P. Hegde

Aerosol characteristics at a high‐altitude location in central Himalayas: Optical properties and radiative forcing

[1]xa0Collocated measurements of the mass concentrations of aerosol black carbon (BC) and composite aerosols near the surface were carried out along with spectral aerosol optical depths (AODs) from a high-altitude station, Manora Peak in central Himalayas, during a comprehensive aerosol field campaign in December 2004. Despite being a pristine location in the Shivalik Ranges of central Himalayas and having a monthly mean AOD (at 500 nm) of 0.059 ± 0.033 (typical to this site), total suspended particulate (TSP) concentration was in the range 15–40 μg m−3 (mean value 27.1 ± 8.3 μg m−3). Interestingly, aerosol BC had a mean concentration of 1.36 ± 0.99 μg m−3 and contributed ∼5.0 ± 1.3% to the composite aerosol mass. This large abundance of BC is found to have linkages to the human activities in the adjoining valley and to the boundary layer dynamics. Consequently, the inferred single scattering albedo lies in the range of 0.87 to 0.94 (mean value 0.90 ± 0.03), indicating significant aerosol absorption. The estimated aerosol radiative forcing was as low as −4.2 W m−2 at the surface, +0.7 W m−2 at the top of the atmosphere, implying an atmospheric forcing of +4.9 W m−2. Though absolute value of the atmospheric forcing is quite small, which arises primarily from the very low AOD (or the column abundance of aerosols), the forcing efficiency (forcing per unit optical depth) was ∼88 W m−2, which is attributed to the high BC mass fraction.

South Asian dust episode in June 2006: Aerosol observations in the central Himalayas

[1]xa0A dust storm blew through the Thar Desert on 12 June 2006, which has significantly influenced aerosol physical and optical properties over the central Himalayas (Nainital, 29.4°N; 79.5°E, 1958 m amsl) on 13 June 2006. Aerosol number concentrations in the coarse and giant modes on 13 June 2006 are found to be five (26 × 106 m−3) and ten (17.2 × 103 m−3) times higher compared to their respective monthly mean values. Aerosol optical depth values also showed two to four times increase, particularly at longer wavelengths suggesting increase in the concentrations of coarse and giant particles. This is supported by three to five times increase in Angstrom turbidity coefficient (β) and significant reduction in Angstrom wavelength exponent (α). Absence of enhancements in black carbon and accumulation mode particles suggests negligible changes in the influences of anthropogenic activities at the site during the study period.

Direct solar ultraviolet irradiance over Nainital, India, in the central Himalayas for clear-sky day conditions during December 2004

[1]xa0From a high-altitude station, Nainital, India (29.4°N, 79.5°E, 1958 masl), located in the central part of lower Himalayas, the observations made during December 2004 using a pair of Sun photometers (Microtops II) at wavelengths ranging from 305 to 1020 nm are reported. The observed parameters are the direct solar UV irradiance, column ozone, water vapor, and aerosol optical depths (AOD). The results are presented for the full day clear-sky conditions that prevailed for about 16 days during the whole month. It is found that Nainital is a comparatively pristine site with average AOD at 500 nm ranging between 0.03 and 0.09 and Angstrom exponent generally close to 1. The high AOD values occurred on 2 and 25 December due to winds from populated north Indian plains as revealed by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectory model. The total column ozone varies between 251 and 308 DU during the entire period of observations. The maximum diurnal UV irradiance values in the 2.4 nm bandwidth centered at 305.5, 312.5, and 320.0 nm varied between 0.027 and 0.049, 0.15 and 0.20, and 0.29 and 0.37 W m−2, respectively. The measured UV irradiances are compared with the Tropospheric Ultraviolet Visible (TUV) radiation model and show a good agreement.

Surface changes in solar irradiance due to aerosols over central Himalayas

During a comprehensive aerosol field campaign as a part of the ISRO-GBP, extensive measurements of radiative fluxes at the surface were made during December 2004 at Manora Peak, in the Shivalik ranges of the Central Himalayas. The surface radiative fluxes were used to estimate aerosol radiative forcing. Our analysis clearly shows that during the clean atmospheric conditions over Manora Peak, the observed aerosol radiative forcing is in good agreement to those of modeled ones, while for the higher aerosol optical depths (AODs), modeled values are significantly smaller than the observed ones. It was observed that at Manora Peak, the anthropogenic aerosols (from valley below) transported upwards by evolution of boundary layer during the daytime provide an atmosphere conducive for ‘mixed’ aerosols. Focused efforts are needed to address this issue for which simultaneous observations at high altitude site with those in nearby valley are essential.

Reply to comment by S. Ramachandran on “Surface changes in solar irradiance due to aerosols over central Himalayas”

[1] Dumka et al. [2006] (hereinafter referred as D06) made extensive measurements of radiative fluxes at the surface during December 2004 at Manora Peak, in the Shivalik ranges of the Central Himalayas during a comprehensive aerosol field campaign as a part of the Indian Space Research Organisations Geosphere Biosphere Programme (ISRO-GBP). The surface radiative fluxes were used to estimate aerosol radiative forcing. Based on the data analysis D06 concluded that the anthropogenic aerosols (from valley below) transported upwards by atmospheric boundary layer (ABL) dynamics during daytime provide an atmosphere conducive for mixed aerosols and suggested that focused efforts are needed to address this issue.