Determining OM/OC Ratio in Fine Particulate Matter Through Functional Group Analysis Over Regionally Representative Site Bhopal, Central India
PANKAJ KHATARKAR, Ramya Sunder Raman, Indian Institute of Science education and Research Bhopal
Abstract Number: 533
Working Group: Carbonaceous Aerosols
Abstract
Fine particulate matter (PM2.5), plays a significant role in atmospheric visibility reduction, radiative forcing, cloud microphysics, and adverse health outcomes. Exposure to ambient fine PM2.5 has been linked to millions of premature deaths annually. Among its major components, organic matter (OM) is particularly challenging to characterize due to its vast chemical complexity, yet it can contribute more than half of the total PM mass. This complexity hampers accurate assessments of its sources, transformation mechanisms, and environmental impacts. A key parameter in evaluating OM behaviour and modelling its climate and health effects is the organic mass to organic carbon (OM/OC) ratio. However, fixed conversion factors commonly used for OM/OC estimation often neglect spatial, seasonal, and compositional variability in ambient organic aerosol.
In this study, we present a functional group-resolved assessment of fine particulate organic aerosols collected on alternate days throughout 2019 at a regionally representative site in Bhopal, Central India. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy was employed to acquire spectra from collected samples, and the AIRSpec model was used for functional group quantification. The analysis focused on polar and aliphatic functional groups, including carboxylic acids (COOH), hydroxyls (OH), carbonyls (CO), and alkane (CH) groups. Distinct seasonal patterns were observed, with higher contributions from oxygenated polar groups during the pre-monsoon and monsoon periods. These periods also exhibited elevated OM/OC ratios, suggesting enhanced atmospheric oxidation and secondary organic aerosol (SOA) formation under high humidity and cloud processing conditions. The study highlights the limitations of applying static OM/OC factors and demonstrates the importance of chemically resolved approaches for capturing real atmospheric variability. Overall, this work offers a robust framework for estimating OM/OC ratios dynamically, contributing to improved carbon accounting in chemical transport models and supporting more accurate assessments of aerosol-climate and aerosol-health interactions.