Characterization of Internal Formaldehyde Production within The Pandora Spectrometer Instrument

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Virginia Tech


Formaldehyde (HCHO), plays an important role in atmospheric chemistry and is an indicator of atmospheric oxidation capacity and surface ozone photo chemistry. The Pandora Spectrometer Instruments are deployed within the NASA/ESA sponsored Pandonia Global Network designed for satellite validation of various gases in atmosphere (e.g. ozone, nitrogen dioxide and formaldehyde). In addition, Pandoras are extensively used during national (e.g. DISCOVER-AQ, OWLETS, LISTOS) and international (CINDI, KORUS-AQ) field campaigns organized to better characterise air pollution and its distribution.

Recently it was discovered and shown in prior research conducted by (Spinei et al. 2020), that Pandora measurements of atmospheric HCHO are impacted by HCHO produced within the telescope assembly due to temperature dependent off-gassing from the Delrin® plastic components. The purpose of the research covered in this thesis is to provide a methodology to correct total HCHO vertical column densities measured during the past field campaigns. The methodology developed through the course of this thesis is first tested on the Pandora simulated measurements derived from the surface concentration HCHO observations during KORUS-AQ (2016) field campaign. The derived correction using synthetic data shows that the proposed methodology is accurate within 30%. The second part of the thesis characterizes heat transfer processes within the telescope assembly to estimate internal temperature as a function of ambient meteorological conditions. Considering that the Pandora instruments have mostly identical design of their telescope assemblies heat transfer coefficients derived from one pandora are expected to be applicable to all Pandoras. Convective heat transfer coefficients were derived at VT wind tunnel as a function of wind speed and telescope assembly position. Internally generated power was measured for several different instruments and averaged at 2.15pm0.38 W. Total long wave emissivity was calculated at 0.63. Surface absorptivities were estimated from the material properties. Semi-empirically derived model is proposed to estimate the internal temperature based on the heat transfer parameters, ambient temperature, relative humidity, solar flux, wind speed and wind direction. The correlation between the estimated and measured internal temperatures is 0.93 R^2.

Finally, the methodology is applied to the actual HCHO data collected during the KORUS-AQ campaign and the results are compared to concurrent in-situ measurements made aboard DC-8 aircraft for eight days in the months of May and June 2016.



DOAS, HCHO, Pandora