Real-time measurements of trace gases using a compact difference-frequency-based sensor operating at 3.5μm

The use of a compact gas sensor based on cw difference-frequency generation in periodically poled LiNbO3 for on-line absorption measurements of H2CO, CH4, and H2O near 3.5 μm is reported. Formaldehyde levels of 30 ppb, corresponding to absorptions of 2×10−4 have been measured using absorption spectroscopy. In this paper we report specifically the performance of this sensor as part of the 1997 Lunar–Mars Life Support Test program at the NASA Johnson Space Center. PACS: 07.88.+y; 42.62.Fi; 42.65.-k The monitoring and detection of trace gases at the level of parts per billion in diverse fields ranging from pollution and greenhouse-gas emission, to applications involving environmental control for the workplace and space habitats has become increasingly important. One such application in which we were involved, was the 90-day Lunar–Mars comprehensive life support test conducted at the NASA Johnson Space Center, Houston. The test was undertaken in a three-level 8-m diameter, human-rated space station simulation chamber, accommodating four mission specialists. One goal of this program was to test new life-support initiatives such as advanced air and water recycling technologies that are to be used for the international space station program. The purpose of our involvement in the test was to ascertain H2CO concentration levels inside the NASA test chamber with a portable and realtime gas sensor. The motivation for monitoring H2CO levels in a sealed, human-rated environment, is that its presence can cause headaches as well as throat and ear irritation at low concentrations (> 100 ppb). There are concerns about more serious adverse health effects at higher H2CO concentration levels. Consequently, NASA has set a stringent spacecraft maximum allowable concentration of 40 ppb for crew exposure from 7 to 180 days [1]. To reduce H2CO levels below this concentration, any outgassing materials and equipment must be identified. Hence, the development of an in situ, real-time, portable gas sensor capable of identifying H2CO emission sources and monitoring concentrations at sub-ppm levels in air was initiated. Further motivation for measuring H2CO concentrations precisely is that it is an important intermediate compound in tropospheric chemistry cycles, and serves as a significant source of CO in the natural troposphere with typical atmospheric levels of 300 ppt [2]. While this concentration level is about two orders of magnitude below the sensitivity limits for the monitoring apparatus described here, H2CO is also a significant byproduct of combustion devices and the present monitor is quite satisfactory for monitoring formaldehyde emissions from combustion. Accurate measurements of combustion emissions are thus important in the reduction of urban air pollution levels of formaldehyde. For real-time measurements of trace gases, optical techniques are the most suitable and include FTIRs [3], tunable infrared laser absorption spectroscopy using either overtone absorption spectroscopy in the near-IR [4, 5], or direct infrared absorption spectroscopy in the mid-IR [6, 7] and photoacoustic spectroscopy [3]. In the mid-IR, differencefrequency generation (DFG)-based sensors have been shown to be particularly suitable for absorption spectroscopy [8, 9]. The criteria that must be considered for a system to be effective as a trace-gas sensor include adequate sensitivity for the concentrations present, the ability to discriminate from any other gases present, and reliable field operation. Considering the monitoring of H2CO specifically, Fried et al. [10] reported a tunable diode laser absorption technique based on cryogenically cooled lead-salt diode lasers which achieved a H2CO detection sensitivity of 0.04 ppb. This is clearly a much higher sensitivity than is reported here, but it comes at the cost of cryogenic operation. Fourier transform infrared spectroscopy (FTIR) is a widespread laboratory and industrial monitoring technique, which might be made suitable for monitoring H2CO, but field FTIR spectrometers generally suffer from inadequate spectral resolution for this purpose. NASA monitors H2CO by using a chemical absorption badge capable of determining average concentrations over an extended period of time with a sensitivity ≈ 20 ppb. The badge is typically exposed for 24 h, and then processed using a wet chemical technique. The result obtained depends