Nitrogen oxide (NOx=NO+NO2) observations were made at the Mt. Bachelor Observatory in central Oregon, USA (MBO; 2.73 km above sea level) during one autumn and three springtime (15 April–20 May) periods. This is the first study to discuss interannual variability in NOx for this region. NOx concentrations (mean1) for spring 2007, 2008 and 2009 were 11965, 11765, and 9154 pptv, respectively. The difference in mean mixing ratios between 2007 and 2008 is not statistically significant, whereas the difference between these years and 2009 is significant (p0.01). We attribute the decline in NOx from 2007–2008 to 2009 to changes in free tropospheric synoptic conditions over the Northeast Pacific and trans-Pacific transport pathways during spring 2009. In 2009, there were: (1) higher geopotential heights over the Gulf of Alaska, (2) warmer temperatures over the Aleutian Islands/Gulf of Alaska and (3) much weaker winds throughout the North Pacific. During the autumn 2008 campaign, NOx concentrations (mean1) were 175548 pptv. The highly non-normal distribution of data (skewness coefficient of 19.1 vs. 2.5, 2.8 and 2.4 in spring 2007, 2008 and 2009, respectively) resulted from periods of very high NOx levels. Using MODIS Rapid Response (Aqua and Terra) results, we show that during autumn our site can be heavily influenced by wildfires in western North America. This is in contrast to springtime, when the smaller positive (i.e., right) tail of the NOx distribution is driven largely by Asian long-range transport (ALRT) events. We developed a novel means of segregating boundary layer (BL)-influenced vs. free tropospheric (FT) air. During spring 2008 we collected "chairlift soundings" of temperature, relative humidity and pressure in an effort to better understand the diurnal pattern of a BL influence at our summit station. Results from this experiment revealed that, on average, a BL influence begins around 10:00 PDT (UTC – 07:00 h) in spring. Using this information to isolate FT air, we characterize probable pollution sources and synoptic conditions for the top 20 FT NOx events over three spring campaigns. Half (n=10) of these 20 events were determined to be "Imported" events characterized by anomalously: (1) high geopotential heights off the west coast of North America, (2) warm temperatures stretching from the Aleutian Islands to Baja California, and (3) strong southwesterly winds in the Asian outflow region. Five events exhibited an influence from the North American continent. These events are characterized by very strong cyclonic behavior off the northwestern USA coast.
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