An Observational and Numerical Study of the Nocturnal Sea Breeze. Part II: Chemical Transport

Abstract

Chemical transport at the Savannah River Site (SRS) in South Carolina during nocturnal sea-breeze passage is examined using simulations from a three-dimensional mesoscale dynamic model [(RAMS) Regional Atmospheric Modeling System] and a Lagrangian particle dispersion model (LPDM) and supplemental surface measurements of sulfur hexafluoride (SF6) obtained during a 1988 field campaign. Plume dispersion and regional transport were characterized by nights with onshore and offshore synoptic winds. For onshore winds, the sea breeze lifts, redirects, and broadens an initially narrow plume but maintains its general structure. Regional calculations reveal particle translations exceeding 100 km under these conditions. On the other hand, with offshore synoptic winds, frontal passage leads to stronger lifting, turbulence, and vertical shearing that fragments the plume. In addition, complicated recirculation of pollutants is possible and may increase chemical concentrations in areas near the source. Observed and model plumes were shown to depend strongly on vertical, horizontal, and temporal wind shear, and vertical motion. These features were simulated by the RAMS and LPDM models but are not possible with Gaussian models. Turbulence and topography also had important effects on plume characteristics but were simulated adequately only in the frontal region. Deficiencies in the model simulation were attributed mainly to insufficient vertical resolution and inadequate resolution of surface features.