Decadal Transition of Summertime PM_2.5-O₃ Coupling and Secondary Organic Aerosol Dominance in Northwest China

Abstract. The Yinchuan metropolitan area in northwest China, situated between the Tengger and Ulan Buh Deserts, is influenced by both natural dust and anthropogenic emissions. However, the evolution of fine particulate matter (PM_2.5) and its interaction with ozone (O₃) under the region’s arid climate remain poorly understood. This study integrates decadal observations (2015–2025) with in-situ measurements using an Aerosol Chemical Speciation Monitor and a Vocus Proton Transfer Reaction Mass Spectrometry during summer 2025 to elucidate the changing PM_2.5-O₃ relationship and sources of organic aerosols. A pronounced shift was identified: Phase I (2015–2018) featured a rapid decline in PM_2.5 accompanied by a sharp O₃ increase, while Phase II (2019–2025) exhibited stabilized PM_2.5 and plateaued O₃, indicating reduced O₃ sensitivity to particulate controls. The average non-refractory PM_2.5 concentration (16.8 µg m^-3) was significantly lower than in eastern Chinese megacities, with organics accounting for ~60 %. Positive matrix factorization resolved three organic aerosol factors, revealing dominant secondary organic aerosols (SOA, ~74 %) derived from prolonged photochemical aging. Volatile organic compound analysis showed that anthropogenic and biogenic precursors, including urban terpenes and aromatic oxidation products jointly contributed to SOA formation. Back-trajectory and potential source analyses indicated that Yinchuan’s summer air masses were mainly locally recirculated, with limited influence from long-range transport. These results demonstrate a regional transition toward SOA-dominated fine particles and decoupled PM_2.5-O₃ dynamics under cleaner conditions, highlighting the need for integrated VOC and oxidant controls to mitigate co-occurring O₃ and PM_2.5 pollution in arid northwest China.