||Elevated O3 concentration has been an important environmental issue in Taiwan. O3 is a secondary air pollutant driven by photochemical reactions involving primary air pollutants such as volatile organic compounds (VOC) and nitrogen oxides (NOx, NO and NO2). Sensitivity analysis is vital for O3 due to the nonlinearity and complex reaction. Unlike Brute Force Method (BFM) where model simulations are repeated with different model inputs, HDDM offers an alternative by directly solving sensitivity equations derived from the governing equations of the model. Thus, HDDM provides an accurate and effective way to perform sensitivity analysis. In this study, the Community Multi-scale Air Quality Modeling system coupled with Higher-Order Direct Decoupled Method (CMAQ-HDDM) was applied for a high O3 event from October 18th to 24th 2011. Three sensitivity experiments were designed to investigate nonlinear response of O3 with respect to its precursor emissions and to quantify the emission contributions from different source regions and categories for the area where high O3 concentration occurs.|
The base-case model result is in a good agreement with the observation on meteorological and pollutant fields. The comparison between BFM and HDDM is in a similar pattern as well, providing a reliable HDDM simulation for further analysis. For the first sensitivity experiment, the aim is to explore how O3 respond to the precursor’s emission. Results indicate most of source region exhibited VOC-limited region, while over the downwind area, both NOx and VOC contributes for O3 production (NOx and VOC transition region). For the second sensitivity experiment, as the northeasterly wind prevailed throughout the event, five possible source region are assigned (including Yulin, Chiayi, Tainan, Kaohsiung, and Pingtung) to estimate the contribution where high O3 concentration occurs. There were stagnant conditions over southern Taiwan on Oct. 23rd, the highest O3 occurred near Tainan city, and most pollutants were from local emissions. However, the local circulation is more pronounced on Oct. 24th, and the highest O3 occurred in downwind area, Pingtung where the emission from Kaohsiung is the main contributor. Furthermore, the importance of various emission source categories is discussed. Among the anthropogenic emissions, the mobile NOx emission and area VOC emission mainly contributes for high O3 concentration. The cross effect between biogenic VOC emission and anthropogenic NOx is also a moderate contributor (about -10 ~ -12%).
In conclusion, due to changes of the meteorological conditions, the emissions were redistributed on Oct. 23rd and 24th. The results from this study can support the policy makers to build an efficient control strategy for reducing O3 concentrations.
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