The evolution of aerosols mixing state derived from a field campaign in Beijing: implications to the particles aging time scale in urban atmosphere

Abstract. The mixing states and aging time scale of aerosol particles play a vital role in evaluating their climate effects. Here, we identified four different real-time mixing patterns of size-resolved particles using the field measurement by a humidity tandem differential mobility analyzer (H-TDMA) in the urban Beijing. We show that the particles with external, transitional and internal mixing state during the campaign account for 20–48 %, 17–24 % and 27–56 % respectively and the fraction highly depends on particles size. The diurnal variation of the mixing states of particles in all sizes investigated (40, 80, 110, 150 and 200 nm) present an apparent aging process from external to internal mixing state, typically spanning a duration of approximately 5–8 hours from 8:00–10:00 to 15:00–17:00. Additionally, the results illustrate that high ambient temperature during daytime or more humid atmosphere accelerates the aging process of aerosol particles, leading to the particles from external to internal mixing on both clear and cloudy days. Also, with the evolution of particulate pollution, the aerosol particles become more internally-mixed. Our result implies that those fine aerosol particles experience aging through both the photochemical process and aqueous growth in the polluted atmosphere of urban Beijing. Furthermore, through a comprehensive review of the aging timescale of particles adopted in current models and derived from observations, we show the great discrepancy between observations and models, highlighting the importance to parameterize their aging time scale based on more field campaigns.