron (HxOy = H2O2, HO2/O2− and HO) chemical system was consistent with laboratory measurements. Thus, the degradation of the carboxylic acids evaluated was closely reproduced by the model. However, photolysis of the Fe(C2O4)+ complex needs to be considered in cloud chemistry models for polluted conditions (i.e., acidic pH) to correctly reproduce oxalic acid degradation. We also show that iron and formic acid lead to a stable complex whose photoreactivity has currently not been investigated. The updated aqueous chemical mechanism was compared with data from irradiation experiments using natural cloud water. The new reactions considered in the model (i.e., iron complex formation with oxalic and formic acids) correctly reproduced the experimental observations.

ON THE FORMATION OF INTERSTELLAR WATER ICE: CONSTRAINTS FROM A SEARCH FOR HYDROGEN PEROXIDE ICE IN MOLECULAR CLOUDS

Recent surface chemistry experiments have shown that the hydrogenation of molecular oxygen on interstellar dust grains is a plausible formation mechanism, via hydrogen peroxide (H2O2), for the production of water (H2O) ice mantles in the dense interstellar medium. Theoretical chemistry models also predict the formation of a significant abundance of H2O2 ice in grain mantles by this route. At their upper limits, the predicted and experimental abundances are sufficiently high that H2O2 should be detectable in molecular cloud ice spectra. To investigate this further, laboratory spectra have been obtained for H2O2/H2O ice films between 2.5 and 200 micron, from 10 to 180 K, containing 3%, 30%, and 97% H2O2 ice. Integrated absorbances for all the absorption features in low-temperature H2O2 ice have been derived from these spectra. For identifying H2O2 ice, the key results are the presence of unique features near 3.5, 7.0, and 11.3 micron. Comparing the laboratory spectra with the spectra of a group of 24 protostars and field stars, all of which have strong H2O ice absorption bands, no absorption features are found that can definitely be identified with H2O2 ice. In the absence of definite H2O2 features, the H2O2 abundance is constrained by its possible contribution to the weak absorption feature near 3.47 micron found on the long-wavelength wing of the 3 micron H2O ice band. This gives an average upper limit for H2O2, as a percentage of H2O, of 9% +/- 4%. This is a strong constraint on parameters for surface chemistry experiments and dense cloud chemistry models.

Modeling natural emissions in the Community Multiscale Air Quality (CMAQ) model – Part 2: Modifications for simulating natural emissions

Abstract. The Community Multiscale Air Quality (CMAQ) model version 4.6 has been revised with regard to the representation of chlorine (HCl, ClNO2) and sulfur (dimethylsulfide, or DMS, and H2S), and evaluated against observations and earlier published models. Chemistry parameterizations were based on published reaction kinetic data and a recently developed cloud chemistry model that includes heterogeneous reactions of organic sulfur compounds. Evaluation of the revised model was conducted using a recently enhanced data base of natural emissions that includes ocean and continental sources of DMS, H2S, chlorinated gases and lightning NOx for the continental United States and surrounding regions. Results using 2002 meteorology and emissions indicated that most simulated "natural" (plus background) chemical and aerosol species exhibit the expected seasonal variations at the surface. Ozone exhibits a winter and early spring maximum consistent with ozone data and an earlier published model. Ozone distributions reflect the influences of atmospheric dynamics and pollutant background levels imposed on the CMAQ simulation by boundary conditions derived from a global model. A series of model experiments reveals that the consideration of gas-phase organic sulfur chemistry leads to sulfate aerosol increases over most of the continental United States. Cloud chemistry parameterization changes result in widespread decreases in SO2 across the modeling domain and both increases and decreases in sulfate. Most cloud-mediated sulfate increases occurred mainly over the Pacific Ocean (up to about 0.1 μg m−3) but also over and downwind from the Gulf of Mexico (including parts of the eastern US). Geographic variations in simulated SO2 and sulfate are due to the link between DMS/H2S and their byproduct SO2, the heterogeneity of cloud cover and precipitation (precipitating clouds act as net sinks for SO2 and sulfate), and the persistence of cloud cover (the largest relative sulfate increases occurred over the persistently cloudy Gulf of Mexico and western Atlantic Ocean). Overall, the addition of organic sulfur chemistry increased hourly surface sulfate levels by up to 1–2 μg m−3 but reduced sulfate levels in the vicinity of high SO2 emissions (e.g., wildfires). Simulated surface levels of DMS compare reasonably well with observations in the marine boundary layer where DMS oxidation product levels are lower than observed. This implies either a low bias in model oxidation rates of organic sulfur species or a low bias in the boundary conditions for DMS oxidation products. This revised version of CMAQ provides a tool for realistically simulating the influence of natural emissions on air quality.

Effect of iron dissolution on cloud chemistry: from laboratory measurements to model results

This study investigates the influence of iron dissolution from aerosol particles on cloud chemistry and presents improvements in modeling of the associated multiphase processes. Iron redox species are important pollutants; they are very reactive in clouds especially through their interactions with HXOy compounds. Solid phase iron is transferred into the aqueous–phase by dissolution. The rate of dissolution of iron drives its concentration in the solid and aqueous–phases and has been determined in laboratory based on dissolution experiments using an urban particles standard. The parameterization of the iron dissolution rate and iron redox speciation as a function of time, stemming from the experimental data, was implemented in the cloud chemistry model M2C2 (Model of Multiphase Cloud Chemistry). A continental cloud event with simplified microphysical properties was simulated. Two simulations were performed: (1) using a predefined fixed iron content, and (2) using iron content values evolving temporally resulting from the dissolution parameterization. Numerical results show that the iron speciation (i.e. the partitioning between iron oxidation states) is driven by the chemical reactivity and not by dissolution. The oxidative capacity of the atmospheric cloud water decreases when dissolution is included in the model. The flux of OH radicals produced is positively correlated to the concentration of iron present that continuously increases with time throughout the simulation. Thus, ignoring the iron dissolution kinetics results in overestimations of net OH production, by factors of 1.7 and 1.2 after 5 minutes and 2 hours of the simulation, respectively. This study concludes that the consideration of the dissolution process is potentially important in estimating radical concentrations and hence cloud oxidative capacity.

Effect of mixed-phase cloud on the chemical budget of trace gases: A modeling approach

A multiphase cloud chemistry model coupling a detailed chemical reactivity mechanism in gas phase and aqueous phase to a cloud parcel model with a two-moment microphysical scheme has been extended to include ice phase processes. This newly developed model is used to study the influence of the ice phase on HCOOH, HNO 3, H 2O 2 and CH 2O in a mixed-phase cloud. Microphysical processes are describing the interactions between the water vapor phase, the liquid phase (cloud and rain water) and the ice phase (pristine ice, snow and graupel) in the cloud and for soluble chemical species, their transfer by mixed-phase microphysical processes has been included. In addition to microphysical transfer between iced hydrometeors, the probable two main processes incorporating soluble chemical species in iced hydrometeors are the retention in ice phase as riming or freezing occurs and the burial in the ice crystal as the crystal grows by vapor diffusion. The model is applied to a cloud event describing a moderate precipitating mixed-phase cloud forming in a continental air mass in winter. The main features of the cloud are described and the evolution of key chemical species as function of time and temperature is discussed. Sensitivity tests are performed: a run without ice to highlight the influence of ice phase on the chemical gas phase composition of the cloud, a run without burial showing that it is a negligible process, a run assuming full retention in ice for all species and a run varying the ice crystal shapes. A detailed analysis of the microphysical rates and chemical rates linked to retention and burial effects show that for this cloud event, the effect of the ice phase on gas phase composition is driven by riming of cloud droplets onto graupels, which leads to retention or not of soluble chemical species in the ice phase. Finally, the impact of crystal geometry on the efficiency of collection is studied together with its impact on the riming of cloud droplets on graupels and also on the retention of chemical species in ice phase.

Analytical estimation of cloud droplet nucleation based on an underlying aerosol population

In this paper, we describe an analytical approach to the approximation of the nucleation rate of cloud condensation nuclei (CCN) in an updraught. This approach is developed as a subgrid parametrization for numerical models whose vertical grid structure prohibits the resolution of supersaturation and thence CCN activation. The method used is a simple extension to the upper bound approximation of Twomey (1959), and follows a similar procedure to that adopted by Cohard et al. (1998). However, unlike these approaches to the problem, we incorporate a dependence upon the physicochemical properties of an underlying dry aerosol population which is not constrained to be chosen a priori. The parametrization outlined here is also able to respond to a multimodal representation of the underlying dry aerosol spectrum. Each mode of this spectrum can further have different chemical characteristics and as such could be suitable for use in a coupled cloud-chemistry model whereby each mode of the aerosol distribution is free to evolve independently. The results of the parametrization calculated using a range of properties of the dry aerosol are validated against those from a more detailed parcel model.

On the early-time chemistry in dark clouds

Context. Agreement between observed and calculated abundances of some molecules in dark clouds is improved when the model gas retains some memory of the assumed initial conditions. Aims. We assess the relevance of commonly-assumed intitial conditions giving rise to so-called early-time chemistry in models of dark cloud chemistry. Methods. Initial conditions for models of dark cloud chemistry are compared with conditions known to exist in the H2-bearing diffuse gas whose evolution they are intended to represent. Results. The initial conditions assumed for dark cloud chemistry differ in at least five important regards from those of H2-bearing diffuse gas. Models typically assume: absolute purity of H2; absence of polycyclic aromatic hydrocarbons; two times smaller gasphase abundance of C, N, and O; near-absence of sulfur and some other easily-ionized metals; total absence of the molecules which accompany the presence of H2 in diffuse gas, for instance, OH, CH, C2H, HCO + . Conclusions. Initial conditions commonly assumed for dark cloud chemical models should be reconsidered and mechanisms for the chemical evolution from diffuse to dark gas should be included in model calculations.

Atmospheric HULIS enhance pollutant degradation by promoting the dark Fenton reaction

Humic‐like substances (HULIS) in the atmosphere are ubiquitous macromolecular substances that comprise a major fraction of the organic component of atmospheric aerosols. In this study we report that HULIS extracted from collected wood burning and urban pollution atmospheric particles enhance aqueous phase oxidation of model organic contaminants (pyrene and phenol), by promoting the dark Fenton reaction under atmospherically relevant conditions. The paucity of radical sources at night makes this reaction, which is not accounted for in cloud chemistry models, potentially quite important for understanding and quantifying in‐cloud degradation of organic pollutants, and for understanding Fe oxidation state speciation in atmospheric waters.

Cloud-scale model intercomparison of chemical constituent transport in deep convection

Abstract. Transport and scavenging of chemical constituents in deep convection is important to understanding the composition of the troposphere and therefore chemistry-climate and air quality issues. High resolution cloud chemistry models have been shown to represent convective processing of trace gases quite well. To improve the representation of sub-grid convective transport and wet deposition in large-scale models, general characteristics, such as species mass flux, from the high resolution cloud chemistry models can be used. However, it is important to understand how these models behave when simulating the same storm. The intercomparison described here examines transport of six species. CO and O3, which are primarily transported, show good agreement among models and compare well with observations. Models that included lightning production of NOx reasonably predict NOx mixing ratios in the anvil compared with observations, but the NOx variability is much larger than that seen for CO and O3. Predicted anvil mixing ratios of the soluble species, HNO3, H2O2, and CH2O, exhibit significant differences among models, attributed to different schemes in these models of cloud processing including the role of the ice phase, the impact of cloud-modified photolysis rates on the chemistry, and the representation of the species chemical reactivity. The lack of measurements of these species in the convective outflow region does not allow us to evaluate the model results with observations.

Comparison of different model approaches for the simulation of multiphase processes

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 the spaccim approach, one- and two-dimensional particle/drop microphysical schemes are used in a time-splitting setup between chemistry and microphysics. The galerkin 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.

A parcel model for the combined treatment of microphysical and multiphase chemical processes

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.

Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling

Molecular hydrogen has an important role in the early stages of star formation as well as in the production of many other molecules that have been detected in the interstellar medium. In this review we show that it is now possible to study the formation of molecular hydrogen in simulated astrophysical environments. Since the formation of molecular hydrogen is believed to take place on dust grains, we show that surface science techniques such as thermal desorption and time-of-flight can be used to measure the recombination efficiency, the kinetics of reaction and the dynamics of desorption. The analysis of the experimental results using rate equations gives useful insight on the mechanisms of reaction and yields values of parameters that are used in theoretical models of interstellar cloud chemistry.

The role of transition metal ions on HO<sub>x</sub> radicals in clouds: a numerical evaluation of its impact on multiphase chemistry

Abstract. 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). Chemical regimes in clouds are analyzed to understand the role of transition metal ions on cloud chemistry and especially, on HxOy 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.