High altitude (∼4520 m amsl) measurements of black carbon aerosols over western trans-Himalayas: Seasonal heterogeneity and source apportionment

The first ever, year-round measurements of aerosol black carbon (BC) over the western part of trans-Himalayas are reported from Hanle (similar to 4520 m above mean sea level). The daily mean BC concentrations varied from as low as 7 ng m(-3) to as high as 296 ng m(-3) with an annual average of 77 +/- 64 ng m(-3), indicating significant BC burden even at free-tropospheric altitudes. Variation with in the day as well as from day to day were highly subdued during winter season (December to February) while they used to be the highest in Spring (March to May). In general, the less frequently occurring high BC values contributed more to the annual and seasonal means, while 64% of the values were below the annual mean. Seasonally, highest BC concentration (109 +/- 78 ng m(-3)) occurred during Spring and lowest (66 +/- 42/66 +/- 62 ng m(3)) during Summer/Winter season(June to August/December to February). Diurnal variations in general were very weak, except during Spring and Summer when the effects of convective boundary layer dynamics is discernible. Back trajectory clustering and concentration weighted trajectory (CWT) analyses indicated that, most time of the year the sampling location is influenced by the advection from West and Southwest Asia, while the contribution from the Indo-Gangetic Plains (IGP) remained very low during Spring and Summer. The seasonal and annual mean BC at Hanle are significantly lower than the corresponding values reported for other Himalayan stations, while they were quite higher than those reported from the South Pole and pristine Antarctic environments.