Characterizing Diurnal CO2 Cycles in the Continental Boundary Layer Using Precise Concentration Measurements and a Simple Numerical Model
Torn, M.S.(a), Riley, W.(a), Rischer, M.L.(a), Biraud, S.(a), and Berry, J.(b), Lawrence Berkeley National Laboratory (a), Carnegie Institution of Washington (b)
Fourteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting
In continental regions, atmospheric CO2 profiles are strongly influenced by atmospheric dynamics as well as ecosystem and anthropogenic fluxes. Relating site level measurements or atmospheric profiles to regional CO2 budgets may require methods to represent or evaluate these influences. At the Southern Great Plains ARM-CART, we are measuring precise CO2 concentrations continuously at 2-60 m and weekly at 300 and 3300 m agl. CO2 flux is measured in individual fields (4 m towers) and at 60 m. The precise CO2 concentrations show strong continental influence in both diurnal and seasonal cycles. There is a vertical gradient in CO2 concentration, with annual average CO2 concentration at 2 m being almost 7 ppm higher than at 60 m, and flasks showing a 1-2 ppm difference between free troposphere and 60 or 300 m samples. To characterize the continental, diurnal cycle in CO2, we have made a box model with three well mixed compartments for the surface layer, boundary layer, and free troposphere. We have precise CO2 measurements in all three compartments. Input data were limited to surface CO2 flux, horizontal wind speed, and PBL height (estimated from discontinuity in potential temperature and vapor profiles measured by ARM radiosonde 4-8 times daily). The characteristic mixing times were tuned to the CO2 data for limited time periods and model performance was evaluated against independent time periods. The model shows that the diurnal cycle in continental regions has three phases, distinguished by different driving mechanisms and trajectories of CO2 concentrations. First, CO2 buildup in the evening follows cessation of photosynthesis and increasing atmospheric stability. Second, the rapid decrease in atmospheric concentrations after sunrise is largely abiotic, driven by atmospheric mixing. Finally, the gradual draw-down of concentrations mid-day is determined by both net ecosystem uptake and the growing boundary layer. For longer time scales, the mass balance requires consideration of fair weather subsidence and consequent dilution of ABL air with air from the free troposphere. In summary, we are able to recreate the measured diurnal structure with a simple numerical model that is driven by regional CO2 flux, PBL height, and horizontal wind speed. We will use this model to evaluate the sensitivity of the diurnal structure to these factors.
Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).
