Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
1680-7324
10
13
2010
10.5194/acp-10-6043-2010
http://www.atmos-chem-phys.net/10/6043/2010/
http://www.atmos-chem-phys.net/10/6043/2010/acp-10-6043-2010.html
http://www.atmos-chem-phys.net/10/6043/2010/acp-10-6043-2010.pdf
6043
6062
2010-07-06
Nitrogen oxides in the boundary layer and free troposphere at the Mt. Bachelor Observatory
D. R. Reidmiller
D. A. Jaffe
E. V. Fischer
B. Finley
Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA
Science & Technology Program, University of Washington-Bothell, Bothell, WA, USA
Nitrogen oxide (NO<sub>x</sub>=NO+NO<sub>2</sub>) 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 NO<sub>x</sub> for this
region. NO<sub>x</sub> concentrations (mean±1σ) for spring 2007,
2008 and 2009 were 119±65, 117±65, and 91±54 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 (<i>p</i><0.01). We attribute the decline in NO<sub>x</sub>
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, NO<sub>x</sub> concentrations (mean±1σ) were
175±548 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 NO<sub>x</sub> 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 NO<sub>x</sub> distribution is driven largely by Asian long-range
transport (ALRT) events.
<br><br>
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 NO<sub>x</sub> events over three spring
campaigns. Half (<i>n</i>=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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