Early springtime O₃ pollution episode in the Sichuan Basin: Transboundary and process analysis

In the context of climate change, there is a trend of earlier onset and longer duration of ambient ozone (O₃) pollution in the Sichuan Basin (SCB). On 21-March 26, 2024, an O₃ exceedance event occurred in the Chengdu Plain and southern SCB during early springtime, when moderate pollution rarely emerged before. The environmental and meteorological observation data, reanalysis dataset, and the WRF-Chem model coupling Integrated Process Rate (IPR) method were employed to examine the synoptic patterns and the transport mechanisms conducive to O₃ pollution episode. The statistical results showed that MDA8 O₃ concentration exceeding 160 μg/m³ amounted to 40 and 63 stations on the 22nd and 23rd in the SCB, respectively. The maximum MDA8 O₃ of 222 μg/m³observed on the 23rd in Zigong located in southern SCB. The synoptic pattern revealed that it was the successive arrival of the southern trough and plateau trough that made the southwestern basin to be controlled by a warm low-pressure system, which consequently led to pollutant accumulation. The strong vertical advection was the reason for the ground-level O₃ surge during the first pollution stage of March 21st to 23rd. Under the influence of northwesterly winds, the O₃-enriched air parcel was subsequently conveyed downstream to southern SCB, with the elevated local temperature exacerbating the O₃ formation. The contribution of horizontal advection was ∼9.03 times higher than the chemical contribution in the southern SCB. During the pollution's later stage, increased wind speed and presence of precipitation were crucial for the enhanced vertical mixing, which ultimately leaded to the O₃ decrease. Our result advances early spring O₃ pollution mechanism and their interactions with distinctive topographic and meteorological conditions over the SCB.