Determining the differences in O-NO-VOCs sensitivity between sauna and roast days.

Temperature is a key meteorological factor that affects tropospheric ozone (O), with both humid-heat (sauna days) and dry-heat (roast days) conditions leading to O exceedances. However, the mechanisms influencing O formation and degradation under these two weather conditions remain unclear. Therefore, experiments were conducted in Beijing from 2019 to 2021 to observe O, its precursors, and related meteorological elements. A total of 18 days with O 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/m greater and the wind speed gradient was 0.018 s greater than those on sauna days, indicating more intense thermal and dynamic turbulence. The strong turbulence enhanced the vertical cycle of nitrogen dioxide (NO) and O, resulting in a 58.2 µg/(m·h) increase in NO upward transport rate and a 1034.4 µg/(m·h) increase in O downward transport rate on roast days than sauna days. Subsequently, a box model analysis was used to examine O formation under the two types of weather conditions, revealing that the NO-O vertical cycling speed dominated the O sensitivity. The O sensitivity was synergistically controlled by nitrogen oxides (NO) and volatile organic compounds on sauna days, while it tended to be NO-limited on roast days. The aim of this study was to provide a scientific theoretical basis for the control of O under different types of high temperature weather conditions.