CMAQ model simulation evaluation

Baseline simulation. For the CMAQ simulation for 1-31 October, the predicted and observed values were compared for every hour during the simulation period (Table B3). Time series and scatter plots of the observed and simulated surface O3 concentration suggest that the simulation tracks the observations well, although peak levels are not fully captured (Figure B2-B3). The average simulated hourly O3 surface concentration (35.6 ppb) is less than the observed (37.8 ppb) when taking into account all available data. When excluding hours when the observations are below 60 ppb, the model average O3 concentration is 76.2 ppb, compared with observed O3 of 80.4 ppb. The NMB and NME are -5.9% and 46.7% for all available hourly measurements, and -5.2% and 20.3% when excluding observations below 60 ppb. The underestimation of model indicates a high bias at low O3 concentrations, including at night (Figure B2), which may result from the difficulty in capturing the vertical diffusivity coefficient in CMAQ or perhaps overestimated nighttime NOX titration. Similar model performance in the same area has been reported by Chang et al (2008), who also indicate that the model tends to underpredict peak O3 values and overestimate nighttime O3 concentration. The modeled MDA1 and MDA8 O3 surface concentrations are predicted to be 74.4 ppb and 60.9 ppb, compared with 83.0 ppb and 65.6 ppb for measured values, respectively (Figure B4 and Table B3). Model performance for the MDA1 O3 surface concentration is -10.3% (NMB) and 25.6% (NME), and -7.1% (NMB) and 24.6% (NME) for MDA8 O3. Furthermore, the model was able to predict 50% of the variability in the MDA8 O3 surface concentration, versus 40% for the MDA1 O3 surface concentration. USEPA (1991) proposed that the model performance goals for unpaired normalized bias of MDA1 O3, normalized bias of 1-hour O3, and gross error of all pairs with O3 > 60 ppb are ≤±20%, ≤±15%

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and ≤35%, respectively. The model for these metrics (-10.3%, -5.9%, and 16.3%), achieve the USEPA suggested model performance goals.

For O3 precursor model performance, the NO2 surface concentration averaged over all valid hourly measurements is predicted to be 30.6 ppb, compared with 17.8 ppb in the observations, and the daily average of hourly VOC surface concentration is predicted to be 364.0 ppb, compared with 265.2 ppb from observations (Table B3). The model has difficulty reproducing the variability of NO2 (30% of R) and VOC (40% of R) concentrations, compared with measurements (Figure B5-B6). Overall, the NMB and NME are 72.1% and 99.4% for NO2 and 37.2% and 77.4% for VOC, respectively. The NMB and NME of NO2 violate TEPA’s model performance criteria (TEPA, 2015).

Episode simulation. As described in section 2.4, we choose an episode with high O3 and a typical synoptic weather pattern of continental high pressure moving toward Taiwan from mainland China, 1 to 5 October 2010, for sensitivity analysis. Whereas this weather system typically causes high O3, there are a few days when low O3 occurs because of a cold front passing by. Figure 3.2 shows the synoptic weather patterns across Taiwan during these episode days and Figure 3.3 shows the predicted O3 concentrations and wind vector field at 2pm (LST) for each episode day. At 8am (LST) on 1 October, a continental high pressure system (1022 hPa, 20N, 120E) forms moving northerly away from Taiwan in the Yellow Sea, resulting in northeasterly wind across Taiwan (Figure 3.2a-b). During northeasterly winds, the winds are split into two air flows by the Central Mountain Range. The Central Mountain Range creates a stable atmospheric condition with dry and warm air on leeward side in southwestern Taiwan. After a sea breeze forms around southwestern Taiwan, strong inward onshore flow brings significant O3 and precursors to the inland areas, resulting in a high O3 episode over KPAB (Figure 3.3a). On that day, 10 of the 15

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observation sites in KPAB had O3 concentration over 120 ppb, and the highest O3 concentration (139.6 ppb) was observed at the Chaozhou site (Figure 3.4). The first episode day is recognized as a typical light northeasterly monsoon. During the following two days (10/2/2010-10/3/2010), a cold front system hit Taiwan (Figure 3.2c-f). The dense pressure gradient force across Taiwan causes strong wind and high humidity over south Taiwan (Figure 3.5). Due to strong land breeze, a vortex forms at 6-7am near coastal central Taiwan on 2 October 2010 (Figure 3.3b). This vortex flow is conducive to air dispersion in KPAB so that O3 dissipates over southeastern Taiwan. Likewise, on the third day, another vortex forming near coastal southeastern Taiwan at 8 am also avoids high O3 accumulation over KPAB, from which it carries the air offshore (Figure 3.3c). Starting from late on the third day, a continental anticyclone system emanating from mainland China forms moving towards Taiwan (Figure 3.2g-j). The following two days (10/4/2010- 10/5/2010) also had prevailing northeasterly winds in middle and northern Taiwan (Figure 3.3d- e). The additional transport of O3 from boundary cause KPAB O3 over 120 ppb observed on the fifth day at Meinong and Linyuan stations (Figure 3.4). During these five episode days, the meteorological parameters show sufficient sunlight, low wind, and high surface pressure, that have been associated with high O3 by previous studies (Chen et al., 2003; Chen et al., 2004; Li et al., 2010; Lin et al., 2007b) (Figure 3.5).

Table 3.2 shows the model performance for O3, NO2 and VOC surface concentration over KPAB sites during these 5 episode days. In general, the model performance during these 5 episode days has slightly higher O3 bias than the month of October (Table B3). The unpaired normalized bias of MDA1 O3, 1-hour O3, and gross error of all pair O3 > 60 ppb are -12.3%, -21.4%, and 9.5%, still meeting the USEPA model performance criteria except for hourly O3. The model well predict O3 variability for these 5 episode days (40%-90% of R) (Figure 3.4) than for the month of October

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(10%-70% of R) (Figure B2). This indicated the acceptable model performance for O3 during these 5 episode days. However, the overestimate of hourly NO2 and VOC surface concentration violate TEPA’s model performance criteria the during these 5 episode days, indicating that the model has difficulty simulating precursor concentrations (Table 3.2).

3.3.2 CMAQ-HDDM sensitivities to Taiwan domain-wide emissions and evaluation with