Characteristics of aerosol spectral optical depths over Manora Peak: A high-altitude station in the central Himalayas

We present, for the first time, spectral behavior of aerosol optical depths (AODs) over Manora Peak, Nainital, located at an altitude of $2 km in the Shivalik ranges of the central Himalayas. The observations were carried out using a multiwavelength solar radiometer during January to December 2002. The main results of the study are extremely low AODs during winter, a remarkable increase to high values in summer, and a distinct change in the spectral dependencies of AODs from relatively steeper spectra during winter to shallower ones in summer. A comparison of the total optical depths of the nighttime measurements taken during the 1970s with the daytime values from the current study underlines the fact that loading of larger size particles during summer also occurred at that time, though less severely than it does today. During transparent days the AOD values usually lie below 0.08, while during dusty (turbid) days they lie between 0.08 and 0.69. The average AOD value during the winter, particularly in January and February, is $0.03 ± 0.01 at 0.5 mm. The mean aerosol extinction law at Manora Peak during 2002 is best represented by 0.10lÀ0.61. However, during transparent days, which covers almost 40% of the time, it is represented by 0.02lÀ0.97. This value of wavelength exponent, representing reduced coarse concentration and the presence of fine aerosols, indicates that the station measures aerosol in the free troposphere at least during part of the year.