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Keywords:

  • aerosol mixing state;
  • aerosol optical properties;
  • urban region;
  • rural region

Abstract

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 m−2) over Kanpur, and is lower (−24 W m−2) 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 ∼0.75 K d−1 and ∼0.5 K d−1 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 © 2012 Royal Meteorological Society