On the influence of the convective boundary layer height on diurnal trace gas variability at a ridgetop in the Appalachian Mountains

Measurements of mountaintop trace gas mixing ratio contain information useful for many applications, including air pollution studies and regional flux estimation. Improving the utility of mountaintop measurements in these applications requires a good understanding of the dominant physical processes affecting trace gas behavior. The height of the daytime convective boundary layer (CBL), for example, has been shown to help explain trace gas variability in flat terrain, but its influence on trace gas variability in mountainous terrain has not been investigated. To this end, we use meteorological and trace gas measurements from two mountaintop sites in the Virginia Blue Ridge Mountains. One mountaintop site is equipped with a 17 m tower outfitted with a suite of instruments, including temperature/humidity sensors, cup and sonic anemometers, radiation sensors, and closed-path CO/CO2 gas analyzer. A UV aerosol LIDAR is located on-site and operated during a selection of fair weather days. A suite of meteorological variables, as well as ozone mixing ratios, are measured at a ridgetop site 14 km to the south. The mountaintop measurements are complemented by meteorological and trace gas observations from a valley site 15 km to the west. CBL heights are estimated using data from the LIDAR and a rawinsonde site 80 km east of the region of interest. Mountaintop trace gas variations exhibit significant diurnal and seasonal variations. Trace gas mixing ratio increases during the daytime coincide with increases in water vapor mixing ratios, suggesting upslope transport from the adjacent valley. The timing and magnitude of these increases varies as a function of CBL height. The increase occurs earlier in the day when the CBL exceeds the ridgetop, and occurs later when the CBL remains below the ridgetop. The magnitude of the trace gas increase is also larger on days when the CBL remains below the ridgetop. These results imply that daytime mountaintop trace gas mixing ratios cannot be assumed to represent free atmospheric trace gas mixing ratios, even on days when the CBL does not exceed the ridgetop.