Is the photochemistry activity weak during haze events? – A novel exploration on the photoinduced heterogeneous reaction of NO₂ on mineral dust

Abstract. Despite the increased awareness of heterogeneous reaction on mineral dust, the knowledge of how the intensity of solar irradiation influences the photochemistry activity remains a crucially important part in atmospheric research. Relevant studies have not seriously discussed the photochemistry under weak sunlight during haze, and thus ignored some underlying pollution and toxicity. Here, we investigated the heterogeneous formation of nitrate and nitrite under various illumination conditions by laboratory experiments and field observations. Observed by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), water-solvated nitrate was the main surface product, followed by other species varying with illumination condition. The growth of nitrate formation rate tends to be slow after the initial fast with increasing light intensity. For example, the geometric uptake coefficient (γ_geo) under 30.5 mW/cm² (5.72 × 10⁻⁶) has exceeded the 50 % of that under 160 mW/cm² (1.13 × 10⁻⁵). This case can be explained by the excess NO₂ adsorption under weak illumination while the excess photoinduced active species under strong irradiation. Being negatively associated with nitrate (R² = 0.748, P < 0.01), nitrite acts as the intermediate and decreases with increasing light intensity via oxidation pathways. Similar negative dependence appears in coarse particles collected during daytime (R² = 0.834, P < 0.05), accompanied by the positive association during nighttime (R² = 0.632, P < 0.05), suggesting illumination a substantial role in atmospheric nitrogen cycling. Overall, for the nitrate formation, the conspicuous response under slight illumination offers opportunities to explain the secondary aerosol burst during haze episodes with weak irradiation. Additionally, high nitrite levels accompanied by low nitrate concentrations may induce great health risk which was previously neglected. Further, Monte Carlo simulation coupled with sensitivity analysis may provide a new insight in the estimations of kinetics parameters for atmospheric modelling studies.