Maud Leriche

Contribution of Microbial Activity to Carbon Chemistry in Clouds

ABSTRACT The biodegradation of the most abundant atmospheric organic C1 to C4 compounds (formate, acetate, lactate, succinate) by five selected representative microbial strains (three Pseudomonas strains, one Sphingomonas strain, and one yeast strain) isolated from cloud water at the puy de Dôme has been studied. Experiments were first conducted under model conditions and consisted of a pure strain incubated in the presence of a single organic compound. Kinetics showed the ability of the isolates to degrade atmospheric compounds at temperatures representative of low-altitude clouds (5°C and 17°C). Then, to provide data that can be extrapolated to real situations, microcosm experiments were developed. A solution that chemically mimicked the composition of cloud water was used as an incubation medium for microbial strains. Under these conditions, we determined that microbial activity would significantly contribute to the degradation of formate, acetate, and succinate in cloud water at 5°C and 17°C, with lifetimes of 0.4 to 69.1 days. Compared with the reactivity involving free radicals, our results suggest that biological activity drives the oxidation of carbonaceous compounds during the night (90 to 99%), while its contribution accounts for 2 to 37% of the reactivity during the day, competing with photochemistry.

A Model for Tropospheric Multiphase Chemistry: Application to One Cloudy Event During the CIME Experiment

In this study, we have used a multiphase box model which takes into account an explicit chemistry mechanism for both gas and aqueous phase for a rural environment and the kinetic of mass transfer between phases (Schwartz, 1986). The model is, then, initialized with multiphase measurements performed by Voisin et al. (2000) during the CIME experiment. The 13th December 1997, the cloud chemistry is mainly governed by high Nox and high formaldehyde levels and by an acidic pH in the droplets. A comparison of the model results is performed versus recent theoretical results from Herrmann et al. (1999), who proposed a slightly different chemical scheme, including C2 chemistry and transition metal chemistry but neglecting some reaction pathways, such as the one involving OHCH2O2 radical and using unrealistic microphysical cloud conditions (cloud duration, constant liquid water content, small droplet radius).

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).

Coupling quasi-spectral microphysics with multiphase chemistry: a case study of a polluted air mass at the top of the Puy de Dôme mountain (France)

Abstract An explicit multiphase chemistry model (Atm. Environ. 34 (29/30) (2000) 5015) has been coupled with quasi-spectral microphysics, based upon Berry and Reinhardts parameterizations (1974a, b). This coupled model has been initialized with polluted conditions as observed at the Puy de Dome mountain in the center of France and for a maritime cloud. The presence of clouds results in two effects on multiphase chemistry: a direct effect through mass transfer, solubility and reactivity, and an indirect effect through microphysical transfer from cloud water into rainwater and redistribution of reactive soluble species among interstitial air, cloud droplets and raindrops. Results demonstrate that microphysical processes are necessary to sketch out the complex, nonlinear multiphase chemistry in a real cloud. In addition to the direct exchange through mass transfer, incorporation of reactive oxidants such as HO x in droplets can arise and consequently make those species no longer available for reacting in the gas-phase. Moreover, microphysical coalescence conversions favor NO x destruction and enhance the chemical nitric acid production. Coalescence of cloud drops to form rain transfers dissolved species into drops that are undersaturated compared to Henrys law equilibrium. The rain becomes a reservoir for these species, allowing aqueous chemistry to produce more nitric acid than would be possible without the presence of rain. Finally, for the different cloud types, the fate of those intermediate and reactive species is investigated, looking at their budget in clear sky situation versus cloudy and/or rainy situations.

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.

Modeling the lava heat flux during severe effusive volcanic eruption: An important impact on surface air quality

The April 2007 eruption of Piton de la Fournaise, one of the most active volcanoes in the world, was the strongest eruption in recent decades with 230Mm3 of lava emitted and more 300KT of SO2 degased. The surface concentrations of SO2 have been measured by the ORA (Air Observatory of the Reunion Island) and showed that many stations exceeded the critic threshold for health. These high concentrations led to important health issues, accompanied by environmental and infrastructure degradations. Realized with MesoNH atmospheric model, our simulations show the transport of sulfur and his component between 2 April and 6 April 2007, with a focus on the influence of heat flow from lava. For this purpose, we have implemented ForeFire, a surface model initially realized to simulate forest fire, by adapting it to reproduce the dynamic of a lava flow. Thus, all flows (SO2, heat, vapor, CO2, CO) are triggered depending on its dynamic. With this first approach, our simulations reproduce quite faithfully the surface field observation of SO2 provides by ORA. Various sensitivity analyzes exhibit that volcano sulfur distribution was mainly controlled by the lava heat flow. Without heat flow parameterization, the surface concentrations are multiplied by a factor 30 compared to the reference simulation. Simulations also put in evidence that the 5 April, during the height of the eruption, changes in meteorological conditions, especially weakening of atmospheric boundary layer stability, led to various pollutants to be transported in higher altitude (8000m). The main consequence is the volcanic pollutants are transported off the east coast of Reunion Island.

Modeling Formation of SOA from Cloud Chemistry with the Meso-NH Model: Sensitivity Studies of Cloud Events Formed at the Puy de Dôme Station

The majority of the organic fraction of aerosols is suspected to be of secondary origin. However, the sources, chemical composition and formation mechanisms of secondary organic aerosols (SOA) remain one of the least understood processes relevant to the atmosphere. Laboratory experiments, in situ measurements and numerical simulations recently highlighted the formation of SOA through aqueous chemistry in humid aerosol particles and in cloud droplets. However, there is still a need to evaluate the relative relevance of SOA formation through aqueous chemistry in comparison to classical gas to particles conversion pathways. Cloud resolving model (CRM) allows simulating the complex aerosols-cloud-chemistry interactions. Meso-NH (Mesoscale Non-Hydrostatic atmospheric model) CRM model includes a cloud chemistry module and an aerosol module, which allows representing the formation of SOA due to aqueous phase reactivity. The model is applied on a cloud event observed at the puy de Dome Mountain. Comparing simulations with or without the cloud chemistry module activated assesses the contribution of the cloud chemistry in the SOA formation. Results show a significant contribution of aqueous phase reactivity in the formation of SOA downstream the Mountain.

Regional Modeling of Aerosol Chemical Composition at the Puy de Dôme (France)

Organic aerosols (OA) represent a large fraction (from 20 to 90 %) of the submicron particulate mass and it is mainly composed of secondary organic aerosols (SOA). Despite the ubiquity of OA in the atmosphere, there are still large uncertainties in understanding the formation pathways of SOA. Consequently, OA sources and physico-chemical transformations during their transport are poorly represented in chemistry-transport models and large gaps still remain between simulated and measured OA concentrations. The ability of the WRF-CHEM model to reproduce the organic aerosol mass concentration originated from anthropogenic or/and biogenic emissions is evaluated. From this perspective, simulations for two contrasted air masses are performed with WFR-Chem using the Volatility Basis Set (VBS) approach dedicated to the formation of SOA. Simulations results are compared to aerosol measurements performed at the puy de Dome station with a compact Time-of-Flight Aerosol Mass Spectrometer for two episodes in autumn 2008 and in summer 2010. Moreover, measurements of both anthropogenic and biogenic VOCs are used to access the capacity of the WRF-Chem model to correctly simulate the concentrations levels of the gas precursors of the SOA.

Chapter 4.9 Modelling of the July 10 STERAO storm with the RAMS model: Chemical species redistribution including gas phase and aqueous phase chemistry

Abstract The meso-scale RAMS model has been applied to the July 10, 1996, STERAO storm observed in Colorado using an idealized horizontally homogeneous sounding with warm bubble initiation. This simulation was done in the framework of the WMO cloud modeling workshop intercomparison on chemistry transport in deep convection led by Mary Barth. The RAMS model coupled with gas and aqueous chemistry simulates CO and O 3 mixing ratios similar to observations and other models. The anvil area, mass flux, CO flux and NO x flux simulated by the RAMS-chemistry model are found to be within 35% of the values deduced from observations. We further examine the simple parameterization of NO production from lightning used in the RAMS simulations, which lead to a good agreement between computed and observed NO x fluxes. Moreover, because the RAMS model allows using either single or double moment microphysical schemes, the impact of the microphysical scheme is examined in terms of chemical species redistribution by the storm. Finally, the effect of gas phase versus aqueous phase chemistry on chemical species redistribution by the storm is also studied.

Transition Metal Ions in Cloud Chemistry

Cloud, fog and rain chemistry have an important effect on both regional and global scales (Lelieveld and Crutzen, 1991; Jacob, 2000). Actually, there are still some remaining questions about processes in the atmospheric liquid phase related, in particular, to the role of transition metal ions, to the presence of VOCs (Volatile Organic Compounds), and to particulate matter that can act as cloud nuclei (Facchini, 2002).