Vertical Distribution of Particulate Matter and its Relationship with Planetary Boundary Layer Structure in Shenyang, Northeast China

Abstract

The impact of the planetary boundary layer (PBL) structure on the vertical distribution of aerosols in Northeast China, which experiences air pollution frequently in autumn and winter, is not well understood. We investigated the characteristics of the vertical distribution of particulate matter (PM 1 , PM 2.5 , and PM 10) mass concentrations and their relationships with PBL structures in Shenyang, a provincial capital city in Northeast China, using balloon sounding data collected during an intensive observation period in November 2018. Aerosols typically decreased with increases in height and were mostly distributed below 400 m at night and reached higher altitudes (~800 m) in the daytime due to convective turbulence. The concentration ratios of PM 1 /PM 2.5 and PM 2.5 /PM 10 measured about 0.6 and 0.8, respectively, below 0.6-1.0 km during the daytime, and below 0.5 km at nighttime. On average, stronger atmospheric stability resulted in greater vertical gradients and higher PM concentrations near the surface. During four air pollution episodes (November 1-4, 7-10, 14-15, and 25-27), when atmospheric stability was strong at night, aerosols tended to remain in a shallow stable surface layer (< 300 m) and at the bottom of a residual layer (250-500 m) due to weak vertical mixing. After sunrise, these aerosols were mixed uniformly in the PBL (the depth increasing from 200 m to more than 1 km), subsequently affecting surface air quality. In addition, strong wind speeds and wind shears caused by nocturnal low-level jets and cold front systems influenced the formation and evolution of air pollution episodes. These processes controlled aerosol transport/dispersion processes and can modify atmospheric stability and PBL height. These results have important implications for understanding the vertical distribution of aerosols, and the crucial roles that PBL structures play in modulating aerosol pollution in Shenyang.

Measurement report: Effect of wind shear on PM&lt;sub&gt;10&lt;/sub&gt; concentration vertical structure in urban boundary layer in a complex terrain

“…Research presenting impact of PBL dynamics, confirms that during convective conditions (mechanical and thermal turbulence) vertical distribution of PM concentrations is uniform (Li et al 2019;Strbova et al, 2017;Wang et al 2018). Mechanical turbulence can be caused by strong wind shear connected to LLJ (Li et al 2019), mountain waves (Zangl, 2003, hydraulic jump (Kishcha et al 2017), rotors (Kunin et al 2019) or passage of an atmospheric front. In the present study, wind shear turned out to be the most important factor in terms of PM10 vertical profile modification.…”

“…black carbon (Zhou et al, 2020) or organic PM10 tracers like levoglucosan (Marynowski et al, 2020). Numerous studies indicate that an important factor that affects the pollution profile is the wind profile (Li and Han, 2016;Zhou et al, 2020), occurrence of low-level jet (Li et al, 2012;Li et al, 2019) or downward flows of pollutants (Han et al, 2018) which may strongly modify diurnal cycle of a pollutant concentration at the lowest part of the troposphere.…”

“…Studies presenting the impact of ABL dynamics on vertical pollutant structure indicate that a low-level jet combined with a strong wind shear affect the transportation of the pollution e.g. by removing it (Trompetter et al, 2013) or bringing it in (pushing into the residual layer), and by favoring the growth of ABL height and weakening the stability of the atmosphere (Li et al, 2019).…”

Key Points in Air Pollution Meteorology

“…However, there is also evidence of quite different results. The formation and evolution of air pollution episodes is influenced by wind speeds and wind shears caused by nocturnal low-level jets and cold fronts [ 29 ]. Moreover, the chemical production of sulphate, nitrate, and ammonium may increase due to the uplift of air pollutant together with humidity, which facilitates the heterogeneous and aqueous-phase oxidation of precursors [ 30 ].…”

The thermodynamic structures of the planetary boundary layer dominated by synoptic circulations and the regular effect on air pollution in Beijing

“…Hu et al (2014) found that westerly warm advection above the Loess Plateau was transported over the NCP and imposed a thermal inversion, which acted as a lid and capped the pollution in the boundary layer (Xu et al, 2019). The dynamic structure of the PBL, including wind shears and turbulence, can modify air quality by influencing the dispersion and transport processes of air pollutants (Li et al, 2019). Zhang et al (2020) found that a much weaker vertical wind shear was observed in the lower part of the PBL under polluted conditions compared with that under clean conditions, which could be caused by the strong ground-level PM 2.5 accumulation induced by weak vertical mixing in the PBL.…”