Determining the differences in O3-NOx-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 (NOx) and volatile organic compounds on sauna days, while it tended to be NOx-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.