Boundary-layer drifting balloons for chemistry-dynamics studies in the Mediterranean region

In dynamics-chemistry coupled boundary-layer studies, specific tools of observation are required. For example, the ageing of pollutant plumes is dependent on transport, on horizontal and vertical diffusion, and on chemistry processes inside the plume. Among the observational tools able to qualify the numerical models performance to adequately describe/forecast the plume behaviour, the drifting balloons offer an invaluable field-truth.

The French space agency (CNES) has developed for a long time the capacity of designing, building and equipping such drifting balloons. The current system consists in a spherical, 2.5-m-diameter envelope, and a gondola in which are embarked the sensors, the acquisition system and the data transmission unit via the Iridium satellite communication network. The isopycnic flight level can be adjusted before launch by varying the air/He ratio inside the balloon. This method allows the internal pressure to be approximately the same whatever the chosen flight level. Besides the basic meteorological instrumentation (including pressure, temperature and moisture sensors), the chemistry-related payload consists of an optical particle counter (OPC) and/or an ozone sensor. The OPC, called LOAC (light optical aerosol counter), is a miniaturized counter (500 g) developed by French groups for measuring the size distribution in 20 classes from 0.4 µm. The ozone probe is an adaptation of a commercial electrochemical unit for which the power source and output signal acquisition are provided by the balloon gondola. Its operation in an intermittent mode extends its lifetime up to several days. The flight level ranges between few hundreds of meters and 2-3 km, and the flight duration could extend up to weeks. However, due to safety constraints, the balloons have to be remotely destroyed when arriving in the vicinity of continental inhabited areas, and the Mediterranean flights are therefore in general restricted to a few days.

During the summer 2012 and 2013, these balloons will be deployed in the frame of the ChArMEx program. ChArMEx (the Chemistry-Aerosol Mediterranean Experiment; http://charmex.lsce.ipsl.fr) is an integrated project which aims at an assessment of the present state of the atmospheric environment in the Mediterranean basin, of its impacts on air quality, regional climate and marine biogeochemistry, and of their evolution in a regional context of intense climate change and increasing anthropogenic pressure. The first phase will be the TRAQA (Transport and Air Quality) campaign, in June-July 2012, during which the balloons will follow and document a pollutant plume originated from a major industrial area close to the French Mediterranean shoreline, and serve as targets for successive visits of the plume by an instrumented aircraft. In summer 2013, two successive campaigns will be performed, the first one (ADRIMED) dedicated to the study of the Saharan dust plumes and their impact on the radiation budget, and the second to the documentation of pollutant plumes chemical ageing. Both campaigns will closely couple aircraft and balloon measurements.

For the studies undertaken from 2014 hereon, the Aeoclipper developed by CNES will also be available in its “Mediterranean” version. It consists of a low altitude streamlined balloon drifting at ~50 m over the surface and moving along the wind but at a lower speed because it is equipped with a cable and a guide-rope in contact with the sea surface. Its instrumentation is distributed on two gondolas, one atmospheric and one oceanic, with the aim to measure surface physical parameters (air and sea surface temperatures, wind, pressure and humidity) in order to derive air-sea fluxes.