Seasonal variation of the chemical content and source identification of PM2.5 in a mixed landuse in Iran

S. Karaj is the fourth largest city upwind of Tehran, Iran’s capital. The complexity of emission sources in and around the city makes the development of targeted mitigation policies more challenging. This study is focused on identifying the mass concentration of PM_2.5, along with its chemical composition and associated sources. Biweekly fine PM samples, started in October 2020 and lasted for one year,were obtained at an air quality monitoring station and analyzed by ion chromatography paired with a mass spectrometer (IC-MS) and thermal-optical analysis (TOA). Annual average concentration of PM_2.5 was 29.61 ± 17.84 µg/m³, ranging from 9.16 µg/m³ (in December 2020) to 99.04 µg/m³ (in November 2020). Based on the chemical mass closure (CMC) results, organic matter (OM) was a principal constituent, accounting for 31% of PM_2.5 mass. The nitrate, dust and non-sea sulfate contributed most significantly to PM mass by accounting for 15%, 13% and 11%, respectively. The diagnostic ratios reflected that, anthropogenic sources (mobile sources and biomass combustion) play a significant role in PM concentration. The study’s findings are significant as they not only influence policy-making for controlling PM in one of the largest cities but also have a broader impact on air pollution in the capital city, located just downwind. The insights provided by this study on primary components in fine particulate matter will be instrumental in leveraging these findings alongside emission inventory and observational data. This integration aims to inform policymaking processes and facilitate the creation of science-based policies, while also furnishing essential data for health impact assessments.

Graphical abstract