Impacts of the high loadings of primary and secondary aerosols on light extinction at Delhi during wintertime S. Tiwari a , A.K. Srivastava a, * , D.M. Chate b , P.D. Safai b , D.S. Bisht a , M.K. Srivastava c , G. Beig b a Indian Institute of Tropical Meteorology (Branch), Prof Ramnath Vij Marg, New Delhi, India b Indian Institute of Tropical Meteorology, Dr Homi Bhabha Road, Pashan, Pune, India c Department of Geophysics, Banaras Hindu University, Varanasi, India highlights  First study on contribution of different species to extinction coefficient.  Scattering type aerosols were dominated by w76% than absorbing type aerosols.  The largest contribution was observed for organic carbon (w46%).  Lowest contribution was observed for ammonium nitrate (w4%). article info Article history: Received 7 November 2013 Received in revised form 27 March 2014 Accepted 29 March 2014 Available online 30 March 2014 Keywords: Particulate mass Carbonaceous aerosols Extinction coefficient Indo-Gangetic plain Meteorological effect abstract High emissions of anthropogenic aerosols over Indo-Gangetic Plain (IGP) inspired continuous mea- surements of fine particles (PM 2.5 ), carbonaceous aerosols (BC, OC and EC), oxides of nitrogen (NO x ) and estimation of light extinction (b ext ) and absorption (b abs ) coefficients over Delhi during high pollution season in winter from December 2011 to March 2012. During study period, the mass concentrations of PM 2.5 , BC and NO x were 186.5  149.7 mgm 3 , 9.6  8.5 mgm 3 and 23.8  16.1 ppb, respectively. The mass concentrations of OC and EC were studied by two different techniques (i) off-line (gravimetric method) and (ii) semi-continuous (optical method) and their mean mass concentrations were 51.1  15.2, 10.4  5.5 mgm 3 and 33.8  27.7, 8.2  6.2 mgm 3 , respectively during the study period. The ratios of mass concentration of OC to EC in both cases were in between 4 and 5. The source contribution of carbonaceous aerosols in PM 2.5 estimated over 24hrs, during day- and night-time where motor ve- hicles accounted for w69%, 90% and 61% whereas coal combustion accounted for w31%, 10% and 39%, respectively. The estimated mean values of b ext and b abs over the station were 700.0  268.6 and 71.7  54.6 Mm 1 , respectively. In day and night analysis, b ext is w37% higher during night-time (863.4 Mm 1 ) than in day-time (544.5 Mm 1 ). Regression analysis between b ext and visibility showed significant negative correlation (r ¼0.85). The largest contribution in the light extinction coefficients was found to be due to organic carbon (w46%), followed by elemental carbon (w24%), coarse mode particles (w18%), ammonium sulfate (w8%) and ammonium nitrate (w4%). The individual analysis of light extinction due to chemical species and coarse mode particles indicates that scattering type aerosols dominated by w76% over the absorbing type. The aforementioned results suggest that the policy- induced control measures at local administration level are needed to mitigate the excess emissions of carbonaceous aerosols over IGP region which ranks highest in India and elsewhere in worldwide. Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved. 1. Introduction The Indo-Gangetic plain (IGP) is the most populous and one of the highly polluted regions in the world, surrounded by various anthropogenic sources such as burning of fossil fuels and agricul- tural residues, vehicular emissions etc. Its unique topography along * Corresponding author. E-mail address: atul@tropmet.res.in (A.K. Srivastava). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv http://dx.doi.org/10.1016/j.atmosenv.2014.03.064 1352-2310/Crown Copyright Ó 2014 Published by Elsevier Ltd. All rights reserved. Atmospheric Environment 92 (2014) 60e68