Development and application of Differential Optical Absorption Spectroscopy (DOAS) for Asian urban air monitoring and atmospheric research

Doctoral thesis, 2004

Urban air pollution is a serious concern in developing countries and there is a need for qualified measurements of a range of compounds on a nation-wide basis. A suitable potential technique for air monitoring is Differential Optical Absorption Spectroscopy (DOAS). DOAS systems are today commercially available, however to prices that hamper the widespread use of the technique in developing countries like China and Nepal. In 1999, a joint AGS (Alliance for Global Sustainability) project named “ChinaSky” was initiated with a 3-partner cooperation (Chalmers University, Fudan University, and Heidelberg University). This project aims at promoting the wide spread use of DOAS and establishing a network of high quality air monitoring stations in China at affordable cost. This dissertation deals with the development and application of instrumentation and measurement strategies based on DOAS techniques for Asian urban air monitoring and atmospheric research. One of the major tasks was to develop a long path DOAS instrument with high quality and reasonable price. The work comprises the technical details like design, selection, and testing of optical components, mechanical construction of the system, and software development. It also includes new instrument approaches such as the mode mixing by air turbulence to improve the residual noise of DOAS measurements, removal the pixel-to-pixel structure, and a method for aerosol measurement. In this work, two field campaigns have been carried out in Shanghai and Kathmandu, which aimed at investigating the air pollution levels and atmospheric chemistry processes. The first DOAS measurement of CS2, to our knowledge, has been demonstrated from industrial sources. This result gives a possibility of applications in geophysical research (e.g. CS2 emission from volcanoes). It also includes the first measurement of HONO in the atmosphere of Asian developing countries. Very high ratios of concentrations of HONO to concentrations of NO2 indicate the unusual chemical process in the Kathmandu atmosphere.The investigations of monocyclic aromatic hydrocarbons in Shanghai and Kathmandu give some interesting results on emission source distribution and atmospheric chemical processes. In addition, this work also includes demonstrations of two novel applications of DOAS: ship emission measurements by solar occultation DOAS and industrial emission measurements by zenith sky scattered light DOAS.