Fourier-Transform Infra-Red (FTIR)

The Optical Techniques lab has been make solar absorption observations for the ACOM Foothills lab since 2010.  In the 2017-2018 time frame targeted funds were secured to make significant upgrades to the interferometer.  These provide for much reduced instrument noise levels, multi-channel recording and ease of operations.  The instrument is now a state of the art Bruker 125HR 600cm interferometer.  The site will become part of the global NDACC network this year and will be the only NDACC FTIR station in the continental US (CONUS).  The site has been and continues to be instrumental for fossil fuel emissions investigations.  Current work led by Dr. Ivan Ortega and supported by the ACOM modeling team, looks at several tropospheric species that have strong local emissions or effect air quality and ozone forming precursors.  Trends in weighted mean VMR along with our current best modeled estimates are given in Figure 1 where we see how well our current understanding is in the case of H2CO or HCN and where we need to improve emissions such as with HCOOH, C2H6 or CH4.  In  particular, the measurements show CH4 increasing over the last 8 years but not as rapidly as is modeled and a larger annual cycle pointing to more local sources or sinks not fully represented in the model.  Although biased slightly high the downward trend in CO is very similar to that at many stations.  To determine emissions sources de-seasonalized anomalies are calculated and shown in Figure 2 where we can readily see in particular high C2H6 from O&NG extraction that can coincide with high CH4 or H2CO and NH3 from feedlots both located to the northwest of Boulder.  Using CO and C2H6 which is solely sourced from O&NG extraction activities we calculate an emission factor of 1.67 tons/day.  Inferring local emissions of CH4 an extremely important and complex greenhouse gas becomes less precise as the accumulated emission ratio between CH4 and C2H6 is not constant.  Future work with university colleagues should help better define this.

FTIR Figure 1
Figure 1.  The retrieval process provides mixing ratio profiles of targeted trace gases. To maximize information content these are converted to a weighted mean mixing ratio for the lower troposphere.  Each panel shows the wVMR for all measurements in grey, daily average in red with the vertical bar representing the 1 sigma day range, a linear and annual cycle are given with the blue and green curves and a CAM-Chem full chemistry model with the yellow circles.
FTIR Figure 2
Figure 2.  To isolate emissions sources wVMR anomalies are calculated by subtracting the fits calculated in Figure 1.  Large episodic hydrocarbon and ammonia emissions originate from bovine feedlots and oil and natural gas extraction regions to the northwest, within 100 mi of our observation site in Boulder.  Using wind analyses, the majority of anomalously high measurements shown in red are tagged to these sources.