Evaluation of Turbulent Mixing in HYSPLIT Using a Tracer of Opportunity Dataset

Turbulent velocity variance is an essential variable in HYSPLIT to determine the mixing of particles. There are different options available in the model to estimate the turbulent velocity variance according to the stability and turbulent variables provided by meteorological fields. The mixing options in HYSPLIT are the Belijaars-Holtslag (BH) method, the Kantha-Clayson (KC) method, the turbulent kinetic energy (TKED) option, and the turbulent exchange coefficient (EXCH) option. In this study, we evaluate turbulent mixing schemes in HYSPLIT using SO₂ as a tracer of opportunity and velocity variance measurements from Doppler Lidar. Dispersion simulations were conducted using different turbulent mixing schemes and WRF meteorological model output for SO₂ plumes measured at measurement sites from known emission sources. We collect SO₂ emissions from U.S. power plants using continuous emissions monitoring systems (CEMs) and hourly ground measurements of SO₂ concentrations for dispersion evaluation. Doppler Lidar data employed in the Southern Great Plains provide continuous measurements of velocity variance covering the entire PBL in 10-minute intervals. These observed velocity variances are used to evaluate velocity variance estimated by different mixing schemes in HYSPLIT. The SO₂ dataset, along with Lidar measurements of velocity variance, presents many cases for model evaluation that may further give insight on how each mixing scheme performs for stable nighttime conditions and unstable daytime conditions.