Solar Absorption Spectroscopy at the Dalhousie Atmospheric Observatory

Abstract

A newly refurbished high-resolution Fourier Transform Spectrometer (FTS) has been installed at the Dalhousie Atmospheric Observatory (DAO) to monitor the outflow of trace gases leaving North America. Regular solar absorption measurements began in June 2011 and trace gas concentrations are determined from the observed spectra via an optimal estimation technique using the standard SFIT4 software package. Significant software development was required to match the FTS output to the software and it was also necessary to tailor the ancillary data inputs to the instrument and location.

The development of a custom-built open-source high-accuracy solar tracker to support the FTS is described. The Community Solar Tracker (CST) can be remotely operated and runs autonomously with a tracking accuracy < 10 arcseconds. Versions of the CST are now operating successfully at a variety of different locations including a high-Arctic research station near Eureka, Nunavut. The high accuracy and comprehensive monitoring possible with this tracker has revealed previously undetected alignment issues with the FTS-tracker combination, but solutions to these issues have been found and are being implemented.

In July 2011, the DAO participated in the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) campaign, and detected biomass burning plumes originating from central Canada. This campaign also involved the Portable Atmospheric Research Interferometric Spectrometer (PARIS), a lower-resolution FTS instrument, which provided a means to validate our trace gas columns. Total column CO differences were < 4% during the extended BORTAS campaign and a significant pollution plume detected in August 2011 is analysed to show the impact of precipitation history on plume composition.

Finally, three years of measurements of CO, C2H6, and CH4 are presented and analyzed showing the expected seasonal trends, as well as an additional summertime enhancements in CO and C2H6 caused by seasonal wildfire emissions being transported across North America and out over the Atlantic Ocean.