[1] We present results from complimentary measurements of physical and optical properties of aerosols carried out at Delhi, as part of the Indian Space Research Organization Geosphere Biosphere Programme's Land Campaign II in December 2004. For the first time we unravel ground truth values of several radiatively important aerosol parameters such as their wavelength dependency in absorption, scattering behavior, singlescattering albedo, number size distribution, and vertical distribution in the atmosphere from this polluted megacity in south Asia. Interesting features are observed in the behavior of aerosol parameters under intermittent foggy, hazy, and clear-sky conditions prevalent during the campaign. All aerosol parameters exhibited a large distribution in their values, with variabilities being particularly higher on hazy and foggy days. The average clear-sky aerosol optical depth (AOD) value is 0.91 ± 0.48, which is higher than the AOD value reported for most other cities in India during this season of the year. Increases in AOD on hazy and foggy days are found to be spectrally nonuniform. The percentage increase in AOD at shorter wavelengths was higher on hazy days compared to clear days. Diurnally averaged BC mass concentration varied from a low of 15 mg/m 3 during clear days to a high of about 65 mg/m 3 on hazy days. The wavelength dependency of aerosol absorption shows signatures of the presence of a significant amount of absorbing aerosols produced from biofuel/biomass burning. Single-scattering albedo at 525 nm is found to vary between 0.6 and 0.8 with an average value of 0.68 for the entire period. Lidar observations reveal that during a fog event there is a subsidence of aerosols to an extremely dense and shallow atmospheric layer of less than 200 m height from the surface. The presence of an aerosol layer at elevated altitudes is also detected. All the results are combined and used for estimating aerosol radiative forcing using a discrete ordinate radiative transfer model. We find a large negative forcing at the surface level in the range of À40 to À86 W/m 2 , while forcing at the top of the atmosphere varied between À2 and +3 W/m 2 .Citation: Ganguly, D., A. Jayaraman, T. A. Rajesh, and H. Gadhavi (2006), Wintertime aerosol properties during foggy and nonfoggy days over urban center Delhi and their implications for shortwave radiative forcing,

[1] Black carbon (BC) aerosol mass concentrations measured using an aethalometer at Ahmedabad, an urban location in western India, from September 2003 to June 2005 are analyzed. BC mass concentrations are found to show diurnal and seasonal variations. Diurnal evolution in BC is marked with two peaks, one in the morning hours, just after the sunrise, and the other in the late evening hours. The peaks occur due to fumigation effect of boundary layer, gradual increase in the anthropogenic activities, and rush hour traffic. January BC values are about a factor of 5 higher than July mass concentrations. During winter the surface boundary layer is shallow resulting in trapping of pollutants in a lesser volume which leads to higher BC concentrations. In July an increase in boundary layer height, surface temperature, convective activity, and rainfall result in lower BC values. BC mass concentrations are about 0.8 mg m

[1] Extensive measurements of various aerosol parameters including single scattering albedo (SSA) were made at various locations over the central Indian region during February 2004 to study their impact on the regional aerosol radiative forcing. An overall increase in the measured value of SSA is noticed (0.75 to 0.9) over the period of this campaign, indicating unequal changes in source strength or removal processes of absorbing and scattering types of aerosols. Diurnally averaged value of direct SW radiative forcing for the region is in the range of À15 to À40 W/m 2 at the surface, about 15% lower compared to that over the Bay of Bengal region and 22% higher than over the Arabian Sea. TOA forcing is in the range of +0.7 to À11 W/m 2 , about 50% lower compared to both these regions. This results in a heating rate of nearly 0.8 K/day for the first 2km in the atmosphere. Citation: Ganguly, D., H. Gadhavi, A. Jayaraman, T. A. Rajesh, and A. Misra (2005), Single scattering albedo of aerosols over the central India: Implications for the regional aerosol radiative forcing, Geophys.

Measurements on spectral dependence of aerosol light absorption and BC mass concentrations were made at various locations between Ahmedabad (23.03°N, 72.50°E) and Hyderabad (17.47°N, 78.45°E) during February 2004. Average concentration of BC was close to 2 μg/m3 which is about 2 to 5% of the total aerosol mass loading measured at the surface. Absorption Angstrom coefficient estimated using a relation of the form βabs(λ) = K.λ−α is found to vary between 1.2 to 2.0 at different locations. Our result shows an excess absorption of up to 30% in lower wavelengths below 880 nm. Deviation from λ−1 dependence, which is typical of BC from fossil fuel, shows that aerosol mixture contains other particles from biofuel/biomass or mineral dust. As there was no major biomass burning observed in the study region as well as no specific enhancement observed in the coarse size aerosols, we attribute the observed excess absorption to the biofuel combustion.

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