Carbon dioxide variability during cold front passages and fair weather days at a forested mountaintop site

This study describes temporal carbon dioxide (CO2) changes at a new meteorological site on a mountaintop in the Virginia Blue Ridge Mountains during the first year of measurements. Continental mountaintop locations are increasingly being used for CO2 monitoring, and investigations are needed to better understand measurements made at these locations. We focus on CO2 mixing ratio changes on days with cold front passages and on fair weather days. Changes in CO2 mixing ratios are largest during cold front passages outside the growing season and on clear, fair weather days in the growing season. 67% (60%) of the frontal passages during the non-growing (growing) season have larger postfrontal than prefrontal CO2 mixing ratios. The increase in CO2 mixing ratio around the frontal passage is short-lived and coincides with changes in CO and O-3. The CO2 increase can therefore be used as an additional criterion to determine the timing of frontal passages at the mountaintop station. The CO2 increase can be explained by an accumulation of trace gases along frontal boundaries. The magnitude and duration of the CO2 increase is affected by the wind speed and direction that determine the source region of the postfrontal air. Southward-moving fronts result in the largest prolonged period of elevated CO2, consistent with the postfrontal advection of air from the Northeastern United States where anthropogenic contributions are relatively large compared to other areas in the footprint of the mountaintop station. These anthropogenic contributions to the CO2 changes are confirmed through concurrent CO measurements and output from NOAA's CarbonTracker model. (C) 2011 Elsevier Ltd. All rights reserved.