Boundary Layer Effects on Atmospheric CO2 Concentrations at Mountaintop Locations

Observations of CO2 concentrations in the atmosphere have provided a basis for an improved understanding of the global carbon cycle. Spatial gradients of CO2 have been used to calculate net surface exchange on large continental scales using inverse modeling approaches. In recent years, measurements on tall towers and mountaintop stations have become available, potentially enabling the estimation of net surface exchange on smaller regional scales. The inclusion of CO2 measurements in mountainous terrain in these inverse modeling studies represent a challenge because the atmospheric models used may not accurately represent the various influences on CO2 concentrations in these complex terrain areas. This is particularly due to a poor representation of atmospheric boundary layer processes over mountainous terrain in these models.

The first step towards inclusion of mountain locations in regional flux studies consists of obtaining a detailed understanding of the temporal variability of CO2 concentrations at these locations. At mountaintop locations around the world, such as Mauna Loa, Hawaii, and Zugspitze, Germany, a selection procedure is commonly performed on the data to filter out the local effects on concentration measurements. This requires a detailed understanding of characteristics of the temporal variability of CO2 concentration and the factors influencing the CO2 concentration.

Using data from the recently established Regional Atmospheric Continuous CO2 Network in the Rocky Mountains (Rocky RACCOON) network and a mesoscale model, the influence of atmospheric boundary layer air on CO2 concentrations at a mountaintop location is investigated. Idealized and realistic simulations show that a mesoscale model is able to capture the observed behavior of spatial and temporal CO2 variability and is able to reveal the responsible physical processes. The degree to which boundary layer air affects the CO2 concentration measurements is shown to be dependent on the boundary layer structure over the adjacent valleys and slopes including the strength and depth of the upslope flows. Using the results from the observations and the simulations, recommendations are given on the use of the mountaintop measurements in regional CO2 flux studies.