Aerosol-type classification based on AERONET version 3 data over two megacities of Pakistan

Abstract

This study investigates the temporal distribution of atmospheric particles over two mega-cities in Pakistan for the period 2015–2023, offering an aerosol-type classification based on the Particle Linear Depolarization Ratio (PLDR) and Single-Scattering Albedo (SSA) at 1020 nm, retrieved by the AERONET (Aerosol Robotic Network) Version 3.0 Level 2.0 inversion products. Results reveal that Lahore has comparatively higher average values for Aerosol Optical Depth (AOD 440nm) and Ångstroṁ Exponent (AE 440-870nm) than Karachi. The contribution of fine (coarse)--mode aerosols to the total volume concentration dominates at lower (higher) PLDR values. The dust ratio (Rd) parameter indicated that coarse-mode aerosols are dominated at both sites with occurrence rates of 86% at Lahore, and 99% at Karachi, respectively. The atmosphere of Lahore city is mainly affected by pure dust (PD), and Karachi city with pollution-dominated mixture (PDM) aerosols with occurrence rates of 43.16% and 49.14%, respectively, while dust-dominated mixture (DDM) has low-frequency rates (33.91% and 29.63%, respectively). The annual incidence rates for dust-free pollution aerosols (NA, WA, SA, and MA) are lower (25.62% at Lahore, 27.87% at Karachi) at both sites as compared to dust-containing (28.47% at Lahore, 30.43% at Karachi) aerosol type. Weakly absorbing (WA) and non-absorbing (NA) aerosols are dominated as compared to strong absorbing (SA) and moderately absorbing (MA) aerosols. In addition, seasonally, winter season is strongly affected by PD (85.42%) and DDM (54.39%), spring seasons by WA (55.53%) and PDM (68.61%), summer by NA (Lahore: 64.94%, and Karachi: 57.22%), and autumn seasons by WA (Lahore: 51.56%, and Karachi: 49.46%) aerosols, over Lahore and Karachi, respectively.