Impact of assimilating CO2 observations from a mountaintop site in the eastern United States on regional carbon fluxes during cold fronts and fair weather days

Measurements of atmospheric CO2 are vital to estimating regional and continental CO2 sources and sinks. CarbonTracker, an inverse model developed by the NOAA Earth System Research Laboratory, assimilates observations from a network of CO2 mole fraction monitoring stations that each represent footprints generally larger than flux towers. Although CO2 measurements from mountaintop sites can provide important constraints on retrieved CO2 fluxes, they have only recently been integrated into the CarbonTracker system. The effect of assimilating such measurements on model performance is currently not well-known. In the present study, we investigate the effects of assimilating data from Pinnacles, a new mountaintop CO2 monitoring site in the eastern United States, on regional CO2 fluxes in CarbonTracker. We run the model for two one-month periods (December, 2008, and July, 2009) to investigate the sensitivity of CarbonTracker's global inversion system to CO2 mixing ratio measurements assimilated from Pinnacles during cold fronts and fair weather days, i.e. conditions when changes in CO2 due to frontal passage may be larger than changes due to flux from the biosphere. We find that CarbonTracker is more sensitive to the assimilation of Pinnacles CO2 measurements in July than in December. The assimilation of Pinnacles suggests greater CO2 uptake in the Midwest in July that occurs both during cold fronts and fair weather days.