Indian Journal of Radio & Space Physics Vol. 38, December 2009, pp 326-337 Number concentration characteristics of ultrafine aerosols (atmospheric nanoparticles / aitken nuclei) during 2008 over western Himalayan region, Kullu-Manali, India Nand Lal Sharma 1,$,* , J C Kuniyal 2 , Mahavir Singh 3 , Ajay K Negi 2 , Kesar Singh 2 & Priyanka Sharma 2 1 Govt Post Graduate College, Kullu 175 101, Himachal Pardesh, India 2 G B Pant Institute of Himalayan Environment & Development, Himachal Unit, Mohal-Kullu 175 126, Himachal Pradesh 3 Himachal Pradesh University, Shimla 171 005, India $ E-mail: nlsharmakullu@hotmail.com Received 29 September 2008; revised 9 June 2009; re-revised received and accepted 9 November 2009 The ultrafine particles of three sizes 200, 30 and 10 A [20 nm (aitken mode), 3 nm, 1 nm (nucleation mode / nano particles)] were monitored with water based condensation particle counter during January-December 2008 at Mohal in Kullu-Manali area of North western Himalayas. The results indicate that diurnal pattern has faint bimodal structure with two peaks, one in morning and other in evening but is not as distinct as found in plains. There is rather constant particle density pattern consistent with vehicular movement from morning till evening. This may also be due to abnormally large bursts of particles (nucleation bursts) which are found to be more in summer than winter, more during sunrise and less during afternoon. The weak nucleation bursts are found to be frequent throughout the year than very strong nucleation bursts which are mostly found during months of June, July and August. The monthly 24 h average density gradually picks up from January, increases rapidly in summer months and then decreases in winter. The density is more in summer than in winter, a trend opposite to plains, may be due to development of warm thermal layer on valley floor while cold layer developed along snowy hilltops in winter leading to convection of fine particles up the slopes of valley during daytime. The relatively more value in September and October is due to month long international Dussehra festival in the valley and unexpected lower values in November and December may be due to low local as well as tourist traffic flow; washout/rainout effects; and higher inversion layer. The vehicular survey conducted agrees well with diurnal, monthly as well as yearly averaged diurnal variation of fine particles. The annual average of 24 h average value of ultrafine particles of three size range is 18045±1212, 16811±2790, and 15407±3109 N cm -3 , respectively. The comparison with earlier results shows significant increase of ultrafine particles indicating impact of vehicular increase in the region. Keywords: Ultrafine aerosol density, Nucleation bursts, Nanoparticle concentration, Aitken nuclei concentration PACS No: 92.60.Mt 1 Introduction The study of ultrafine particles is one of the important aspects of Aerosol Science at a particular place, as the impact of these particles on health and climate change are scientifically proven the world over. Further, this study in Himalayan region of Kullu-Manali is of particular interest as the ecology of Himalayan is under serious threat from various forms of pollutants 1-2 . Himalayan glaciers are receding; 44 snow lakes swollen only in Nepal and Bhutan part of Himalayan region; and biggest river systems of which Himalayan glaciers are source will dry up by 2035 if environmental degradation continues at present pace 3-5 . This can lead to starvation and wiping of 2.5 billion people living in the drain system of these rivers. Moreover, the environmental factors of hill areas are used as background levels for urban environments. The region specific and season specific studies of ultrafine aerosol characteristics and significance of these characteristics are required to be done in Himalayan region 6 . The Indian Space Research Organization - Geosphere Biosphere program (ISRO-GBP) is a step in this direction but it mostly covers urban/semi urban, oceans and coastal areas 7 . Further, it mainly focuses on aerosol optical depth (AOD), Angstrom parameters, trace gases and radiation budgeting while studies on ultrafine particles are being done at fewer stations and in hilly Himalayan regions of India, this study is non- existent 8-9 .The anthropogenic activities such as increasing vehicular traffic due to increased tourism related activities, biomass burning, forest fires, cow dung and fuel wood burning for cooking may alter the pollutants level in the region.