| dc.description.abstract | Recent rapid urbanization has had a profound impact on local-scale atmospheric circulation but its impacts on the physical and chemical processes controlling the tropospheric ozone remain poorly resolved. In Taiwan, due to the strict emission policy, the ambient concentrations of nitrogen oxides (NOx) and volatile organic compounds (VOCs) have reduced by nearly 60% since 1994. However, such reduction in precursors has not been linearly reflected on the annual mean ozone concentration, but an increasing or flattening trend is seen in the last decade. Therefore, a comprehensive investigation on the urban impacts on tropospheric ozone chemistry is necessary for prescribing an effective ozone abatement strategy. Our study area focuses on southern Taiwan, a complex region of coastal urban and industrial parks and inland mountainous areas, where high ozone episode often occurs during the seasonal transition period (i.e. Apr-May and Oct-Nov). In this thesis, we modelled the spatial and temporal distribution of ozone and its precursors (i.e. NOx and VOCs) using WRF-CMAQ model at urban scale resolution 1.0 x 1.0 km.
Firstly, we investigated the impacts of urban land-surface forcing and its interaction with local circulations on local meteorology and ozone air quality. Two simulations were performed with the same emissions but different land cover designations: URBAN scenario represents the current urbanized condition and NO-URBAN scenario replaces all urban grid cells with cropland. It was shown that when the urban-heat-island (UHI) convergent flow stalls over the city, a circulation flow is formed and traps the pollutants at an elevated height, increasing the reaction of hydroxyl radical with VOCs by 2.0-4.0 ppbv h-1 at 1000-1500 m. At nighttime, the deeper boundary layer of URBAN scenario diluted NOx mixing ratio by 17 ppbv and weakened the titration effect, causing higher O3 concentration by 15 ppbv in the urban area. When the UHI vertical mixing diminished, the O3 aloft diffused downward to the surface level and further degraded the nighttime air quality.
Secondly, we examined the budget analysis of boundary-layer O3, NOx and NMHC over the urban and inland area of southern Taiwan. In the near-surface budget, chemical process and dry deposition are the main sink of O3 with the contribution more than 10 ppbv h-1 and 15 ppbv h-1, respectively; the major source of near-surface O3 is vertical diffusion exceeding 30 ppbv h-1. In the boundary-layer budget, chemical process is the main source while vertical diffusion becomes the sink for O3. The physiochemical circulation involves the vertical transport of near-surface pollutants and enhances photochemical production of O3 in the upper PBL level is dominant in urban areas. This vertical exchange is mainly attributed to the vertical diffusion process and gradually decreases with heights. Our results highlighted the important role of daytime sea breeze circulation pushing the polluted urban air masses into the inland region which greatly enhanced the inland O3 production due to the NOx-limited condition. Thus, control of NOx emission in inland area may be ineffective due to the dynamics role of land-sea breeze; whereas most of the urban areas are characterized by VOC-limited condition where control of VOCs emission is helpful to reduce urban O3 concentration.
Thirdly, we developed a CMAQ-PMF-based composite index to identify the key VOC source-species for effective ozone abatement strategy. 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 using CMAQ-Higher Direct Decoupled Method (HDDM). 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. Most of the urban areas 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 non-linear 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 8 factors including mixed industry (21%), vehicle emissions (22%), solvent usage (17%), biogenic (12%), plastic industry (10%), aged air mass (7%), motorcycle exhausts (7%), and manufacturing industry (5%). Based on the CMAQ-PMF-based composite index, our results indicate that VOC control measures should prioritize (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.
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