Identifying sensitive precursors of ozone formation using photochemical indicators and transition values comprised by continuous automatic monitoring data

Abstract

The O3 photochemical indicator is a tool used to identify the sensitive precursors of O3 formation. Establishing a reliable photochemical indicator and its transition value composed of continuous automatic monitoring of chemical components can not only analyze sensitive precursors of O3 formation in real-time but also analyze the time and space distribution characteristics of sensitive precursors of O3 formation at a relatively lower cost. Among the photochemical indicators that are often mentioned at present, the ratio of ozone to nitrogen oxides (O3/NOx, O3/NOZ, and O3/NOY) should have the greatest potential for continuous monitoring. This study used the Community Multiscale Air Quality Modeling System to evaluate whether these three indicators are applicable to environmental conditions in Taiwan. Research results show that the indicator O3/NOZ was less suitable for Taiwan, while the indicator O3/NOY (transition value = 5.3) and the indicator O3/NOx (transition value = 29.3) could more accurately evaluate the ozone sensitivity of Taiwan's overall grid, and had no obvious bias in judging the ozone sensitivity of rural and urban grids. This study also found that it is not appropriate to directly use the photochemical indicator transition values from the literature. Usually, because the value is too high or too low, there is an obvious bias in judging the ozone sensitivity of rural and urban grids. This result also demonstrates the need to consider local meteorological and emission conditions when estimating transition values of photochemical indicators. In addition, when using the photochemical indicator O3/NOY, attention should be paid to the calculation of NOY species composition. The more complete the NOY composition, the better its ability to correctly judge ozone sensitivity.