An Investigation of the Effect of Valley Boundary Layer Structure on Atmospheric CO2 Concentrations at a Mountain Top Location

Mountain top locations are frequently used as background locations but local effects can be significant and should be studied to ensure high data quality of the background concentrations. The effect of cold air pool dynamics on mountain top CO2 concentrations has, to our knowledge, not been studied before. Data from continuous CO2 monitoring stations are used to investigate the effects of persistent cold air pools in the Salt Lake Valley on CO2 concentrations in the Valley and an adjacent mountain top location. The current study examines a few case studies in 2007, the first winter for which CO2 concentrations are available for a mountain top station adjacent to the Salt Lake Valley. The CO2 concentrations exhibit a characteristic diurnal variability, both at the mountain top and at the valley location. A peak in the CO2 concentration is found at the mountain top location in the afternoon. The difference between the maximum and daily mean CO2 concentration usually fluctuates between 0 and 2 ppm with a significant larger difference found on a number of days in the winter. A close examination of these days shows that the peak at the mountain top is highly correlated with a sharp decrease of the CO2 concentration at the Valley Station. The Valley Station exhibits a gradual build-up of the CO2 concentrations prior to the large decrease. It is hypothesized that the depth and strength of the cold air pool determines the amount of CO2 accumulated in the valley and the magnitude of the CO2 peak at the mountain top location after the cold air pool breaks up. Mesoscale analysis of the weather situation provides an in-depth explanation of the evolution of CO2 concentration at the valley and mountain top location during one of the persistent cold air pool cases. Changes in the CO2 concentrations are related initially to the passage of a weak upper-level trough, which weakens the strength and expands the depth of the cold air pool. A frontal passage breaks the inversions and produces a pulse of high CO2 at the mountain top location. This study provides a good example of local and mesoscale influences on CO2 concentration at a mountain top location.