Anbar A D, Yung Y L, Chavez F P
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA.
Global Biogeochem Cycles. 1996 Mar;10(1):175-90. doi: 10.1029/95gb02743.
The oceans play an important role in the geochemical cycle of methyl bromide (CH3Br), the major carrier of O3-destroying bromine to the stratosphere. The quantity of CH3Br produced annually in seawater is comparable to the amount entering the atmosphere each year from natural and anthropogenic sources. The production mechanism is unknown but may be biological. Most of this CH3Br is consumed in situ by hydrolysis or reaction with chloride. The size of the fraction which escapes to the atmosphere is poorly constrained; measurements in seawater and the atmosphere have been used to justify both a large oceanic CH3Br flux to the atmosphere and a small net ocean sink. Since the consumption reactions are extremely temperature-sensitive, small temperature variations have large effects on the CH3Br concentration in seawater, and therefore on the exchange between the atmosphere and the ocean. The net CH3Br flux is also sensitive to variations in the rate of CH3Br production. We have quantified these effects using a simple steady state mass balance model. When CH3Br production rates are linearly scaled with seawater chlorophyll content, this model reproduces the latitudinal variations in marine CH3Br concentrations observed in the east Pacific Ocean by Singh et al. [1983] and by Lobert et al. [1995]. The apparent correlation of CH3Br production with primary production explains the discrepancies between the two observational studies, strengthening recent suggestions that the open ocean is a small net sink for atmospheric CH3Br, rather than a large net source. The Southern Ocean is implicated as a possible large net source of CH3Br to the atmosphere. Since our model indicates that both the direction and magnitude of CH3Br exchange between the atmosphere and ocean are extremely sensitive to temperature and marine productivity, and since the rate of CH3Br production in the oceans is comparable to the rate at which this compound is introduced to the atmosphere, even small perturbations to temperature or productivity can modify atmospheric CH3Br. Therefore atmospheric CH3Br should be sensitive to climate conditions. Our modeling indicates that climate-induced CH3Br variations can be larger than those resulting from small (+/- 25%) changes in the anthropogenic source, assuming that this source comprises less than half of all inputs. Future measurements of marine CH3Br, temperature, and primary production should be combined with such models to determine the relationship between marine biological activity and CH3Br production. Better understanding of the biological term is especially important to assess the importance of non-anthropogenic sources to stratospheric ozone loss and the sensitivity of these sources to global climate change.
海洋在甲基溴（CH3Br）的地球化学循环中起着重要作用，甲基溴是破坏臭氧层的溴向平流层输送的主要载体。每年海水中产生的CH3Br数量与每年从自然和人为来源进入大气的量相当。其产生机制尚不清楚，但可能是生物作用。大部分CH3Br在原位通过水解或与氯化物反应而被消耗。逃逸到大气中的部分的大小难以确定；海水和大气中的测量结果被用来支持向大气输送大量海洋CH3Br通量以及海洋净吸收量小的观点。由于消耗反应对温度极其敏感，温度的微小变化会对海水中CH3Br的浓度产生很大影响，进而影响大气与海洋之间的交换。CH3Br的净通量对CH3Br产生速率的变化也很敏感。我们使用一个简单的稳态质量平衡模型对这些影响进行了量化。当CH3Br产生速率与海水叶绿素含量呈线性比例关系时，该模型再现了Singh等人[1983年]和Lobert等人[1995年]在东太平洋观测到的海洋CH3Br浓度的纬度变化。CH3Br产生与初级生产之间明显的相关性解释了两项观测研究之间的差异，强化了最近的观点，即开阔海洋是大气CH3Br的一个小的净吸收源，而不是一个大的净源。南大洋被认为可能是大气中CH3Br的一个大的净源。由于我们的模型表明大气与海洋之间CH3Br交换的方向和幅度对温度和海洋生产力都极其敏感，并且由于海洋中CH3Br的产生速率与该化合物被引入大气的速率相当，即使温度或生产力的微小扰动也会改变大气中的CH3Br。因此，大气中的CH3Br应该对气候条件敏感。我们的建模表明，假设人为源占所有输入的不到一半，气候引起的CH3Br变化可能大于人为源小幅度（±25%）变化所导致的变化。未来对海洋CH3Br、温度和初级生产的测量应与此类模型相结合，以确定海洋生物活动与CH3Br产生之间的关系。更好地理解生物因素对于评估非人为源对平流层臭氧损耗的重要性以及这些源对全球气候变化的敏感性尤为重要。
