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首页> 外文期刊>Atmospheric chemistry and physics >Modelling marine emissions and atmospheric distributions of halocarbons and dimethyl sulfide: the influence of prescribed water concentration vs. prescribed emissions
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Modelling marine emissions and atmospheric distributions of halocarbons and dimethyl sulfide: the influence of prescribed water concentration vs. prescribed emissions

机译：模拟海洋排放物和卤代烃和二甲基硫的大气分布：规定水浓度与规定排放量的影响

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摘要

Marine-produced short-lived trace gases such as dibromomethane (CH2Br2), bromoform (CHBr3), methyliodide (CH3I) and dimethyl sulfide (DMS) significantly impact tropospheric and stratospheric chemistry. Describing their marine emissions in atmospheric chemistry models as accurately as possible is necessary to quantify their impact on ozone depletion and Earth's radiative budget. So far, marine emissions of trace gases have mainly been prescribed from emission climatologies, thus lacking the interaction between the actual state of the atmosphere and the ocean. Here we present simulations with the chemistry climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry) with online calculation of emissions based on surface water concentrations, in contrast to directly prescribed emissions. Considering the actual state of the model atmosphere results in a concentration gradient consistent with model real-time conditions at the ocean surface and in the atmosphere, which determine the direction and magnitude of the computed flux. This method has a number of conceptual and practical benefits, as the modelled emission can respond consistently to changes in sea surface temperature, surface wind speed, sea ice cover and especially atmospheric mixing ratio. This online calculation could enhance, dampen or even invert the fluxes (i.e. deposition instead of emissions) of very short-lived substances (VSLS). We show that differences between prescribing emissions and prescribing concentrations (28% for CH2Br2 to +11% for CHBr3) result mainly from consideration of the actual, time-varying state of the atmosphere. The absolute magnitude of the differences depends mainly on the surface ocean saturation of each particular gas. Comparison to observations from aircraft, ships and ground stations reveals that computing the air-sea flux interactively leads in most of the cases to more accurate atmospheric mixing ratios in the model compared to the computation from prescribed emissions. Calculating emissions online also enables effective testing of different air-sea transfer velocity (k) parameterizations, which was performed here for eight different parameterizations. The testing of these different k values is of special interest for DMS, as recently published parameterizations derived by direct flux measurements using eddy covariance measurements suggest decreasing k values at high wind speeds or a linear relationship with wind speed. Implementing these parameterizations reduces discrepancies in modelled DMS atmospheric mixing ratios and observations by a factor of 1.5 compared to parameterizations with a quadratic or cubic relationship to wind speed.
机译：海洋产生的短寿命痕量气体，例如二溴甲烷（CH2Br2），溴仿（CHBr3），甲基碘（CH3I）和二甲基硫醚（DMS），极大地影响了对流层和平流层化学。尽可能准确地在大气化学模型中描述它们的海洋排放，以量化其对臭氧消耗和地球辐射预算的影响。迄今为止，海洋中痕量气体的排放主要是根据排放气候规定的，因此缺乏大气的实际状态与海洋之间的相互作用。在这里，我们用化学气候模型EMAC（ECHAM5 / MESSy大气化学）进行模拟，并根据地表水浓度在线计算排放量，而不是直接规定排放量。考虑模型大气的实际状态会导致浓度梯度与海洋表面和大气中的模型实时条件一致，从而确定计算通量的方向和大小。这种方法具有许多概念上和实践上的好处，因为模拟的排放可以一致地响应海表温度，表面风速，海冰覆盖以及尤其是大气混合比的变化。这种在线计算可以增强，抑制甚至反转极短寿命物质（VSLS）的通量（即沉积而不是排放）。我们表明，规定的排放量与规定的浓度之间的差异（CH2Br2为28％，CHBr3为+ 11％）主要是由于考虑了大气的实际时变状态。差异的绝对大小主要取决于每种特定气体的表面海洋饱和度。与飞机，轮船和地面站的观测结果进行比较后发现，与根据规定的排放量进行计算相比，在大多数情况下，以交互方式计算海气通量会导致模型中更准确的大气混合比。在线计算排放量还可以有效测试不同的海气传输速度（k）参数，在此对八个不同的参数进行了测试。这些不同的k值的测试对于DMS尤为重要，因为最近发布的通过使用涡率协方差测量的直接通量测量得出的参数化建议在高风速下减小k值或与风速成线性关系。与与风速具有二次或三次关系的参数化相比，实施这些参数化可将模拟DMS大气混合比和观测值的差异减少1.5倍。

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