Wintertime ClNO₂ Observations

Reveals vertical exchange

First observation of boundary layer-residual layer vertical exchange of ClNO₂ in the troposphere, sustaining reactive chlorine through daytime entrainment, providing key data for atmospheric oxidation studies.

Yuxuan, Song · Tan, Zhaofeng · Xuezhen, Xu · Tham, Yeejun · Shuyang, Xie · Ce, Chen · Xinghan, Zhu · Wen-Xin, Zhou · Hongxia, Liu · Lou, Shengrong · Zhanyun, Liu · Yuhang, Wu · Wang, Haichao · Chen, Xiaorui · Ma, Xuefei · Lu, Keding · Zhang, Yuanhang

Journal of Geophysical Research D: Atmospheres 2026

Specifications

Peak ClNO₂ concentration

{'zh': '931.0', 'en': '931.0'} ppt

Diurnal average peak concentration

{'zh': '112.6', 'en': '112.6'} ppt

NO₃-to-ClNO₂ production efficiency

{'zh': '0.3', 'en': '0.3'} %

Residual layer ClNO₂ concentration

{'zh': '720.2', 'en': '720.2'} ppt

Advantages

Captures extreme peaks

A maximum ClNO₂ concentration of 931.0 ppt was recorded in Beijing, far exceeding diurnal average peaks, providing evidence for short-lived chlorine activation events.

Reveals vertical entrainment

The BL-RL model shows that the morning ClNO₂ surge in Beijing resulted from entrainment of ClNO₂ stored in the residual layer, indicating that vertical exchange is critical for wintertime chlorine chemistry.

Identifies controlling factors

Inter-city comparison shows that production efficiency is driven by temperature, particulate chloride, and N₂O₅ partitioning, with Beijing chloride-saturated and coastal cities chloride-supply-limited.

Applications

• Atmospheric chemistry observations：Provides ClNO₂ data from three cities for validating halogen chemistry mechanisms.
• Air quality monitoring：Reveals ClNO₂ contribution to secondary pollutants, aiding monitoring strategies.
• Environmental sensing：Offers high-time-resolution ClNO₂ measurements as a reference for sensor system design.