Discoveries from the Mt. Bachelor Observatory: Findings from 6 years of observations (2004-2009)

The Mt Bachelor Observatory (MBO: 43.979N, 121.687W, 2.73 km ASL) in central Oregon has observed a variety of pollution plumes in the free troposphere (FT) since it was established in 2004. We present results from the entire 2004-09 dataset. First, we isolated times when FT air was sampled as determined by a climatological analysis of water vapor (WV) from the twice-daily Medford, OR ( 42.36°N, 122.86°W, 405 m ASL) soundings. We found there to be no statistically significant difference (p<0.05) between mean WV values during the 0 UTC and 12 UTC soundings in any season. Using this Medford sounding data, we retained a "FT dataset" from MBO that had the same WV distribution as the Medford soundings at 730 hPa (the approximate pressure altitude of MBO). This resulted in 53-66% of the MBO data being retained as part of a FT dataset, depending on season, with 36-58% of those times occurring during daylight hours and 32-56% occurring overnight.

There are statistically significant differences in the means of the FT vs. ALL datasets for almost all chemical and meteorological parameters measured, with the most significant differences occurring during spring and summer. We present results from an event analysis that quantified the frequency and most likely season for the three main types of pollution events sampled at MBO: (1) Asian long-range transport, (2) upper troposphere / lower stratosphere intrusions and (3) local or regional biomass burning. Our study shows that MBO consistently samples the FT no matter the season and over the 6 years since its inception has observed dozens of long-range pollution transport events.