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.