Free Tropospheric Peroxyacetyl Nitrate (PAN) and Ozone at Mount Bachelor: Causes of Variability and Timescale for Trend Detection
We report on the first multi-year springtime measurements of PAN in the free troposphere over the U.S. Pacific Northwest. The measurements were made at the summit of Mount Bachelor (43.979°N, 121.687°W; 2.7 km ASL) by gas chromatography with electron capture detector during spring 2008, 2009, and 2010. This dataset provides an observational estimate of the month-to-month and springtime interannual variability of PAN mixing ratios in this region. Springtime seasonal mean (1 April – 20 May) PAN mixing ratios at Mount Bachelor varied from 100 pptv to 152 pptv. The standard deviation of the three seasonal means was 28 pptv, 21% of the springtime mean. We focus on three factors that we expect to drive PAN variability: biomass burning, transport efficiency over the central and eastern Pacific, and transport temperature. There was an early and unusually strong fire source in southeastern Russia in spring 2008 due to early snow melt, and several fire plumes were observed at Mount Bachelor. Colder air mass transport from higher altitudes in April 2009 is consistent with the higher average PAN mixing ratios observed at MBO during this month. A trough located off the U.S. Pacific Northwest coast in April 2010 caused reduced transport from the north in spring 2010 as compared to previous years. It also facilitated more frequent transport to Mount Bachelor during spring 2010 from the southwest and from lower elevations. Zhang et al., (2008) used the GEOS-Chem global chemical transport model to show that rising Asian NOx emissions from 2000 to 2006 resulted in a relatively larger positive trend in PAN than O3 over western North America. However the model results only considered monotonic changes in Asian emissions, whereas other factors, such as biomass burning, isoprene emissions or climate change can complicate the atmospheric concentrations. We combined the observed variability in PAN and O3 at Mount Bachelor with a range of possible trends in these species to determine the observational requirements to detect the trends. Though the relative increase in PAN is expected to be nearly four times larger than that of O3, PAN is more variable. If PAN mixing ratios are currently increasing at a rate of 4% per year due to rising Asian emissions, we would detect a trend with 13 years of measurements at a site like Mount Bachelor. If the corresponding trend in O3 is 1% per year, the trends in O3 and PAN should be detected on approximately the same timescale.