1. LOAC (Light Optical Particle Counter) a new aerosols counter for the determination of their sizes and their main nature under meteorological balloons P. Charpentier MeteoMODEM company, Ury, France J.-B. Renard, G. Berthet, F. Jegou, M. Jeannot LPC2E-CNRS, Orléans, France F. Dulac LSCE/CEA, Gif-sur-Yvette, France M. Mallet LA, Toulouse, France C. Thaury, S. Salles, J.-L. Mineau, B. Fournier Environnement-SA company, Poissy, France N. Verdier CNES, Toulouse, France

2. Different types of aerosols in the troposphere and the stratosphere, having different origins Interplanetary grains Pollution Volcanoes Transported sands Disintegrated meteorites Biomass burning (and pyroconvection)

3. Optical aerosols measurements (counter, lidar, photometer) Sparse in situ measurements with strong constraints:  - High costs facilities and complex installation (aircrafts and balloons)  - Technical limitations (how to fly during strong volcanic eruption?) Aerosol counter operated in the middle atmosphere, below stratospheric balloons

4. There is a need for an aerosol counter that can be launched under all kinds of balloons (including meteo balloons) : - “Low cost” (the instrument can be lost after the flight) - Light instrument - Insensitive to the aerosols nature (accurate determination of size), which is not the case with the usual aerosol counters - Concentration measurements in different size classes

5. LOAC : Light Optical Aerosol Counter (patented concept) Measurements at 2 scattering angles : Where the scattered light is insensitive to the aerosols nature Where the scattered light is strongly sensitive to the aerosols nature Combining the measurements Accurate determination of the size distribution Estimation of the main nature of the aerosols Aerosols Laser

6. LOAC was design and tested in French by research institutes and by the Environnement-SA company Meteo balloon gondola (including PTU sensors and telemetry) MeteoMODEM company

7. LOAC 20 size classes in the 0.4 - 100  m range Optical chamber and pump: 250 grams 4 hours with 2 alkaline batteries of 4.5 V Meteo balloon gondola : 850 grams Long duration stratospheric balloon Meteo balloon Low altitude troposheric balloon

8. Kiruna, Northern Sweden, 1 April 2011 (large stratospheric balloon, onboard German gondola TWIN, CNES launch) Strong variability of aerosol content

9. West Iceland, 20 July 2011 (meteo balloon, Iceland meteo office launch) Clean stratosphere event

10. Identification of the main nature of aerosols in 4 size classes (typically 20  m) from the ratio of the 2 channels measurements - Liquid/ wet aerosols - Carbon particulates (soot) - Mineral dust (sand)

11. Sausset-les-Pins (France), 28 June 2012 (meteo balloon, CNES) Flights during an strong episode of sand above Mediterranean sea Size grains : 5-30  m Sand Soot (pollution) Sea droplets

12.  Candillargues (France), 23 September 2012 (MeteoFrance)  Hymex campaign Sand Small liquid background aerosols  Clouds/haze

13. Applications (under meteo balloons) Better determination of the size distribution and concentration for : - Pollution aerosols - Transported atmospheric sand - Volcanic ashes (aviation) Determination of the main nature of the aerosols in the various parts of the atmosphere

14. LOAC is already involved in several scientific projects Winter 2012-2013: ParisFog, study of fog, tethered balloon Summer 2013: Charmex campaign, study of aerosols transport above Mediterranean sea (30 LOAC) End of 2013: Aerowave, stratospheric aerosols content and variability, large stratospheric balloons 2018 : Strateole phase 2, tens of long-duration stratospheric balloons above equatorial regions