Sensitivity of air quality model prediction to parameterization of vertical eddy diffusivity

This paper investigates the effects of vertical eddy diffusivities derived from the 3 different planetary boundary layer (PBL) schemes on predictions of chemical components in the troposphere of East Asia. Three PBL schemes were incorporated into a regional air quality model (RAQM) to represent vertical mixing process and sensitivity simulations were conducted with the three schemes while other options are identical. At altitudes <2km, all schemes exhibit similar skill for predicting SO₂ and O₃, but more difference in the predicted NO_x concentration. The Gayno–Seaman scheme produces the smallest vertical eddy diffusivity (Kz) among all schemes, leading to higher SO₂ and NO_x concentrations near the surface than that from the other 2 schemes. However, the effect of vertical mixing on O₃ concentration is complex and varies spatially due to chemistry. The Gayno–Seaman scheme predicts lower O₃ concentrations than the other two schemes in the parts of northern China (generally VOC-limited) and higher ones in most parts of southern China (NO_x-limited). The Byun and Dennis scheme produces the largest mixing depth in the daytime, which bring more NO_x into upper levels, and the mixing depth predicted by the Gayno–Seaman scheme is the smallest, leading to higher NO_x and lower O₃ concentrations near the surface over intensive emission regions.