Maximizing ozone control by spatial sensitivity-oriented mitigation strategy in the Pearl River Delta Region, China.

The Pearl River Delta (PRD) has long been plagued by severe O pollution, particularly during the autumn. A regional O pollution episode influenced by the Western Pacific Subtropical High in September 2021 was characterized by near-surface O escalation due to strong photochemical reactions within the planetary boundary layer. This event was targeted to develop effective control strategies through investigation of precursor control type and scope based on the high-order decoupled direct method (HDDM) and integrated source apportionment method (ISAM) of CMAQ. Generally, the majority of areas (67.0 %) were under NO-limited regime, which should strengthen afternoon NO control inferred by positive convex O responses. However, high emission and heavily polluted areas located in central PRD were under VOC-limited regime (11.6 %) or mixed regime (15.0 %). The remaining areas (6.4 %) were under NO-titration or insensitive conditions. Regarding source apportionment, Guangdong province contributed 32.3 %-58.4 % to MDA8 O of PRD, especially higher proportion (>50 %) to central areas. Overall, local-focused NO/VOC emission reductions had limited effects on O mitigation for receptor cities compared to regional-cooperative regulation. When region-wide VOC emission reduction was implemented, MDA8 O in VOC-limited grids exhibited the largest declines (2.3 %-4.1 %, 3.9- 7.0 μg·m). However, unified NO control contributed to increasing MDA8 O in VOC-limited grids (most stations located for air quality evaluation) whereas decreased MDA8 O by 2.1 %- 5.7 %, 3.0- 8.2 μg·m in large-scale NO-limited grids. The sensitivity-oriented regional control avoided O rebound and achieved the greatest decline of 3.4 %- 5.0 %, 5.7- 8.4 μg·m in VOC-limited grids; additionally, time-refined dynamic aggressive NO control decreased peak O by an extra 1.2- 6 μg·m, both of which facilitate the regulation for the forecasting O episodes. These findings suggest that in heavily polluted environments, the enhancement of O regulation benefits requires meticulous, coordinated, and dynamic NO and VOC controls spanning the entire region based on high-resolution analysis of heterogeneous O-NO-VOC sensitivity. Furthermore, emission reduction gains should be more reasonably reflected through increasing in-situ observations covering multi-sensitivity regions.