Long-term variation in aerosol optical properties and their climatic implications over major cities of Pakistan

The present study investigates long-term variation in aerosol optical properties (AOP) and their associated climatic implications over selected cities of Chitral, Gwadar, Karachi, Lahore, Peshawar and Quetta in Pakistan for the period 2005-2018. For this purpose, aerosol optical depth (AOD) and aerosol index (AI) are retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI). Results revealed annual increasing trend of AOD with maximum values in summer during the study period over all study regions, except for Chitral. Similar annual trend was observed for AI but with minimum values in summer. Further, temperature and relative humidity (RH) showed significant relationships with AOD along with evidences of precipitation influences. In addition, the trajectory analysis of Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model confirmed the arrival of both short and long-range air masses to the receptor sites. Similarly, the direct aerosol radiative forcing (DARF) and direct aerosol radiative forcing efficiency (DARFE) were calculated using Clouds and the Earth's Radiant Energy System (CERES) data. The multiyear mean monthly atmospheric DARF values of 8.80, 16.32, 10.74, 21.64, 9.10 and 10.94 W/m(2) were observed over Chitral, Gwadar, Karachi, Lahore, Peshawar and Quetta, respectively. Consequently, the maximum heating rate (HR) of 0.48 K/day (Chitral), 0.93 K/day (Gwadar), 0.51 K/day (Karachi), 1.23 K/day (Lahore), 0.44 K/day (Peshawar) and 0.56 K/day (Quetta) showed a net warming effect during 2005-2018. These results can give an insight into aerosol concentration and can form the basis for aerosol-induced climatic implications in the study area.