[1] Dust samples were collected and analyzed during May 2004 in Kanpur (in Ganga basin), Northern India for the first time. Chemical evidence along with the air mass trajectories suggests three major sources of the mineral dusts transported to the Ganga basin. High concentration of Pb and Cd indicates possible mixing with the anthropogenic pollution. The composite aerosol model reveals low single scattering albedo ($0.74) due to the presence of black carbon. During the dust storms, shortwave (SW) clear-sky diurnally averaged top of the atmosphere (TOA) and surface forcing come out to be +11 ± 0.7 and À26 ± 3 W m À2 , respectively. Corresponding forcings in the longwave (LW) region are +1.9 ± 0.6 and +1.6 ± 0.4 W m À2 . Net atmospheric forcing (i.e., SW+LW, 36 W m À2 ) corresponds to heating rate of $1.02°K/day in the lower atmosphere. Dust alone has resulted in the net TOA and surface forcings of +7 and À12 W m À2 .

[1] This paper attempts to analyze the chemical compositions of the near surface aerosols at a typical location in the Ganga basin with an emphasis on delineating the source of aerosols in foggy/hazy conditions. Collocated measurements of a number of atmospheric and aerosol parameters along with simultaneous sampling of near surface aerosols of size less than 10 mm (PM 10 ) were made as part of an intense field campaign launched under the Indian Space Research Organization Geosphere Biosphere Program (ISRO-GBP) in December 2004. PM 10 and black carbon (BC) mass concentration was found to be significantly higher during the foggy/hazy period. Much of the PM 10 mass ($81%) was due to fine/accumulation mode particles (0.1-0.95 mm). associated with very low values (<5 ppmv) of SO 2 despite considerable plausible emissions due to fossil fuel and biomass burning in the region suggests that loading of fine mode aerosols in the region could have been enhanced through reactions of gaseous pollutants on the solid surfaces. These results along with the findings presented in the companion paper indicate that prolonged foggy/hazy conditions in the region may be due to the increased anthropogenic emissions.

This paper attempts to characterize the physical and optical properties of the aerosols along with relevant meteorological parameters at a typical location in the Ganga basin. The emphasis is on delineating the prolonged foggy/hazy conditions, a phenomenon believed to be of relatively recent origin, faced by millions of people during the winter months of December and January. Collocated measurements of a number of aerosol and atmospheric parameters were made using ground‐based instruments as part of an intense field campaign launched under the Indian Space Research Organization Geosphere Biosphere Programme in December 2004. The meteorological conditions suggest limited mixing due to shallow boundary layer thickness and essentially calm wind conditions. Monthly mean aerosol optical depth was high (0.77 ± 0.3 at 0.5 μm wavelength) and showed high spectral variation (first‐order Ångström exponent for all wavelengths, α = 1.24 ± 0.24). The second‐order Ångström exponent α′ derived for 0.34, 0.5, and 1.02 μm wavelengths showed much higher curvature in the aerosol optical depth spectrum on the hazy/foggy days (0.93 ± 0.36) as compared to that during the clear days (0.59 ± 0.3). Single‐scattering albedo (0.87–0.97) showed strong spectral variation. Aerosol mass concentration was high with monthly average 125.9 ± 47.1 μg m−3. Fine mode particles (<1 μm) contributed ∼75% to the total mass of aerosols. Similarly, aerosol number concentration was found to vary in the range 1.5–2 × 103 cm−3, with fine mode particles contributing to ∼99.6%. The hazy/foggy conditions typically prevailed when higher daytime relative humidity, lower maximum temperature, and higher fine/accumulation mode particles were observed. The companion paper suggests that the rise in aerosol mass/number concentration could be attributed to the aqueous‐phase heterogeneous reactions mediated by anthropogenic pollutants and the associated reduction in boundary layer thickness and suppressed mixing.

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