Long Open Path Fourier Transform Spectroscopy Measurements of Greenhouse Gases in the Near Infrared

Abstract:

Atmospheric composition measurements are an important tool to quantify local and regional emissions and sinks of greenhouse gases. Most in situ measurements are made at a point, but how representative are such measurements in an inhomogeneous environment? Open path Fourier Transform Spectroscopy (FTS) measurements potentially offer spatial averaging and continuous measurements of several trace gases (including CO₂, CH₄, CO and N₂O) simultaneously in the same airmass. Spatial averaging over kilometre scales is a better fit to the finest scale atmospheric models becoming available, and helps bridge the gap between models and in situ measurements. In this paper we assess the precision, accuracy and reliability of long open path measurements by Fourier Transform Spectroscopy in the near infrared from a 5-month continuous record of measurements over a 1.5 km pathlength.

Direct open-atmosphere measurements of trace gases CO₂, CH₄, CO and N₂O as well as O₂ were retrieved from several absorption bands between 4000 and 8000 cm-1 (2.5 – 1.25 micron). At one end of the path an in situ FTIR analyser simultaneously collected well calibrated measurements of the same species for comparison with the open path-integrated measurements. The measurements ran continuously from June – November 2014. We introduce the open path FTS measurement system and present an analysis of the results, including assessment of precision, accuracy relative to co-incident in situ measurements, reliability. Short term precision of the open path measurement of CO₂ was better than 1 ppm for 5 minute averages and thus sufficient for studies in urban and other non-background environments. Measurement bias relative to calibrated in situ measurements was stable across the measurement period. The system operated reliably with data losses mainly due to weather events such as rain and fog preventing transmission of the IR beam. In principle the system can be improved to provide longer pathlengths and higher precision, and we present recent progress in improving the original measurements.