Key Role of Nitrate in Phase Transitions of Urban Particles: Implications of Important Reactive Surfaces for Secondary Aerosol Formation
Key Role of Nitrate in Phase Transitions of Urban Particles: Implications of Important Reactive Surfaces for Secondary Aerosol Formation
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DOI：
10.1002/2017jd027264
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发表时间：
2018-01-27
影响因子：
4.4
通讯作者：
Li, Weijun
中科院分区：
文献类型：
--
作者：
Sun, Jiaxing;Liu, Lei;Li, Weijun
硫酸铵（AS）和硝酸铵（AN）是城市细颗粒物的重要组成部分。现场和模式研究均表明，SO2、NO2和NH3与湿气溶胶的非均相反应加速了北方灰霾的形成。然而，很少有人知道的AS-AN含雾粒子的相变。在这里，实验室产生的AS-AN颗粒和在城市站点的灰霾事件期间收集的单个颗粒的吸湿特性进行了研究，使用单个颗粒吸湿系统。AS-AN颗粒在相互潮解相对湿度（MDRH）和完全潮解相对湿度（DRH）下呈现两阶段潮解过程，并呈现三种物理状态：MDRH前为固态，MDRH和DRH之间为固-水态，DRH后为水态。在水化过程中，城市灰霾颗粒物在相对湿度为60-80%时表现为固体核和水相壳，在相对湿度> 80%时表现为水相。在脱水过程中，大部分颗粒在RH>50%的水相中。结果表明，颗粒中AS含量决定了颗粒的DRH，AN含量决定了颗粒的MDRH。AN含量的增加可以降低MDRH，这表明在较低的RH下会出现水壳。硝酸盐丰富的城市颗粒物的湿度依赖的相变是重要的，以提供二次气溶胶形成在污染的air.Plain语言摘要的反应表面最近，气溶胶水受到了更多的关注，因为SO2，NO2，和NH3上的湿颗粒的非均相反应加速严重的雾霾形成在中国北方。硫酸铵和硝酸铵（AS-AN）是城市细颗粒物的重要组成部分。特别是我国污染空气中硝酸盐浓度持续升高。研究表明，在较低的相对湿度下，AN含量的增加促进了水壳的形成。研究发现，在相对湿度为60-80%时，大多数城市颗粒物基本保持固体核和水相壳，而在相对湿度> 80%时，大多数城市颗粒物基本保持水相。这些结果可以解释硝酸盐在相变过程中的作用，并补充了北方重霾天气中颗粒表面非均相反应的讨论。颗粒物的湿度相关相态对于解释严重霾中二次气溶胶的形成以及在模式研究中是有用的。
Ammonium sulfate (AS) and ammonium nitrate (AN) are key components of urban fine particles. Both field and model studies showed that heterogeneous reactions of SO2, NO2, and NH3 on wet aerosols accelerated the haze formation in northern China. However, little is known on phase transitions of AS-AN containing haze particles. Here hygroscopic properties of laboratory-generated AS-AN particles and individual particles collected during haze events in an urban site were investigated using an individual particle hygroscopicity system. AS-AN particles showed a two-stage deliquescence at mutual deliquescence relative humidity (MDRH) and full deliquescence relative humidity (DRH) and three physical states: solid before MDRH, solid-aqueous between MDRH and DRH, and aqueous after DRH. During hydration, urban haze particles displayed a solid core and aqueous shell at RH=60-80% and aqueous phase at RH>80%. Most particles were in aqueous phase at RH>50% during dehydration. Our results show that AS content in individual particles determines their DRH and AN content determines their MDRH. AN content increase can reduce MDRH, which indicates occurrence of aqueous shell at lower RH. The humidity-dependent phase transitions of nitrate-abundant urban particles are important to provide reactive surfaces of secondary aerosol formation in the polluted air.Plain Language Summary Recently, aerosol water has received more attention because heterogeneous reactions of SO2, NO2, and NH3 on wet particles accelerate the severe haze formation in north China. Ammonium sulfate and ammonium nitrate (AS-AN) are key components of fine urban particles. Especially, nitrate concentration keeps increasing in polluted air in China. Our study indicates that the increase of AN content promotes the occurrence of aqueous shell at lower RH. Here we find that most of urban particles generally keep solid core and aqueous shell at RH=60-80% and aqueous phase at RH>80%. These findings can clearly explain the role of nitrate in phase transitions and make up the discussion about heterogeneous reactions on particle surfaces during the severe hazes in north China. Humidity-dependent phase states of particles are useful for interpreting the secondary aerosols' formation in severe hazes as well as in modeling studies.