Abstract:
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
~1.00 7 0.12 µg m⁻³ to 12.50 73.06 µg m⁻³, with a remarkable annual pattern of winter high and
monsoon low. The BC values maximize during winter (December–January), ~6.6770.22 µg m⁻³, and
drop during summer (June–August), ~2.36 70.09 µg 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 Ångström exponent (σabs) estimated from
the spectral values of absorption coefficient (σabs) 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.
