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Detection of airborne chemical releases in densely populated urban environments requires precise sensors with high temporal and spatial resolution capable of covering large areas. For this purpose, we present a mobile mid-infrared quantum cascade laser dual-comb spectrometer for identification and quantification of chemical plumes. Field tests with the remote sensor were conducted during daytime in the downtown Boston area over a five day period during which chemical releases were simulated by intermittently emitting non-toxic substances. Open-air sensing was performed with retroreflectors positioned at up to 230 m distance and with sensitivities in the ppm m range for one second of averaging time. The field campaign demonstrates a step toward a semiconductor dual-comb spectroscopic sensor in the mid-infrared fingerprint region, suitable for long-term deployments. These types of sensors will be valuable complements to existing optical sensors for urban hazardous gas leak monitoring, air quality assessments, and localization of clandestine chemical production. [ABSTRACT FROM AUTHOR]

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 ‰ in 1 s, 0.02 ‰ in 60 s) and stability (drift in 1 h of<0.2 ‰), 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 CO2 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. [ABSTRACT FROM AUTHOR]