Sulfur Dioxide Transported From the Residual Layer Drives Atmospheric Nucleation During Haze Periods in Beijing

New particle formation (NPF) is a global phenomenon that significantly influences climate. NPF also contributes to haze, with pronounced negative impacts on human health. Theory and observations both show nucleation favored during clean days inhibited haze episodes due high pre-existing condensation sink (CS). Here we the surprising occurrence of in Beijing associated concentration sulfuric acid dimers. With field model simulations, demonstrate downward mixing sulfur dioxide (SO2) from residual layer aloft enhances ground level SO2, which turn elevates dimer after rapid SO2 oxidation polluted air. Our results address key gap between source its atmospheric products conditions megacity, Beijing, providing missing link complete chain describing atmosphere.