Decisive role of atmospheric circulation in the troposphere on ozone pollution in the Beijing-Tianjin-Hebei region

Using ozone observations and the ERA5 reanalysis, general circulation conditions between high and low ozone periods in the Beijing-Tianjin-Hebei (BTH) region in June are compared. The atmospheric circulation pattern is contrastingly different in midlatitudes between high and low ozone concentration periods. During high ozone periods in the BTH region, the East Asia-Pacific jet center shifts further eastward in the upper troposphere. Additionally, positive geopotential height anomalies develop at 500 hPa over the BTH region, accompanied by anomalously warm near surface conditions. In contrast, during low ozone periods in BTH, a similar circulation pattern is observed but with the sign reversed. During high (low) ozone periods, the temperature in the lower troposphere usually rises (reduces), solar ultraviolet radiation increases (decreases), and cloud cover, precipitation, and humidity decreases (increases). Moreover, during high (low) ozone periods in BTH, warm (cold) anomalies develop in the low troposphere, extending from the surface upward. This meteorological condition is (not) conducive to photochemical reactions in the near surface layer. The tropopause over the BTH region is anomalously higher (lower) during high (low) ozone periods, and the upper tropospheric ozone is -20% less (-17.5% higher) than the average. Ozone from the upper levels might not consistently contribute to an increase (decrease) in the near surface ozone concentration in BTH, while negative (positive) ozone concentration anomalies prevail in upper levels. Existing evidence suggests that the frontal zone in the troposphere offsets the dynamic contribution of stratospheric transport to changes in ozone content in BTH.