Effects of Light Intensity and Relative Humidity on the Heterogeneous Formation Processes of Nitrogen-Containing Organic Compounds on Mineral Dust Particles

Abstract An in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reaction system was employed to investigate the effects of light intensity and relative humidity (RH) on the heterogeneous reactions of toluene/NO2/NH3 on α-Fe2O3/α-Al2O3 particles. The results show that the characteristic peak of the R–ONO2 group appears at 1292 cm–1 under dark and dry conditions, confirming benzyl nitrate as the dominant nitrogen-containing organic compounds (NOCs) product. With light intensity increasing from 0 to 36 mW·cm–2, the NOC formation rate rises from (1.131 ± 0.003) × 1018 ions·g–1·s–1 to (1.773 ± 0.023) × 1018 ions·g–1·s–1, but drops to (0.848 ± 0.004) × 1018 ions·g–1·s–1 when light intensity further increases to 100 mW·cm–2. This nonmonotonic trend indicates that moderate light intensity can promote the formation of reactive oxygen species (ROS), accelerate the transformation from toluene to relative radical intermediates, and further promote the uptake of NO2 to form R–ONO2 groups of benzyl nitrate. However, high light intensity is unfavorable for NOC formation due to the photolysis effect of NOCs. The roles of water vapor in the initial stage and steady stage are different with increasing RH. In the initial reaction stage, the uptake coefficient can reach (3.014 ± 0.311) × 10–7 when at 80.8% RH, which is higher than the value of (1.341 ± 0.099) × 10–7 under dry conditions. The promotion at high RH is attributed to water-facilitated ROS generation and the activation of an additional aromatic ring oxidation pathway for R–ONO2 formation. This study provides experimental evidence and theoretical support for understanding NOC formation mechanisms under complex air pollution conditions.