Aerosol mixing over an urban region: radiative effects

Abstract

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 μ m are different from those obtained for columnar SSA at 0.50 μ 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.