Record High Free Tropospheric Ozone in the Spring of 2015 over the Western US and its Influence of Surface Air Quality

Abstract:

In the Western US (WUS) background ozone is a significant fraction of the health and regulatory thresholds. Several researchers have previously shown that background free tropospheric O₃ can significantly impact surface sites, including urban areas in the WUS (e.g. Jaffe 2011; Langford et al 2012; Lin et al 2012; Wigder et al 2013).

The Mt Bachelor Observatory (MBO) has measured ozone in the free troposphere since 2004. Between 2004-2014, the median springtime ozone mixing ratio was found to be 44 ppbv, with a positive trend of 0.7 ppbv per year (Gratz et al 2014). Superimposed on this trend are daily and inter-annual variations. Up until 2014, the spring of 2012 had the highest median ozone mixing ratio at MBO (48 ppbv) and this had significant influence on many surface sites across the WUS including, for example, Boise ID, Salt Lake City UT and Reno NV. Each of these sites demonstrated an unusual number of days in spring 2012 with an 8-hour maximum ozone mixing ratios greater than 70 ppbv. In 2012, this influence reflected an enhanced flux from the upper troposphere/lower stratosphere to the lower free troposphere and ultimately the surface.

In spring of 2015, we recorded our highest median ozone mixing ratio ever observed at MBO (56 ppbv). In contrast to 2012, enhanced ozone in the spring 2015 was associated with transport of emissions from Siberian wildfires. We expect that surface ozone at urban sites in the WUS will be significantly enhanced in the spring of 2015, as we saw in 2012.

The combined influence of increasing baseline ozone and interannual variations on top of this rising baseline means that surface ozone in the WUS will continue to be an increasing challenge for air quality managers. While observations at MBO are extremely useful in this regard, we propose that additional observations of baseline ozone would prove useful to better understand the influence of background ozone on urban air quality in the WUS.