Evaluation of the optimal schemes for O₃-NO_x-VOC sensitivity utilizing regridding TROPOMI observations

The ratio of formaldehyde (HCHO) to nitrogen dioxide (NO₂) column density observed by satellites has been widely utilized to probe O₃-NO_x-VOC sensitivity. Numerous studies have proposed threshold schemes based on this ratio to delineate NO_x-limited and VOC-limited regimes. However, a critical question remains unresolved that which threshold scheme is optimal. This research employs Guangdong Province as a demonstration area. Leveraging TROPOMI-observed HCHO and NO₂ column density, we developed an O₃-NO_x-VOC sensitivity threshold evaluation model, which is used to assess several internationally reported threshold schemes and to investigate the evolution of O₃-NO_x-VOC sensitivity thresholds in Guangdong Province, potentially identifying the optimal threshold. Prior to model execution, uncertainties in satellite observations were reduced using a self-developed TROPOMI level-2 pixel regridding algorithm. Evaluation results reveal that the J20 threshold scheme demonstrated superior performance in Guangdong Province compared to other schemes. Between 2019 and 2021, the optimal threshold for the NO_x-limited regime in Guangdong Province was approximately 3.6, while the optimal threshold for the VOC-limited regime was around 2.0. Determining the optimal thresholds necessitates careful consideration of sample number. As a demonstration application, we diagnosed O₃-NO_x-VOC sensitivity in Guangdong Province from 2019 to 2021 based on J20 scheme. Research shows FNR in Guangdong exhibits a distinct monthly periodicity, peaking in July to September and bottoming in December to January. At the urban scale, FNR distribution is centered on developed areas such as Guangzhou and Shenzhen, increasing outward with an obvious annular distribution. The aerosol-ozone sensitivity relationship is complex, and raise in PM_2.5, PM₁₀, and SO₂ tend to lower FNR, driving O₃-NO_x-VOC sensitivity to shift towards VOC-limited regimes. Compared to 2019, the year 2020 saw the highest proportion of shift from the transitional regimes to the NO_x-limited regimes, while the most frequent transition in 2021 was from the NO_x-limited regimes to the transitional regimes.