Rapid growth of new atmospheric particles by nitric acid and ammonia condensation
Rapid growth of new atmospheric particles by nitric acid and ammonia condensation
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DOI：
10.1038/s41586-020-2270-4
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发表时间：
2020-05-01
期刊：
影响因子：
64.8
通讯作者：
Donahue, Neil M.
中科院分区：
文献类型：
--
作者：
Wang, Mingyi;Kong, Weimeng;Donahue, Neil M.
作者及其所属机构的列表出现在论文的最后。新粒子的形成是城市烟雾的主要贡献者（1，2），但它如何在城市中发生往往令人困惑（3）。如果城市颗粒物的增长率与清洁环境中的增长率相似（每小时1-10纳米），那么现有的理解表明，新的城市颗粒物应该被高浓度的预先存在的颗粒物迅速清除。在这里，我们通过在欧洲核子研究中心的云室中在大气条件下进行的实验表明，在大约+5摄氏度以下，硝酸和氨蒸气可以凝结成直径小到几纳米的新核粒子。此外，当温度足够低（低于-15摄氏度）时，硝酸和氨可以通过酸碱稳定机制直接成核，形成硝酸铵颗粒。考虑到这些蒸汽的含量通常是硫酸的1000倍，因此产生的颗粒生长速率可能非常高，达到每小时100纳米以上。然而，这些高生长速率要求气体-颗粒硝酸铵系统不平衡，以维持气相过饱和。鉴于我们测量的气相过饱和度对温度的依赖性很强，我们预计这种瞬态条件会发生在不均匀的城市环境中，特别是在冬季，由垂直混合和强大的本地来源，如交通。尽管硝酸和氨冷凝的快速生长可能只持续几分钟，但它仍然足够快，可以引导新成核的颗粒通过最小的尺寸范围，在那里它们最容易受到清除损失，从而大大增加了它们的生存概率。我们还预计，硝酸和氨的成核和快速增长在相对清洁和寒冷的上部自由对流层中是重要的，在那里氨可以从大陆边界层对流，而硝酸是丰富的电风暴（4，5）。
A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog(1,2), but how it occurs in cities is often puzzling(3). If the growth rates of urban particles are similar to those found in cleaner environments (1-10 nanometres per hour), then existing understanding suggests that new urban particles should be rapidly scavenged by the high concentration of pre-existing particles. Here we show, through experiments performed under atmospheric conditions in the CLOUD chamber at CERN, that below about +5 degrees Celsius, nitric acid and ammonia vapours can condense onto freshly nucleated particles as small as a few nanometres in diameter. Moreover, when it is cold enough (below -15 degrees Celsius), nitric acid and ammonia can nucleate directly through an acid-base stabilization mechanism to form ammonium nitrate particles. Given that these vapours are often one thousand times more abundant than sulfuric acid, the resulting particle growth rates can be extremely high, reaching well above 100 nanometres per hour. However, these high growth rates require the gas-particle ammonium nitrate system to be out of equilibrium in order to sustain gas-phase supersaturations. In view of the strong temperature dependence that we measure for the gas-phase supersaturations, we expect such transient conditions to occur in inhomogeneous urban settings, especially in wintertime, driven by vertical mixing and by strong local sources such as traffic. Even though rapid growth from nitric acid and ammonia condensation may last for only a few minutes, it is nonetheless fast enough to shepherd freshly nucleated particles through the smallest size range where they are most vulnerable to scavenging loss, thus greatly increasing their survival probability. We also expect nitric acid and ammonia nucleation and rapid growth to be important in the relatively clean and cold upper free troposphere, where ammonia can be convected from the continental boundary layer and nitric acid is abundant from electrical storms(4,5).