Air pollution and global climate change are important environmental issues that affect our society. In order to address these problems and to develop control strategies, advanced detection techniques and deeper understanding of atmospheric chemistry are required. This presentation focuses on (1) developments and applications of cavity ringdown spectroscopy (CRDS) and cavity enhance absorption spectroscopy (CEAS) to monitor atmospheric trace gases and their reaction processes; and (2) application of photoionization mass spectrometry to investigate reactive intermediates in the ozonolysis reactions of alkenes.