Investigations of aerosol types classification and PM2.5 concentrations: A case study of two major cities in Pakistan

In the atmosphere of Earth, aerosols are highly variable components that significantly affect Earth’s climate and human health. Due to their complex nature, these particles vary greatly in their form. Thus, in-depth analysis of aerosol classification is crucial for air quality and climate studies. Therefore, this study investigated the temporal distribution of atmospheric particles over two megacities of Pakistan (Lahore and Karachi). The study offers an aerosol-type classification based on the particle linear depolarization ratio (PLDR) and the single scattering albedo (SSA) at 1020 nm, retrieved from the Aerosol Robotic Network (AERONET) Version 3.0 Level 2.0 inversion products. The study also analyzed particulate matter (PM_2.5) levels in Lahore and Karachi, which has recently experienced alarming air pollution. Temporal trends of PM_2.5 in both cities were assessed using ground-based remote sensing. The dust ratio parameter (R_d) calculated from PLDR indicated that coarse-mode aerosols dominated, with occurrence rates of 86% in Lahore and 99% in Karachi. Pure dust (PD) and pollution-dominated mixture (PDM) plumes were more frequent in Lahore (43.16%) and Karachi (44.14%). Annually, dust-containing aerosols occur more frequently (28.47% in Lahore, 30.43% in Karachi) than dust-free types (25.62% in Lahore, 27.87% in Karachi). PM_2.5 levels peak in autumn and winter, especially December 2021 (369.17 µg/m³). The highest seasonal (winter) PM_2.5 averages were 284.99 µg/m³ (2022), 269.05 µg/m³ (2021), 264.99 µg/m³ (2023), and 222.34 µg/m³ (2020), contributing to smog and respiratory diseases in Lahore. Low PLDR values over Lahore are due to the large anthropogenic emissions, resulting in poor air quality as compared to Karachi.