Methane photochemistry on Neptune : ethane and acetylene mixing ratios and haze production
Methane photochemistry on Neptune : ethane and acetylene mixing ratios and haze production
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海王星上的甲烷光化学：乙烷和乙炔的混合比例和雾霾的产生
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发表时间：
1993
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通讯作者：
S. Atreya
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作者：
P. Romani;J. Bishop;B. Bézard;S. Atreya
摘要 我们使用一维甲烷光化学模型来分析航行者号对海王星平流层中碳氢化合物和雾霾的观测结果。 Vayager IRIS 光谱提供有关 p > 0.1 mbar 时的全球平均 C 2 H 2 和 C 2 H 6 混合比的信息。除了C 2 H 2 和C 2 H 6 之外，UVS 光曲线还对CH 4 和C 2 H 4 提供约束，但仅限于掩星纬度和较低压力。模型预测的碳氢化合物对涡流扩散系数 (K) 的高度剖面非常敏感。对于这两个数据集，K 与大气数密度成反比变化到一定程度会产生较差的结果。与数据的良好一致性要求 K 在平流层下部较弱（对于 p⪊2 mbar，K ≃ 2 × 10 3 cm 2 sec -1），但在平流层上部相当活跃（对于 p⪉ 0.5 mbar，K > 5 × 10 7 cm 2 sec -1），即，快速混合的上部平流层覆盖在停滞的下部平流层上，其间有快速过渡。模型C 2 H 6 和C 2 H 2 混合比也对C 2 H 4 + H 和CH 3 + C 2 H 3 的反应速率常数敏感。值得注意的是，我们必须使用当前的 C 2 H 4 + H 速率上限，以使模型结果与观测结果最佳拟合。我们能够很好地再现 IRIS C 2 H 2 和 C 2 H 6 发射特征，但 UVS 掩星光曲线则较差。由于甲烷光解产生的C 2 H 2 、C 2 H 6 和其他碳氢化合物是通过冰雾形成和沉积从平流层输送出来的，因此我们将模型雾霾预测与PPS和IRIS观测结果进行了比较。对于太阳最大通量（航行者号遭遇条件），模型批量生产率为 1 × 10 -14 g cm 2 sec -1 。 C 2 H 6 是主要的雾度成分(75%)，其余部分来自C 2 H 2 以及C 3 和C 4 化合物。通过 0.25 μm 半径颗粒的沉降速率（PPS 观测的颗粒半径上限）平衡上述雾度产生速率，产生略高于 PPS 上限的总雾度柱负荷。然而，寿命分析表明，模型雾霾产生率应在太阳最小和最大条件下进行平均。在这些条件下，模型雾度密度与 PPS 数据一致。预测的 C 4 H 2 和 C 2 H 6 雾柱密度与 IRIS 光谱中缺乏冰特征一致。
Abstract We have used a one-dimensional methane photochemical model to analyze Voyager observations of hydrocarbons and hazes in the stratosphere of Neptune. Vayager IRIS spectra provide information about the global average C 2 H 2 and C 2 H 6 mixing ratios for p > 0.1 mbar. The UVS lightcurves provide constraints on CH 4 and C 2 H 4 in addition to C 2 H 2 and C 2 H 6 but only at the solar occultation latitudes and for lower pressures. The model-predicted hydrocarbons are very sensitive to the height profile of the eddy diffusion coefficient ( K ). For both data sets K varying inversely with the atmospheric number density to some power produced poor results. Good agreement with the data requires that K be weak in the lower stratosphere ( K ≃ 2 × 10 3 cm 2 sec -1 for p⪊2 mbar) but fairly vigorous in the upper stratosphere ( K > 5 × 10 7 cm 2 sec -1 for p⪉ 0.5 mbar), i.e., a rapidly mixed upper stratosphere overlying a stagnant lower stratosphere with a rapid transition in between. The model C 2 H 6 and C 2 H 2 mixing ratios are also sensitive to the reaction rate constants of C 2 H 4 + H and CH 3 + C 2 H 3 . Notably, we must use the present upper limit for the C 2 H 4 + H rate to best fit the model results to the observations. We are able to reproduce the IRIS C 2 H 2 and C 2 H 6 emission features well, less so the UVS occultation lightcurves. Since the transport of C 2 H 2 , C 2 H 6 , and other hydrocarbons produced from methane photolysis out of the stratosphere is by ice haze formation and sedimentation, we compared model haze predictions to PPS and IRIS observations. For solar maximum fluxes (Voyager encounter conditions) the model mass production rate is 1 × 10 -14 g cm 2 sec -1 . C 2 H 6 is the dominant haze component (75%), with the remainder coming from C 2 H 2 and C 3 and C 4 compounds. Balancing the above haze production rate by the sedimentation rate for 0.25-μm radius particles (upper limit to particle radius from PPS observations) yields a total haze column burden slightly above the PPS upper limit. However, lifetime analysis indicates that the model haze production rate should be averaged over solar minimum and maximum conditions. Under these conditions the model haze density is consistent with the PPS data. The predicted C 4 H 2 and C 2 H 6 haze column densities are consistent with the lack of ice signatures in the IRIS spectra.