Identification of Free-Tropospheric Air at the Mount Bachelor Observatory

There are few observatories where free tropospheric air is sampled on a regular basis. The Mount Bachelor Observatory (MBO) is a new mountaintop site in central Oregon, U.S.A (44.0°N, 121.7 °W, 2763 m above mean sea level), where chemical, aerosol, and meteorological measurements are ongoing and the results from the first year of data (April 2004 - June 2005) are presented. During this first year, we are evaluating when and under what conditions the site samples free tropospheric (FT) air. To do this, monthly averaged water vapor mixing ratio (WV), temperature and wind speed from MBO are compared to rawinsondes from nearby Medford, Oregon, and monthly averaged ozone (O3) at MBO is compared with ozonesondes from Trinidad Head, California and Boulder, Colorado. These comparisons suggest that MBO samples air from the free troposphere as well as the boundary layer (BL) under specific meteorological conditions. Overall, the monthly mean WV and O3 values are 19% higher and 18% lower, compared to the sonde data at a similar altitude. Small diurnal cycles of these parameters suggest that daytime heating can mix BL air upward to the summit. By selecting hours with WV less than the monthly average rawindsonde data at the same altitude, we obtain a ``dry'' dataset that gives O3 values in closer agreement to the ozonesonde measurements. Additionally, these dry data include at least 5 long-range transport events from Asia which have significantly enhanced CO, O3, aerosols and mercury. Regional biomass burning events (n=3) were observed during the summer and fall, and displayed very different enhancement ratios between chemical species than Asian industrial pollution. Kinematic back trajectories show good agreement with our local meteorological observations in confirming a lack of BL-influence during periods of dry air. For real-time data from MBO, see the URL listed below.