Role identification of NH3 in atmospheric secondary new particle formation in haze occurrence of China

Abstract

Haze pollution in China, dominated by the aerosol particulate matters smaller than 2.5 μm (PM2.5) has raised much concern over the past decade. PM2.5 is principally derived from secondary new particulate formation (NPF) with main precursors SO2, NO2, NH3 and organic compounds (OC), where NH3 is the only alkaline inorganic gas. However, due to the lack understanding of NH3's role in NPF, few attention has been paid to NH3, which hinders the haze mitigation in China. In this work, the role of NH3 in NPF is investigated theoretically with a new kinetic model. Combining the oxidation of SO2/NO2 in SO2/NO2/NH3/H2O/air system and the aggregation of clusters in H2SO4/HNO3/NH3/OC system, this model has been established based on gas-kinetic theory, and tested upon uncertainty analysis with Monte Carlo method. The results of NPF calculations at the atmospheric concentration of each precursor show that NH3 is able to enhance NPF indirectly by facilitating conversions of SO2 and NO2, and directly by promoting aggregations of H2SO4, HNO3, NH3 and OC. The major effect of NH3 on NPF is found to be the enhancement of conversion fractions for SO2 and NO2 during oxidation processes. In addition, the promotion in contribution of HNO3 to NPF due to NH3 has been particularly observed. Therefore, controlling the emission of NH3 strictly as current restrictions on SO2 and NOx is reasonable to mitigating the haze pollution in China which has much higher atmospheric NH3 concentration over the global average level.