Figure S2. Estimates of tracer background variability. The magnitude and variability of atmospheric tracer concentrations are determined by a mixture of background air plus underlying terrestrial and ocean surface influences. Surface influences (Figure 4 ) represent a significant but incomplete component of observed variability. The background component is typically calculated by sampling the end points of particle back trajectories, which consists of a mixture of air within the Hysplit domain and reaching the boundary (Figure S1 ), and taking the average across particles. The background can also be estimated by sampling air in the free troposphere (FT), representing the start point of HYSPLIT particels, which is primarily influenced by large scale advection, and less so by underlying regional surface exchange. This figure compares these two estimates (BL particle end points in solid, FT start points in markers) as sampled from atmospheric tracer concentration fields from GEOS-Chem/TM5-4DVAR (for COS), as a function of tracer (panel), season (x-axis), and region (color). Results using flask samples during fair weather days are shown in the top row, and using cold and warm air masses in the bottom row. The root mean squared error (RMSE) between the two estimates, averaged across seasons and regions, is provided within each panel. In general, the two approaches agree well at seasonal scale with lowest RMSE during fair weather days, suggesting that removing FT values provides a viable approach for estimating observed BL enhancements in the ACT-America data.