Mixing Processes in the Nocturnal Atmospheric Boundary Layer and Their Impacts on Urban Ozone Concentrations and Heat Island Intensity

The known prevalence of southerly winds and low-level jet formations in the central Great Plains during summer, make this region an ideal site to investigate the structure of the nocturnal boundary layer (NBL) and its impacts on NBL mixing processes. Data sets collected during the Joint Urban 2003 (JU2003) campaign, which took place in July 2003 in Oklahoma City (OKC), were used to classify the boundary-layer structure and dynamics. Additionally, ozone concentrations measured at regulatory monitoring sites in and near the OKC metro area were used as quasi tracers to study possible mixing and transport regimes for various atmospheric conditions. Ozone time series measured at the regulatory monitoring sites in OKC during six exemplarily chosen days of the JU2003 campaign in general show the expected behavior with lower O₃ mixing ratios at night than during the day, most likely due to the NO titration reaction. However, high O₃ values and secondary peaks are often seen at night. As O₃ is not formed photochemically at night these peaks are most likely due to downward mixing of O₃ transported in the ABL aloft. Low-level jets developed on most nights during JU2003; associated with strong wind shears. Wind shear strength and duration varied, but a clear relationship between these variations and the O₃ patterns is not obvious. However, some trends suggest that O₃ levels at night and in the early morning are correlated with the flux Richardson number Ri_fl and friction velocity u_*. On more stable nights (larger Ri_fl), lower O₃-concentrations are observed at night, which could indicate less vertical mixing as also suggested by the lower u_*- values measured. During a second period, larger concentrations of O₃ at night coincide with less stable conditions (lower values for Ri_fl), during which higher u_*- values and thus stronger vertical mixing persisted. Despite the variability of atmospheric stability in the early morning hours, the tendency for increasing O₃ concentrations with decreasing atmospheric stability, which promotes more mixing in the ABL at night and in the early morning, can clearly be noted. The NBL mixing processes also impact the urban heat island intensity. During nights with stronger vertical mixing (indicated by higher O₃-values and lower values for Ri_fl) the heat island intensity tends to be lower than during more stable nights with limited vertical mixing. Our analysis demonstrates that detailed meteorological information can provide new insights into nocturnal mixing processes and related air-quality problems, but the lack of vertical concentration profiles still make it difficult to further study the interplay between the BL structure and air chemistry.