Remote Sensing of Glyoxal by Differential Optical Absorption Spectroscopy (DOAS): Advancements in Simulation Chamber and Field Experiments

摘要

Air pollution in many large cities is linked with the photochemical transformation of primary pollutants like VOCs (volatile organic compounds) and NO_x, which in the presence of sunlight foster the formation of secondary pollutants including ozone (O_3) and secondary organic aerosol (SOA) (Finlayson-Pitts and Pitts 2000; Molina and Molina 2002). 'Photochemical smog' has adverse effects on human health (Kunzli et al. 2000; Evans et al. 2002), the ecosystem (Middleton et al. 1950; Gregg et al. 2003) and regional climate (Lelieveld et al. 2001; Ramanathan and Crutzen 2003). While the crucial role of the atmospheric chemistry of VOCs in the formation of urban smog is well established, the performance of traditional indicators for VOC chemistry like O_3, O_x (sum of O_3 and nitrogen dioxide (NO_2)), formaldehyde (HCHO), and methylvinylketone (MVK) is affected by direct vehicle emissions, which contribute significantly to the atmospheric concentrations of these species; in the case of O_3, high concentrations of nitric oxide (NO) efficiently scavenge this gaseous pollutant during morning rush hour (Finlayson-Pitts and Pitts 2000). Hence, these trace gases may be ambiguous indicators for VOC chemistry in urban air, where the simultaneous presence of multiple pollutant sources complicates the development of robust control strategies to reduce O_3 and aerosol levels. Recently, glyoxal (CHOCHO) was detected for the first time directly in the atmosphere by means of differential optical absorption spectroscopy (DOAS) (Volkamer et al. 2005a). These time-resolved glyoxal measurements revealed atmospheric glyoxal concentrations to be essentially unaffected by vehicle emissions in Mexico City, and demonstrated that glyoxal is a unique indicator of fast VOC photochemical oxidation in urban air.