Aerosols in urban regions have a distinct character as they can exhibit significant seasonal and interannual variabilities owing to variations in local emissions and long-range transport. This diversity in aerosol sources can give rise to a complex aerosol mixture over urban regions which can influence resultant aerosol optical properties and their radiative effects. Seasonal variation in aerosol mixing state and aerosol radiative effects over Ahmedabad, an urban region in western India, are deduced from measured aerosol optical properties and a radiative transfer model. Aerosol mixing states for near-surface single scattering albedo (SSA) at 0.55 m are different from those obtained for columnar SSA at 0.50 m in each season, emphasizing that aerosol mixing state can vary with altitude determining differences in SSA. In winter, mineral dust coated by water-soluble aerosols emerges as the mixing state for near-surface SSA (0.69), while black carbon coated by water-soluble aerosols emerges as the probable mixing state for columnar SSA (0.94). Aerosol radiative forcing at the surface estimated for probable mixing states of columnar SSA follows observations during all seasons except the monsoon. Heating rate is higher for near-surface SSA than for column SSA. Heating rates for near-surface SSA during winter and post-monsoon are 1 K day(-1) below 3 km, but 0.5 K day(-1) for columnar SSA. A secondary peak in heating rate profile is seen between 2 and 4 km during pre-monsoon which is due to presence of absorbing aerosols at these altitudes. External mixing emerges as a probable mixing state during pre-monsoon and monsoon, indicating that mixing depends on aerosol types, their abundance and meteorological conditions. These results can serve as standard regional representatives for aerosol mixing and radiative effects over urban regions. The proposed approach can be applied to other environments to determine extreme bounds of core-shell mixing of aerosols.

Seasonal variations in mixing states of aerosols over an urban (Kanpur) and a rural location (Gandhi College) in the Indo-Gangetic Plain (IGP) are determined using the measured and modelled optical properties, and the impact of aerosol mixing state on radiative forcing is examined. Different fractions of black carbon (BC) and water-soluble aerosols in core-shell mixing emerged as the probable mixing state during winter, monsoon and post-monsoon over Kanpur. The degree of mixing, i.e. the percentage mass fraction of aerosols involved in core-shell mixing, is found to exhibit seasonal variations. Owing to the abundance of mineral dust (MD) during the pre-monsoon, MD coated by BC emerges as the most probable mixing state. Top-of-atmosphere (TOA) forcing changes its sign from positive for external mixing to negative for different probable mixing states during the pre-monsoon over both locations, as single scattering albedo is lower for external mixing. However, for other seasons, the TOA forcing is negative for external and different probable core-shell mixing states of aerosols. Surface aerosol forcing for probable mixing state during the post-monsoon is higher (44 W m2) over Kanpur, and is lower (24 W m2) over Gandhi College. A regression between instantaneous model-derived aerosol forcing and AERONET-measured forcing yielded r2 > 0.9, which confirms the robustness of the methodology adopted to retrieve aerosol optical properties and estimate forcing. Heating rates over Kanpur and Gandhi College during the pre-monsoon and monsoon are approximate to 0.75 K d1 and approximate to 0.5 K d1 respectively. Differences exist between measured and model-derived asymmetry parameter, g, owing to the non-sphericity of aerosols. However, aerosol radiative forcing is found to be weakly sensitive to the variation in g due to high (> 0.2) surface albedo. The modelling study provides new insights into the state of aerosol mixing, and indicates that aerosol mixing can vary depending on the type and abundance of aerosol species. Copyright (c) 2012 Royal Meteorological Society