The determination of carbon dioxide flux in the atmosphere using atmospheric pressure ionization mass spectrometry and isotopic dilution

Brendan G. DeLacy

doi:10.17918/etd-868

An atmospheric pressure ionization mass spectrometry/isotopically labeled standard (APIMS/ILS) method has been developed for the determination of carbon dioxide (CO₂) flux in the atmosphere. Initial method development involved the enhancement of the APIMS/ILS sensitivity for CO₂. This work included the optimization of ion optics, ionization chemistry, and flow rates. Additionally, the impact that analog and counting mode detectors had on sensitivity was explored. This led to an investigation of how detector signal was amplified. A novel analog amplifier was created in-house and subsequently evaluated. The analog amplifier made for the negative ion mode of detection, first converted detector current to voltage, and subsequently converted voltage to frequency. The analog mode of signal amplification was then compared to a pulse generating amplifier. It was concluded through a series of experiments that analog amplification in conjunction with an analog multiplier produced higher signal to noise ratios and increased the life of the detector. The newly developed APIMS/ILS method was used to determine CO₂ flux over an open field in Lewes, Delaware during August and September of 2004. The CO₂ flux was calculated using the gradient flux method. CO₂ fluxes of -1,879 to +4,925 mol m-2 yr-1 were obtained during the study. Additionally, negative flux values were obtained during the late afternoon and positive flux values were obtained in the early morning to early afternoon hours, exhibiting a diurnal variability in flux. The magnitude of the fluxes and the diurnal variability in flux were consistent with results previously observed in the literature. The last phase of research involved the derivation of an equation describing errors in CO₂ measurements that are introduced by dryers. Formulas quantifying this error were derived from the Ideal Gas Law, the conservation of mass, and Dalton's Law. Laboratory experiments were conducted to verify the errors created solely by the dryer in CO₂ concentration measurements post-dryer. The atmospheric pressure ionization mass spectrometry/isotopically labeled standard (APIMS/ILS) method was used to detect CO₂ post-dryer. The errors observed in these laboratory experiments verified the theoretical errors predicted in the derived equations. Real fast water vapor data obtained from research flight #7 of the DYCOMS II field campaign were also used to evaluate the effect. This data analysis revealed systematic errors in CO₂ concentration measurements and CO₂ flux measurements when a dryer is used. Such errors can be avoided by using the APIMS/ILS technique which measures mixing ratios.

The determination of carbon dioxide flux in the atmosphere using atmospheric pressure ionization mass spectrometry and isotopic dilution

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The determination of carbon dioxide flux in the atmosphere using atmospheric pressure ionization mass spectrometry and isotopic dilution

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