Time variability of black carbon (BC) aerosols over different timescales (daily, weekly and annual) is studied over a tropical urban location Hyderabad in India using seven channel portable Aethalometer. The results for the 2-year period (January 2009-December 2010) show a daily-mean BC variability from similar to 1.00 +/- 0.12 mu g m(-3) to 12.50 +/- 3.06 mu g m(-3), with a remarkable annual pattern of winter high and monsoon low. The BC values maximize during winter (December-January), similar to 6.67 +/- 0.22 mu g m(-3), and drop during summer (June-August), similar to 2.36 +/- 0.09 mu g m(-3), which establishes a large seasonal variation. Furthermore, the BC mass concentration exhibits a well-defined diurnal variation, with a morning peak and early afternoon minimum. The magnitude of the diurnal variations is seasonal dependent, which maximizes during the winter months. Air mass back trajectories indicated several different transport pathways, while the concentration weighted trajectory (CWT) analysis reveals that the most important potential sources for BC aerosols are the Indo-Gangetic plain (IGP), central India and some hot spots in Pakistan, Arabian Peninsula and Persian Gulf. The absorbing Angstrom exponent (alpha(abs)) estimated from the spectral values of absorption coefficient (sigma(abs)) ranges from 0.9 to 1.1 indicating high BC/OC ratio typical of fossil fuel origin. The annual average BC mass fraction to composite aerosols is found to be (10 +/- 3) % contributing to the atmospheric forcing by (55 +/- 10) %. The BC radiative forcing at the atmosphere shows strong seasonal dependency with higher values in winter (33.49 +/- 7.01) and spring (31.78 +/- 12.89) and moderate in autumn (18.94 +/- 6.71) and summer (13.15 +/- 1.66). The BC radiative forcing at the top of the atmosphere (TOA) is positive in all months, suggesting an overall heating of the regional climate over Hyderabad. (C) 2013 Elsevier Ltd. All rights reserved.

Regular measurements of spectral Aerosol Optical Depth (AOD) at ten wavelengths, obtained from multi-wavelength radiometer (MWR) under cloudless conditions in the outskirts of the tropical urban region of Hyderabad, India for the period January 2008 to December 2009, are examined. In general, high AOD with a coarse-mode abundance is seen during the pre-monsoon (March to May) and summer monsoon (June to September) with flat AOD spectra and low angstrom ngstrom wavelength exponent (), while in post-monsoon (OctoberNovember) and winter (DecemberFebruary) seasons, fine-mode dominance and steep AOD spectra are the basic features. The aerosol columnar size distribution (CSD) retrieved from the spectral AOD using King's inversion showed bimodal size distributions for all the seasons, except for the monsoon, with an accumulation-mode radius at 0.120.25 mu m and a coarse-mode one at 0.861.30 mu m. On the other hand, the CSD during the monsoon follows the power law for fine mode and the unimodal distribution for coarse mode. The fine-mode aerosols during post-monsoon and winter appear to be associated with air masses from continental India, while the coarse-mode particles during pre-monsoon and monsoon with air masses originating from west Asia and western India. The single-scattering albedo (SSA) calculated using the OPAC model varied from 0.83 +/- 0.05 in winter to 0.91 +/- 0.01 during the monsoon, indicating significant absorption by aerosols due to larger black carbon mixing ratio in winter, whereas a significant contribution of sea-salt in the monsoon season leads to higher SSAs. Aerosol radiative forcing (ARF) calculated using SBDART shows pronounced monthly variability at the surface, top of atmosphere (TOA) and within the atmosphere due to large variations in AOD and SSA. In general, larger negative ARF values at the surface (65 to 80 W m2) and TOA (approximate to 17 W m2) are observed during the pre-monsoon and early monsoon, while the atmospheric heating is higher (approximate to 5070 W m2) during January-April resulting in heating rates of approximate to 1.62.0 K day1. Copyright (c) 2012 Royal Meteorological Society