Ramp‐Like PM Accumulation Process and Z‐Less Similarity in the Stable Boundary Layer

Shi, Yu; Hu, Fei

doi:10.1029/2019gl086530

Cited by 9 publications

(2 citation statements)

References 45 publications

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“…According to conventional theory, TKE generally decreases with altitude but when there is mechanical turbulence at higher altitudes associated with strong wind shear, the TKE at a higher altitude is often greater than that at ground level, and TKE can be transported toward the surface, which is called an “upside‐down” structure (Mahrt & Vickers, 2002). Observation data from a 7‐layer ultrasonic anemometer mounted on the 325 m tower in Beijing have also revealed the existence of upside‐down structures (Shi & Hu, 2020). Strong wind shear was observed or simulated near the LLJ and since wind shear is a source of turbulent mixing, the presence of the LLJ often affects the vertical distribution of TKE.…”

Section: Resultsmentioning

confidence: 99%

Observed Evidence That Subsidence Process Stabilizes the Boundary Layer and Increases the Ground Concentration of Secondary Pollutants

et al. 2022

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Subsidence in atmospheric boundary layer (ABL) appears to be rare and short‐lived, so it is very difficult to observe. In this paper, field experimental data from a large tethered balloon (1,900 m3) synchronously measuring meteorological parameters and air pollutants, a ground‐based aerosol lidar, a Doppler wind lidar, and some other ground observations were used to analyze the formation mechanism of subsidence and its influences on the variation of meteorological parameters and pollutant concentrations of the ABL. Results show that the occurrence of subsidence was closely correlated with the cold advection accompanied by cold front. Pollutants were horizontally transported to the observation site driven by southwest low‐level jet. Under the influence of subsidence behind the cold front and turbulent kinetic energy transported toward the surface, the vertical downward transport of pollutants aloft lasted for nearly 5 hr. The observation data of the tethered balloon suspending at 500 m show that the concentrations of PM2.5, NO3−, SO42−, and NH4+ increased simultaneously. O3 concentration in the lower atmosphere also increased. Subsidence had significant effects on the secondary inorganic aerosol concentrations but the concentration of organic aerosols was not affected. Subsidence led to a stronger inversion layer, further suppressing the vertical dispersion of pollutants. Both the stronger subsidence‐induced inversion layer and the downward transport of pollutants have led to an increase in the ground PM2.5 concentration. Our findings demonstrate the close connection between subsidence and air pollution process and may provide the scientific reference for air quality potential forecast.

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Section: Resultsmentioning

confidence: 99%

Observed Evidence That Subsidence Process Stabilizes the Boundary Layer and Increases the Ground Concentration of Secondary Pollutants

et al. 2022

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“…Since most of the pollution sources are distributed near the ground, in most cases, such as when there are no obvious elevated pollution sources or regional transportation of pollutants, previous vertical profiles of particles observed show that the concentration of particulate matter near the ground was highest (Lyu et al 2018, Sun et al 2021. The surface PM 2.5 during heavy pollution in Beijing often experiences slow accumulation for about 3-4 days (Wu et al 2017), and then under strong winds, its concentration decreases rapidly within a few hours, showing obvious 'ramp-like' structures (Shi and Hu 2020).…”

Section: Introductionmentioning

confidence: 96%

Important role of turbulent wind gust and its coherent structure in the rapid removal of urban air pollution

Shi

Qincun

Liu

et al. 2022

Environ. Res. Commun.

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The removal process of urban air pollution is very rapid even under the strong obstruction of various buildings in the city. In this study, based on high-accuracy ultrasonic anemometer observations at seven different altitudes on a 325 m tower, we proposed a physical mechanism in which turbulent wind gust and its coherent structure may play important roles in this phenomenon. Pollutants suspended within the urban canopy layer UCL cannot be quickly removed by means of basic flow but diffuse and crosse the UCL by means of the coherent structure of gust wind and then propagate further under strong basic flows above the UCL. With a continuous increase in wind speed in the whole layer, combined with gust-wind and turbulence mixing, the rapid transformation from heavily polluted days to clean days in urban areas can take place in just a few hours. Turbulent wind gust and its coherent structure were extracted by third-order Daubechies wavelet transformation. The downwind wind was decomposed into three separate components, basic flow ¯u(t), gust-wind 〖u^'〗_g (t), and turbulence 〖u^'〗_t (t). The results show that in the early rapid drop of urban surface pollutant concentration, horizontal wind speed at 8 m was still low, and sinking movements dominated in the lower layer. Friction velocity calculated from gust-wind u_(*g) significantly affected the surface PM2.5 concentration, and was the most obvious factor with an inverse correlation coefficient -0.56. The correlation coefficient of coherent structure of gustiness Rugwg at 8 m basically changed to positive values before the rapid decline of the surface PM2.5 concentration. The concentration of PM2.5 decreased significantly when Rugwg>0, and exhibited an increasing trend when Rugwg<0. The close relationship between the concentration of PM2.5 and Rugwg indicated the important role of the coherent structure of gustiness in the rapid removal process of urban pollutants. The research deepens the vertical exchange process of matter and energy in the UCL, and provides a scientific reference for the rapid dissipation mechanism of urban air pollut

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Different Turbulent Regimes and Vertical Turbulence Structures of the Urban Nocturnal Stable Boundary Layer

Shi

Qingcun

et al. 2023

Adv. Atmos. Sci.

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Ramp‐Like PM Accumulation Process and Z‐Less Similarity in the Stable Boundary Layer

Cited by 9 publications

References 45 publications

Observed Evidence That Subsidence Process Stabilizes the Boundary Layer and Increases the Ground Concentration of Secondary Pollutants

Observed Evidence That Subsidence Process Stabilizes the Boundary Layer and Increases the Ground Concentration of Secondary Pollutants

Important role of turbulent wind gust and its coherent structure in the rapid removal of urban air pollution

Different Turbulent Regimes and Vertical Turbulence Structures of the Urban Nocturnal Stable Boundary Layer

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