Atmospheric Nitrate Formation through Oxidation by Carbonate Radical

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.