Reshaping emissions and chemical composition of semi-volatile and intermediate-volatility organic compounds from urban vehicle fleets: Implications of electrification and stricter emission standards

Vehicular organic emissions remain key precursors to urban ozone and secondary organic aerosol (SOA), yet their phase-resolved characteristics and evolution under rapid fleet electrification are poorly understood. Here, gas- and particle-phase semi-volatile and intermediate-volatility organic compounds (S/IVOCs) from real-world vehicle fleets were characterized via comprehensive two-dimensional gas chromatography–mass spectrometry (GC × GC-MS) during two urban tunnel campaigns in 2021 and 2025. An approach combining supervised machine learning (PLS-DA) and independent univariate hypothesis testing (t-tests) was used to identify fuel-sensitive molecular tracers. Our results showed that gas- and particle-phase organic emission factors (EFs) derived from the 2025 campaign were 14.08 ± 4.69 and 4.42 ± 2.34 mg km⁻¹ veh⁻¹ , respectively. Gas-phase emissions were dominated by VOCs, particularly single-ring aromatics, while particle-phase emissions were enriched in S/IVOCs, nitrogen-containing compounds, and long-chain n -alkanes. Short-chain alkanes and alkyl benzenes were identified as potential indicators of gasoline vehicles, while long-chain alkanes are potential diesel indicators and naphthalene is also a potential diesel indicator. Aromatics dominated both ozone formation potential (OFP) and SOA formation potential (SOAFP). Notably, intermediate-volatility organic compounds (IVOCs) accounted for only 29.0% of gas-phase EFs yet contributed 42.2% of estimated SOAFP. The organic EFs declined by 31.2% from 2021 to 2025, coinciding with greater proportions of electric, hybrid, and China VI vehicles; however, residual emissions became relatively enriched in aromatics. These findings highlight that future traffic pollution control must extend beyond reducing total organic emissions through fleet renewal and stricter standards, to targeted management of aromatics and S/IVOCs, which can drive secondary pollution beyond regulated VOCs and particulate mass.