S. Cautenet

Large-scale advection of continental aerosols during INDOEX

In this paper, we present passive and active remote sensing measurements of atmospheric aerosols over the North Indian Ocean (NIO) during the Intensive Field Phase (IFP, January to March 1999) of the Indian Ocean Experiment. The variability of the aerosol load over NIO is discussed based on three-dimentional numerical simulations made at a local scale by use of Regional Atmospheric Modeling System (RAMS) and at a regional scale using the zoomed Laboratoire de Meteorologie Dynamique global circulation model (LMD-Z version 3.3). Ground-based measurements of the columnar aerosol optical thickness (AOT) and of surface black carbon (BC) concentration were carried out at two different sites in India: Goa University on the NIO coast and Dharwar 150 km inland. Local-scale investigations point out that the trend in BC concentration at the ground is not correlated with AOT. Lidar profiles obtained both from the surface at Goa and in the NIO from the Mystere-20 research aircraft indicate that a significant contribution to the total AOT (more than 50%) is due to a turbid monsoon layer located between 1 and 3 km height. RAMS simulation shows that the advection of aerosols in the monsoon layer is due to the conjunction of land-sea breeze and topography. We present the regional-scale extent of the aerosol plume in terms of AOT derived from the visible channel of Meteosat-5. During March, most of the Bay of Bengal is overcast by a haze with a monthly average AOT of 0.61±0.18, and a spatially well-defined aerosol plume is spreading from the Indian west coast to the Intertropical Convergence Zone with an average AOT of 0.49±0.08. Those values are bigger than in February with AOT at 0.35±0.18 and 0.37±0.09 for the Bay of Bengal and the Arabian Sea, respectively. One of the principal findings of this paper is that a significant contribution to the aerosol load over the NIO is due to the advection of continental aerosols from India in a well-identified monsoon layer above the marine boundary layer. Moreover, it is suggested that the increase in biomass burning plays a significant role in the increasing trend in AOT during the winter dry monsoon season.

Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO): An overview

This paper presents an overview of the Experiment for Regional Sources and Sinks of Oxidents (EXPRESSO) including the objectives of the project, a detailed description of the characteristics of the experimental region and of field instrumentation deployed, and a summary of the main results of all components of the experiment. EXPRESSO is an international, multidisciplinary effort to quantify and better understand the processes controlling surface fluxes of photochemical precursors emitted by vegetation and biomass burning along a tropical forest to savanna gradient in central Africa. The experiment was conducted at the beginning of the dry season in November-December 1996. Three main research tools were deployed during this period: (1) the French research aircraft (Avion de Recherche Atmospherique et de Teledetection, Fokker 27), instrumented for chemistry and flux measurements (CNRS- France), (2) two satellite receivers for in situ acquisition of National Oceanic and Atmospheric Administration-advanced very high resolution radiometer (NOAA-AVHRR) imagery for fire detection (EC-JRC, Ispra, Italy), and (3) a 65-m walkup tower installed at a tropical forest site in the Republic of Congo (National Center for Atmospheric Research, Boulder, Colorado). Average dynamic and turbulence characteristics over savanna and forest ecosystems were retrieved from aircraft measurements. They illustrate the complex atmospheric circulation occurring in this region in the vicinity of the Intertropical Convergence Zone. Satellite receivers were operated three times a day to produce maps of fire distribution. Statistics and mapping of burned surfaces from NOAA-AVHRR and ERS-Along Track Scanning Radiometer space systems have been developed. The influence of biogenic and biomass burning sources on the chemical composition of the lower atmosphere was studied through both aircraft and tower measurements. The EXPRESSO field campaign was followed by modeling efforts (regional and global scales) in which model components are evaluated using the experimental data.

Quantification of Coastal New Ultra-Fine Particles Formation from In situ and Chamber Measurements during the BIOFLUX Campaign

Environmental Context. Secondary processes leading to the production of ultra-fine particles by nucle- ation are still poorly understood. A fraction of new particles formed can grow into radiatively active sizes, where they can directly scatter incoming solar radiation and, if partly water soluble, contribute to the cloud condensation nuclei population. New particle formation events have been frequently observed at the Mace Head Atmospheric Research Station (western Ireland), under low tide and sunny conditions, leading to the hypothesis that new particles are formed from iodo-species emitted from macroalgae. Abstract. New particle formation processes were studied during the BIOFLUX campaign in September 2003 and June 2004. The goals were to bring new information on the role of I2 in new particle formation from seaweeds and to quantify the amount of I2 emitted and new particles formed by a given amount of seaweed. These two goals were achieved by using a simulation chamber filled with selected species of seaweeds from the Mace Head area and flushed with particle-free atmospheric air. It was found that total particle concentrations and particles in the 3-3.4 nm size range produced in the chamber are positively correlated with gaseous I2 concentrations emitted by the seaweeds, with a typical source rate of 2800 particles cm −3 ppt −1 (I2) in the 3-3.4 nm size range. In fact, I2 and particle concentrations are also both directly positively correlated with the seaweed mass (64 300 particles cm −3 formed per kg of seaweed, and 24 ppt of I2 per kg of seaweeds) until saturation was reached for a seaweed biomass of 7.5 kg m −2 . From the chamber experiments, the flux of 3-3.4 nm particles was calculated to be 2.5 × 10 10 m −2 s −1 for a seaweed loading of 2.5 kg m −2 (representative of a typical seaweed field density), decreasing to 1 × 10 10 m −2 s −1 for a seaweed loading of 1 kg m −2 . At a seaweed loading of 2.5 kg m −2 , the growth rate of particles produced in the chamber was calculated to be 1.2 nm min −1 . The source rates and growth rates determined from the chamber experiments were used in conjunction with seaweed coverage in and around Mace Head to produce local emission inventories for a meso-scale dispersion model. Comparison of the resulting aerosol size distributions from the model simulations and those observed exhibited good agreement suggesting that the chamber fluxes and growth rates are consistent with those associated with the tidal emission areas in and around Mace Head.

Regional transport and dilution during high-pollution episodes in southern France: Summary of findings from the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE)

In the French Mediterranean basin the large city of Marseille and its industrialized suburbs (oil plants in the Fos-Berre area) are major pollutant sources that cause frequent and hazardous pollution episodes, especially in summer when intense solar heating enhances the photochemical activity and when the sea breeze circulation redistributes pollutants farther north in the countryside. This paper summarizes the findings of 5 years of research on the sea breeze in southern France and related mesoscale transport and dilution of pollutants within the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE) program held in June and July 2001. This paper provides an overview of the experimental and numerical challenges identified before the ESCOMPTE field experiment and summarizes the key findings made in observation, simulation, and theory. We specifically address the role of large-scale atmospheric circulation to local ozone vertical distribution and the mesoscale processes driving horizontal advection of pollutants and vertical transport and mixing via entrainment at the top of the sea breeze or at the front and venting along the sloped terrain. The crucial importance of the interactions between processes of various spatial and temporal scales is thus highlighted. The advances in numerical modeling and forecasting of sea breeze events and ozone pollution episodes in southern France are also underlined. Finally, we conclude and point out some open research questions needing further investigation.In the French Mediterranean basin the large city of Marseille and its industrialized suburbs (oil plants in the Fos-Berre area) are major pollutant sources that cause frequent and hazardous pollution episodes, especially in summer when intense solar heating enhances the photochemical activity and when the sea breeze circulation redistributes pollutants farther north in the countryside. This paper summarizes the findings of 5 years of research on the sea breeze in southern France and related mesoscale transport and dilution of pollutants within the Field Experiment to Constraint Models of Atmospheric Pollution and Emissions Transport (ESCOMPTE) program held in June and July 2001. This paper provides an overview of the experimental and numerical challenges identified before the ESCOMPTE field experiment and summarizes the key findings made in observation, simulation, and theory. We specifically address the role of large-scale atmospheric circulation to local ozone vertical distribution and the mesoscale processes driving horizontal advection of pollutants and vertical transport and mixing via entrainment at the top of the sea breeze or at the front and venting along the sloped terrain. The crucial importance of the interactions between processes of various spatial and temporal scales is thus highlighted. The advances in numerical modeling and forecasting of sea breeze events and ozone pollution episodes in southern France are also underlined. Finally, we conclude and point out some open research questions needing further investigation.

Thermal Impact of Saharan Dust over Land. Part I: Simnulation

Abstract Simulations are carded out to verify a mesoscale model in order to perform sensitivity tests of satellite response to atmospheric dust content. The model chosen is the mesoscale model of Colorado State University with a modified radiation parameterization in order to take atmospheric dust content into account. Downward and upward longwave irradiances are estimated using a 25-interval model. The shortwave pan of the spectrum is processed by a very fast, highly parameterized, single-interval code. Tests using experimental data gathered during the Etude de la Couche Limite Atmospherique Tropicale Seche (ECLATS) experiment performed during the 1980 dry season near Niamey (Niger, West Africa) prove that dust content is satisfactorily handled. Three 24-h simulations performed under various meteorological and turbidity conditions show that ground surface energy exchanges are satisfactorily described, so that surface temperature is predicted with a standard deviation of about 1°C. Vertical profiles of co...

A case study of extreme tropospheric ozone contamination in the tropics using in-situ, satellite and meteorological data

Since 1992, more than 150 radiosoundings have been performed from Reunion Island in the Indian Ocean in Southern tropics. On August 25, 1995, tropospheric ozone amounts of over 200 ppbv through the entire troposphere were encountered. Typical values during that period do not usually reach more than 80 ppbv. In order to explain these disturbances, we performed an analysis using satellite and ECMWF data. Results indicate that several mechanisms are working together, including photochemical effect of biomass burning associated with convection, and stratosphere-troposphere exchange near the subtropical jet stream. Based on results shown in this paper, it is difficult to estimate the relative contribution of biomass burning and stratospheric intrusions. Quantitative estimates should stem from other chemical component measurements and from a more detailed dynamical analysis.

Simulation of ozone production in a complex circulation region using nested grids

During the ESCOMPTE precampaign (summer 2000, over Southern France), a 3-day period of intensive observation (IOP0), associated with ozone peaks, has been simulated. The comprehensive RAMS model, version 4.3, coupled on-line with a chemical module including 29 species, is used to follow the chemistry of the polluted zone. This efficient but time consuming method can be used because the code is installed on a parallel computer, the SGI 3800. Two runs are performed: run 1 with a single grid and run 2 with two nested grids. The simulated fields of ozone, carbon monoxide, nitrogen oxides and sulfur dioxide are compared with aircraft and surface station measurements. The 2-grid run looks substantially better than the run with one grid because the former takes the outer pollutants into account. This on-line method helps to satisfactorily retrieve the chemical species redistribution and to explain the impact of dynamics on this redistribution.

Simulation of carbon monoxide redistribution over central Africa during biomass burning events (Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO))

The Regional Atmospheric Modeling System (RAMS) is used to assess a realistic estimate of emissions by savanna fires from NOAA advanced very high resolution radiometer (AVHRR) satellite imagery during the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) campaign. The daily amounts of gases released by biomass burning depend on burnt biomaterial, the mass of which is estimated from two independent methods: classification of vegetation formations (CV) or net primary productivity (NPP). The area of the burnt zone is derived from satellite imagery. The percent of burnt biomass or combustion factor (α) is determined by an adjustment procedure using two model runs and assumed to be 40%, a value consistent with the biomass wetness. The simulated CO redistribution is compared with aircraft measurements (tracks and profiles). Two events of 4 and 5 days are examined, where flights above forest and savanna zones have been performed. In this study we show that the main mechanisms of the chemical species spatial redistribution originates from mesoscale features. The RAMS results are in rather good agreement with the aircraft measurements (vertical profiles or horizontal legs) over forest and savanna. The difference between the CO simulated fields obtained from CV or NPP is weak (about 20% or 30%), and it is difficult to discriminate the best method because this difference is generally inferior to the gap between model and observations. One may consider this study as a preliminary approach to improve the estimate of released gases by biomass burning with the help of mesoscale modeling and the knowledge of the real sources.

Mesoscale Atmospheric Modeling Using a High Horizontal Grid Resolution over a Complex Coastal Terrain and a Wine Region of South Africa

Abstract The Regional Atmospheric Modeling System (RAMS) was used to assess local air circulation patterns over the wine-producing Stellenbosch region of South Africa. Numerical simulations using four nested grids (25, 5, and 1 km, and 200 m of horizontal resolution) were performed for each day of February 2000 (during the grape-ripening period) over southern Western Cape Province. Modeled hourly data were extracted from the analysis files and used to produce mean hourly fields (temperature, relative humidity, wind speed, and radiation). Three runs with increasing horizontal resolutions for the finer grid were performed (run 1 with two nested grids of 25 and 5 km; run 2 with three nested grids of 25, 5, and 1 km; run 3 with four nested grids of 25, 5, and 1 km, and 200 m). For each event, the simulations of 1-km and 200-m resolution were superior to the 5-km-resolution simulation, especially in reproducing the local air circulations (sea and slopes breezes) because of a better representation of the local ...

Contrasting behavior of gas and aerosol scavenging in convective rain: A numerical and experimental study in the African equatorial forest

A two-dimensional convective cloud model has been coupled with a chemical model consisting of the explicit prediction of five chemical species: SO2, SO42−, NH4+, O3, and H2O2. The model takes the scavenging processes into account. We examine the relationship between the liquid water content (LWC) and the chemical concentrations of atmospheric trace elements in convective precipitation. The model results compare favorably with observations (ABLE 2B and DECAFE experiments). The modeled dilution curves were found to be nonlinear, in agreement with the DECAFE data. The model also accounts for the large differences in dilution effects that exist between gases and aerosols. More generally, this study shows that within the African equatorial forest there are (1) a reduction of aerosol scavenging efficiency with increasing rain intensity (or LWCg); (2) a strong impact of vertical profiles of atmospheric trace elements on ground rain concentrations; (3) a difference in scavenging efficiencies according to the origin of the elements (gas or aerosol) and (4) a depletion of atmospheric concentrations during rainfall.