Study of aerosol black carbon during different weather conditions over Delhi Tarannum Bano, Sachchidanand Singh , R.K.Roshan, Kirti Soni, R.S.Tanwar and Shambhu Nath National Physical Laboratory, New Delhi – 110012, India Email: banot@mail.nplindia.ernet.in Abstract Black Carbon (BC) aerosols are the main sunlight-absorbing component of atmospheric aerosols. In this paper we present results of BC aerosols concentration at typical weather conditions over Delhi during June 2005 to June 2006 using an Aethalometer. We studied for foggy, rainy, dusty and very clear days. The BC concentration shows pronounced variations with weather changes. The annual average BC concentration was 16.32 µg/m 3 of which foggy days concentrations were maximum while the clear days were minimum. The average BC concentration during foggy days reaches up to ~50.39 µg/m 3 in comparison to clear days which peaks only upto ~6.8 µg/m 3 , so foggy days values is 7.4 times more. Followed by foggy days, rainy days show the higher BC concentration. The average values are ~6.82 µg/m 3 and 6.9 µg/m 3 for dusty days and dust storm period respectively. The BC concentration shows strong anti-correlation with the mixing layer height measured simultaneously using an acoustic SODAR. Stronger anti- correlation (correlation coefficient –0.618) was found with the ventilation coefficients. The effect of BC aerosols over Radiative forcing has also been estimated using OPAC and SBDART model. Keywords: Black carbon, Aethalometer, mixing layer height, Ventilation coefficients. 1. Introduction Climate change poses potentially major challenges to social and economic development in all countries. Black carbon aerosols are one of the important factors in the global climate change phenomenon. This is mainly due to their absorptive nature, which directly accounts for the reduction in incoming short wave solar radiation. It is the graphitic form of carbonaceous aerosols emitted into the atmosphere as by-product of all combustion processes such as industrial pollution, traffic, outdoors fires, household burning of coal and biomass fuels The ability of carbonaceous aerosols to modify local meteorology and climatology in regions where emissions are high, like china and India, has been postulated ( Ramanathan et al., 2001; Menon et al., 2002). Most of the BC are fine accumulation size particles (radius <0.2micrometer) which are hydrophobic or weakly hydrophilic (Jacobson, 2001) and have global residence times of about 1 week. It ultimately change the dynamics of the atmospheric boundary layer. 2. Location and General meteorology of the sampling site The study area is Delhi, the capital city of India, which is densely populated as well as highly polluted mega city of the world. Delhi lies in the latitude 28.38 N, longitude 77.12 E. Area is 1483 Km² and density is 10340/Km². Delhi has a semi-arid climate. It has extreme weather conditions during peak summers temperature rises even beyond 45 o C. and in winter temperature may drop to below 3 o C. In the pre-monsoon period (April- June), frequent dust storms from western and north- western desert regions cause large scale loading of aerosols over Delhi. The rainy season is from July to September when the climate sometimes becomes very humid. The average annual rainfall is approximately 670 mm (27 inches). The monitoring have been carried out from June 2005 to June 2006 in the premises of NPL ,New Delhi. 3. Measurement Technique Continuous observations on BC aerosols have been carried out by using an Aethalometer (Model AE-21, Magee Scientific, USA).In this method, atmospheric air is pumped through an inlet at the flow rate of about 2 litre/min -1 , which impinges on a quartz micro fiber strip. A light beam from a high intensity LED lamp is transmitted through 1