The Influence of Orographic Flows on PICO-NARE Trace-Gas Measurements

The PICO-NARE station is located at a mountaintop on a remote island (Pico, Azores Islands, North Atlantic). Meteorological variables, atmospheric trace gases and aerosols are measured to study background concentrations and the frequency and impact of intercontinental transport of air pollution in the free troposphere (FT). This study evaluates the effects of upslope flows at this station. Orographic flows influence the origin of the airmasses sampled at the station. Mechanically forced lifting in strong winds or buoyant forced lifting in daytime and weak winds can cause upslope flow near the mountain which can potentially bring marine boundary layer (MBL) air to the station. To determine the origin of mechanically lifted air upstream of the mountain, a balance of upstream kinetic energy with potential energy as air flows over or around the mountain (Sheppard's model) was applied to upstream velocity and temperature profiles. The original height of an airmass was below the MBL height 25% of the time during July 2001 - December 2004, predominantly in the winter months (January through March). However, ozone mixing ratios at the summit during these periods were not significantly different than during FT episodes. Buoyant upslope flows were quantified through meteorological measurements on the mountain slope in summer 2004. Diurnal cycles consistent with daytime upslope and nighttime downslope flow on the mountain slope were found 28% of the time during July and August 2004. However, the water vapor mixing ratio was significantly smaller at the mountaintop than on the slope, indicating turbulent mixing during ascent or vertical decoupling of airmasses. Impacts of buoyant upslope flow on ozone or nitrogen oxides mixing ratios at the mountaintop station were rare or extremely small, and no clear diurnal cycle of ozone (expected if daytime upslope flow of MBL air occurred regularly) was present. The small size of Pico island, its high latitude, and the steep slope of the mountain seem to prevent frequent transport of MBL air to the PICO-NARE station.