Atmospheric chemistry of Polycyclic Aromatic Hydrocarbons (PAHs) and role of gas-partitioning in the formation of secondary Nitro-PAHs

Polycyclic Aromatic Hydrocarbons (PAHs) and Nitro-PAHs are ubiquitous semi-volatile organic pollutants. Their high concentration in the ambient air is a severe cause for concern because they are carcinogenic, mutagenic, and teratogenic to humans. This study elucidates the atmospheric chemistry and gas-particle partitioning mechanisms of PAHs and Nitro-PAHs, as well as their role in the formation of secondary aerosols. 16 priority PAHs and two nitro-PAHs were analysed using gas chromatograph-mass spectrometry (GC-MS) from dual-phase (gas and particle) aerosol samples that were simultaneously collected in a rural and traffic-dominated region of Agra. At the traffic and rural sites, the overall concentration of PAHs (gas + particulate) was 2481 and 1011 ng m^-3, respectively, while the total concentration of nitro-PAHs was 90 and 28 ng m^-3. The dual model governs the gas-particle partitioning of PAHs in Agra's ambient air, demonstrating how the concentration of PAHs is affected by the concentrations of OC and EC in the environment. Regression statistics (R2 > p<0.01) of the dual model, along with a statistically significant negative correlation between 1-NPyr (R²= 0.73, p<0.01)and 3-NFla (R²= 0.78, p<0.01)and their parent compounds, i.e., Pyr and Fla, confirm the formation of nitro-PAHs in the ambient air of Agra. A statistically significant correlation (R² > 0.75, p<0.01) for Clausius–Clapeyron plots was obtained, indicating the temperature dependency of gas-phase PAHs at both sites. Source analysis of PAHs and Nitro-PAHs reveals that the PAH concentration at the traffic site is primarily attributed to traffic and combustion sources, whereas at the rural site, the PAH concentration is largely due to biomass combustion and pyrogenic sources. However, the Nitro-PAHs concentration at the traffic site is due to both primary and secondary sources. ILCR values of PAHs and Nitro-PAHs show that humans are prone to cancer risk from the dermal exposure pathway, followed by ingestion and inhalation.