BATAL: The Balloon measurement campaigns of the Asian Tropopause Aerosol Layer

International audienceWe describe and show results from a series of field campaigns using balloon-borne instruments launched from India and Saudi Arabia during the summers 2014-2017 to study the nature, formation and impacts of the Asian Tropopause Aerosol Layer (ATAL). The campaign goals were to i) characterize the optical, physical and chemical properties of the ATAL, ii) assess its impacts on water vapor and ozone, and iii) understand the role of convection in its formation. In order to address these objectives, we launched 68 balloons from 4 locations, one in Saudi-Arabia and 3 in India, with payload weights ranging from 1.5 kg to 50 kg. We measured meteorological parameters, ozone, water vapor, and aerosol backscatter, concentration, volatility and composition in the Upper Troposphere and Lower Stratosphere (UTLS) region. We found peaks in aerosol concentrations of up to 25 part/cm3 for radius > 75 nm, associated with Scattering Ratio at 940 nm of ~1.9 near the cold point tropopause. During medium-duration balloon flights near the tropopause, we collected aerosols and found, after offline ion chromatography analysis, the dominant presence of nitrate ions with a concentration of about 100 ng/m3. Deep convection was found to influence aerosol loadings 1 km above the cold point tropopause. The BATAL project will continue for the next 3-4 years and the results gathered will be used to formulate a future NASA-ISRO airborne campaign with NASA high altitude aircraft