Atmospheric Nitrate Formation through Oxidation by Carbonate Radical
Xiaozhong Fang, Yangyang Liu, Kejian Li, Tao Wang, Yue Deng, Yiqing Feng, Yang Yang, Hanyun Cheng, Jianmin Chen, Liwu Zhang
Abstract
The carbonate radical (CO3•–) has been proved to be of great significance in the water environment. However, a few studies concerned with its contribution to atmospheric heterogeneous chemistry. Here, we report the pathway for the first time for CO3•– generation by interfacial photogenerated hole/hydroxyl radical (•OH) transfer between the photoactive component and (bi)carbonate (CO32–/HCO3–) of mineral dust under atmospherically relevant actinic irradiation. By combining laboratory simulation, nanosecond transient absorption spectroscopy (NTAS), and field observation, we demonstrate that (bi)carbonate in the atmosphere not only has a pH buffering effect in the uptake of NO2 but more importantly directly participates in the photochemical reactions by generating CO3•– under irradiation. CO3•– can play an important role by facilitating nitrite transformation to nitrate, especially under a lower light intensity (15 mW/cm2) and a higher humidity (60% RH). Diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) was used to detect the surface species generated on simulated mineral dust. Under irradiation, the time-dependent reactive uptake coefficient for the first-order reaction of NO2 uptake on simulated mineral dust was found to be 2.817 × 10–8 × e–t/6559. These results highlight the important role of CO3•– in atmospheric photochemistry, which could act as a reactive radical and influence secondary aerosol formation.