Quantitative analysis of high accuracy greenhouse gas standards for ambient and emission levels, in support of air quality monitoring in South Africa

Abstract

The measurement of greenhouse gases (CO2, CH4, N2O and reactive gas CO) emitted in the environment is popular worldwide as these gas compounds influence global warming which poses risk in the society. The observed rise in the global temperature has been linked to the increase in the amount of greenhouse gases in the atmosphere. Therefore, the accurate measurement of atmospheric greenhouse gases is needed to understand the climate and various human activities. There is a greater demand for stable and internationally traceable gas standards with low uncertainty for accurate and comparable measurements of greenhouse gases worldwide. The need to accurately measure these gaseous pollutants is required to ensure quality air pollution monitoring. In this thesis, the development of greenhouse gas primary standard gas mixtures with amount fractions reflected in the atmosphere was carried out by static gravimetric method. The gas mixtures were prepared in different matrix such as N2 and artificial air (Ar + O2 + N2). Because it was the first time the greenhouse gas mixtures were prepared in artificial air at trace level, the gas mixtures were first prepared in N2 balance first to develop the method and accurately identify the peak of interest. The gas mixtures in artificial air were then prepared after the analysis of the gas mixtures in N2 balance. The development of these gaseous pollutants is tricky as the accuracy of these gas mixtures is affected by the impurities and matrix effect. Purity analysis of the starting materials or highly pure gas (N2, Ar, O2, CO2, CH4, N2O, and CO) was carried out to obtain accurate composition of the final gas mixture. Various analytical technique such as gas chromatography with pulse discharge helium detector, flame ionization detector, cavity ring-down and non - dispersive infrared spectroscopy was used. The N2 impurity in CO2, CH4, N2O and CO was quantified as 823.4 ± 82.3 µmol.mol-1, 237 ± 11.85 µmol.mol-1,16.5 ± 0.85 µmol.mol-1 and 1.05 ± 0.10 µmol.mol-1 respectively. A full purity table of each starting material, where the impurities in each starting material was quantified, is obtained under each paper discussed in this thesis. The methodology used in the development of the gas mixtures included several dilutions steps from the raw material to the trace amount fraction. In the final preparation step, the gas mixtures were then prepared in different matrix for accurate quantification. The greenhouse gas mixtures prepared for this study, were gravimetrically prepared at a nominal amount fraction of 380 ± 0.11 µmol.mol-1, 480 ± 0.13 µmol.mol-1, 800 ± 0.11 µmol.mol-1 for CO2, 2 ± 0.007 µmol. mol-1 for CH4, 330 ± 0.032 nmol.mol-1 for N2O, 5 ± 0.0030 µmol.mol-1 and 0.350 ± 0.0050 µmol.mol-1 where the standard uncertainty lies at k=1 coverage factor. After gravimetric preparation, the metrological characterization of the primary standards was conducted to verify the gravimetric values of greenhouse gas mixtures. The gas chromatography, cavity ring-down and non-dispersive infrared spectroscopy were the methods used to verify the amount fraction of the gas mixtures. This was confirmed by comparison method where the gas mixture with similar amount fraction were compared against each other following a single point calibration method. A linear calibration curve of the gas chromatography and the two-spectroscopic technique used was also checked. Based on the analytical response of the instrument, the measurement criteria were set to meet the metrological targets of less than 1% of the instrumental drift. For the gas chromatography analysis, the parameters such as precision, sensitivity, instrumental drifts, relative deviation between the analytical and gravimetric values were checked. Meanwhile, for the spectroscopic methods linearity, reproducibility and precision of the analytical instrument were evaluated. The prepared standards in artificial air were then applied to check the amount of CO2, CH4, and N2O in the background air samples. Only N2O standard was also used to check the amount of N2O in the filter air samples. The amount of CO in the background air samples was not checked as CO content is less than the prepared value of 0.350 µmol.mol-1. The CO2 contents in the background air samples was quantified as (299.06, 419.91, 403.02 and 425.02) µmol.mol-1. The analyzed background air samples is reportedly within the gravimetric values of the CH4 in artificial vi air standards. The value of N2O quantified in the background air samples ranged from 329 to 330 nmol.mol-1. The N2O content in the filtered air was obtained as 135 nmol.mol-1. A good accuracy of less than 1.2% was obtained between the gravimetric and the analytical results for each gas standards. The greenhouse gas standards were certified with a final amount fraction of 380 ± 2.00, 480 ± 1.61, 800 ± 1.03 µmol.mol-1 for CO2, 2.00 ± 0.042 for CH4, 330 ± 0.066 nmol.mol-1 for N2O and 0.350 ± 4.00 µmol.mol-1 for CO where the combined uncertainty was estimated as expanded uncertainty (2δ) at 95% confident level. In this study, the greenhouse gas primary standards were successfully developed and used to identify and quantify the greenhouse gases present in the background and filtered air samples.