Calculating ambient aerosol surface area concentrations using aerosol light scattering enhancement measurements
Calculating ambient aerosol surface area concentrations using aerosol light scattering enhancement measurements
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DOI：
10.1016/j.atmosenv.2019.116919
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发表时间：
2019-11
影响因子：
5
通讯作者：
Y. Kuang;J. Tao;Wanyun Xu;Yingli Yu;Gang Zhao;Chuanyang Shen;Yuxuan Bian;Chunsheng Zhao
中科院分区：
文献类型：
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作者：
Y. Kuang;J. Tao;Wanyun Xu;Yingli Yu;Gang Zhao;Chuanyang Shen;Yuxuan Bian;Chunsheng Zhao
气溶胶表面积浓度（SA）是研究气溶胶水或气溶胶表面发生的大气化学反应的关键。然而，没有商业仪器可以提供环境SA的直接测量。在本文中，我们提出了一种方法来计算环境S A的基础上，只有一个三波长加湿浊度计系统，测量在干燥状态下，不同的相对湿度（RH）条件下的气溶胶光学特性在三个波长的测量。在该方法中需要两个关键步骤：（1）使用仅基于由“干”浊度计测量的光学性质的训练的随机森林机器学习模型来计算干燥状态下的环境气溶胶的表面积浓度SA（干）。建议的机器学习方法进行了评估与颗粒数尺寸分布（PNSD）数据集，从8个领域的运动在华北平原在不同的季节。PNSD的SA（干）预测值与计算值之间的相关系数平方约为0.99，SA（干）预测值与计算值之间的平均比值为1.01，70%的数据点的相对误差小于10%。(2)利用所提出的大气气溶胶粒子表面积增长因子fs（RH）参数化方案fs（RH）=（1+ κ S RH 100− RH）2 3计算大气气溶胶粒子吸水引起的表面积增长因子fs（RH），利用实测气溶胶光散射增强因子和朗格斯特罗姆指数计算吸湿性参数κ S。应用该方法计算了华北平原两次野外观测（望都观测和古城观测，分别为夏季和冬季）的环境S_A值。望都战役期间大气SA值为42 ~ 1871 μ m2/cm 3，平均值为319 μ m2/cm 3;古城战役期间大气SA值为19 ~ 4156 μ m2/cm 3，平均值为788 μ m2/cm 3。在这两个运动期间，观察到环境SA的剧烈日变化。结果表明，气溶胶吸湿增长的影响显着的环境SA的变化，特别是在高RH条件下。望都战役fS（RH）值在1 ~ 4之间，平均值为1.4;谷城战役fS（RH）值在1 ~ 2.7之间，平均值为1.3。结果表明，相对湿度越大，fS（RH）对κ S的变化越敏感，这表明气溶胶吸湿性的实时测量是准确计算环境SA所必需的。该方法的优点是可以单独基于三波长增湿浊度计系统的测量来获得环境SA，便于实时测量环境SA，促进气溶胶非均相反应的研究。
Aerosol surface area concentration (S A) is crucial for studying atmospheric chemical reactions happened in aerosol water or on aerosol surface. However, there is no commercial instrument that can provide direct measurements of ambient S A. In this paper, we propose a method to calculate ambient S A based only on measurements of a three-wavelength humidified nephelometer system, which measures aerosol optical properties at three wavelengths under dry state and different relative humidity (RH) conditions. Two critical steps are required in this method:(1) Calculating surface area concentration of ambient aerosols in dry state, S A (dry), using a trained random forest machine learning model based only on optical properties measured by the “dry” nephelometer. The proposed machine learning method is evaluated with particle number size distributions (PNSD) datasets from eight field campaigns conducted on the North China Plain during different seasons. The square of correlation coefficients between predicted and calculated S A (dry) for PNSD is about 0.99, the average ratio between predicted and calculated S A (dry) is 1.01 and 70% of data points has a relative difference less than 10%.(2) Calculating the surface area growth factor f S (RH) of ambient aerosol particles due to water uptake using the proposed f S (RH) parameterization scheme f S (RH)=(1+ κ S R H 100− R H) 2 3, and the hygroscopicity parameter κ S can be calculated using measured aerosol light scattering enhancement factor and Å ngstr o¨ m exponent. The ambient S A values during two field campaigns which are conducted on the North China Plain (Wangdu campaign and Gucheng campaign, in summer and winter respectively) are calculated using the proposed method. The ambient S A ranges from 42 to 1871 μ m 2/c m 3 with an average of 319 μ m 2/c m 3 during Wangdu campaign, and ranges from 19 to 4156 μ m 2/c m 3 with an average of 788 μ m 2/c m 3 during Gucheng campaign. Drastic daily variations of ambient S A are observed during these two campaigns. The results demonstrate that aerosol hygroscopic growth impacts significantly on variations in ambient S A especially under high RH conditions. The f S (RH) ranges from near 1 to 4 with an average of 1.4 during Wangdu campaign, and ranges from near 1 to 2.7 with an average of 1.3 during Gucheng campaign. The results indicate that the larger RH, the more sensitive f S (RH) becomes to variations in κ S which highlights that real-time measurements of aerosol hygroscopicity are required for accurate calculations of ambient S A. The advantage of the proposed method is that the ambient S A can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating real-time measurements of ambient S A and promoting studies in aerosol heterogeneous reactions.