Optical Properties of Aerosols from the Central Himalayas during Rawex - Gvax Field Campaign

The Indo-Gangetic Plain (IGP) region in the Northern India is one of the most populated regions of the world and it encompasses a variety of anthropogenic and biogenic source of aerosols and pollutants due to rapidly growing industrialization and expanding urbanization in recent years. The observed aerosol layer covers the vast expansion during the winter and early spring of each year extending across the Indo-Gangetic Plain (IGP) to the Bay of Bengal (BoB). However, the ground based observations are very limited to verify the same. Based on the modeling, the vertical lifting of aerosols/ pollutants from this region and the wide spread transport of aerosols and pollutants during the prevailing higher wind speed, hence influencing the radiation budget and thereby climate change over the wide region. In view of this, the first Atmospheric Radiation Measurement mobile facility (AMF1) has been set-up at ARIES, Nainital (29.4oN, 79.5oE, 1958m above mean sea level) under the Regional Aerosol Warming Experiment (RAWEX)-Ganges Valley Aerosol Experiment (GVAX) project. Under this program the observations of physical, optical and microphysical properties of atmospheric aerosols, radiation and meteorological parameters are being made during June 2011 to March 2012. The aerosol optical properties derived from the measurements carried out with Aerosol Observation System (AOS) are presented in this paper. The preliminary analysis of the data obtained during the above said period shows significant signature of the transport of atmospheric aerosols and pollutants to the observational site with the prevailing and episodic winds over the region and suggest different origins for the air masses arriving at the observational site. The nominal particle concentration ranges from 200 to 5000 cm-3 during the study period and such values are typical for a high altitude site. A distinct diurnal variation of aerosol optical properties is observed, with highest values during late evening and minimum values appear at night and early morning hours. The low values of scattering coefficient are observed during Indian summer monsoon period and it showed increasing trend after the monsoon period. Considering the large increase in the aerosol scattering coefficient between the Indian summer monsoon and post monsoon seasons, there is little variability in the aerosol intensive properties. In general, aerosols observed over the observational site have a relatively high value of single scattering albedo and low value of Ångström exponent, indicating relatively large particles with the low absorption. The hygroscopic growth factors (ratio of scattering at 85/40% Relative Humidity) are moderate to low and indicate that the aerosol isn't highly oxidized. The observed surface radiative forcing efficiency ranges from -3 to -20 W m-2. Detailed discussion will be made during the presentation.