The evolution of aerosol mixing state derived from a field campaign in Beijing: implications for particle aging timescales in urban atmospheres

The mixing states and aging timescale of aerosol particles play a vital role in evaluating their climate effects. Here, by using field measurements at a site in urban Beijing, we have identified four different real-time mixing patterns of size-resolved particles, which are defined as less hygroscopic (LH) internally mixed, externally mixed, transitional externally mixed and more hygroscopic (MH) internally mixed particles, with atmospheric fractions of 0 %–10 %, 20 %–46 %, 17 %–24 % and 27 %–56 %, respectively. The fraction depends on particle size, with the maximum fraction of MH internally mixed particles at 80 and 110 nm and the minimum fraction of LH internally mixed particles across all sizes, implying rapid mixing and aging of ambient particles during the observational period. The diurnal variations of the mixing states of particles in all sizes investigated (40, 80, 110, 150 and 200 nm) present an apparent aging process from externally mixed to MH internally mixed, which typically spans a duration of approximately 5–10 h from 08:00–10:00 to 15:00–17:00 local time (LT), revealing the mixing (aging) timescale of aerosols in a polluted urban atmosphere. Additionally, our results suggest that those fine aerosol particles experience aging through both the photochemical process and non-photochemical growth during the campaign. Furthermore, through a comprehensive review of the mixing–aging timescale of particles adopted in current models and derived from observations, we show a great discrepancy between observations and models, highlighting the importance of parameterizing their aging timescale based on more field campaigns.