Long-term continuous sampling of ¹²CO₂, ¹³CO₂ and ¹²C¹⁸O¹⁶O in ambient air with a quantum cascade laser spectrometer

A recently developed laser spectroscopic instrument allows real-time continuous measurements of the stable isotopologues of carbon dioxide at ambient concentrations. This compact instrument offers sufficient precision (0.2 per thousand in 1 s, 0.02 per thousand in 60 s) and stability (drift in 1 h of<0.2 per thousand), to allow isotopic studies on a variety of timescales and to study a variety of processes. During the development of the instrument, a prototype was set up to sample ambient air nearly continuously for more than 10 months, in a heterogeneous urban area northwest of Boston, MA. During this long sampling experiment, we continued to improve and modify the instrument and sampling system. In this paper, we present data collected during that long sampling experiment in order to demonstrate some of the possibilities provided by such real-time continuous monitoring. We have observed distinct isotopic signatures in CO(2) variations from timescales of seconds to seasons. We also present a method of performing continuous Keeling regressions on a cascade of timescales and show some results in application of that method to the continuous sampling data set.