Photochemical production of ozone and control strategy for Southern Taiwan

An observation-based method (OBM) is developed to evaluate the ozone (O₃) production efficiency (O₃ molecules produced per NO_x molecule consumed) and O₃ production rate (P(O₃)) during a field campaign in southern Taiwan. The method can also provide an estimate of the concentration of OH. A key step in the method is to use observed concentrations of two aromatic hydrocarbons, namely ethylbenzene and m,p-xylene, to estimate the degree of photochemical processing and amounts of photochemically consumed NO_x and NMHCs by OH. In addition, total oxidant (O₃+NO₂) instead of O₃ itself turns out to be very useful for representing ozone production in the OBM approach. The average O₃ production efficiency during the field campaign in Fall (2003) is found to be about 10.2±3.9. The relationship of P(O₃) with NO_x is examined and compared with a one-dimensional (1D) photochemical model. Values of P(O₃) derived from the OBM are slightly lower than those calculated in the 1D model. However, OH concentrations estimated by the OBM are about a factor of 2 lower than the 1D model. Fresh emissions, which affect the degree of photochemical processing appear to be a major cause of the underestimate. We have developed a three-dimensional (3D) OBM O₃ production diagram that resembles the EKMA ozone isopleth diagram to study the relationship of the total oxidant versus O₃ precursors. The 3D OBM O₃ production diagram suggests that reducing emissions of NMHCs are more effective in controlling O₃ than reducing NO_x. However, significant uncertainties remain in the OBM, and considerable more work is required to minimize these uncertainties before a definitive control strategy can be reached. The observation-based approach provides a good alternative to measuring peroxy radicals for evaluating the production of O₃ and formulating O₃ control strategy in urban and suburban environments.