Boundary-Layer Meteorol (2018) 168:81–102
https://doi.org/10.1007/s10546-018-0343-9
The Impact of the Afternoon Planetary Boundary-Layer
Height on the Diurnal Cycle of CO and CO
2
Mixing
Ratios at a Low-Altitude Mountaintop
Temple R. Lee
1,2
· Stephan F. J. De Wekker
1
·
Sandip Pal
1
Received: 20 December 2016 / Accepted: 6 February 2018 / Published online: 27 February 2018
© Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Mountaintop trace-gas mixing ratios are often assumed to represent free atmo-
spheric values, but are affected by valley planetary boundary-layer (PBL) air at certain times.
We hypothesize that the afternoon valley–PBL height relative to the ridgetop is important
in the diurnal cycle of mountaintop trace-gas mixing ratios. To investigate this, we use, (1)
4-years (1 January 2009–31 December 2012) of CO and CO
2
mixing-ratio measurements and
supporting meteorological observations from Pinnacles (38.61
◦
N, 78.35
◦
W, 1017 ma.s.l.),
which is a monitoring site in the Appalachian Mountains, (2) regional O
3
mixing-ratio mea-
surements, and (3) PBL heights determined from a nearby sounding station. Results reveal
that the amplitudes of the diurnal cycles of CO and CO
2
mixing ratios vary as a function of
the daytime maximum valley–PBL height relative to the ridgetop. The mean diurnal cycle for
the subset of days when the afternoon valley–PBL height is at least 400m below the ridgetop
shows a daytime CO mixing-ratio increase, implying the transport of PBL air from the valley
to the mountaintop. During the daytime, on days when the PBL heights exceed the moun-
taintop, PBL dilution and entrainment cause CO mixing ratios to decrease. This decrease in
CO mixing ratio, especially on days when PBL heights are at least 400 m above the ridgetop,
suggests that measurements from these days can be used as with afternoon measurements
from flat terrain in applications requiring regionally-representative measurements.
Keywords Carbon dioxide · Carbon monoxide · Mountaintop monitoring · Planetary
boundary-layer height
B
Temple R. Lee
temple.lee@noaa.gov
1
Department of Environmental Sciences, University of Virginia, 291 McCormick Road,
Charlottesville, VA 22904, USA
2
Present Address: NOAA ARL Atmospheric Turbulence and Diffusion Division and Cooperative
Institute for Mesoscale Meteorological Studies, 456 S. Illinois Avenue, Oak Ridge, TN 37830, USA
123
@Phil_Robichaud
@deepthiw
@JoseServera