title: Columnar aerosol characteristics and radiative forcing over the Doon Valley in the Shivalik range of northwestern Himalayas.
abstract: Spectral aerosol optical depth (AOD) measurements obtained from multi-wavelength radiometer under cloudless conditions over Doon Valley, in the foothills of the western Himalayas, are analysed during the period January 2007 to December 2012. High AOD values of 0.46 ± 0.08 and 0.52 ± 0.1 at 500 nm, along with low values of Ångström exponent (0.49 ± 0.01 and 0.44 ± 0.03) during spring (March-May) and summer (June-August), respectively, suggest a flat AOD spectrum indicative of coarse-mode aerosol abundance compared with winter (December-February) and autumn (September-November), which are mostly dominated by fine aerosols from urban/industrial emissions and biomass burning. The columnar size distributions (CSD) retrieved from the King's inversion of spectral AOD exhibit bimodal size patterns during spring and autumn, while combinations of the power-law and unimodal distributions better simulate the retrieved CSDs during winter and summer. High values of extinction coefficient near the surface (∼0.8-1.0 km-1at 532 nm) and a steep decreasing gradient above are observed via CALIPSO profiles in autumn and winter, while spring and summer exhibit elevated aerosol layers between ∼1.5 and 3.5 km due to the presence of dust. The particle depolarisation ratio shows a slight increasing trend with altitude, with higher values in spring and summer indicative of non-spherical particles of dust origin. The aerosol-climate implications are evaluated via the aerosol radiative forcing (ARF), which is estimated via the synergy of OPAC and SBDART models. On the monthly basis, the ARF values range from ∼ -30 to -90 W m-2at the surface, while aerosols cause an overall cooling effect at the top of atmosphere (approx. -5 to -15 W m-2). The atmospheric heating via aerosol absorption results in heating rates of 1.2-1.6 K day-1during March-June, which may contribute to changes in monsoon circulation over northern India and the Himalayas.
authors.affiliation: Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital, Uttarakhand, 263 001, India. dumka@aries.res.in.|Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science and Engineering, Institute of Atmospheric Sciences, Fudan University, Shanghai, 200 433, China. dumka@aries.res.in.
authors.forename: U C
authors.initials: UC
authors.lastname: Dumka
authors.affiliation: Department of Space, Indian Institute of Remote Sensing, ISRO, Dehradun, Uttarakhand, 248 001, India.
authors.forename: Shaik Darga
authors.initials: SD
authors.lastname: Saheb
authors.affiliation: Atmospheric Research Team, Institute for Environmental Research and Sustainable Development, National Observatory of Athens, Athens, 11810, Greece.
authors.forename: D G
authors.initials: DG
authors.lastname: Kaskaoutis
authors.affiliation: Department of Space, Indian Institute of Remote Sensing, ISRO, Dehradun, Uttarakhand, 248 001, India.
authors.forename: Yogesh
authors.initials: Y
authors.lastname: Kant
authors.affiliation: Department of Space, Indian Institute of Remote Sensing, ISRO, Dehradun, Uttarakhand, 248 001, India.
authors.forename: D
authors.initials: D
authors.lastname: Mitra
mesh_terms: D000336:Aerosols / Q000737:chemistry*; D000392:Air Movements; D000393:Air Pollutants / Q000032:analysis; D000531:Altitude; D001272:Atmosphere; D002980:Climate; D004391:Dust / Q000032:analysis*; D004777:Environment; D004784:Environmental Monitoring / Q000379:methods*; D007194:India; D008962:Models, Theoretical; D013696:Temperature
chemical_list: D000336:Aerosols; D000393:Air Pollutants; D004391:Dust
keywords: Aerosol; Aerosol radiative forcing; Columnar size distribution; Doon valley; India; Vertical profiles