Seasonal effects of additional HONO sources and the heterogeneous reactions of N2O5 on nitrate in the North China Plain

We coupled the heterogeneous hydrolysis of N₂O₅ into the newly updated Weather Research and Forecasting model with Chemistry (WRF-Chem) to reveal the relative importance of the hydrolysis of N₂O₅ and additional nitrous acid (HONO) sources for the formation of nitrate during high PM_2.5 events in the North China Plain (NCP) in four seasons. The results showed that additional HONO sources produced the largest nitrate concentrations in winter and negligible nitrates in summer, leading to a 10% enhancement of a PM_2.5 peak in southern Beijing and a 15% enhancement in southeastern Hebei in winter. In contrast, the hydrolysis of N₂O₅ produced high nitrate in summer and low nitrate in winter, with the largest contribution of nearly 20% for a PM_2.5 peak in southeastern Hebei in summer. During PM_2.5 explosive growth events, the additional HONO sources played a key role in nitrate increases in southern Beijing and southwestern Hebei in winter, whereas the hydrolysis of N₂O₅ contributed the most to a rapid increase in nitrate in other seasons. HONO photolysis produced more hydroxyl radicals, which were greater than 1.5 μg m^-3 h^-1 in the early explosive stage and led to a rapid nitrate increase at the southwestern Hebei sites in winter, while the heterogeneous reaction of N₂O₅ contributed greatly to a significant increase in nitrate in summer. The above results suggest that the additional HONO sources and the heterogeneous hydrolysis of N₂O₅ contributed the most to nitrate formation in NCP in winter and summer, respectively.