Chemical compositions and possible transportation of PM_2.5 during two haze periods in a coastal city of the North China Plain

Although numerous studies have been carried out to study haze formation in the North China Plain (NCP) and Northeastern Plain (NP), little is known about the chemical compositions and haze formations in Qinhuangdao city, which is located in the northeast edge of the NCP and adjacent to the NP. In this study, water‐soluble inorganic ions, organic carbon and elemental carbon (EC) were analysed offline during two haze periods from October 29 to November 7, 2021. The results showed that NO₃⁻ and organic matter (OM) accounted for 34.81% and 21.15% of total PM_2.5 mass concentration, respectively, followed by NH₄⁺ (14.06%) and SO₄²⁻ (12.33%). The sulphur oxidation ratio and nitrogen oxidation ratio as well as OM/EC ratio on haze days were higher than those on non‐haze days, suggesting both high secondary inorganic and organic aerosol formation on haze days under high relative humidity. Source apportionment results showed that marine aerosols contributed small of total aerosol particle loads, while the combustion‐related sources were the most, possibly coal combustion, biomass burning and vehicle emissions. Air masses during haze periods mainly came from southwest and west directions in this case. Further calculated meteorological data of 2018–2021 showed high PM_2.5 mass in Qinhuangdao were mainly affected by low‐speed southwest wind in autumn. The results highlighted the important role of long‐range transport of air pollutants from the NCP instead of the NP on haze formation in autumn in Qinhuangdao.