Physical and optical properties of aerosols in a free tropospheric environment: Results from long-term observations over western trans-Himalayas

•Long term aerosol observations from a high altitude location (4.5 km asl) in trans-Himalayas.•Black carbon and AOD showed significant seasonal variation with spring maxima.•BC contribute significantly to the size range of 200-400 nm of the submicron aerosols size spectra.•CWT analysis revealed the role of long-range transport in modulating the aerosol loading.

Simultaneous and collocated estimates of near-surface mass concentration of black carbon (BC), number size distribution (NSD) and total number concentration (NT) of composite aerosols, columnar spectral aerosol optical depth (AOD) and local meteorological parameters were made from Aug 2009 to July 2012 at the high altitude (4520 m amsl), remote location Hanle in western trans-Himalayas. Even though all the aerosol parameters remain quite low, large annual variations are seen in the monthly mean values of BC (25 ng m−3-181 ng m−3), total number concentrations of composite aerosols (628 cm−3-1500 cm−3) and AOD (0.05-0.16). Size segregated correlation analysis reveals that the BC mass contributes significantly to the size range of 200-400 nm of the submicron aerosol size spectra. The diurnal variation of BC mostly has been dampened; yet seems to be prominent during spring, showing the presence of weak boundary layer dynamics. In contrast, the diurnal fluctuations in total number concentration have been mostly controlled by the new particle formation events, leading to bursts of large concentrations of ultrafine particles, which subsequently undergo coagulation. Spectral dependence of AOD also shows large monthly variations, with the Angstrom exponent varying from ∼0.52 to 1.3, with a mean value of ∼0.95 ± 0.21. The vertical distribution of extinction coefficients, obtained from CALIPSO data, indicates the presence of elevated aerosol layers, attributed mainly to the influence of long range transport of aerosols from the west Asian and Indian desert region.