Unraveling the Formation and Evolution of Winter Haze in Beijing: Insights from Vertical Measurements of Particle Size Distributions and Aerosol Composition

Understanding the dynamics of haze formation is essential for addressing air quality and public health concerns in megacities. In this study, we present vertical measurements of size-resolved particle number concentrations and aerosol composition in urban Beijing during winter haze events from 2018 to 2019. By analyzing aerosol properties at ground level, 140 m, and 240 m, we characterized the vertical distributions, particle growth, and chemical evolution of haze episodes. Our results show significant vertical gradients in aerosol properties, especially for smaller particles (<100 nm), which exhibit strong height-dependent variations, while larger particles remain relatively well-mixed. Organic aerosols (OA), accounting for 44–45% of total PM2.5, are dominated by secondary organic aerosols (SOA, 82–86%) and show increased photochemical processing at higher altitudes. Particularly, we observe distinct phases in the evolution of haze episodes, characterized by varying rates of particle growth and changes in aerosol composition, and influenced by meteorological conditions, regional transport, and local emissions. Specifically, nitrate and photochemical SOA drive the initial stage of particle growth and haze formation, while regional transport plays a significant role in the later stage. This study advances our understanding of aerosol dynamics in the urban boundary layer and provides valuable insights for improving air quality management strategies in megacities.