The Influence of PBL Parameterizations on the Distributions of Chemical Species in a Mesoscale Chemical Transport Model, WRF-Chem

The turbulent mixing and chemical transformation in the planetary boundary layer (PBL) play a crucial role in the distribution of chemical species. Thus, the reasonable representation of the PBL processes in a mesoscale chemical transport model is a key to the accurate prediction of concentrations of chemical species, e.g., ozone. In this study, we utilize WRF-Chem model (Weather Research and Forecasting - Chemistry model) to examine the effects of the PBL modeling on the distribution of the chemical species, such as O3, CO, NO2, NOy, SO2, and isoprene. WRF-Chem simulations are evaluated with the surface air quality monitoring network, aircraft observation, and ship measurement during ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) field campaign (7/15/04-8/15/04). Two PBL models are used: YSU (K-profile method) and MYJ (2.5 turbulence closure) models.

Both PBL models produce much higher near-surface concentrations of CO, NO2, NOy, and SO2 than observations. Under stable condition (nighttime over land, daytime over lake or ocean) boundary layer height and eddy diffusivity are too low, which traps the chemical species near the surface. For unstable conditions, biases in O3 and other species are found to be related to PBL dynamics and cloud predictions in each PBL model. For example, the vertical distribution of isoprene from YSU PBL are compared better to the aircraft measurements during daytime. The important factors affecting the vertical distribution of isoprene are examined. Necessary changes in the parameterizations of PBL height and eddy diffusivity are identified in order to improve predictions of near-surface mixing of various gas-phase species.