Characterization of the vertical evolution of urban nocturnal boundary layer by UAV measurements: Insights into relations to cloud radiative effect.

The complex structure of the nocturnal boundary layer (NBL) and its impact on air pollution remain poorly understood. In this study, we present in-situ nocturnal flight measurements onboard an unmanned aerial vehicle (UAV) during the wintertime of 2022 at an urban site in Hefei, China. Besides, co-located measurements of radiation intensity and total amount of cloud were conducted. The vertical distribution of temperature, particle number concentration, and relative humidity were obtained to study the structure of NBL and the key factors driving the evolution of the NBL. A multi-layer inversion boundary layer was observed during haze and fog episodes, which affects the vertical diffusion of particles near the surface and leads to a vertical gradient of particle number concentrations. The particle size distribution demonstrates a drastic vertical variation over different sections of the nocturnal boundary layer: homogeneously mixed in the SBL and the RL layer, sharply reduced in the IL. It is found that the temperature and particle number concentration differences between near-surface and at 500 m are highly related to variations of the radiation intensity and the amount of cloud. The decreased cloud cover enhances the surface cooling, creating a shallow NBL with multiple inversion layers, which reinforces the suppression of vertical diffusions and consequently promotes the accumulation of aerosols within the NBL. This reveals an important mechanism for the impact of NBL evolution modulated by cloud radiative effect on the formation of urban haze.

Vertical profile of aerosol number size distribution during a haze pollution episode in Hefei, China.

The vertical distribution of aerosols has important implications on haze formation as development, which is manifested to some extent by the planetary boundary layer (PBL)-aerosol interactions. Information on the number concentration and size of particles is essential to understand these processes, but studies on vertical profiles of particle number-size distribution are limited. Herein, an unmanned aerial vehicle (UAV) equipped with a custom-built optical particle counter (0.4-10 µm) was used to investigate the vertical profiles of particle number-size distribution in Hefei (China) during January 20-30, 2021. Combining ground-based scanning mobility particle sizer and meteorological data, the pollution accumulation and diffusion mechanisms were analyzed in depth. Results showed that as the pollution episode developed, the vertical distribution of the particle number concentration changed from a flat profile to a sharp vertical gradient. Under polluted conditions, a three-layer structure was clearly evident: uniform distribution in a mixed layer near the ground, a sharply reduced transition layer, and a low number concentration layer in the free atmosphere. Analysis revealed that fundamental to this conversion is that aerosols are highly affected by the PBL dynamics. Concurrent on-UAV and ground-based observations revealed that the ratio of particle numbers in the accumulation mode to that in the Aitken mode was 0.92 ± 0.05 in polluted days, which was almost three times that of clean days. This difference in the ratio of large to small particles suggests that hygroscopic growth of aerosol particles under high humidity conditions played an important role in haze development. Moreover, the sharp vertical gradient of the particle number concentration in the transition layer was identified as an important parameter for characterizing PBL height. The findings in this study highlight the importance of PBL dynamics on the under-studied vertical profiles of particle number-size distribution, especially during heavy pollution episodes.

In Situ Quantitative Observation of Hygroscopic Growth of Single Nanoparticle Aerosol by Surface Plasmon Resonance Microscopy.

Aerosol particle hygroscopicity is an important factor in visibility reduction, cloud formation, radiation forcing, and the global climate. The high number concentration of nanoparticles (defined as particles with diameters below 100 nm) means that their hygroscopic growth abilities and potential contributions to the climate and environment are significant. Therefore, a rapid and accurate in situ analysis method for single nanoparticle hygroscopic growth in an atmospheric environment is important to characterize the effects of the particle's physical and chemical properties in this process. In this work, surface plasmon resonance microscopy with azimuthal rotation illumination (SPRM-ARI) is used to observe the hygroscopic growth and water content of single nanoparticles in situ. The hygroscopic growth results of a single-component nanoparticle are well matched with the extended aerosol inorganic model (E-AIM) results, and the proposed method remains reliable even when the relative humidity (RH) exceeds 90%. For a bicomponent nanoparticle (with NaCl as the primary content), the presence of a component without deliquescence phase transitions under increasing humidity conditions causes the measured data to differ from both the Zdanovskii-Stokes-Robinson (ZSR) model and E-AIM predictions in the low RH range. However, because of their complete liquefaction, the growth factor (GF) variation of the bicomponent nanoparticle is close to the model predictions in the high RH range. Finally, based on the positive correlation between particle volume and the gray intensity of SPRM-ARI, GF values can be obtained from the cube root of the gray intensity and the actual water content of single nanoparticles can then be derived.