Enhanced sulfate formation through SO 2 +NO 2 heterogeneousreactions during heavy winter haze in the Yangtze River Deltaregion, China

Abstract. Rapid sulfate formation is recognized as key characteristics of severe winter haze in China. However, air quality models tend to underestimate sulfate formation during heavy haze periods and heterogeneous formation pathways have been proposed as promising mechanisms to reduce gaps between observation and model simulation. In this study, we implemented a reactive SO 2 uptake mechanism through the SO 2 +NO 2 heterogeneous reactions in the Comprehensive Air Quality Model with extensions (CAMx) to improve simulation of sulfate formation in the Yangtze River Delta (YRD) region for the first time. Parameterization of the SO 2 +NO 2 heterogeneous reactions is based on observations in Beijing and considered both impact of relative humidity and aerosol pH on sulfate formation. Ammonia is reported to be critical for the formation of secondary inorganic aerosols and estimation of ammonia emissions is usually associated with large uncertainties. Sensitivity tests were conducted to evaluate the importance of the SO 2 +NO 2 heterogeneous reactions as well as ammonia emissions on modelled sulfate concentrations during a period with several heavy haze episodes in the YRD region. Base case model results show large underestimation of sulfate concentrations by 36 % under polluted conditions in the YRD region. Adding the SO 2 +NO 2 heterogeneous reactions or doubling ammonia emissions alone leads to slight model improvement (~ 6 %) on simulated sulfate concentrations in the YRD region. However, model performance significantly improved when both the SO 2 +NO 2 heterogeneous reactions and doubled ammonia emissions were included in the simulation: predicted sulfate concentrations during polluted periods increased from 23.1 µg m −3 in the base scenario to 29.1 µg m −3 (representing an increase of 26 %). Aerosol pH is crucial for the SO 2 +NO 2 heterogeneous reactions and our calculated aerosol pH is always acidic and increased by 0.7 with doubled ammonia emissions. Modelling results also show that this reactive SO 2 uptake mechanism enhanced sulfate simulations by 1 to 5 µg m −3 for the majority of eastern and central part of China, with more than 20 µg m −3 increase of sulfate concentrations over the north-eastern plateau. These findings suggest that the SO 2 +NO 2 heterogeneous reactions could be important for sulfate formation in the YRD region as well as other parts of China. In addition, ammonia emissions need to be carefully estimated. More studies are needed to improve the parameterization of the SO 2 +NO 2 heterogeneous reactions based on local data further evaluate this mechanism in other regions. Substantial efforts are needed to improve the accuracy of ammonia emissions inventory.