Characteristics of Secondary Aerosols and Ozone During Intense Biomass Burning Episodes in Northeast China: Insights From High‐Resolution Time‐of‐Flight Aerosol Mass Spectrometer

Abstract Springtime haze in the intensive agricultural regions of Northeast China is closely associated with large‐scale open biomass burning (OBB), yet the resulting chemical evolution remains poorly understood. Based on the field campaign utilizing a High‐Resolution Time‐of‐Flight Aerosol Mass Spectrometer (HR‐ToF‐AMS) in Harbin during 2022, we identified OBB as the primary driver of intense haze episodes, characterized by sharp increases in biomass burning organic aerosol (BBOA) and synergistic surges in associated inorganic species. Our results revealed that massive OBB plumes significantly reshaped urban atmospheric chemistry, for example, increasing daytime O 3 levels by a factor of up to ∼1.6. Further analysis highlighted that photochemistry primarily drove organic aerosol aging within these OBB episodes. Concurrently, the elevated oxidative capacity promoted nitrate formation via photochemical pathways during the daytime, whereas nocturnal nitrate formation was driven by two pathways involving dark reactions and heterogeneous processes. These findings provided critical insights for addressing pollution in OBB‐impacted regions.