Variation between near-surface and columnar aerosol characteristics during the winter and summer at Delhi in the Indo-Gangetic Basin A.K. Srivastava a , Sachchidanand Singh b,n , S. Tiwari a , V.P. Kanawade c , D.S. Bisht a a Indian Institute of Tropical Meteorology (Branch), Prof. Ramnath Vij Marg, New Delhi, India b Radio & Atmospheric Sciences Division, National Physical Laboratory, CSIR, New Delhi, India c Department of Chemistry, Kent State University, Kent, OH 44240, USA article info Article history: Received 29 July 2011 Received in revised form 5 November 2011 Accepted 20 November 2011 Available online 30 November 2011 Keywords: PM 10 PM 2.5 Aerosol optical depth Vertical winds (omega) Aerosol backscatter abstract Aerosol characteristics were studied over Delhi, a typical urban station in the Ganga basin in Northern India, during two contrasting weather conditions: winter and summer, to explain the changes in columnar and surface aerosol characteristics with the help of ground based measurements and CALIPSO satellite data. The near-surface mean aerosol mass concentrations of PM 10 and PM 2.5 ( 7standard deviation) were observed to be  200 ( 724) and 118 ( 733) mgm 3 , respectively, during the winter and 168 ( 731) and 55 ( 712) mgm 3 , respectively, during the summer. PM 2.5 was found to be about two times higher than the PM 10 concentration during the winter period. Aerosol mass size distribution showed bi-modal nature during both the periods, with relative dominance of fine-particle mass concentrations during the winter, having low R eff value (0.63 70.05 mm) and coarse-particle mass concentrations during the summer, having large R eff value (1.52 70.60 mm). The concurrent measure- ment of columnar aerosol optical depth (AOD) showed high values ( 40.60 at 500 nm) during both the periods. The ˚ Angstr ¨ om exponent (a) over the station, however, also suggests relatively large contribu- tion of fine-mode particles during the winter (a 1.02) and coarse-mode dust particles during the summer (a 0.51). The observed features in the surface and columnar measured aerosol characteristics during two different seasons are explained using the vertical winds coupled with the vertical profile of aerosols. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Atmospheric aerosols influence the Earth-atmosphere system directly by scattering and absorbing sunlight (Schwartz et al., 1995) and indirectly by changing radiative properties and mod- ifying microphysical properties, and lifetime of clouds (Twomey, 1991). They are considered to be one of the largest uncertain components, estimating the radiative forcing in the assessments of global climate change (IPCC, 2007). Recent research reveals that assessing aerosol effects on climate requires knowledge of not only the regional and global distribution of aerosol amount, but also the various properties of aerosols such as physical (e.g. size distribution), chemical (e.g. composition) and optical proper- ties, including especially the absorption (Myhre, 2009 and refer- ences therein). The Indo-Gangetic Basin (IGB) region, encompassing most of the northern part of India, is the world’s most populated river basin where more than 700 million people are living and exposed to the enormous pollution from various anthropogenic and natural sources. The region extensively experiences two extreme weather conditions every year such as winter and summer and is of particular research interest due to large seasonal heterogeneity in aerosol characteristics including aerosol load during these periods (Dey et al., 2004; Jethva et al., 2005; Dey and Di Girolamo, 2010; Srivastava et al., 2011a). The heterogeneity in various aerosol characteristics may be different for near-surface and columnar measured aerosol characteristics, which is highly associated with the surface synoptic conditions over the region, apart from the variability in aerosol emission sources in and around the region. During the winter, low surface convection occurs due to low surface temperature and high pressure system persists over the region, which results in shallow atmospheric boundary layer (ABL) condition due to its highly stable nature (Stull, 1999). On the other hand, during the summer, strong surface convection occurs due to high surface temperature and low pressure system over the region, which results in deep ABL condition due to its highly unstable nature. Studies undertaken so far, over the IGB region, indicate large aerosol emissions from different anthropogenic and natural sources in and around the region (Reddy and Venketaraman, 2002a, 2002b; Ramanathan Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.11.009 n Corresponding author. Tel.: þ911145608243; fax: þ911145609310. E-mail address: ssingh@nplindia.org (S. Singh). Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 57–66