Figure 7. Atmospheric
plane slices from NARR at 500 hPa at 18:00 UTC (a) 21 August 2013 (b) 30
August 2013. Vertical Cross sections at 90˚ W, 20-60˚ N 18:00 UT (c) 21
August 2013 (d) 30 August 2013. Blue contour lines show Potential
Vorticity (PV) 106 PVU, Relative Humidity (RH) below
30% is shaded from light to dark, with darker shades representing the
dryer air. The black lines on (a) and (b) are pressure height contours
in m. While, the black lines on (c) and (d) represent potential
temperature (isentropic surfaces) in K. Biomass burning sources tend to
have low PV and can have moderate RH levels for pyroCbs, while
stratospheric air masses have high PV and low RH. The blue dashed line
PV contour (1.5 PV) shows the approximate boundary between stratospheric
and tropospheric air masses. Arrows (green) show air-mass transport
patterns from Stratospheric-Tropospheric Transport (STT) and from
Biomass Burning (BB).