Aerosol optical and radiative properties during summer and winter seasons over Lahore and Karachi

The study of aerosol optical and radiative properties presented here focuses on a geographic region in which there exist significant gaps in our knowledge. These properties have been analyzed through the ground-based Aerosol Robotic Network (AERONET) over the two megacities of Lahore and Karachi for summer (April–June) and winter (December–February) of 2010–11. During the study period the monthly mean aerosol optical depth (AOD) at 500 nm over Lahore ranged from 0.57 ± 0.18 to 0.76 ± 0.38, and the monthly mean Ångström exponent (<alpha>) ranged from 0.39 ± 0.17 to 1.22 ± 0.13. Likewise, over Karachi the monthly mean AOD ranged from 0.33 ± 0.11 to 0.63 ± 0.28 and the <alpha> values varied between 0.29 ± 0.08 to 0.95 ± 0.22. The average AOD values in summer and winter are 0.66 ± 0.30, 0.50 ± 0.18 and 0.67 ± 0.40, 0.34 ± 0.12 in Lahore and Karachi respectively. The relationship between the Absorption Ångström Exponent (AAE) and the Extinction Ångström Exponent (EAE) provided an indication of the relative proportions of urban-industrial and mineral dust aerosols over the two sites. The volume size distributions were higher over Lahore than over Karachi during both seasons. The single scattering albedo (SSA) ranged from 0.83 ± 0.02 (440 nm) to 0.91 ± 0.05 (1020 nm) over Lahore and from 0.88 ± 0.02 (440 nm) to 0.97 ± 0.01 (1020 nm) over Karachi. The lower SSA values over Lahore suggest that absorbing aerosols are more dominant over Lahore than over Karachi. The average aerosol radiative forcing (ARF) values in summer at the surface and the top of atmosphere (TOA) are −101.6 ± 8.2, −63.3 ± 9.5 and −19 ± 4.35, −20 ± 3.1 over Lahore and Karachi respectively. Likewise, the average ARF values in winter at the surface and TOA are −90.3 ± 21.03, −57 ± 6.35 and −26 ± 7, −16 ± 2.3 over Lahore and Karachi respectively. The averaged aerosol ARF values over Lahore and Karachi for the entire period covered by the observations were −22.5 ± 5.9 W m−2 and −18 ± 2.2 W m−2 at the TOA and −96 ± 13 W m−2 and −60 ± 6.8 W m−2 at the surface, respectively, giving an averaged atmospheric forcing of 74.56 ± 16.8 W m−2 over Lahore and 41.85 ± 6.4 W m−2 over Karachi, which indicates significant heating of the atmosphere at both sites. The average heating rate during summer was 2.3 ± 0.1 and 1.2 ± 0.2 K day−1 and during winter was 1.8 ± 0.4 and 1.1 ± 0.1 K day−1 over Lahore and Karachi respectively.