Nicole Audiffren

Deviations from the Henry's law equilibrium during cloud events : a numerical approach of the mass transfer between phases and its specific numerical effects

Cloud drops perturb the gaseous-phase concentrations of pollutants through aqueous-phase reactions and also mass transfer between the two phases. Regional models of pollution require an accurate description of the mass transfer from gaseous to aqueous phase or from aqueous to gaseous-phase. We show that the most soluble species deviate from the Henrys law equilibrium at the sudden apparition of the aqueous-phase due to increasing phase equilibration times higher than chemical and dynamical time scales. Using Henrys law equilibrium for all species result in spurious predictions of the concentrations even for moderate soluble species. Mass transfer between phases must, therefore, be described in a real kinetic form. We also show that simple kinetic solvers (QSSA) with constant timestep are not accurate enough to resolve the increasing stiffness due to the sudden formation of a cloud when the mass transfer is modelled by kinetic equations. Depletion or overestimation of concentrations are artificially generated when the aqueous chemistry and mass transfer are taken into account. As regional models of pollution require simple, rapid chemical solvers, we have preserved the simplicity and low memory storage of the QSSA code by developing a variable timestep version of QSSA code. This modified version was revealed to be more accurate than the basic version of QSSA and about 30% faster than a Gear code.

Numerical simulation of aqueous-phase atmospheric models: use of a non-autonomous Rosenbrock method

We present in this article an efficient numerical solver for the time integration of atmospheric multiphase chemical kinetics. This solver is based on a second-order Rosenbrock scheme, that has been proposed by Verwer et al. (SIAM J. Sci. Comput. 20 (4) (1999) 1456) for gas-phase chemical kinetics. We show that the stiff time dependence of cloudy events (through liquid water content) has to be solved by the numerical scheme and a non-autonomous version has to be used. We benchmark the non-autonomous ROS2 scheme with the classical LSODE solver for two kinetic schemes. For detailed schemes such as RADM2, the speed-up is of magnitude 5 for the same accuracy.

Reduction of Multiphase Atmospheric Chemistry

The aim of this article is to investigate the dynamical behaviour ofmultiphase atmospheric chemical mechanisms. Reducing procedures areapplied to a multiphase chemical box model including gas-phasereactions, aqueous-phase reactions and interfacial mass transfer. The lumping of species is computed in an automatic wayusing an efficient algorithm (apla). The computed lumped species arerelated to the fast behaviour of chemical and microphysical processessuch as Chapman cycle, ionic dissociations within the cloud drops andinterfacial Henrys equilibria. Depending on some parameters (liquidwater content, droplet radius) mixed lumped species (including both phases) may also becomputed. We show the existence of hierarchical reduced models due to the existence ofmultiple timescales. We use a special algorithm (dan2) in order tosolve the reduced models. Such models are accurate and the relative errorremains under the threshold of 1%. The speed-up is up to a factor 5comparedwith a fully implicit method (Gear) for the same accuracy. The key pointis that it provides a good qualitative understanding for the behaviourof the kinetic scheme.

Modeling of scavenging processes in clouds: some remaining questions about the partitioning of gases among gas and liquid phases

Clouds can play an important role by affecting the chemical composition of the troposphere through modification of photolysis rates, and by redistributing compounds through the vertical transport of species and their removal by wet deposition and finally by aqueous phase chemical reactions within cloud water or precipitation water. Several examples of the effects of clouds on tropospheric chemistry are shown, using a box model or a mesoscale model illustrating the role of clouds on hydrogen peroxide: its partitioning between the gas and aqueous phases, including deviations from Henrys law. The main results are that deviations from Henrys law exist even for small droplets, which are located on the edges of orographic clouds while equilibrium is attained in the center of the cloud. The partitioning of gases is a function of the cloud development conditions such as the air mass in which the cloud has been formed (continental vs. maritime), the microphysical properties (cloud water content, rainwater content).

A modeling study of the influence of ice scavenging on the chemical composition of liquid-phase precipitation of a cumulonimbus cloud

Abstract Evidence of the efficient removal of chemicals by ice particles has been deduced from past field experiments and laboratory studies. However, the ice phase has been poorly represented in prior cloud chemistry modeling. This paper uses a two-dimensional Eulerian cloud model to address the impact of ice-phase processes on the chemistry of precipitation in the context of a simulated cumulonimbus cloud. Riming of graupel and the freezing of supercooled rain are the main processes for the transfer of species toward graupel. Even when freezing is the main mode for this transfer, riming still plays an important role by providing a feedback effect that limits the diluting influence of rain. When riming is the only process, sulfate production is more efficient in rainwater, whereas when freezing dominates a decrease in sulfate production is observed. During the decaying stage, the precipitation (glaciated and/or liquid) has higher concentrations of the hydrogen peroxide and sulfates that originated from t...

A sensitivity analysis study for radm2 mechanism using automatic differentiation

A sensitivity analysis of an atmospheric multiphase mechanism is performed using an automatic differentiation tool. The sensitivity of some key concentrations is computed with respect to some input parameters (kinetic rates, microphysical parameters). The package odyssee is used in order to obtain the so-called linear tangent model giving the derivatives of outputs with respect to inputs. The direct model takes into account gas-phase reactions, aqueous-phase reactions and interfacial mass transfer and is based on the radm2 mechanism. Local sensitivity coefficients are computed for two different scenarii, rural and sub-urban. We focus in this study on the sensitivity of the gas-phase O3–NOx–HOx system with respect to some aqueous phase reactions and we investigate the influence of the reduction in the photolysis rates in the area below the cloud region. This preliminary work illustrates how powerful automatic differentiation tools may be for the study of large chemical mechanisms. We show for instance that the oxidation of trace metals (FeII, FeIII, Cu+ and Cu2+) in the case of low S(IV) polluted area is not always in disfavor of HOx gaseous concentrations, as it is usually claimed.

Effects of a polydisperse cloud on tropospheric chemistry

Effects of a polydisperse cloud on tropospheric chemistry have been studied in the framework of a two-dimensional model where dynamical, microphysical, and chemical processes are fully interactive. The chemical module describes the tropospheric photochemistry of ozone precursors in both gaseous and aqueous phases for a remote atmosphere. Impacts of the cloud polydisperse feature have been obtained by comparing the results in the case of a monodisperse cloud created under the same meteorological conditions. The [NO]/[NO2] ratio decreases more sharply in the case of the polydisperse cloud. The partitioning of the most soluble species does not follow the Henrys law equilibrium except in the middle of the cloud. This result has implications for airborne measurements made within clouds. Deviations from Henrys law found in samples are usually explained only by the effect of variations of the liquid water content with time, assuming that no real deviations exist in the real cloud. Here, it is shown that deviations from Henrys law equilibrium may exist even for clouds consisting of small droplets.

A numerical study of the seasonal variations for tracer redistribution by clouds over West Africa

The sensitivity of the horizontal and vertical transport of an inert tracer to seasonal variability of the complex West Africa circulations is explored by means of a 2D mesoscale meteorological model including explicit microphysics and describing the cloud dynamics of these circulations. The seasonal variations of the location and spatial extent of the cloudy masses associated with the Inter Tropical Convergence Zone (ITCZ) are reproduced in a meridional cross section over West Africa. The redistribution of the inert tracer is shown to be due to the northward migration of the ITCZ from January to July and also to the enhancement of the convective activity of the ITCZ in July. The redistribution of carbon monoxide during the dry and wet seasons is discussed, highlighting the importance of the biomass burning during the dry season as a source of trace gas enrichment in the upper levels of the tropical troposphere over West Africa.

Modelling of Chemical Processes in Clouds: Scavenging and Partitioning of Species Among Gas and Liquid Phases

More than 50% of the earth’s surface is covered by clouds and theoretical calculations of Ravishankara (1997) have shown that clouds can alter the composition on a global scale. Clouds interact in many ways with chemicals and on a wide range of scales from micrometers up to thousand of kilometers.

Modeling of Atmospheric Multiphase Chemistry: Numerical Integration and Sensitivity Analysis

The presence of clouds in the atmosphere can substantially modify the chemical kinetics and thus the rates of destruction and production of chemical species. Multiphase models are suffer and rather more difficult to integrate than pure gas models. They contain moreover many physical and microphysical parametres whose values are often known with poor accuracy. In this paper we focus on two major points. The first one is time integration. We propose the nonautonomous second-order Rosenbrock method in order to solve such heterogenous models. In the second part we focus on the sensitivity analysis of the model outputs with respect to the input parameters using an automatic differentiation tool (ODYSSEE).