Abstract
Abstract. The NASA In Situ Airborne Formaldehyde (ISAF) instrument is a high-performance laser-based detector for gas-phase formaldehyde (HCHO). ISAF uses rotational-state specific laser excitation at 353 nm for laser-induced fluorescence (LIF) detection of HCHO. A number of features make ISAF ideal for airborne deployment, including (1) a compact, low-maintenance fiber laser, (2) a single-pass design for stable signal response, (3) a straightforward inlet design, and (4) a stand-alone data acquisition system. A full description of the instrument design is given, along with detailed performance characteristics. The accuracy of reported mixing ratios is ±10% based on calibration against IR and UV absorption of a primary HCHO standard. Precision at 1 Hz is typically better than 20% above 100 pptv, with uncertainty in the signal background contributing most to variability at low mixing ratios. The 1 Hz detection limit for a signal / noise ratio of 2 is 36 pptv for 10 mW of laser power, and the e fold time response at typical sample flow rates is 0.19 s. ISAF has already flown on several field missions and platforms with excellent results.
Highlights
Formaldehyde (HCHO) is a ubiquitous constituent of the Earth’s atmosphere
HCHO is lost via photolysis and reaction with OH, with a typical daytime lifetime of 2–3 h
All hardware interfacing is handled with an all-solid-state CompactRIO (National Instruments) with realtime operating system and a field programmable gate array (FPGA)
Summary
Formaldehyde (HCHO) is a ubiquitous constituent of the Earth’s atmosphere. It is primarily produced during hydrocarbon oxidation, with an estimated source strength of 250 ± 54 Tg yr−1 (Fortems-Cheiney et al, 2012). HCHO is emitted through fuel combustion (Olaguer et al, 2009; Luecken et al, 2012), biomass burning (Yokelson et al, 2013) and vegetation (DiGangi et al, 2011) These sources are generally minor globally compared to secondary production, but they may be significant locally. The number of fluorescence photons per unit of laser power is proportional to the mixing ratio of HCHO in the sample gas This relationship is readily quantified with a known standard. The offline position is only 0.005 nm away from the peak This wavelength difference provides a high differential cross-section for HCHO, but the differential is negligible for species that have non-structured absorptions in this region. This allows for precise and selective measurements of HCHO. We have not identified any interfering species that has a measurable differential absorption and fluorescence between the online and offline positions
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