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Effect of Polyoxymethylene (POM-H Delrin) offgassing within Pandora head sensor on direct sun and multi-axis formaldehyde column measurements in 2016–2019
Citation:
Spinei, E., M. Tiefengraber, M. Müller, M. Gebetsberger, A. Cede, Luke Valin, J. Szykman, A. Whitehill, A. Kotsakis, F. Santos, N. Abbuhasan, X. Zhao, V. Fioletov, S. Lee, AND R. Swap. Effect of Polyoxymethylene (POM-H Delrin) offgassing within Pandora head sensor on direct sun and multi-axis formaldehyde column measurements in 2016–2019. Atmospheric Measurement Techniques. Copernicus Publications, Katlenburg-Lindau, Germany, NA(NA):NA, (2021). https://doi.org/10.5194/amt-14-647-2021
Impact/Purpose:
This article discusses a temperature-dependent bias in Pandora spectrometer measurements of formaldehyde made between 2016 and 2019. This is relevant to the expanding Pandora spectrometer network and influences the validity of the data acquired prior to the discovery of the issue. It was discovered that parts of the sensor were manufactured from a formaldehyde-derived polymer that decomposes to formaldehyde, producing large formaldehyde concentrations in the optical path and biasing the resulting formaldehyde column measurements. This is significant because it affects data that has been acquired (and was publicly available) between 2016 and 2019. This would be of interest to scientists and air quality managers who use Pandora formaldehyde column data.
Description:
Analysis of formaldehyde measurements by the Pandora spectrometer systems between 2016 and 2019 suggested that there was a temperature dependent process inside Pandora head sensor that emitted formaldehyde. Some parts in the head sensor were manufactured from thermal plastic polyoxymethylene homopolimer (E.I. Du Pont de Nemour & Co., USA: POM-H Delrin®) and were responsible for formaldehyde production. Laboratory analysis of the four Pandora head sensors showed that internal formaldehyde production had exponential temperature dependence with a damping coefficient of 0.0911 ± 0.0024 °C−1 and the exponential function amplitude ranging from 0.0041 DU to 0.049 DU. No apparent dependency on the head sensor age and heating/cooling rates was detected. The total amount of formaldehyde internally generated by the POM-H components and contributing to the direct sun measurements were estimated based on the head sensor temperature and solar zenith angle of the measurements. Measurements in winter, during cold days in general and at high solar zenith angles (> 75 °) were minimally impacted. Measurements during hot days and small solar zenith angles had up to 1 DU contribution from POM-H parts. Multi-axis differential slant column densities were minimally impacted (< 0.01 DU) due to the reference spectrum collected within a short time period with a small difference in head sensor temperature. Three new POM-free Pandora head sensors (manufactured in summer 2019) were evaluated for temperature dependent attenuation across the entire spectral range (300 to 530 nm). No formaldehyde or any other absorption above the instrumental noise was observed across the entire spectral range.
URLs/Downloads:
DOI: Effect of Polyoxymethylene (POM-H Delrin) offgassing within Pandora head sensor on direct sun and multi-axis formaldehyde column measurements in 2016–2019