Free tropospheric peroxyacetyl nitrate (PAN) and ozone at Mount Bachelor: potential 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 US Pacific Northwest. The measurements were made at the summit of Mount Bachelor (43.979 degrees N, 121.687 degrees W; 2.7 km a.s.l.) 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 summarize the interannual variability in three factors expected to drive PAN variability: biomass burning, transport efficiency over the central and eastern Pacific, and transport temperature. 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 O-3 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 induce greater variability in the atmospheric concentrations and thus extend the time needed for trend detection. We combined the observed variability in PAN and O-3 at Mount Bachelor with a range of possible future trends in these species to determine the observational requirements to detect such trends. Though the relative increase in PAN is expected to be larger than that of O-3, 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 O-3 is 1% per year, the trends in O-3 and PAN would be detected on approximately the same timescale.