Aerosol Optical Properties Over the High Altitude Station Hanle in the Western Himalayas

As a part of an astronomical site characterization program, studies of atmospheric aerosols were carried-out at the Indian Astronomical Observatory (IAO) Hanle (32047' N and 78058' E, 4500 m amsl), Ladakh region in the Western Himalayas, using Skyradiometer (Prede, Japan) from direct solar and diffuse sky irradiance measurements at 400, 500, 675, 870, and 1020 nm. Aerosol optical properties are retrieved using Skyrad.pack to calculate the aerosol optical depth (AOD), single scattering albedo (SSA), volume size distribution, and phase function. The AOD at 500 nm was found to be between 0.02 and 0.10, which is consistent with reported values for high altitude stations across the globe. Value of the Angstrom exponent (α), which relates to size particles, fluctuates between 0.5 and 1.5. The SSA is close to 1.0 for each wavelength, indicating the dominance of scattered light in the observed spectral region. The estimated daily mean aerosol asymmetry parameter (g) from the retrieved phase function varies in between 0.68 and 0.72 for the different wavelengths, indicating predominantly forward scattering than the backward scattering. The value of g decreases at the visible region and slightly increases in the near-infrared region. The retrieved g parameter strongly depends on both the wavelength and the Angstrom parameter. We find that the AOD value peaks during the spring season, and this is likely to be due to aerosol transport from the deserts to the west. The HYSPLIT back Trajectory analysis indicates the passage of air mass transported predominantly from Sahara in the African region towards the observing station. There is a bimodal as well as tri-modal volume size distribution with particle population sizes in the range 0.15 to 10 microns. The bi-modal features are more common during winter and autumn seasons, while the tri-modal features are seen in all the seasons with a marginal predominance during spring and summer. The temporal response of the two populations of particles is clearly different and due to wind lifting of near surface aerosols. We compared our Skyradiometer data with satellite data, specifically CALIPSO and MODIS, for validation. The satellites have trouble retrieving AOD at pristine locations such as the Himalayas where the AOD is usually less than 0.1. However, we find that the satellite data, when present, is generally consistent with the radiometer results in both the AOD magnitude and the seasonal cycle.