Nadine Chaumerliac

Potential impact of microbial activity on the oxidant capacity and organic carbon budget in clouds.

Within cloud water, microorganisms are metabolically active and, thus, are expected to contribute to the atmospheric chemistry. This article investigates the interactions between microorganisms and the reactive oxygenated species that are present in cloud water because these chemical compounds drive the oxidant capacity of the cloud system. Real cloud water samples with contrasting features (marine, continental, and urban) were taken from the puy de Dôme mountain (France). The samples exhibited a high microbial biodiversity and complex chemical composition. The media were incubated in the dark and subjected to UV radiation in specifically designed photo-bioreactors. The concentrations of H2O2, organic compounds, and the ATP/ADP ratio were monitored during the incubation period. The microorganisms remained metabolically active in the presence of ●OH radicals that were photo-produced from H2O2. This oxidant and major carbon compounds (formaldehyde and carboxylic acids) were biodegraded by the endogenous microflora. This work suggests that microorganisms could play a double role in atmospheric chemistry; first, they could directly metabolize organic carbon species, and second, they could reduce the available source of radicals through their oxidative metabolism. Consequently, molecules such as H2O2 would no longer be available for photochemical or other chemical reactions, which would decrease the cloud oxidant capacity.

Scavenging of acidic gases (HCOOH, CH3COOH, HNO3, HCl, and SO2) and ammonia in mixed liquid‐solid water clouds at the Puy de Dôme mountain (France)

In order to study scavenging processes of chemical species in mixed phase clouds, in-cloud field measurements were conducted in December 1997 at the Puy de Dome mountain (center of France, 1465 m above sea level). Soluble species including NH+4, Cl−;, NO3−3, SO−4, HCOO−, CH3COO−, and C2O−4 present in the different phases (supercooled water droplets, rimed snowflakes, interstitial gases, and aerosols) of cold clouds have been investigated. Conducted in parallel to microphysical studies of clouds (liquid water and ice contents, and size distribution of hydrometeors), these chemical investigations allow us to examine the partitioning of strong (HNO3 and HCl) and weak (SO2, HCOOH, and CH3COOH) acids as well as ammonia between interstitial air and the condensed phases (liquid and solid water particles) in mixed clouds present during winter at midlatitude regions. From that, we discuss the processes by which these key atmospheric species are taken up from the gas phase by the condensed phases (liquid and ice) in these cold clouds. We examine several factors which are of importance in driving the final composition of cloud ice. They include the partitioning of species between gaseous and supercooled liquid phases, the amount of rimed ice collected by snowflakes, and the retention of gas during shock freezing of supercooled droplets onto ice particles. Strong acids (HCl and HNO3) as well as NH3, being sufficiently soluble in water, are mainly partitioned into supercooled water droplets. Furthermore, being subsaturated in liquid droplets, these species are well retained in rimed ice. For these species, riming is found to be the main process driving the final composition of snowflakes, direct incorporation from the gas phase during growth of snowflakes remaining insignificant because of low concentrations in the gas phase. For light carboxylic acids the riming process mainly determines the composition of the snowflakes, but an additional significant contribution by gas incorporation during the growth of snowflakes cannot be excluded. SO2 is also present at significant levels in the interstitial air and is poorly retained in ice during riming of supercooled water droplets. However, hydroxymethanesulfonate (HMSA) was likely present in supercooled liquid droplets, making it difficult to evaluate by which mechanism S(IV) (i.e., HMSA plus SO2) has been incorporated into snowflakes.

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).

Ozone nighttime recovery in the marine boundary layer: Measurement and simulation of the ozone diurnal cycle at Reunion Island

We describe the diurnal cycle of ozone in the marine boundary layer measured at Reunion Island (21°S, 55°E) in the western part of the Indian Ocean in August-September 1995. Results from a box chemistry model are compared with ozone measurements at Reunion Island. We focus on the peak-to-peak amplitude of ozone concentration, since our measurements show a variation of about 4 parts per billion by volume, which is close to the value obtained by Johnson et al. [1990] during the Soviet-American Gases and Aerosols (SAGA) 1987 Indian Ocean cruise. Different dynamical mechanisms are examined in order to reproduce such a variation. We conclude that the most important one is the exchange between the ozone-rich free troposphere and the ozone-poor boundary layer. This exchange is supposed to be more important during the night than during the day, allowing ozone nighttime recovery. This is the key point of the observed diurnal cycle, since daytime ozone photochemistry is well described by the model. Then we assume an entrainment velocity equal to 1 mm s−1 during the day and 14 mm s−1 during the night to closely match our measurements. Topography influences, together with clouds, are presumed to be responsible for this difference between nighttime and daytime entrainment velocities of free tropospheric air into the boundary layer at Reunion Island. Over the open ocean the difference of the turbulent flux of sensible heat between the day and the night explains the strong ozone nighttime recovery observed by us and by Johnson et al. [1990].

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.

Impact of cloud dynamics on tropospheric chemistry: Advances in modeling the interactions between microphysical and chemical processes

A chemical module describing the tropospheric photochemistry of ozone precursors in both gaseous and aqueous phases for a remote continental atmosphere has been developed within the framework of a two-dimensional cloud model. Dynamical, microphysical and chemical processes are fully interacting in order to study the influence of clouds on ozone chemistry and to quantify the relative importance of the different processes on the budget and evolution of 12 chemical species. Whereas the concentrations of highly soluble species are strongly affected by evaporation and sedimentation, less soluble species are affected primarily by accretion. The model reproduces previously observed chemical phenomena such as the enrichment of formic acid at the top of the cloud.

Numerical Simulation of Orographic Enhancement of Rain with a Mesoscale Model

Abstract Orographic precipitation enhancement associated with the feeder mechanism proposed by Bergeron has been simulated using a two dimensions model based upon primitive equations including detailed parameter microphysics. A case-by-case comparison is made between model results and each of 14 well-documented precipitation episodes in southern Wales. The model reproduces the observed strong dependence of the precipitation enhancement on the low-level wind speed, as well as the weak dependence on the upwind precipitation rate. Model results also demonstrate that a satisfactory treatment of orographically enhanced precipitation requires the linking of the dynamical, thermodynamical and microphysical processes.

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.

Effects of Different Rain Parameterizations on the Simulation of Mesoscale Orographic Precipitation

Abstract A detailed comparison is made between the results obtained from two microphysical parameterizations capable of simulating cloud and precipitation processes in a mesoscale model. The behavior of each microphysical scheme is first investigated in the context of a mountain wave simulation. Major differences are found in raindrop sill distributions as well as in the rates associated with various microphysical processes. An assessment of the accuracy of each scheme is then obtained by comparing model predictions with observational data from well-documented orographically enhanced precipitation episodes in South Wales. The parameterization of Berry and Reinhardt does a better job of reproducing the observed dependency of the precipitation enhancement on the low-level windspeed than does Kesslers.