A model investigation of summertime diurnal ozone behavior in rural mountainous locations

The STEM-II pollutant transport, transformation, and deposition model has been used to simulate diurnal patterns of surface ozone concentration at idealized high-elevation mountaintop locations and low-elevation plains and valley sites. Two-dimensional hydrostatic mesoscale simulations, initialized with atmospheric conditions representative of sunrise over the southern Appalachians on 19 June, were used to generate meteorological data to drive STEM-II simulations. Sensitivity of surface ozone concentrations has been investigated against input initial and background vertical profiles, synoptic wind speed, photochemistry, and surface removal. The model reproduced typical surface ozone concentration diurnal patterns observed at several high- and low-elevation sites in the United States and central Europe. Results indicate that the nighttime high ozone concentrations at high-elevation mountainous locations are primarily due to the occurrence of local topographically induced wind systems which transport ozone-rich air from aloft down mountain slopes. The simulations indicate that in some situations higher ozone concentrations may also be observed at mountaintop locations due to transport of residual ozone-rich air masses above the nocturnal boundary layer to high-elevation locations.