respec tively (Atkinson, 1990). The value of the ratio Eq/Xq is about 0.4 for the emission sources in southern Taiwan. This value is usually observed in the early morning, around 7-8 a.m. In fact, even around 9-10 a.m., when we collect the first daily VOC samples, the E/X ratios are fairly close to 0.4 because the photochemical activities are relatively weak before 9-10 a.m. Using the observed value of E/X at 11 a.m. -12 noon, JQ [OH] dt between early morning and 11 a.m.-12 noon can be calculated from Eq. (5). Then, using observed values of NOx at 11 a.m.-12 noon, initial values of NOx in the early morning (along the backward Lagrangian trajectory) can be calculated from Eq. (4), and the photochemically consumed values can be obtained from the difference between initial values and the values at 11 a.m.-12 noon. Similarly, NMHCs consumed by OH and initial values can be calculated from Eq. (5) (Chang et al., 2007).

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