Wang Yan, Liu Pengfei, Li Yong Jie, Bateman Adam P, Martin Scot T, Hung Hui-Ming
Department of Environmental Health , Harvard T.H. Chan School of Public Health , Boston , Massachusetts United States.
Department of Civil and Environmental Engineering, Faculty of Science and Technology , University of Macau , Macau , China.
J Phys Chem A. 2018 Sep 27;122(38):7739-7747. doi: 10.1021/acs.jpca.8b06685. Epub 2018 Sep 13.
The atmospheric reactions of secondary organic material (SOM) with gaseous reactants alter its composition and properties, which can further impact the Earth system. To investigate how water content and precursor affect the reactivity of SOM, the reaction between toluene-derived SOM and ammonia for variable relative humidity (RH) was investigated. A Fourier transform infrared spectrometer was used to monitor the absorbance change of the functional groups as a function of exposure time. There was a fast response to water vapor compared with a gradual spectral variation associated with ammonia uptake. When RH is higher than 25 ± 5%, the spectral changes across 1500-1900 cm showed a decreasing trend for carboxylic acids and an increasing trend for carboxylates, suggesting a neutralization reaction by ammonia uptake. The observed increasing trend for the region of 1270-1360 cm might be associated with amines and suggests the formation of organonitrogen compounds for the toluene-derived SOM aging by ammonia at high RH. The corresponding intensity change of C-O groups (1000-1260 cm) with the increased liquid water content as RH increases at the first 6 min suggested that the possible chemical reactions, such as hydrolysis of acetals and hemiacetals to aldehydes and alcohols or esters to carboxylic acids and alcohols, might change the diffusivity of particles and affect the ammonia uptake. The threshold point of ammonia uptake at 30% RH was consistent with a more significant absorbance change of liquid water content and C-O groups at RH ≥ 35 ± 5%. For comparison between anthropogenic and biogenic precursor gases, an isoprene-derived SOM film was also studied. It was more volatile and reactive to ammonia than the toluene-derived SOM. This result implies that the diffusion of ammonia was faster inside isoprene-derived SOM. Overall, the chemical reactions of SOM particles during their atmospheric residence time are precursor- and RH-dependent, which may alter the current understanding of their impact on the Earth system.
二次有机物质（SOM）与气态反应物的大气反应会改变其组成和性质，进而可能对地球系统产生进一步影响。为了研究含水量和前体如何影响SOM的反应活性，研究了甲苯衍生的SOM与氨在不同相对湿度（RH）下的反应。使用傅里叶变换红外光谱仪监测官能团的吸光度随暴露时间的变化。与氨吸收相关的光谱逐渐变化相比，对水蒸气有快速响应。当RH高于25±5%时，1500 - 1900 cm范围内的光谱变化显示羧酸呈下降趋势，羧酸盐呈上升趋势，表明氨吸收发生了中和反应。在1270 - 1360 cm区域观察到的上升趋势可能与胺有关，这表明在高RH下，甲苯衍生的SOM因氨老化而形成有机氮化合物。在前6分钟内，随着RH增加，液态水含量增加，C - O基团（1000 - 1260 cm）的相应强度变化表明，诸如缩醛和半缩醛水解为醛和醇或酯水解为羧酸和醇等可能的化学反应，可能会改变颗粒的扩散率并影响氨吸收。30%RH下氨吸收的阈值点与RH≥35±5%时液态水含量和C - O基团更显著的吸光度变化一致。为了比较人为和生物源前体气体，还研究了异戊二烯衍生的SOM膜。它比甲苯衍生的SOM更易挥发且对氨更具反应活性。这一结果表明氨在异戊二烯衍生的SOM内部扩散更快。总体而言，SOM颗粒在大气停留时间内的化学反应取决于前体和RH，这可能会改变目前对其对地球系统影响的理解。
