Changing ozone sensitivity in Fujian Province, China, during 2012-2021: Importance of controlling VOC emissions.

In the troposphere, ozone (O) formation can be limited by NOx, VOCs, or both, complicating efforts to reduce O by controlling its precursors. This study used formaldehyde (HCHO) data and nitrogen dioxide (NO) data from the Ozone Monitoring Instrument (OMI) to analyze O formation sensitivity in Fujian from 2012 to 2021. Over the past decade, an 8.7% reduction in NO VCDs and a 9.91% increase in HCHO VCDs were observed. Due to differences in the primary driving factors, HCHO VCDs exhibit a characteristic seasonal pattern with higher in summer and lower in winter, whereas NO VCDs show the opposite trend. O formation chemistry was accurately diagnosed by combining satellite-based data and ground-based O data. A new threshold value (3.3-4.6) was derived to determine the transition from VOC-limited to NOx-limited O formation regimes. Results showed that O sensitivity exhibited pronounced seasonal variations. The VOC-limited regime predominates throughout the entire Fujian region in winter, whereas it occupies only 5% of the area in summer. A VOC-limited region was found widely across Fujian on an annual average, but it decreased by 24% over 10 years. Transitional areas experienced a 19% increase. In two natural emission reduction cases (reductions during the Chinese Lunar New Year holiday and reductions in weekend traffic emissions compared to weekdays), ground-level O effectively captured the impacts of sensitivity changes. The impact suggests that when Fujian is in the VOC control region, a significant reduction in NOx, without effective VOC control, might lead to an O increase. The importance of controlling VOC emissions is highlighted in Fujian. This study enhances the understanding of O formation regimes in southeastern China, which is crucial for developing O prevention and control strategies.