# Upper-layer ozone intrusion promotes wintertime secondary aerosol formation on the ground

## TL;DR

This study introduces a new method to detect ozone intrusion events and shows how they increase ground-level ozone and pollution in China during winter.

## Contribution

A novel method to identify upper-layer ozone intrusion events using ground observations is proposed.

## Key findings

- ULOI events occur 22%–74% of the time in China, especially in coastal regions.
- ULOI increases nighttime ozone by 13–43 ppbv and daytime ozone by 3–14 ppbv.
- ULOI boosts atmospheric oxidation capacity, promoting sulfate and organic aerosol formation.

## Abstract

Upper-layer ozone (O3) intrusion (ULOI) is an important source of surface O3, affecting gas pollutants and secondary aerosol formation. However, no robust method has been reported to identify ULOI events based on ground observations and assess their effects on surface atmospheric chemistry. We propose a novel method to identify ULOI events by ranking O3 concentrations before dawn and evaluate their contributions to ground-level O3 and aerosol formation across China. Our results show that ULOI events occur at a rate of 22%–74% across China, with higher frequency in eastern and southern coastal regions. ULOI enhances ground-level O3 by 13–43 ppbv at night and 3–14 ppbv during the day. This increases atmospheric oxidation capacity (AOC) and enhances the contribution of the O3 oxidation path to sulfate and secondary organic aerosol (SOA) formation. This study emphasizes the importance of atmospheric layer interactions and the impact of ULOI events on surface atmospheric chemistry.

This study proposed a novel method to identify upper-layer O3 intrusion (ULOI) events using ground-based observations. It is found that ULOI occurs frequently and significantly contributes to both nighttime and daytime surface O3 levels, boosting atmospheric oxidation capacity and promoting secondary pollution in winter in China.