Underestimated benefits of NOx control in reducing SNA and O3 based on missing heterogeneous HONO sources

Substantial NO_x emission mitigation is crucial for the synergistic reduction of particulate matter and ozone (O₃) pollution in China. The traditional air quality model does not consider heterogeneous HONO chemistry, leading to uncertainties in estimating the benefits of NO_x control. Previous studies have shown that the parameterization of heterogeneous HONO formation increases both the simulated value of sulfate–nitrate–ammonium (SNA) and that of O₃, thus adding the heterogeneous reactions of HONO into air quality models inevitably leads to changes in the estimated benefits of NO_x abatement. Here we investigated the changes in SNA and O₃ concentrations from NO_x emission reduction before and after adding heterogeneous HONO reactions in the Community Multi-Scale Air Quality (CMAQ) model. Including heterogeneous HONO reactions in the simulation improved the benefits of NO_x reduction in terms of SNA control in winter. With 80% NO_x reduction, the reduction in SNA increased from 36.9% without considering heterogeneous HONO reactions to 42.8% with heterogeneous HONO chemistry. The reduction in the maximum daily 8h average (MDA8) O₃ in summer caused by NO_x reduction increased slightly from 4.7% to 5.2% after adding heterogeneous HONO reactions. The results in this study highlight the enhanced effectiveness of NO_x controls for the reduction of SNA and O₃ after considering heterogeneous HONO formation in a complex chemical ambient, demonstrating the importance of NO_x controls in reducing PM_2.5 and O₃ pollution in China.