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 ¿m wavelength) and showed high spectral variation (first-order ¿ngstr¿m exponent for all wavelengths, α = 1.24 ¿ 0.24). The second-order ¿ngstr¿m exponent α' derived for 0.34, 0.5, and 1.02 ¿m 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 ¿g m-3. Fine mode particles (<1 ¿m) 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.