Aerosol mass size distribution and black carbon over a high altitude location in western Trans-Himalayas: impact of a dust episode

Abstract

The information on the aerosol properties from remote locations provides insights into the background and natural conditions against which anthropogenic impacts could be compared. Measurements of the near surface aerosol mass size distribution from the high altitude remote site help us to understand the natural processes, such as, the association between Aeolian and fluvial processes that have a direct bearing on the mass concentrations, especially in the larger size ranges. In the present study, the total mass concentration and mass-size distribution of the near surface aerosols, measured using a 10-channel Quartz Crystal Microbalance (QCM) Impactor from a high altitude location-Hanle (32.78°N, 78.95°E, 4520 m asl) in the western Trans-Himalayas, have been used to characterize the composite aerosols. Also the impact of a highly localized, short-duration dust storm episode on the mass size distribution has been examined. In general, though the total mass concentration (M_t) remained very low (∼0.75 ± 0.61 μg m⁻³), interestingly, coarse mode (super-micron) aerosols contributed almost 72 ± 6% to the total aerosol mass loading near the surface. The mass-size distribution showed 3 modes, a fine particle mode (∼0.2 μm), an accumulation mode at ∼0.5 μm, and a coarse mode at ∼3 μm. During a localized short duration dust storm episode, Mt reached as high as ∼13.5 μg m⁻³ with coarse mode aerosols contributing to nearly 90% of it. The mass size distribution changed significantly, with a broad coarse mode so that the accumulation mode became inconspicuous. Concurrent measurements of aerosol black carbon (BC) using twin wavelength measurements of the aethalometer showed an increase in the wavelength index of absorption, from the normal values of ∼1 to 1.5 signifying the enhanced absorption at the short wavelength (380 nm) by the dust.