Atmospheric ultrafine aerosol number concentration and its correlation with vehicular flow at two sites in the western Himalayan region: Kullu-Manali, India

The concentration of ultrafine aerosol particles of aitken and nucleation mode having size in the range of 1-20 nm was monitored with water-based Condensation Particle Counter. The monitoring was carried out from midnight-to-midnight in every alternate day on a fortnightly basis to represent summer, monsoon and winter (autumn) seasons of 2008 at Mohal (1154 m amsl) and Kothi (2530 m amsl) in Kullu-Manali area of the northwestern Himalayan region of India. The results indicate that diurnal pattern has faint bimodal structure with two peaks, one in morning and the other in evening at both the sites but it is not as distinct as found in plains. There is rather a constant particle density pattern of large magnitude consistent with vehicular movement from morning till evening. The monthly 24 h average particle density gradually picks up from January, increases rapidly in summer months and then decreases in monsoon season at Mohal but at Kothi it keeps on rising from April to October with a slight more increase in September. The particle density is more in summer than in monsoon season at Mohal, a trend opposite to plains. It may be due to the development of warm thermal layer on valley floor while a cold layer develops along snowy hilltops in winter leading to convection of fine particle up the slopes of valley during daytime. At Kothi, the trend is same as it is in continental plains but opposite to Mohal. The relatively more value of particle density in September and October at both the sites may be due to month long International Kullu Dussehra fair in the valley. The vehicular survey conducted agrees well with entire study period averaged diurnal variations and monthly 24 h averaged value of fine particle density. The average value of ultrafine particle density at each hour of a day for entire study period is 20369 ± 1230 Ncm ^{- 3} and 14389 ± 1464 Ncm ^{- 3} at Mohal and Kothi sites, respectively. The comparison with earlier results shows a significant increase indicating impact of vehicular onslaught on pure air of this hilly region.