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

Mountaintop trace-gas mixing ratios are often assumed to represent free atmospheric 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 m a.s.l.), which is a monitoring site in the Appalachian Mountains, (2) regional O ₃ mixing-ratio measurements, 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 400 m 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 mountaintop, 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.