Ozone variability in the atmospheric boundary layer in Maryland and its implications for vertical transport model

Although much research has focused on daytime ozone (O₃) distribution in the atmospheric boundary layer, there remain many unresolved processes related to O₃ transport in the residual layer. To address such unresolved questions, a field study was conducted in Beltsville, MD during the summer of 2010 to study the spatial and temporal distribution of O₃ and other pollutants using ground-based gas analyzers and ozonesondes. During elevated pollution events in the daytime, the convective boundary layer, which reached a maximum depth of about 2 km, had nearly uniform O₃ levels of almost 100 parts per billion (ppbv). Due to intermittent and intense vertical turbulent motion, the residual layer became "leaky" and permitted vertical transport to enhance ground-level O₃ mixing ratios by as much as 10-30 ppbv in a span of 0.5-3 h. Model simulations, using the Weather Research and Forecasting model with Chemistry (WRF/Chem), were carried out to investigate the impact of different treatments of vertical mixing on the simulation of O₃ in the nocturnal boundary layer and residual layer. WRF/Chem model simulations provided realistic O₃ vertical distribution during the daytime. During the nighttime, in the residual layer, model outputs resulted in higher O₃ levels compared with the in-situ observations. Model sensitivity analyses showed that increasing the turbulent length scales and improved stability functions yielded improvements in the vertical transport of O₃ within the residual layer. One key conclusion of this study is that models such as WRF/Chem require improved numerical algorithms to properly account for the nocturnal vertical transport of O₃ in the residual region of the atmospheric boundary layer.