Carbon Dioxide in the Free Troposphere and Boundary Layer at the Mt. Bachelor Observatory

ABSTRACT

We use data for 2012–2014 from the Mt. Bachelor Observatory (MBO) in central Oregon to understand variations in carbon dioxide (CO₂) in the free troposphere (FT) and boundary layer (BL). The goals of this analysis are to identify and examine CO₂ in FT and BL air, events due to wildfires, the role of transport patterns, and the variation of CO₂ seasonally. For all seasons, we found that FT air has higher average CO₂ mixing ratios compared to BL air. FT air was most often seen during the night and early morning at MBO (20-8 PST) while BL air was most often observed during the afternoon and evening (12-20 PST). Winter and spring showed the highest mixing ratios of CO₂ while summer and fall showed the lowest mixing ratios. The maximum diurnal change in CO₂ was found during spring and summer. HYSPLIT back-trajectories and a cluster analysis of those trajectories were initiated for spring months. Based on this analysis, the spring clusters with the highest ozone and lowest water vapor mixing ratios were associated with the highest mixing ratios of CO₂. Four case studies of CO₂ variations are presented: a long-range transport event observed at MBO and three wildfire events. In one large fire event, CO₂ showed a large enhancement and was well correlated with CO. In another fire event, CO₂ was observed to decrease, suggesting that depletion in BL air by surface uptake can counteract the enhancements from wildfire emissions.