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NARL Studies tropical tropopause aerosol layer using in-situ observations in collaboration with NASA

Aerosols, Radiation and Trace Gases Group (ARTG) of National Atmospheric Research Laboratory is engaged in studying atmospheric aerosols, trace gases, radiations, clouds and their interactions. Aerosols, the sub-micron size particles suspended in air are produced from a variety of man-made as well as natural processes such as vehicle exhaust, waste-burning, wind blown dust, volcanic eruptions etc. These aerosols are mostly restricted to the first few kilometers from the surface of Earth's atmosphere. It is natural to detect them in the Upper Troposphere and Lower Stratosphere (UTLS) only during episodes like volcanic eruptions.

However, recently an intense aerosol layer persisting at the tropical tropopause altitude (~16-17km) over South Asia is discovered in the satellite observations. They are perceived to be result of increased pollutant gases and aerosols reaching UTLS altitudes due to strong convective activity over this region during summer months. These pollutants on entering stratosphere reside there for long duration and can affect the Earth’s energy budget, stratospheric ozone through heterogeneous chemistry and the formation of cirrus clouds thereby having climate impact. Observed increasing pollution trends is a matter of concern as these trace gases and aerosols could affect ozone in the lower stratosphere and modulate radiative properties of cirrus clouds. To date, no attempt has been made to prove it as a aerosol layer due to its complicity in nature.

In order to ascertain this as aerosol layer, Scientists from University of Wyoming and Science Systems and Applications Inc., NASA from USA, in collaboration with NARL carried out balloon borne in-situ measurements of Asian Tropopause Aerosol Layer (ATAL) since 2014. Under Implementing Arrangement between NASA and ISRO, in-situ measurement campaign to characterise the ATAL in UTLS region was carried out in July-August 2017. A series of balloon field campaigns were carried out during the Asian Summer Monsoon to understand the nature, formation and impacts of the ATAL better.

The overall campaign comprised of 6 small balloon launches to measure ozone, water vapor and aerosol from NARL, Gadanki, and 9 medium to big balloon launches from TIFR balloon facility, Hyderabad. Several ground based instruments like MST radar, Mie Lidar, surface aerosol and trace gas instruments are being operated from Gadanki during the campaign period. Among the instruments flown on balloons are COBALD (Compact Optical Backscatter Aerosol Detector), CFH (Cryogenic Frost point Hygrometer), Ozonesonde, (OPC) Optical Particle Counter, and Impactor / Filter. OPC and Impactor are relatively heavy instruments which require plastic balloons. Payloads weight ranged from 2 kgs to 30 kgs with overall weight as high as 80 kgs. All the payloads were recovered with 100% success rate. Particle sampling using Impactor with controlled ascent rate is the highlight of this campaign.

These measurements confirmed the presence of aerosol layer seen in satellite measurements over Asian region as shown in the figure. Sharp increase of aerosol concentration (r > 0.075 micron) near 16.5-18.5 km is observed. This layer has distinct size distribution as opposed to that from background mineral aerosol dust that is naturally present in the atmosphere. This  layer contains particles of size less than 0.25 micron and are 90% volatile. It appears the aerosol is formed from precursor pollutant gases in the UTLS region which are transported via convection from the ground. The layer is strongly correlated with cold point tropopause and influenced by convective moisture indicating meso-scale convective systems role in transport of pollutant gases and moisture to UTLS region.

Preliminary chemical analysis of samples collected in this campaign indicates dominant presence of nitrate, which is a new finding. Converging air masses over northern part of India, particularly centered around head Bay of Bengal during Asian Monsoon which is generally active during July – August months is found to be the main forcing for vertical transport of the aerosols and trace gases to the UTLS region along with long-range transport from northern parts of India.

However, detailed analysis yet to be carried out with all the data collected during the campaign. One additional campaign during winter months for obtaining the background conditions is planned. Presence of different pollutants which results from interplay of different circulation patterns and pollutant source regions also necessitates multiple campaigns spread over a few years (until 2020) to comprehensively characterise the aerosol layer in the UTLS region and study its impact on radiation budget and ozone chemistry. Additionally, this campaign seeks to use balloon-borne measurements of aerosol, water vapor, and ozone to validate measurements from NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite and NASA’s Stratospheric Aerosol and Gas Experiment (SAGE) III instrument along with Cloud-Aerosol Transport System (CATS) on the International Space Station.

Zero Pressure flight launched on  Aug 21-22, 2017 from TIFR balloon facility, Hyderabad



Profile of scattering ratio at 940 nm and Color Index (Source: Jean-Paul Vernier/SSAI-NASA Langley)