Interdecadal Changes in Aerosol Optical Depth Over Pakistan Based on the MERRA‐2 Reanalysis Data during 1980–2018

The spatiotemporal evolution and trends in aerosol optical depth (AOD) over environ‐
mentally distinct regions in Pakistan are investigated for the period 1980–2018. The AOD data for
this period was obtained from the Modern‐era retrospective analysis for research and applications,
version 2 (MERRA‐2) reanalysis atmospheric products, together with the Moderate‐resolution
imaging spectroradiometer (MODIS) retrievals. The climatology of AODMERRA‐2 is analyzed in three
different contexts: the entire study domain (Pakistan), six regions within the domain, and 12 cities
chosen from the entire study domain. The time‐series analysis of the MODIS and MERRA‐2 AOD
data shows similar patterns in individual cities. The AOD and its seasonality vary strongly across
Pakistan, with the lowest (0.05 ± 0.04) and highest (0.40 ± 0.06) in the autumn and summer seasons
over the desert and the coastal regions, respectively. During the study period, the annual AOD
trend increased between 0.002 and 0.012 year‐1. The increase of AOD is attributed to an increase in
population and emissions from natural and/or anthropogenic sources. A general increase in the
annual AOD over the central to lower Indus Basin is ascribed to the large contribution of dust
particles from the desert. During winter and spring, a significant decrease in the AOD was ob‐
served in the northern regions of Pakistan. The MERRA‐2 and MODIS trends (2002–2018) were
compared, and the results show visible differences between the AOD datasets due to theuseof
different versions and collection methods. Overall, the present study provides insight into the
regional differences of AOD and its trends with the pronounced seasonal behavior across Pakistan.