Abstract
Abstract. NASA Compact Airborne Formaldehyde Experiment (CAFE) is a nonresonant laser-induced fluorescence instrument for airborne in situ measurement of formaldehyde (HCHO). The instrument is described here with highlighted improvements from the predecessor instrument, COmpact Formaldehyde FluorescencE Experiment (COFFEE). CAFE uses a 480 mW, 80 kHz laser at 355 nm to excite HCHO and detects the resulting fluorescence in the 420–550 nm range. The fluorescence is acquired at 5 ns resolution for 500 ns and the unique time profile of the HCHO fluorescence provides measurement selectivity. CAFE achieves a 1σ precision of 160 pptv (1 s) and 90 pptv (10 s) for [HCHO] = 0 pptv. The accuracy of CAFE, using its curve-fitting data processing, is estimated as ±20 % of [HCHO] + 100 pptv. CAFE has successfully flown on multiple aircraft platforms and is particularly well-suited to high-altitude research aircraft or small air quality research aircraft where high sensitivity is required but operator interaction and instrument payload is limited.
Highlights
Formaldehyde (HCHO) in Earth’s atmosphere is primarily produced during the oxidation of volatile organic compounds (VOCs) including methane and isoprene, with minor direct emissions from biomass burning and fossil fuel combustion (Fortems-Cheiney et al, 2012)
In this paper we present the instrumental details of Compact Airborne Formaldehyde Experiment (CAFE), including modifications from the original COmpact Formaldehyde FluorescencE Experiment (COFFEE) design that stem from an improved understanding of the HCHO NR-laser-induced fluorescence (LIF) technique
Signal from each PMT is counted with 5 ns resolution by two channels of a NI 9402 high-speed digital I/O module, which is triggered by a transistor–transistor logic (TTL) pulse from the laser
Summary
Formaldehyde (HCHO) in Earth’s atmosphere is primarily produced during the oxidation of volatile organic compounds (VOCs) including methane and isoprene, with minor direct emissions from biomass burning and fossil fuel combustion (Fortems-Cheiney et al, 2012). A new approach, nonresonant LIF (NR-LIF), was recently developed to provide a more compact, lower-cost instrument with simpler operation than typical HCHO LIF instruments (St. Clair et al, 2017). St. Clair et al.: CAFE instrument for airborne in situ formaldehyde wavelength 355 nm laser that excites multiple HCHO absorption features. The NR-LIF HCHO instrument described in St. Clair et al (2017), Compact Formaldehyde FluorescencE Experiment (COFFEE), was designed for operation on the Alpha Jet stationed at NASA Ames Research Center and is not integrated into other aircraft platforms. The Compact Airborne Formaldehyde Experiment (CAFE) instrument described here uses the same general measurement approach as COFFEE but with updated components and design changes for operation under the demanding environmental conditions of the NASA ER-2 aircraft. In this paper we present the instrumental details of CAFE, including modifications from the original COFFEE design that stem from an improved understanding of the HCHO NR-LIF technique
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