Optimization of a Thermal Desorption - Gas Chromatography/ Mass Spectrometry method for the determination of Polycyclic Aromatic Hydrocarbons in ambient PM2.5 over Bhopal, central India

Organic aerosols (OA) constitute a large fraction of fine particulate matter (PM2.5). Quantitative analysis of OA is essential for understanding the sources and atmospheric evolution of fine PM, which necessitates precise quantification of organic compounds also known as markers/tracers. In this study analytical approach - thermal desorption coupled with gas chromatography and mass spectrometry (TD-GC/MS) was optimized for the measurement of particle-bound 16 polycyclic aromatic hydrocarbons (PAHs). One year-long measurement of PM2.5 was conducted every other day at Bhopal - a regional representative site in COALESCE network (Lekinwala et al., 2020). Ambient PM2.5 samples were collected on a quartz fiber filter having a QbT (Quartz behind Teflon) and QbQ (Quartz behind Quartz) filter configuration to account for organic artefacts (Bhardwaj and Raman, 2022). The performance of the optimized method was compared with solvent extraction method, showing a good recovery (57.2-109.8 %), repeatability (80.7-96.7 %), and reproducibility (87.1-96.7 %). For all the optimized parameters of thermal desorption technique, limit of detection was superior than the solvent extraction method. The optimized method was then applied for the quantification of PAHs in ambient PM2.5 samples. The distribution of the target PAHs in the PM2.5 were given in Figure 1. Among target PAHs, B[b]F (0.25 ± 0.13 ng m-3) was the major contributor to total PAHs, followed by B[k]F (0.10 ± 0.05 ng m-3). Further, diagnostic ratios between PAHs were used to evaluate the contributions of particulate PAHs sources. Based on values given in literature, B[a]P/ B[ghi]P ratio <0.6 indicates a higher contribution from vehicle emissions, B[a]A/B[a]A + Chr was 0.4 and 0.2 for coal combustion and vehicular emissions. These ratios suggested that vehicular emissions were the dominant source of PAHs during Jan, Feb, and Mar. The diagnostic analysis of Flr/Flr + Pyr and InP/ InP + B[ghi]P indicates that PAHs were primarily from petroleum combustion and coal combustion during Jan, Feb, and Mar. However, during Apr the PAHs were jointly influenced by vehicular emissions and biomass burning (grass and wood combustion).