Abstract
The characteristics of turbulence in the planetary boundary layer (PBL) and the aerosol optical and radiative properties during haze and haze–fog mixed episodes on 22–27 January 2021, in Shenyang, a provincial city in Northeast China, were analyzed using meteorological and aerosol observations. During the haze episode, the hourly mean PM2.5 concentration reached a maximum of 337 µg m−3 and visibility decreased to 1.6 km. The PM2.5 concentration decreased gradually during the haze–fog mixed episode as a result of the scavenging effects of fog, but visibility mostly remained below 1 km owing to high ambient relative humidity (>90%). During the haze–fog mixed episode, an increasing proportion of PM2.5 led to a higher ratio of the backward to the total scattering coefficient. As fog occurred, downward shortwave radiation arriving at the surface was significantly reduced, and upward longwave radiation increased and almost equaled the downward longwave radiation, which can be used as a good indicator for distinguishing haze and fog. Mechanical turbulence was weak during both episodes, and latent heat flux varied within a wider range during the haze–fog mixed episode. The PBL dynamic structure affected the vertical distribution of aerosols/fog droplets. Aerosol-rich layers appeared at altitudes below 0.5 km and above 0.6 km during the haze episode. The elevated aerosol layer was related to the aerosol transport from upstream polluted areas caused by strong upper-level turbulence, and it began to mix vertically after sunrise because of convective turbulence. Aerosols and fog droplets were mostly trapped in a shallower PBL with a height of 0.2–0.4 km during the haze–fog mixed episode because of weaker turbulence.
Highlights
IntroductionHaze and fog events typically characterized by high PM2.5 (particulate matter with aerodynamic diameters of 2.5 μm and smaller) concentrations have occurred frequently during the past few decades in China, in the most developed and highly populated regions [1,2,3]
Haze and fog events typically characterized by high PM2.5 concentrations have occurred frequently during the past few decades in China, in the most developed and highly populated regions [1,2,3]
Turbulence observations during two haze episodes in Shenyang showed that the PM2.5 concentration was well correlated with the friction velocity (u* ; positive and negative correlations in local-accumulation- and long-transport-related haze episodes) but did not have an obvious relationship with the scaling potential temperature, which means that the haze events were affected more by the dynamic effects of turbulence and less affected by its thermal effects [21]
Summary
Haze and fog events typically characterized by high PM2.5 (particulate matter with aerodynamic diameters of 2.5 μm and smaller) concentrations have occurred frequently during the past few decades in China, in the most developed and highly populated regions [1,2,3]. Severe haze and fog events can damage human and ecological health [4,5,6], cause low visibility and traffic safety issues [7,8,9], and even produce weather and climate change [10,11]. To understand the formation mechanisms and climate/radiation effects of haze and fog, researchers have investigated the physical and chemical properties of aerosols/fog droplets and meteorological conditions during severe haze and fog events over different regions in China [12,13,14,15,16,17]. As haze and fog events both occur in the planetary boundary layer (PBL), turbulence in the PBL can directly affect the horizontal transport and vertical mixing of aerosols/fog droplets and alter the near-surface visibility and air quality [18,19,20].
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