Development of a CMAQ–PMF-based composite index for prescribing an effective ozone abatement strategy: a case study of sensitivity of surface ozone to precursor volatile organic compound species in southern Taiwan

Photochemical ozone pollution is a serious air quality problem under weak synoptic conditions in many areas worldwide. Volatile organic compounds (VOCs) are largely responsible for ozone production in urban areas where nitrogen oxide (NOx) mixing ratios are high while usually not a limiting precursor to ozone (O3). In this study, the Community Multiscale Air Quality model higher-order decoupled direct method (CMAQ-HDDM) at an urban-scale resolution (1.0 km×1.0 km) in conjunction with positive matrix factorization (PMF) was used to identify the dominant sources of highly sensitive VOC species to ozone formation in southern Taiwan, a complex region of coastal urban and industrial parks and inland mountainous areas. First-order, second-order, and cross sensitivities of ozone concentrations to domain-wide (i.e., urban, suburban, and rural) NOx and VOC emissions were determined for the study area. Negative (positive) first-order sensitivities to NOx emissions are dominant over urban (inland) areas, confirming ozone production sensitivity favors the VOC-limited regime (NOx-limited regime) in southern Taiwan. Furthermore, most of the urban areas also exhibited negative second-order sensitivity to NOx emissions, indicating a negative O3 convex response where the linear increase of O3 from decreasing NOx emissions was largely attenuated by the nonlinear effects. Due to the solidly VOC-limited regime and the relative insensitivity of O3 production to increases or decreases of NOx emissions, this study pursued the VOC species that contributed the most to ozone formation. PMF analysis driven by VOCs resolved eight factors including mixed industry (21 %), vehicle emissions (22 %), solvent usage (17 %), biogenic sources (12 %), plastic industry (10 %), aged air mass (7 %), motorcycle exhausts (7 %), and manufacturing industry (5 %). Furthermore, a composite index that quantitatively combined the CMAQ-HDDM sensitivity coefficient and PMF-resolved factor contribution was developed to identify the key VOC species that should be targeted for effective ozone abatement. Our results indicate that VOC control measures should target (1) solvent usage for painting, coating and the printing industry, which emits abundant toluene and xylene; (2) gasoline fuel vehicle emissions of n-butane, isopentane, isobutane, and n-pentane; and (3) ethylene and propylene emissions from the petrochemical industry.