Vertical transport of ultrafine particles and turbulence evolution impact on new particle formation at the surface & Canton Tower

Abstract

New particle formation (NPF) is a key process occurring in the planetary boundary layer (PBL). Newly formed particles are an important source of aerosols and cloud condensation nuclei (CCN) that influence clouds and climate. However, the distribution of these new particles at different altitudes has rarely been studied. In-situ measurements of ultrafine particles (UFP) observed at the ground level and at the top of the Canton Tower (454 m) located in downtown Guangzhou in southern China were analyzed, along with measurements of multiple meteorological and physicochemical quantities observed during a field campaign carried out from December 2019 to March 2020 and simulated with the Weather Research and Forecasting (WRF-Chem) model. We found that turbulence and NPF characteristics vary considerably with height, with UFP concentrations diminishing by half from the surface to the tower top. This indicates that UFP are transported upward from the ground in the lower boundary layer. A consistent relationship is established between the occurrences of NPF and the evolution of turbulence. The correlation between the exchange ratio at the tower top correlates well with nucleation growth, suggesting that turbulence can play an important role in episodes of NPF growth. The growth rate is closely related to the turbulence exchange ratio, effectively dictating the UFP concentration before and during the COVID-19 lockdown period. A new mechanism is thus hypothesized: NPF happens earlier near the surface and grows faster in the upper PBL, attributed to condensable vapor being transported by turbulent vertical mixing in the boundary layer. Model simulations using the WRF-Chem model reveal that the exchange ratio changed NPF parameters, supporting the proposed mechanism that the evolution of the PBL has a significant impact on NPF. This should not be neglected in NPF research because this physical factor may be the dominant one underlying the NPF mechanism.