Characteristics and ecological significances of methyl halides emissions by terrestrial ecosystems
陆地生态系统卤甲烷释放特点及其生态意义

Methyl halides are the primary carriers of halogens between the biosphere and the atmosphere, and therefore play an important role in the effects of halogens on the chemistry of atmosphere, hence on the integrity of the ozone layer. Methyl bromide (CH 3Br) and methyl chloride (CH 3Cl) are both generated mainly from natural processes. These gases can reach the stratosphere where these halogen atoms can be released through photolysis and destroy ozone through catalytic reactions. Methyl iodide plays an important role in global iodide recycle and is replacing methyl bromide (CH 3Br), because CH 3Br is phasing out from being used as a fumigant. When the anthropogenic contributions to the emissions of methyl halides decrease, biogenic and other natural inputs will become the sole contributors to the global halogen radicals, and a steady state will eventually be reached. However, scientists are confused by the unclear budgets of CH 3Cl, CH 3Br and CH 3I in the atmosphere. The known sources of CH 3Br only account for 60% of the its known sinks. Only about half to two\|thirds of the known sinks for CH 3Cl can be balanced with known sources. CH 3I is required a more accurate estimating of both source and sink terms to fully explain its atmospheric budget. Some conclusions can be drawn from the previous works. The missing sources of the three gases are not oceanic origin. High emission of methyl halides from terrestrial ecosystems such as salt marshes, rice paddies, and tropical forests were identified recently. Together with other findings on methyl halides productions by plants, fungi and inorganic processes of soil, it shows that the natural processes of releasing methyl halides may play important roles in biological competition, metabolism and air pollution. Evidences for both biotic and abiotic mechanisms of organic halide production in terrestrial ecosystems were also introduced. Ozone recovery will be a long\|term process. The recommended fields for the study of the emission of methyl halides include the detection of unknown sources, the exploration of the mechanisms of emission, and the development of methods for assessment, prediction and reduction of the emission.