Multi-year trends and source shifts of representative nonpolar I/SVOCs: Insights from gas-particle partitioning in a central Chinese city

Intermediate-volatility and semi-volatile organic compounds (I/SVOCs) constitute an important fraction of organic pollutants in urban atmospheres. They can partition between the gas and particle phases and are key precursors of secondary organic aerosol (SOA). As representative nonpolar I/SVOCs (NP-I/SVOCs), n-alkanes and polycyclic aromatic hydrocarbons (PAHs) provide valuable molecular fingerprints for investigating the atmospheric behaviour and sources of I/SVOCs. Here, we conducted a multi-year data analysis (2014-2015, 2019, 2022 and 2024) to characterise long-term pollutant trends and improve source attribution by accounting for gas-particle partitioning. Seasonal data of n-alkanes (C₈-C₄₀) and PAHs were analysed in a central Chinese city, where the measurements of gaseous NP-I/SVOCs remain scarce. Positive Matrix Factorisation (PMF) model was employed to apportion source contributions. The results revealed a 58% reduction in PM_2.5 and a 50% reduction in PM_2.5 bounded n-alkanes in 2022 compared to 2014-2015, reflecting the positive impact of past pollution control measures. Gas-particle partitioning of NP-I/SVOCs was largely governed by absorption into organic matter; however, partitioning models showed limitations in reproducing observed partitioning behaviour. PMF results indicated that motor vehicle emissions overtook coal combustion as the primary anthropogenic contributor to PM_2.5 bounded n-alkanes in recent years, and remained a major source of intermediate-volatility/semi-volatile n-alkanes and PAHs in 2024. Notably, particle-only and dual-phase (gas + particle) PMF analyses yielded significantly different source contributions and PAH health risk metrics, underscoring the importance of gas-particle partitioning in both source attribution and risk assessment. Incorporating gas-phase data enables a more comprehensive assessment of the NP-I/SVOC sources, particularly for sources enriched in low-molecular-weight homologues. These findings deepen the understanding of long-term NP-I/SVOC profiles and support more targeted air pollution control strategies.