Impact of stratospheric intrusions on surface ozone enhancement in Hong Kong in the lower troposphere: Implications for ozone control strategy.

Understanding the impact of stratospheric intrusion (SI) is crucial for elucidating atmospheric complexities and implications for ozone (O 3) control. However, current studies have not focused on the influence of SI on surface O 3 , thus limiting the effectiveness of control strategies. This study delves into an SI event that occurred from March 5 to 12, 2022, employing a comprehensive integration of the Weather Research and Forecasting model coupled with Chemistry and multi-reanalysis data. The lower tropospheric O 3 enhancement in Hong Kong has been noticed, despite the absence of SIs in the upper troposphere. This study employed a comprehensive methodology, integrating the use of a box model to estimate the stratosphere–troposphere exchange (STE) O 3 flux, integrated process rate (IPR) analysis to quantify the contributions from individual physical and chemical processes, and tracer methods to detect stratospheric O 3. During the deep SI episode, the STE flux peaked at −81.33 × 10−8 kg m−2·s−1, surpassing the monthly average by 15.2-fold. The IPR results indicate that vertical transport within the 0–16 km range contributes between 18.4 and 39.0 ppbv h−1 during the SI event, whereas horizontal advection shows a negative contribution. Stratospheric O 3 tagging revealed that SI contributes 15.5–31.3 ppb (29.6–50.2%, respectively, of surface O 3) when stratospheric O 3 mixes down. Five emission reduction paths were designed in response to the negative impacts of SI. The "anthropogenic VOC (AVOC) only" path was the most efficient; however, when SI contributed 31 ppb on March 9, the "NOx only" path required a 69% reduction, while the "AVOC only" path required an 85% reduction for efficiency. This research not only elucidates the complex interplay between SI and surface O 3 concentrations but also emphasizes the significance of refining existing emission reduction paths. [Display omitted] • A distinctive stratospheric intrusion (SI) event in Hong Kong was evidenced. • Tagging method revealed that SI contributes 15.5–31.3 ppb to surface O 3 enhancement. • It is crucial to offset the adverse effects of SI on emission reduction strategies. • The optimal emission reduction path needs to be adjusted as the impact of SIs changes. [ABSTRACT FROM AUTHOR]