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Measurement of atmospheric formaldehyde with a diffusion scrubber and light‐emitting diode–liquid‐core waveguide based fluorometry

A new automated instrument for the near-real-time measurement of atmospheric formaldehyde is described. The chemistry involves the cyclization reaction of formaldehyde with 1,3-cyclohexanedione (CHD) In the presence of ammonium ions to form a fluorescent dihydropyridine derivative. A GaN-based light-emitting diode (LED) emitting in the near UV was used as the excitation source in a miniature flowthrough fluorescence detector based on a transversely illuminated liquid-core waveguide. The Instrument is configured to operate in a periodic autozero mode where the exhaust from the sampling pump is chemically treated to provide zero gas for automated periodic checks of the baseline. The liquid-phase portion of the system provides a S/N = 3 limit of detection (LOD) of 10-nM aqueous formaldehyde. A thermostated Nafion®-membrane-based diffusion scrubber is used to collect atmospheric formaldehyde into pure water with an absorption efficiency of ∼70%, which results in an LOD of 30 pptv HCHO. (In cases where the H 2 O 2 to HCHO ratio is very high, as in background polar atmospheres, the LOD will deteriorate markedly.) Design, performance details, and illustrative results from a 1999 field campaign (Atlanta Supersite Study) are presented. Interference from H 2 O 2 is discussed in detail.