Insights into long-term variations of Black carbon over various metropolitans in Pakistan

Black carbon (BC), despite their small contribution in atmospheric aerosol loads, have growing attention for air quality, human health, and climate change implications. This study aims to investigate the long- term spatio-temporal trends of BC over various metropolitan cities in Pakistan through MERRA-2 reanalysis datasets ranging from 2001 to 2022. In addition, statistically significant spatial clusters (hotspots) of BC in Pakistan have been assessed through a geospatial statistical tool (Getis-Ord 𝐺^∗𝑖) and finally, the hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model has been applied to identify the path and direction of BC. The increased trend of BC has been observed in winters due to low PBH (planetary boundary layer) and increased anthropogenic activities during this season. The decreased trends of BC were observed in summers due to precipitation and the washout process. Among the metropolitans of Pakistan, the highest values of BC concentration were recorded in Karachi while lowest values have been observed in Islamabad. The findings showed that most of the hotspot regions are in the southern region along with some central areas. The results demonstrate that BC concentration in Pakistan rises annually because of increased biomass burning, vehicle emissions, and transboundary air pollution. It is anticipated that our study will furnish valuable insights for assessing the hotspots of BC along with their local and remote sources across Pakistan.