Aerosol Characteristics and Radiative Forcing during Pre-Monsoon and Post- Monsoon Seasons in an Urban Environment

The present study reports on aerosol characteristics and radiative properties utilizing ground based Aerosol Robotic Network (AERONET) data for the pre-monsoon (March, April, May) and post-monsoon (September, October, November) seasons over Lahore, Pakistan, for the two years during 2009–2010. The Aerosol Optical Depth (AOD) data from AERONET and a Moderate Resolution Imaging Spectro-radiometer (MODIS) were compared in order to validate both systems. The correlation coefficient for the post-monsoon season was > 0.68 in comparison to > 0.66 for the pre-monsoon season. In the pre-monsoon season, AERONET and MODIS AOD values were in the range of 0.2 to 1.2, and 0.2 to 1.67, respectively. For the post-monsoon season, these values were in the range of 0.17 to 2.46, and 0.15 to 2.45 for AERONET and MODIS, respectively. Strong dust loading resulted in higher values for the coarse particles during the pre-monsoon period, followed by an increase in the absorbing anthropogenic aerosols with the change from the pre-monsoon to post-monsoon season. The higher dust loading corresponded to the higher values of the real part of the refractive index in the pre-monsoon season, causing a relatively large single scattering albedo (SSA) (0.85–0.915) and thus a higher value for the asymmetry parameter (ASY). Similarly, the higher value of absorbing anthropogenic aerosols resulted in a higher value for the imaginary part of the refractive index in the post-monsoon period, followed by relatively lower values for SSA (0.88–0.911) and ASY. The averaged aerosol radiative forcing (ARF) for the pre-monsoon period at the top of the atmosphere was –19 ± 6 W/m 2 , while at the surface it was –93 ± 22 W/m 2 leading to an atmospheric forcing of +74 ± 16 W/m 2 . Likewise, the averaged ARF for the post-monsoon period at the top of the atmosphere was –28 ± 8 W/m 2 , while at the surface it was –98 ± 24 W/m 2 leading to an atmospheric forcing of +70 ± 15 W/m 2 , indicating significant heating of the atmosphere.

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