Photochemical Reaction Characteristics and Source Apportionment of VOCs Based on Estimation of Initial Volume Mixing Ratios during Summer in Dalian
This study estimated initial volume mixing ratios of volatile organic compounds (VOCs) in Dalian from June 1 to August 31, 2024, using a photochemical age-based parameterization method, and performed source apportionment with positive matrix factorization (PMF). Observed average TVOCs concentration was 12.49×10⁻⁹, comprising alkanes (84.2%), alkenes (10.4%), and aromatics (5.4%). Corrected initial TVOCs was 14.93×10⁻⁹, indicating a loss rate of 16.4%. Loss rates were highest for alkenes (53.2%), followed by aromatics (23.3%) and alkanes (6.8%). Ozone formation potential (OFP) averaged 21.31×10⁻⁹ (observed) and 38.75×10⁻⁹ (initial), with an OFP loss rate of 45.0%, distributed as alkenes (56.4%), aromatics (32.7%), and alkanes (10.3%). During ozone pollution episodes, TVOCs chemical loss was 1.9 times that of non-pollution periods, with alkene loss reaching 61.6%; OFP loss was 1.2 times higher, with alkenes contributing 88.4% to TVOCs loss. Secondary organic aerosol (SOA) formation potential from 08:00–17:00 was 1.51×10⁻¹ μg·m⁻³, with 99.4% from aromatics and toluene contributing 68.3%. PMF identified five sources: motor vehicles (49.6%), oil and gas volatilization (20.7%), petrochemical enterprises (12.6%), industrial processes (11.2%), and solvent use (5.9%). OFP modeling indicated motor vehicles contributed most to ozone formation (41.1%), followed by petrochemical enterprises (35.8%). During ozone pollution, PMF based on initial concentrations showed petrochemical sources had the highest OFP contribution (42.5%), whereas observed concentrations indicated motor vehicles as the top contributor (42.5%). This discrepancy underscores the necessity of correcting for photochemical losses in source apportionment studies.