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Figure 5. Linear trends of O3NO2 and O3 + NO2 in the KaoPing area from 1994 to 2003. The increase in O3 is nearly equal to the decreased titration by NO.

accounted for by the reduction in NO2, i.e. by the reduction of titration. We also use the 1D model to generate a surface similar to the diagram in Fig. 4 (not shown). The slope of Ox against initial NMHCs is more than a-factor-of-3 greater than the slope against initial NOx, consistent with the OBM result. Therefore the balance of evidence suggests that the O3 production rate is likely to be NMHCs-sensitive and that controlling NMHCs is more efficient than controlling NOx in reducing urban ozone production.

The effectiveness of our OBM can be demonstrated by comparing Fig. 4 with Fig. 6, in which we replace the initial concentrations of precursors with the concentrations of precursors observed at the daily maximum values of O3 + NO2. In addition, the vertical coordinate is replaced with the daily maximum values of O3 + NO2 instead of A[O3 + NO2]. One can see that no meaningful relationship can be derived. The change to A[O3 + NO2] does not affect the value of the correlation coefficient. The difference between Fig. 6 and Fig. 4 clearly shows that the daily maximum concentrations of the total oxidant are photochemically controlled by the initial early morning abundances of precursors which are derived by the OBM along Lagrangian trajectories, but are affected little by the concurrent concentrations of precursors.

2.6. Applying the OBM to Beijing and PRD experiments

A plot corresponding to Fig. 4 for observations at the station at Peking University during the summer 2006 campaign is shown in Fig. 7 (Chang et al., 2008a). It can be seen that the Ox production at the Peking University station is even more NMHCs-sensitive as the slope of delta Ox against initial NOx is negative, meaning that Ox production will decrease with NOx. The increased sensitivity to NMHCs at Peking University compared with southern Taiwan is probably due to the 50%-smaller ratio

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