ISRO-NASA efforts towards resolving the issue of Asian Tropopause Aerosol Layer (ATAL)

Atmospheric aerosol and clouds play important role in weather and climate. A recent discovery of high altitude (~ 16km) Aerosol layer occurring during monsoon in the south Asian region using CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) has started puzzling the atmospheric scientists. Very little is known on the composition and the formation mechanisms of this intense aerosol layer. This layer is of concern since it could play an important role on the climate and weather. To understand this enigmatic layer, balloon borne experiments along with ground based observations are being conducted under a ISRO-NASA collaborative program – "Balloon Borne measurement campaigns of Asian Tropopause Aerosol Layer (BATAL)". Tropopause is the interface between the Troposphere and the Stratosphere. From Indian side, National Atmospheric Research Laboratory (NARL) has taken lead in conducting unique balloon borne observations in collaboration with Tata Institute of Fundamental Research (TIFR) Balloon Facility, Hyderabad and Banaras Hindu University, Varanasi, to study the physical, chemical and radiative properties of this layer. In addition to these, NARL has made special effort to study this layer using its powerful MST radar and Lidar and other collocated experiments located at Gadanki near Tirupati. Several observational data have already been gathered since 2014.

Nearly 35 Scientists from NARL, NASA (USA), CNRS (France), and other national institutes and universities met at NARL, Gadanki in a two-day workshop during February 1-2, 2018 to discuss and plan the future course of action to address the scientific issues and the implications of ATAL on weather and climate. Based on the deliberations made using the in-situ observations collected during BATAL campaigns, it is now clear that ATAL is comprised of mostly small (r < 0.25 μm), liquid (~ 80-95%) aerosols with relatively low scattering ratio  (< 1.2) at 532 nm. Chemical analysis of particles collected near the tropopause indicates the dominance of nitrate aerosol with concentration of about 100 ng/m3, which is puzzling. Interestingly, no sulfate aerosol is noticed as thought to be the major composition, thus opening new direction in further studies of ATAL.

This result is in contrast with the expectation of increasing influence of sulfur emissions in Asia over the past few decades on aerosol trends in the Upper Troposphere and Lower Stratosphere (UTLS) region. A regional chemical transport model WRF-Chem (Weather Research and Forecasting) coupled with meteorology from NARL also show the presence of Nitrates within the Asian Summer Monsoon anticyclone region. Long-range transport from northern part of India through tropical easterly jet is seen as a main source for the observed ATAL besides convection. Existence of Anti-cyclonic flow in the UTLS region makes the confinement of aerosols and trace gases. Thus, dynamics play a major role in the formation and maintenance of the ATAL. The possibility of gas to aerosol formation is also being examined.

At present, it is not clear on the relative roles of convection verses long-range transport and natural verses anthropogenic sources on the formation of ATAL. Towards this, a winter campaign was conducted during February 5-13, 2018 at TIFR Balloon Facility, Hyderabad, to test these hypothesis including testing the new sensors for probing the UTLS aerosol composition which will be used in summer campaign of BATAL.

 

Participants of the BATAL Workshop held at NARL, India.

Participants of the BATAL Workshop held at NARL, India.

 

BATAL Winter Campaign conducted at TIFR Balloon Facility during February 5-13, 2018 carrying 10 different payloads.

BATAL Winter Campaign conducted at TIFR Balloon Facility during February 5-13, 2018 carrying 10 different payloads.