Comprehensive Characterization and Source Analysis of VOC Fluxes in a Chinese Megacity

Volatile organic compounds (VOCs) are key precursors of secondary air pollutants such as ozone and PM_2.5, which have significant health and environmental impacts. In urban areas, the complex emission patterns of VOCs─driven by diverse anthropogenic activities─present major challenges for accurate source characterization and quantification. In this study, we conducted eddy covariance flux measurements of VOC in a Chinese megacity to elucidate strengths and origins of VOC in urban region. The results revealed pronounced temporal and compositional differences between VOC fluxes and concentrations. By linking compositional differences between VOC fluxes (fresh emissions) and concentrations (chemically processed air), we provide direct evidence of the critical role of the chemical evolution in shaping atmospheric compositions of VOCs in urban air. We demonstrate the advantages of using flux data in source apportionment for emissions by comparing positive matrix factorization (PMF) analyses based on flux and concentration data sets. Flux-based source apportionment revealed that urban VOC tracers, such as toluene, are primarily associated with volatile chemical product (VCP) rather than traffic. The temperature-driven dynamics of biogenic and anthropogenic (average: 53% vs 47%) contributions to the total monoterpene fluxes are quantified, highlighting the shifting importance of anthropogenic and biogenic sources in urban environments. These findings underscore the value of integrating flux and concentration data sets to enhance understanding of urban VOC sources and their atmospheric evolution.