Classification of Aerosols in an Urban Environment on the Basis of Optical Measurements

The present study investigates various types of aerosol in the Lahore city of Pakistan using Aerosol Robotic Network (AERONET) data over a six year period from 2007 to 2012. Aerosol optical depths (AODs) observed was in the range 0.2–1.12. An analysis of seasonal variations in AOD has indicated that the highest AOD values occurred in summer and the lowest in winter. The urban aerosols of the study area were classified on the basis of optical parameters such as AOD, Extinction Angstrom Exponent (EAE), Absorption Angstrom Exponent (AAE), Single Scattering Albedo (SSA), Asymmetry Parameter (ASY) and Refractive Index (RI). The AAE values were in the range from 0.25 to 3.2. Real Refractive Index (RRI) and Imaginary part of the Refractive Index (IRI) values were in the range from 1.5 to 1.6 and 0 to 0.005 respectively. The major contributions to the atmospheric aerosols over Lahore were from urban industrial emissions, fossil fuel burning and road/soil dust. Higher RRI values reflected larger re-suspended road dust particles and long-range transported particles, while lower values reflected increased anthropogenic absorbing carbonaceous aerosols over the area. The AERONET retrieved SSA (0.80–0.89) and ASY (0.70–0.83) values suggested a predominance of urban industrial, vehicular and dust aerosols over Lahore. The Derivative of Angstrom Exponent (DAE) was derived at a wavelength of 500 nm and was found to indicate a predominance of fine aerosols across all seasons, particularly during summer and autumn seasons. Back-trajectory analyses using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model revealed that the major air masses over Lahore originated from India, Iran and Afghanistan.