环境介质中左旋葡聚糖记录与植被火燃烧变化研究进展

Vegetation fire, also called biomass burning, wildfire, landscape fire, bushfire, forest fire, scrub fire, crop residue burning, peat bog fire, savanna fire and grass fire, are spontaneous combustion of plants in a natural setting. Vegetation fires began soon after the appearance of terrestrial plants in the Silurian about 420 million years ago. The regime of vegetation fire has varied with long-term changes in global climate and shorter-term regional changes in climate and vegetation since then, and human beings have altered fire activity since the mid-Holocene, especially in recent years. Vegetation fire affects vegetation distribution, ecological structure, biogeochemical and hydrologic cycles, geophysical processes, and the climate system at local to global scales. Vegetation fire is one of the largest sources of trace gases and aerosols in the Earth’s atmosphere. It contributes more than half of black carbon and about 80% of primary organic aerosol worldwide. Besides, smoke aerosol from vegetation fire is composed of hundreds of chemical components, and many of which (e.g., polycyclic aromatic hydrocarbons) are known to be harmful to human health. Fire smoke causes more than 600000 premature deaths annually worldwide. Therefore, understanding the spatial-temporal changes of vegetation fire is of great importance. Using some specific biomarkers for tracing changes and emissions of vegetation fire has become a research focus in recent years. Among various chemical components, anhydro-sugars are mainly emitted by pyrolysis of cellulose and hemicellulose when burning temperature exceeds 300°C, whereas the contribution of fossil fuel emissions seems to be negligible. Levoglucosan accounts for more than 90% of anhydro-sugars in the nature. Levoglucosan has been reported with a lifetime longer than ten days under most atmospheric conditions, and thus could be transported far from the fire sources. The annual total levoglucosan emission is estimated at about 3.8 Tg (million ton) per year worldwide, however, when atmospheric degradation is considered, the atmospheric budget of levoglucosan is only about 1.9 Gg (kiloton) as calculated by the GEOS-Chem global 3-D chemical transport model. Eventually, levoglucosan is scavenged out the atmosphere to be deposited on earth surfaces by precipitation or gravitational processes. Its presence has been detected in various environmental medias including atmospheric aerosols, precipitation, lake/sea water, snowfall, glacier snow/ice, lake/marine sediments. High levoglucosan concentration/flux is usually found in middle and low latitudes where intense fire sources are nearby. In comparison, extreme low concentration/flux are found on highmountains, polar regions and marine sites where levoglucosan can only arrive by long distance transportation. Levoglucosan has been widely used as a biomarker for tracing fire emissions ranging from event to glacial-inter glacial timescales. For example, levoglucosan has been detected in lake sediments older than 430 ka years before present under anoxic environmental conditions. This work summarized the recent progress of levoglucosan records and vegetation fire changes in various environmental media. Levoglucosan in atmospheric aerosols can reflect fire emissions from event to seasonal scales, and levoglucosan records in glacial ice and lake/marine sediments can reveal fire evolution from seasonal to glacial-interglacial timescales. Overall, the current knowledge supports that levoglucosan should be an ideal biomarker for tracing vegetation fire changes on Earth’s surface. However, more evidences are needed in the near future to better understand its stability, interpret its records in ice core and sediments, as well as its relation with other proxies.