Application of Passive DOAS for Studies of Megacity Air Pollution and Volcanic Gas Emissions

Doctoral thesis, 2009

Optical Remote Sensing Techniques offer the opportunity to, in real-time, obtain line averaged measurements of gases in the lower and upper atmosphere. These techniques easily offer themselves to automatization, which increases their usability for e.g. risk-assessment of remotely situated volcanoes and routine monitoring of air pollution levels in urban areas. This thesis deals with the application of two instruments based on passive Differential Optical Absorption Spectroscopy (DOAS) for studying air pollution in Megacities, applied to México City and Beijing, and for monitoring of emissions of volcanic gases. The mobile mini-DOAS instrument is designed for emission measurements from a mobile platform whereas the other instrument used, the scanning mini-DOAS instrument is designed to perform automatic measurements from a fixed point. Measurements have been performed to estimate the production of SO2 and NO2 in the city of Beijing. The first measurement of HCHO using mobile mini-DOAS has been performed in Mexico City. The measured outflow of NO2 and HCHO has been compared with the results of a chemical model showing good agreement flux but a disagreement in the spatial distributions. A technique has been developed to determine the speed and altitude of a gas plume using a modified standard instrument. The possibility to reconstruct the distribution of SO2 inside a gas plume by combination of data from two or more scanning instruments has been demonstrated. An algorithm for calculating the flux of a gas plume from measurements using the novel conical scanning mini-DOAS instrument has been developed and a partial error sensitivity of the instrument has been performed and compared to the flat scanning mini-DOAS instrument. Several algorithms have been developed with the purpose of making emission measurements using the scanning mini-DOAS instrument fully automatic. The newly developed routines have been implemented in software, the NovacProgram. This software is currently used for routine monitoring of volcanic gas emission on 17 volcanoes around the world.