Identifying the spatiotemporal variations of ozone formation regimes across China from 2005 to 2019 based on polynomial simulation and causality analysis

Abstract

Abstract. Ozone formation regimes are closely related to the ratio of VOCs to NOx. Different ranges of HCHO/NO2 indicate three formation regimes, including VOCs-limited, transitional and NOx-limited regimes. Due to the unstable interactions between a diversity of precursors, the range of transitional regime, which plays a key role in identifying ozone formation regimes, remains unclear. To overcome the uncertainties from single models and the lack of reference data, we employed two models, polynomial simulation and Convergent Cross Mapping (CCM), to identify the ranges of HCHO/NO2 across China based on ground observations and remote sensing datasets. The ranges of transitional regime estimated by polynomial simulation and CCM were [1.0, 1.9] and [1.0, 1.8]. Since 2013, ozone formation regime has changed to the transitional and NOx-limited regime all over China, indicating ozone concentrations across China were mainly controlled by NOx. However, despite the NO2 concentrations, HCHO concentrations continuously exert a positive influence on ozone concentrations under transitional and NOx-limited regimes. Under the circumstance of national NOx-reduction policies, the increase of VOCs became the major driver for the soaring ozone pollution across China. For an effective management of ozone pollution across China, the emission-reduction of VOCs and NOx should be equally considered.