Determining the differences in O₃-NO_x-VOCs sensitivity between sauna and roast days

Temperature is a key meteorological factor that affects tropospheric ozone (O₃), with both humid-heat (sauna days) and dry-heat (roast days) conditions leading to O₃ exceedances. However, the mechanisms influencing O₃ formation and degradation under these two weather conditions remain unclear. Therefore, experiments were conducted in Beijing from 2019 to 2021 to observe O₃, its precursors, and related meteorological elements. A total of 18 days with O₃ exceedances were selected, including 10 sauna days and 8 roast days. The results of this study revealed that on roast days, the sensible heat flux was 143.5 W/m² greater and the wind speed gradient was 0.018 s^-1 greater than those on sauna days, indicating more intense thermal and dynamic turbulence. The strong turbulence enhanced the vertical cycle of nitrogen dioxide (NO₂) and O₃, resulting in a 58.2 µg/(m²·h) increase in NO₂ upward transport rate and a 1034.4 µg/(m²·h) increase in O₃ downward transport rate on roast days than sauna days. Subsequently, a box model analysis was used to examine O₃ formation under the two types of weather conditions, revealing that the NO₂-O₃ vertical cycling speed dominated the O₃ sensitivity. The O₃ sensitivity was synergistically controlled by nitrogen oxides (NO_x) and volatile organic compounds on sauna days, while it tended to be NO_x-limited on roast days. The aim of this study was to provide a scientific theoretical basis for the control of O₃ under different types of high temperature weather conditions.