Black carbon aerosols variation in Kathmandu valley, Nepal

Black carbon (BC) aerosols were measured using an aethalometer (AE-31) at Pulchowk Campus, an urban location of Kathmandu valley, Nepal from May 2009 to April 2010. The amount of BC in the total mass of the suspended particulate matter (TSP) were estimated by comparing the average BC with mass of particulate matter simultaneously measured with frm Omni Ambient Air Sampler for the last two months (March–April, 2010). BC concentration shows significant temporal variation with monthly mean concentration varying between 14.9 μg m−3 and 3.0 μg m−3. The highest daily mean concentration was recorded as high as 39.9 μg m−3. The highest concentration of BC occurred during winter and pre-monsoon seasons. The lowest concentration of BC was found during monsoon and post-monsoon seasons. A distinct diurnal variation of BC with two high peaks occurred; one in around 09:00 morning and another late evening around 20:00 local time. Minimum BC concentration was also observed in the afternoon. The annual average of BC for observation period was 8.6 ± 4.4 μg m−3. The seasonal, monthly and diurnal variation of BC were found to be significantly influenced by the local traffic movements, the domestic and industrial fuel consuming activities, wind speed, wind direction and rain. The amount of BC in the total mass of particulate matters during the observed period varied between 11.6 and 56.2 percent. This variation is attributed to the high amount of re-suspension of dust particles in the atmosphere of the valley.