Development, validation, and application of a high performance tunable diode laser absorption spectrometer for airborne formaldehyde measurements

A tunable diode laser absorption spectrometer (TDLAS) was modified to produce more precise, higher time resolution atmospheric formaldehyde (CH ₂O) measurements. Precisions of 15–50 pptv (1σ) were typically achieved in the field for 1 min of averaging, corresponding to absorbances of 0.5–1.7 × 10−6. These precisions are a factor of 10–15 better than achieved with the first field version of the instrument (1993), due largely to implementation of background subtraction, improved mechanical and thermal stability of the entire optical system, and better cell and laser dewar pressure control. Instrument precisions during airborne and groundbased field deployments were stable, except in the case of brief cabin pressure changes. Extensive field and laboratory tests found that CH₂O was efficiently transmitted through the TDLAS inlet under a range of conditions representative of the lower to mid troposphere. Instrument accuracy was verified by calibration cross checks, interference tests, and field comparisons with a DOAS system.; During the Texas Air Quality Study 2000 (TexAQS 2000), 1 and 10 s time resolution airborne CH₂O measurements were acquired over the Houston, Texas area. Throughout the campaign, the highest levels of CH₂O precursors and VOC reactivity were measured in petrochemical plumes. Correspondingly, CH₂O and ozone production were greatly enhanced in petrochemical plumes, compared with plumes dominated by power plant and urban (mobile source) emissions. Substantial direct CH₂O emissions from petrochemical facilities were not detected. The photochemistry of several isolated petrochemical facility plumes was accurately modeled using three non-methane hydrocarbons (NMHCs): ethene, propene (both anthropogenic), and a smaller amount of isoprene (biogenic), and was in accord with standard OH-initiated chemistry. Measurement-inferred facility emissions of ethene and propene were far larger than reported by inventories. The rapid production of CH₂O and ozone observed in a large, highly polluted plume (30+ ppbv CH₂O, 200+ ppbv ozone) originating over Houston was well replicated by a model employing only two NMHCs, ethene and propene.