Upslope flows caused by mechanical forcing in strong synoptic winds or by buoyant forcing driven by solar heating under weak synoptic winds can influence the air composition at mountaintop observatories. Using meteorological and trace gas measurements at the PICO-NARE observatory on Pico mountain (Azores Islands, North Atlantic Ocean), the frequency and impact of such orographic flows on a small, volcanic, subtropical island was examined. To determine the origin of mechanically lifted air, upstream kinetic energy was balanced against potential energy gained during uplift (Sheppard's model). Mechanically forced upslope flow is most important during October through April, when the calculated probability of observing marine boundary layer (MBL) air at the observatory near the summit ranges from 35 to 60% per month. In contrast, lower synoptic wind speeds and a more stable lower free troposphere during May-September result in a reduced frequency of MBL impacts (