Coupling detrended fluctuation analysis of the relationship between O3 and its precursors -a case study in Taiwan

The relationship between ozone and its main precursors exhibits complex non-linear feature in the atmospheric photochemical system. Therefore, sophisticated mathematical and nonlinear methods are very important for studying the O-3 sensitivity to its precursors and improving the O-3 prediction model. In this study, the coupling detrended fluctuation analysis (CDFA) method has been used to investigate the multi-temporal scaling pattern of coupling correlations between O-3 and its precursors. Hourly concentrations of ozone (O-3), nitrogen oxide (NOx) and non-methane hydrocarbons (NMHCs) from 1 March 2016 to 28 February 2017 of two traffic monitoring sites in Taiwan are analyzed. Coupling multifractality features are confirmed in the temporal distribution of coupling correlations between O-3 and its precursors. And the sources of coupling multifractal features are investigated by the phase randomized surrogates and the shuffling procedures. Moreover, in order to confirm the ozone sensitivity to its precursors at different time scales, a novel dynamical index, namely the coupling sensitivity degree (CSD), is introduced based on CDFA method. High temporal scale resolution of the O-3-NMHCs-NOx sensitivity is presented based on the concept of sliding window. And the O-3-NMHCs-NOx sensitivity regime can be shown visually by the temporal evolutions of the CSD on the whole year. Furthermore, statistics of seasonal CSD values are obtained by averaging the hourly sliding values. Result shows that the CSD values for both NMHCs and NOx are greatest in summer due to the more active photochemical reaction, compared to the other seasons. Meanwhile, the photochemical system is more sensitive to NMHCs for the two sites, which is consistent with the characteristics of pollutant emissions in the urban traffic area. At last, the diurnal pattern of the CSD for NMHCs and NOx are investigated. It is found that the greatest CSD for both NMHCs and NOx series are synchronized to the maximum O-3 generation. This study provides a feasible and effective dynamical index to describe O-3-NOx-NMHCs sensitivity, and suggests a new approach to improve the ozone prediction models in the photochemical system.