Analysis of atmospheric visibility degradation in early haze based on the nucleation clustering model

The initial nucleation of ultrafine particle recently attracts more attention as a key factor in visibility degradation during early haze. In this study, we have performed Density Functional Theory (DFT) calculation to investigate the particle-induced initial nucleation process during early haze. Our calculation results show that the water molecules aggregate surrounding nuclei to form heterogeneous cluster under a certain RH, the synergistic effect of multiple-bonding interactions makes heterogeneous cluster incline to form and stably exist with lower total energy as well as approximately 3-10 times expansionary volume, the hygroscopic growth of aerosol particles would play an important role in affecting the atmospheric visibility. The electron cloud density transferring from the exterior H2O molecule to the interior nuclei in the cluster is characterized by migration effect or nonuniform distribution. The decreasing energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the heterogeneous cluster would lead to a red-shifted absorption wavelength λmax. It exerts the blue, indigo and violet visible-light absorption of corresponding wavelength for NO, NO2, NO3− and O3 heterogeneous clusters to enhance light extinction, thus contributing to visibility degradation, during early haze periods. It helps to intensify the color-enhancing effect of visible light with longer wavelength, such as red, orange as well as yellow light, to promote the evolution of early haze in clean weather period. This study shows that one of the important mechanisms of atmospheric visibility degradation in haze event is the heterogeneous cluster formation in the size range of about 2-4 nm during initial nucleation stage. Our results highlight that prevention of gas-phase chemical species (NO, NO2 and SO2) emission, and also regulatory controls of the concentrations of NO3−, O3 in atmosphere are necessary to prevent deterioration of the air quality in future.