Occurrence of upslope flows at the Pico mountaintop observatory: A case study of orographic flows on a small, volcanic island

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 (<20%). Buoyant upslope flows (BUF) were quantified through meteorological measurements on the mountain slope in summer 2004. Diurnal cycles of wind direction on the mountain slope consistent with daytime upslope and nighttime downslope flow were found on 24% of the days during late June, July, and August 2004. Buoyant forcing can also occur in the presence of moderate synoptic winds, resulting in enhancement of the mechanically forced upslope flow on the windward side of the mountain. Such conditions were found on 15% of the summer days in 2004. However, on BUF days the specific humidity at the mountaintop was significantly smaller than on the slope, indicating turbulent mixing during ascent or vertical decoupling of air masses. Impacts of BUF or a mixture of buoyant and mechanical upslope flow on O3 or nitrogen oxides mixing ratios at the mountaintop station were rare or extremely small, and no significant diurnal cycle of O3 (expected if daytime BUF of MBL air occurred regularly) was present. Midday increases in isoprene concentrations, a nonmethane hydrocarbon (NMHC) expected to be emitted from vegetation more than 700 m below the PICO-NARE station, were found on 54% of the days during May– August 2005. No corresponding increase in n-butane (used for heating and cooking at sea level residences) was detected, suggesting that the air did not originate from as low as sea level. These results indicate that the latitude, size, and topography of Pico island combine to prevent frequent transport of MBL air to the PICO-NARE station in the summer.