Formic and acetic acids are ubiquitous in the troposphere.Although weak in acidity, they contribute significantly to the free acidity of the atmospheric environment, especially in remote regions (Fornaro and Gutz, 2003; Galloway et al., 1982), thus influencing the terrestrial ecosystem as well as the atmospheric reactions dominated by pH conditions (Prospero, 1985).They account for about 20% of volatile organic carbon in the troposphere (Loflund et al., 2002), and thus help reduce the hydroxyl (OH) in the atmosphere (Jacob, 1986), which is the most important oxidant of the greenhouse gases CH4 and CO (Keeling, 2004).As important cloud condensation nuclei (Yu, 2000), formic and acetic acids influence the amount and location of precipitation, and thus the regional climate.Furthermore, being prevalent corrosive reagents in the air, they play important roles in the rusting of metal equipment (Tennent and Cannon, 1993), corrosion of limestone material in buildings and statues (Tennent and Baird, 1985), as well as in the deterioration of paper documents in museums (Dupont and Tetreault, 2000).Due to the environmental importance, studies have long been performed toward understanding the biogeochemical cycles of the organic acids in the troposphere.