Photochemical Reaction Characteristics and Source Apportionment of VOCs Based on Estimation of Initial Volume Mixing Ratios during Summer in Dalian

• • Photochemical loss correction increased TVOCs from 12.49×10⁻⁹ to 14.93×10⁻⁹ (16.4% loss), with alkenes losing 53.2%—critical for accurate emission inventory and control strategy design. • • OFP loss rate reached 45.0% (from 21.31×10⁻⁹ to 38.75×10⁻⁹), with alkenes contributing 56.4% of the loss—underscoring the need to account for reactivity in ozone mitigation. • • During ozone pollution, TVOCs chemical loss was 1.9× higher and OFP loss 1.2× higher than non-pollution periods, with alkenes accounting for 88.4% of TVOCs loss—highlighting the role of alkenes in episodic ozone formation. • • PMF source apportionment based on initial concentrations shifted the dominant OFP source from motor vehicles (42.5% by observed) to petrochemical enterprises (42.5% by initial), demonstrating that ignoring photochemical losses misidentifies key ozone precursors.