Gao, Jie, Li, Yiming, Li, Jiayun, Shi, Guoliang, Liu, Zirui, Han, Bo, Tian, Xiao, Wang, Yuesi, Feng, Yinchang, and Russell, Armistead G.
Secondary inorganic aerosol (especially SNA: sulfate, nitrate, and ammonium) formed from different pathways, account for significant proportions of ambient particulate matter during haze events. For the first time, we quantitatively explored contributions from different SNA formation pathways by an online monitoring AMS instrument and 3‐dimension factor analysis PMF3 model, which is called "SNA formation pathway apportionment". Three formation process were identified: NH4NO3 heterogeneous phase reactions (NH4NO3‐heter), (NH4)2SO4 gas phase reactions ((NH4)2SO4‐gas), and mixed NH4NO3 and (NH4)2SO4 aqueous phase processes (S&N‐aque). Their overall contributions to SNA were 25.7%, 22.1%, and 52.2%, respectively. Moreover, (NH4)2SO4‐gas contributed mainly in clean (52.1%); S&N‐aque got the highest contribution in haze episode (69.6%); while sulfate gas phase formations were also found in haze stage (10.3%). The quantitative findings from this work provide hard evidence that aqueous phase processes (including sulfate aqueous phase reactions and nitrate gas‐particle partition) were major drivers for SNA increasing. Plain Language Summary: Secondary inorganic aerosol (particularly SNA: sulfate, nitrate and ammonium) was found to be the main component of particulate matter in northern China. SNA can be formed through different reaction pathways, such as gas phase, aqueous phase, and heterogeneous reactions pathways. However, quantitative assessments of different pathways are still lacking, especially during different pollution processes. In this work, AMS data with high time resolution and three‐dimension receptor model (PMF3) were used to study the characteristics of secondary aerosols in winter in Beijing and make SNA formation pathway contribution apportionment. Three possible formation pathways were identified by PMF3: Heterogeneous reactions pathway of NH4NO3, gas phase reactions pathway of (NH4)2SO4, and aqueous phase processes of mixed NH4NO3 & (NH4)2SO4, respectively. We found that the main dominant factors were diverse during different periods: (NH4)2SO4 gas phase reactions pathway contributed mainly to SNA in clean period; mixed NH4NO3 and (NH4)2SO4 aqueous phase processes contributed mostly in haze episode. The methods and findings of this work enable and further our understandings on the formations of secondary inorganic aerosol in haze. Key Points: Mixed NH4NO3 and (NH4)2SO4 (aqueous phase process) had the highest contribution during winter haze period (69.6%)(NH4)2SO4 (gas phase formation pathway) contributed mainly in clean (52.1%)(NH4)2SO4 (gas phase formation pathway) was also found during haze pollution stage (10.3%) [ABSTRACT FROM AUTHOR]