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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Homogeneous ice nucleation from aqueous inorganic/organic particles representative of biomass burning: Water activity, freezing temperatures, nucleation rates
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Homogeneous ice nucleation from aqueous inorganic/organic particles representative of biomass burning: Water activity, freezing temperatures, nucleation rates

机译：由代表生物质燃烧的含水无机/有机颗粒形成的均匀冰核：水活度，冻结温度，成核速率

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Homogeneous ice nucleation plays an important role in the formation of cirrus clouds with subsequent effects on the global radiative budget. Here we report on homogeneous ice nucleation temperatures and corresponding nucleation rate coefficients of aqueous droplets serving as surrogates of biomass burning aerosol. Micrometer-sized (NH4)2SO4/ levoglucosan droplets with mass ratios of 10:1, 1:1, 1:5, and 1:10 and aqueous multicomponent organic droplets with and without (NH4) 2SO4 under typical tropospheric temperatures and relative humidities are investigated experimentally using a droplet conditioning and ice nucleation apparatus coupled to an optical microscope with image analysis. Homogeneous freezing was determined as a function of temperature and water activity, aw, which was set at droplet preparation conditions. The ice nucleation data indicate that minor addition of (NH4) 2SO4 to the aqueous organic droplets renders the temperature dependency of water activity negligible in contrast to the case of aqueous organic solution droplets. The mean homogeneous ice nucleation rate coefficient derived from 8 different aqueous droplet compositions with average diameters of ～60μm for temperatures as low as 195 Kand awof 0.82-1 is 2.18 × 106 cm-3 s-1. The experimentally derived freezing temperatures and homogeneous ice nucleation rate coefficients are in agreement with predictions of the water activity-based homogeneous ice nucleation theory when taking predictive uncertainties into account. However, the presented ice nucleation data indicate that the water activity-based homogeneous ice nucleation theory overpredicts the freezing temperatures by up to 3 Kand corresponding ice nucleation rate coefficients by up to ～2 orders of magnitude. A shift of 0.01 in aw, which is well within the uncertainty of typical field and laboratory relative humidity measurements, brings experimental and predicted freezing temperatures and homogeneous ice nucleation rate coefficients into agreement. The experimentally derived ice nucleation data are applied to constrain the water activity-based homogeneous ice nucleation theory to smaller than ±1 order of magnitude compared to the predictive uncertainty of larger than ±6 orders of magnitude. The atmospheric implications of these findings are discussed.
机译：均匀的冰成核作用在卷云的形成中起着重要作用，随后对全球辐射预算产生影响。在这里，我们报告了均匀的冰成核温度和作为生物质燃烧气溶胶替代物的水滴的相应成核速率系数。在典型的对流层温度和相对湿度下，质量比为10：1、1：1、1：5和1:10的微米级（NH4）2SO4 /左旋葡聚糖滴和含有和不含（NH4）2SO4的含水多组分有机滴为使用液滴调节和冰成核装置与光学显微镜结合图像分析进行实验性研究。确定均质冻结与温度和水活度aw的关系，该温度和水活度aw被设置为液滴制备条件。冰成核数据表明，与水性有机液滴的情况相比，向水性有机液滴中少量添加（NH 4）2 SO 4使得水活度的温度依赖性可忽略不计。在低至195 K且awof 0.82-1的温度下，由8种平均直径约为60μm的不同水滴成分组成的平均冰成核率系数为2.18×106 cm-3 s-1。当考虑到预测不确定性时，实验得出的冻结温度和均匀冰成核速率系数与基于水活度的均匀冰成核理论的预测一致。然而，现有的冰成核数据表明，基于水活度的均质冰成核理论高估了高达3 K的冰点温度，而相应的冰成核率系数则高估了约2个数量级。 0.01的aw偏移完全在典型的田间和实验室相对湿度测量的不确定性之内，这使实验和预测的冻结温度以及均匀的冰成核速率系数一致。实验得出的冰成核数据可用于将基于水活度的均质冰成核理论限制在小于±1个数量级，而预测不确定性大于±6个数量级。讨论了这些发现对大气的影响。

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