Important contribution of N₂O₅ hydrolysis to the daytime nitrate in Xi'an, China during haze periods: Isotopic analysis and WRF-Chem model simulation

Nitrate, as one of the major components of tropospheric aerosols, plays a crucial role in winter haze formation. While, the formation mechanism of the high production of nitrate in Chinese megacities is still not fully understood. To quantify the contributions of major formation pathways to nitrate, airborne particles in Xi'an, inland China during the winter of 2017 were measured and analyzed for the water-soluble ions and stable nitrogen/oxygen isotope compositions of nitrate in PM_2.5, followed by a WRF-Chem model simulation. The oxygen isotopic results indicated that N₂O₅ hydrolysis was an important formation pathway for the daytime nitrate in the haze episodes. The model simulation further revealed that N₂O₅ hydrolysis contribution increased from 8.2% to 20.5% of the total nitrate over 14:00–16:00 p.m., clearly showing that N₂O₅ formation followed by a heterogeneous hydrolysis to nitrate can effectively proceed in daytime under the abundantly co-existing O₃, NO₂ and NH₃ conditions.