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<strong class="journal-contentHeaderColor">Abstract.</strong> An observation-based method (OBM) is developed to investigate the sensitivity of ozone formation to precursors during two persistent elevated ozone episodes observed at 77 stations in Guangdong. Average OH concentrations derived at the 77 stations between 08:00 and 13:00 local time stay within a narrow range of <span class="inline-formula">2.5Ã10<sup>6</sup></span> to <span class="inline-formula">5.5Ã10<sup>6</sup></span>âcm<span class="inline-formula"><sup>â3</sup></span> with a weak dependence on the NO<span class="inline-formula"><sub><i>x</i></sub></span>. These values are in good agreement with OH values observed at a rural station in the Pearl River Delta (PRD). They also agree well with a box model constrained by the ambient conditions observed during the two episodes. The OBM has been used to evaluate the ozone production efficiency, <span class="inline-formula"><i>ε</i></span>(NO<span class="inline-formula"><sub><i>x</i></sub></span> or volatile organic compound, VOC), defined as the number of O<span class="inline-formula"><sub>3</sub></span> molecules produced per molecule of NO<span class="inline-formula"><sub><i>x</i></sub></span> (or VOC) oxidized. Average values of <span class="inline-formula"><i>ε</i></span>(NO<span class="inline-formula"><sub><i>x</i></sub></span>) and <span class="inline-formula"><i>ε</i></span>(VOC) determined by the OBM are 3.0 and 2.1âppbâppb<span class="inline-formula"><sup>â1</sup></span>, respectively, and both compared well with values in previous studies. Approximately 67â% of the station days exhibit ozone formation sensitivity to NO<span class="inline-formula"><sub><i>x</i></sub></span>, and approximately 20â% of the station days are in the transitional regime sensitive to both NO<span class="inline-formula"><sub><i>x</i></sub></span> and VOC, and only approximately 13â% of the station days are sensitive to VOC. These results are in semi-quantitative agreement with the ozone formation sensitivity calculated by the box model constrained by ambient conditions observed during the two episodes. However, our OBM results differ from those of most previous investigations, which suggested that limiting the emission of VOC rather than NO<span class="inline-formula"><sub><i>x</i></sub></span> would be more effective in reducing ozone reduction in Guangdong.