In this paper, the ''Expérience sur Site pour COntraindre les Modèles de Pollution atmosphérique et de Transport d'Emissions'' (ESCOMPTE) program is presented. The ESCOMPTE program is used to produce a relevant set of data for testing and evaluating regional pollution models. It includes high-resolution (in space and time) atmospheric emission inventories and field experiments, and covers an area of 120 Â 120 km, centered over the Marseilles-Berre area in the southeast of France during Summer 2001. This region presents a high occurrence of photochemical pollution events, which result from numerous industrial and urban sources of primary pollutants. From the dynamical characteristics of the area, sea-breeze circulation and channeling effects due to terrain features highly influence the location of the pollutant plumes. ESCOMPTE will provide a highly documented framework for dynamics and chemisty studies.Campaign strategies and experimental set up are described. During the planning phase, existing modeling results helped defining the experimental design. The campaign involved surface measurement networks, remote sensing, ship-borne, balloon-borne, and airplane measurements. Mean standard meteorological parameters and turbulent fluxes, ozone, ozone precursors, photochemically active trace gases, and aerosols were measured. Five intensive observation periods (IOPs) were documented using a wide spectrum of instruments, involving aircraft (7)

The uptake of N2O5 by pure water and NaCl solution was studied as a function of temperature in the range from 262 to 278 K with the droplet train technique, where a highly controlled beam of droplets was exposed to N2O5 in a low-pressure flow tube reactor, and the formation of nitrate in the liquid phase was determined by ion chromatography. The uptake coefficients, y, for N2O5 on pure water are observed to decrease from 0.03 to 0.0 13 with increasing temperature. This behavior corresponds to the expected negative temperature dependence of mass accommodation leading to an enthalpy m o b , = (-9.6 f 1.6) kcal mol-' and to an entropy m o b s = (-43 f 6) cal mol-' K-l for the phase transfer, corresponding to a continuous nucleation process with a critical cluster size N* of about 2.4. A significantly lower yield of nitrate than with pure water is observed in the experiments on NaCl solution (1 mol/L), indicating that nitrogen compounds (such as ClN02) are formed after the uptake of N2O5 by subsequent reactions with NaCl and escape from the droplets. After correction for the known yield for the formation of CIN02, the results exhibit a slight systematic tendency for the uptake coefficient on NaCl solution to be greater than on pure water, indicating that the uptake of N2O5 by these aqueous media might be reaction-controlled. This assumption leads to a lower limit of 800 mol L-I atm-I s-l12 for the product Hk112 from a simple steady state model (where H is the Henry's law constant for N2O5 and k is the first-order hydrolysis rate constant).

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The uptake of nitryl chloride C1NO 2 by pure water and NaI solutions was studied as a function of temperature in the range from 268 to 279 K with the droplet train technique. An upper limit of 10 -5 was derived for the uptake on pure water, while the uptake rate was observed to be greatly enhanced in the presence of 10 -3 -10 -2 M iodide, yielding uptake coefficients in the range from 1.1x10 -3 to 6.6x10 -3. This enhancement is a consequence of a reaction between I-and C1NO 2 where we suggest that a transfer of C1 + to iodide is involved. We also suggest that bulk kinetics alone is unable to describe the measured uptake rate which is influenced by surface reactions. These results show that heterogeneous chemistry in concentrated aerosols may play an important role for the fate of C1NO2, and may affect the concentration of atomic chlorine in the marine boundary layer.