Photochemical Aging of Beijing Urban PM_2.5: HONO Production

Photochemical aging represents an important transformation process of aerosol particles in the atmosphere, which greatly influences the physicochemical properties and the environmental impact of aerosols. In this work, we find that Beijing urban PM_2.5 aerosol particles release substantial HONO, a significant precursor of •OH radicals, into the gas phase during the photochemical aging process. The generation of HONO exhibits a high correlation with the amount of nitrate in PM_2.5. The formation rate of HONO becomes gradually decreased with the irradiation time, but can be restored by introducing the acidic proton, indicative of the essential role of the acidic proton in the HONO production. Other environmental factors such as relative humidity, light intensity, and reaction temperature also possess important influences on HONO production. The normalized photolysis rate constant for HONO ( J_HNO3→HONO) is in the range of 1.22 × 10^-5 s^-1 ∼ 4.84 × 10^-4 s^-1, which is 1-3 orders of magnitude higher than the reported photolysis rate constant of gaseous HNO₃. The present study implies that the photochemical aging of Beijing PM_2.5 is an important atmospheric HONO production source.