Seasonal contrast in the vertical profiles of aerosol number concentrations and size distributions over India: Implications from RAWEX aircraft campaign
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Aircraft measurements of the vertical profiles of aerosol total number concentrations and size distributions (in the size range of 0.5–20 μm) were made over seven geographically diverse locations of the Indian mainland during two contrasting seasons, winter (December 2012) and spring (April–May 2013), as a part of the regional aerosol warming experiment (RAWEX). Our observations revealed an increase in the vertical extent of aerosol loading during spring having a significant enhancement in coarse mode aerosols in the lower free-troposphere (FT) over western and central parts of India and the Indo-Gangetic plains (IGP). The particulate depolarisation ratio (PDR) derived from the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) over the same region showed the presence of dust (including polluted dust) at higher altitudes in spring. Concurrent and collocated measurements of aerosol scattering and absorption properties aboard the aircraft revealed that the FT enhancement in coarse mode aerosol loading during spring is associated with a decrease in single scattering albedo and an increase in columnar absorption aerosol optical depth. This confirms that the elevated layers of coarse mode aerosols seen during spring are absorbing in nature, especially over the IGP. The presence of such coarse-mode absorbing aerosols plays a crucial role in governing the radiation balance over the IGP in spring through the diabatic heating of the upper atmosphere.
KeywordsAerosol number concentration size distribution coarse mode fraction single scattering albedo CALIPSO vertical profile
This study was carried out as part of the Regional Aerosol Warming Experiment (RAWEX) under the Aerosols and Radiative Forcing over India (ARFI) project of ISRO-Geosphere Biosphere Programme (ISRO-GBP). We acknowledge the National Remote Sensing Centre (NRSC), Hyderabad for the support with the aircraft operation and measurements. S. Suresh Babu acknowledges the Department of Science Technology for the Swarna Jayanti Fellowship. We also acknowledge NOAA Air Resources Laboratory for the provision of the HYSPLIT transport and dispersion model and READY website (http://www.arl.noaa.gov/ready.html) used in this publication. The CALIPSO data were obtained from the NASA Langley Research Centre and Atmospheric Sciences Data Centre.
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