Formulating the relationship between ozone pollution features and the transition value of photochemical indicators

Abstract

Abstract Indicator transition value represents the value when O3 sensitivity transfers from VOC-sensitive to NOx-sensitive conditions. The applicability of the indicator transition value for O3 sensitivity is inconsistent in the past model studies. Thus, this study is undertaken to analyze the variation of the indicator transition value under different conditions. Initially, a three-dimensional (3D) Eulerian air quality model is adopted to predict spatial and temporary O3 concentrations during a particular 4-day episode in Taiwan. The methodology for determining indicator transition values, which is different from those used in other studies, is as follows. First, the peak O3 isopleth profile for each target grid cell in the modeling domain is established to determine the transition line based on 25 combinations of NOx and VOC emission reduction. The results are then compared with the isopleth profiles of concurrent indicator to determine the transition value of eight indicators for each target grid cell. The daily spatial distribution of indicator transition values for all grid cells in the modeling domain is different during the 4-day simulation period. Secondly, the ratio of NOz/NOy, NOy and O3 are used to describe the pollution features of air mass, and then a set of equations is developed to illustrate their relationship with the indicator transition value. Finally, the capability of these equations to assess O3 sensitivity is evaluated. The H2O2/HNO3 ratio is found to be the best indicator among the eight indicators evaluated. The indicator transition value equation for air mass (ITVEAM) for H2O2/HNO3 is then constructed and the results described well the spatial variation during the 4-day simulation period. The ITVEAM along with the observed H2O2/HNO3 can be used to determine which precursor controls O3 production, especially during the O3 episode period. As a result, it can guide an emission reduction strategy to improve O3 concentration.