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.