Carbonaceous Composition and Multi-Wavelength Optical Properties of Particulate Matter in Lahore, Pakistan

Fine particulate matter (PM2.5) is a major air pollutant in South Asian cities; however, information on its carbonaceous composition and multi-wavelength optical properties remains limited for Lahore, Pakistan. This study investigated PM2.5 collected at an urban site in Lahore between 15 March and 29 April 2025. A total of 30 quartz-fiber filters were collected for PM2.5 sampling during the campaign. Of these, 25 filter samples were available for gravimetric and optical analyses, while the first 15 available filter samples were analyzed for OC/EC as a complementary component of the study. Aerosol light absorption was determined using the Multi-Wavelength Absorbance Analyzer (MWAA) at five wavelengths (375, 407, 532, 635, and 850 nm). PM2.5 concentrations ranged from 40 to 417 μg m−3, with an average of 137 ± 84 μg m−3, indicating severe particulate pollution throughout the sampling period. Within the 15 filters analyzed for OC/EC, organic carbon was the dominant carbonaceous component, contributing 76.6% of total carbon, whereas elemental carbon accounted for 23.4%. The MWAA measurements showed the expected decrease in aerosol absorption with increasing wavelength, reflecting the spectral behaviour of carbonaceous aerosols. The average Absorption Ångström Exponent (AAE) was 1.17 ± 0.30, indicating generally weak-to-moderate wavelength dependence, with occasional enhancement of short-wavelength absorption. Based on empirical AAE intervals, 56% of the samples had values between 1.0 and 1.5, 32% had values below 1.0, and 12% exhibited values above 1.5, indicating enhanced short-wavelength absorption during a limited number of events. These intervals provide qualitative information on spectral variability rather than unambiguous source attribution. Overall, this study provides new multi-wavelength optical observations of PM2.5 from a six-week field campaign conducted in Lahore during March–April 2025 and contributes to the characterization of carbonaceous aerosols in the Indo-Gangetic Plain. The generated dataset provides a useful basis for future source-apportionment studies, air-quality management, and investigations of the radiative implications of light-absorbing aerosols in highly polluted South Asian urban environments.