Development and application of optical absorption spectroscopic measurement techniques for atmospheric measurements

Abstract

Absorption spectroscopy is a powerful tool for environmental study, especially in the field of atmospheric science. This technique can be applied in a wide range of measurement settings. This thesis focus on using optical absorption spectroscopic measurement techniques for atmospheric trace constituents detection and monitoring, including atmospheric trace gases and greenhouse gases. These measurement techniques cover a wide spectral range from the ultraviolet, visible all the way to the infrared wavelength range. Several different measurement techniques and experimental setups were developed and investigated. These measurement techniques include long-term open path measurement of atmospheric nitrogen dioxide (NO2) using a light emitting diode (LED) based long path differential optical absorption spectroscopy (LP-DOAS), mobile cavity enhanced differential optical absorption spectroscopy (CE-DOAS) measurement of NO2 spatial distribution over Hong Kong, passive scattered sunlight measurement of NO2 vertical column densities using multi-axis differential optical absorption spectroscopy (MAX-DOAS), in-situ measurement of atmospheric carbon dioxide (CO2) using dispersive infrared spectroscopy (DIRS) with scanning Fabry-Pérot interferometer (FPI) detector and aqueous state trace metal detection using LED based liquid waveguide broad wavelength band optical absorption spectroscopy. These measurement techniques cover the ultra-violet (UV), visible (Vis) and infrared (IR) wavelength range, measuring greenhouse gases and some major atmospheric pollutants in both gas and particle phase. Detailed description of the developments, improvement as well as the new applications of the measurement techniques are presented. Long-term measurements of atmospheric NO2 were performed by using a LED based long path DOAS instrument. Combined with chemical transport model simulations, the measurement results were used to validate satellite measurement. In addition, the measurement results were analyzed to identify different emission sources of NO2. The long path DOAS measurements were also combined with the mobile cavity enhanced DOAS measurements to investigate the spatial distribution of NO2 over Hong Kong. Vertical column densities of NO2 were measured by using several MAX-DOAS instruments located in Shanghai. The measured data were used to compare with satellite observations. The satellite data were then used for investigation of the spatial distribution of NO2 in Shanghai. The measurements were also used to evaluate the reduction of NO2 levels during the Expo 2010 Shanghai. The diurnal pattern of the MAX-DOAS NO2 measurements were also analysis for emission source identification. Measurement of atmospheric CO2 were achieved by using dispersive infrared spectroscopy (DIRS) with scanning Fabry-P´erot interferometer (FPI) detector. The developed DIRS system and retrieval algorithm features with auto correction for both non-linear effect of absorption and interference from other co-existing infrared absorbers. By using a micro electro mechanical system (MEMS) of FPI sensor, the physical size and the cost of the system were greatly reduced by the newly developed experimental setup and retrieval algorithm for easy field deployment. In addition, this new measurement technique is not limited to CO2 measurement by simultaneously including other infrared absorbing gas measurements, e.g. methane (CH4), carbon monoxide (CO), nitrous oxide (N2O) etc., which makes it an ideal option for applications of atmospheric gas monitoring and indoor air quality measurements. Aqueous state trace metal detection were performed by using LED based liquid waveguide broad wavelength band optical absorption spectroscopy. This proposed measurement technique solves the problem of the interference from the light source and other instrumental effect on the intensity levels. The developed data retrieval algorithm is possible to perform simultaneous measurement of multiple metal species by adding the reference absorption cross section spectrum of other metal species into the fitting algorithm. The development of the absorption spectroscopic measurement techniques demonstrated in the thesis provide alternative better options for the atmospheric trace constituent measurement. Further effort is still necessary to simplify the experimental setups in order to reach wider community. The measurement results presented in the thesis can be helpful for future air pollution and climate studies.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhi NING (Supervisor) & Mark Oliver WENIG (Supervisor)