Atmospheric Methyl Bromide: Agricultural Source, Atmospheric Amounts and Variations

甲基溴(CH3Br2或MeBr2)在大气化学中很重要，它是平流层中溴原子的最大来源，也可能是自由对流层中溴原子的最大载体。大气中甲基溴的来源包括自然过程和工业过程，但比例尚不清楚。在人类甲基溴的使用中，农业植物前熏蒸是最大的应用，而且在世界范围内不断增长，但正在考虑禁止进一步使用甲基溴，以限制对臭氧层的破坏。这项研究的目标是获得关于大气甲基溴的定量信息--其来源、大气数量和变化。通过实地测量，将调查农田熏蒸作为潜在来源的情况。从这些土壤中逸出的甲基溴的通量将通过静态箱测量，而气态甲基溴的垂直剖面将在0到90厘米深的土壤中测量。这些现场熏蒸研究将作为实际商业熏蒸的一部分进行；每年将进行几次这样的实验，以便在使用甲基溴的大州加利福尼亚州的一系列实践中进行。大气总量将根据新的NIST绝对校准标准(独立于以前的标准)进行测量。加利福尼亚州和澳大利亚格里姆角的两个地点将每两周进行两次采样，以确定甲基溴的南北比例、季节变化以及通过一个农业山谷与太平洋海岸的差异。火焰电离气相色谱将用于现场熏蒸测量，GC质谱仪将用于高精度大气测量。低分辨率全球大气模式也将被用来研究地表源和混合比模式以及假设源和汇的作用。模型计算将使用扩展的2D模型和3D模型执行。这项研究的结果预计将改善甲基溴法规(或缺乏法规)的基础，并对我们对大气溴循环的理解做出重大贡献。

Methyl bromide (CH3Br, or MeBr) is important in atmospheric chemistry as the single largest source of Br atoms for the stratosphere and probably as the largest carrier of Br atoms in the free troposphere. Sources of atmospheric MeBr include both natural and industrial processes, but the proportions are not clear. Of human MeBr usages, agricultural pre-plant fumigation is the largest application and it is growing world wide but national and international bans on further usage are being considered so as to limit ozone-layer damage. The goals of this research are to obtain quantitative information on atmospheric MeBr - its sources, its atmospheric amounts and variations. Through field measurements, fumigation of agricultural fields will be investigated as a potential source. Fluxes of MeBr escaping from these soils will be measured through the use of static chamber, while vertical profiles of gaseous MeBr will be measured in the soil form zero to 90 cm depth. These field-fumigation studies will be performed as part of actual commercial fumigations; several such experiments will be performed yearly so as to span a range of practices in California, a large MeBr-using state. Atmospheric amounts will be measured against a new absolute NIST calibration standard (independent of previous standards). Two California sites and Cape Grim, Australia will each be sampled twice biweekly so as to determine the north/south ratio of MeBr, its seasonal variations and differences from the Pacific Coast through an agricultural valley. Flame-ionization gas chromatography will be used in field-fumigation measurements and a GC mass spectrometer will be used for high-precision atmospheric measurements. Low resolution global atmospheric models will also be used to investigate surface source and mixing ratio patterns and the roles of hypothesized sources and sinks. Model calculations will be performed with an extended 2D model and with a 3D model. The results of this research are expected to improve the basis for MeBr regulations (or the lack of them) and to contribute significantly to our understanding of atmospheric bromine cycling.