• aerosol types;
  • Sun photometer;
  • monsoons;
  • back trajectories;
  • Hyderabad;
  • India

[1] Aerosol measurements over the tropical urban site of Hyderabad, India, provide a way of determining the variability of the aerosol characteristics over a duration of 1 year (October 2007 to September 2008). The meteorological pattern over India, mainly driven by the regional monsoons, has a great effect on the amount and size distribution of the aerosols. Higher aerosol optical depth (AOD) values are observed in premonsoon, while the variability of the Ångström exponent (α) seems to be more pronounced, with higher values in winter and premonsoon and lower values in the monsoon periods. The AOD at 500 nm (AOD500) is very large over Hyderabad, varying from 0.46 ± 0.17 in postmonsoon to 0.65 ± 0.22 in premonsoon periods. A discrimination of the different aerosol types over Hyderabad is also attempted using values of AOD500 and α380–870. Such discrimination is rather difficult to interpret since a single aerosol type can partly be identified only under specific conditions (e.g., anthropogenic emissions, biomass burning or dust outbreaks), while the presence of mixed aerosols, without dominance of the coarse or accumulation mode, is the usual situation. According to the analysis the three individual components of differing origin, composition and optical characteristics are (1) an urban/industrial aerosol type composed of aerosols produced locally and all year round by combustion activities in the city or long-range transported (mainly in spring) biomass burning, (2) an aerosol type of mineral origin raised by the wind in the deserts (mainly in premonsoon) or constituting coarse-mode aerosols under high relative humidity conditions mainly in the monsoon period, and (3) an aerosol type with a marine influence under background conditions occurring in monsoon and postmonsoon periods. Nevertheless, the mixed or undetermined aerosol type dominates with percentages varying from 44.3% (premonsoon) to 72.9% (postmonsoon). Spectral AOD and α data are analyzed to obtain information about the adequacy of the simple use of the Ångström exponent for characterizing the aerosols. This is achieved by taking advantage of the spectral variation of lnAOD versus lnλ, the so-called curvature. The results show that the spectral curvature can be effectively used as a tool for aerosol types discrimination, since the fine-mode aerosols exhibit negative curvature, while the coarse-mode particles are positive.