School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China.
College of Chemistry and Material Engineering, Quzhou University, Quzhou 324000, China.
J Environ Sci (China). 2023 May;127:320-327. doi: 10.1016/j.jes.2022.05.048. Epub 2022 Jun 3.
The high NO concentration in fine particulate matters (PM) during heavy haze events has attracted much attention, but the formation mechanism of nitrates remains largely uncertain, especially concerning heterogeneous uptake of NO by aqueous phase. In this work, the heterogeneous uptake of NO by sodium acetate (NaAc) droplets with different NO concentrations and relative humidity (RH) conditions is investigated by microscopic Fourier transform infrared spectrometer (micro-FTIR). The IR feature changes of aqueous droplets indicate the acetate depletion and nitrite formation in humid environment. This implies that acetate droplets can provide the alkaline aqueous circumstances caused by acetate hydrolysis and acetic acid (HAc) volatilization for nitrite formation during the NO heterogeneous uptake. Meanwhile, the nitrite formation will exhibit a pH neutralizing effect on acetate hydrolysis, further facilitating HAc volatilization and acetate depletion. The heterogeneous uptake coefficient increases from 5.2 × 10 to 1.27 × 10 as RH decreases from 90% to 60% due to the enhanced HAc volatilization. Furthermore, no obvious change in uptake coefficient with different NO concentrations is observed. This work may provide a new pathway for atmospheric nitrogen cycling and secondary nitrite aerosol formation.
在重霾事件中，细颗粒物（PM）中高浓度的 NO 引起了广泛关注，但硝酸盐的形成机制仍在很大程度上不确定，特别是关于通过水相的非均相吸收。在这项工作中，通过微观傅里叶变换红外光谱仪（micro-FTIR）研究了具有不同 NO 浓度和相对湿度（RH）条件的乙酸钠（NaAc）液滴对 NO 的非均相吸收。水相液滴的 IR 特征变化表明，在潮湿环境中，乙酸盐会耗尽，亚硝酸盐会形成。这意味着在 NO 的非均相吸收过程中，乙酸盐液滴可以提供由乙酸盐水解和乙酸（HAc）挥发引起的碱性水相环境，从而促进亚硝酸盐的形成。同时，亚硝酸盐的形成将对乙酸盐水解产生中和 pH 的作用，进一步促进 HAc 的挥发和乙酸盐的耗尽。由于 HAc 的挥发增强，非均相吸收系数从 90% RH 时的 5.2×10降至 60% RH 时的 1.27×10。此外，在不同的 NO 浓度下，吸收系数没有明显变化。这项工作可能为大气氮循环和二次亚硝酸盐气溶胶的形成提供了一条新途径。
