Observed Interactions Among Haze, Fog and Atmospheric Boundary Layer during a Haze-fog Episode in the Yangtze River Delta Region, Eastern China

Abstract

A severe haze-fog episode occurred in the Yangtze River Delta region of eastern China during 22-30 November, 2018. In this period, the PM2.5 mass concentration and meteorological parameters at the surface were collected at the Station for Observing Regional Processes of the Earth System site in Nanjing. The vertical distributions of PM2.5, humidity and potential temperature below 500 m were observed simultaneously by an unmanned aerial vehicle, and the profile of potential temperature at 1400 local standard time on each day was also observed by radiosonde at the same site. During the first four days, the PM2.5 mass concentration increased, the maximum convective planetary boundary layer height (CBLH) decreased, and the air humidity increased. These are favorable conditions for fog formation. In the latter five days, fog formed on four days, with a lowering of the CBLH and a further increase in PM2.5 mass concentration. We found that the fog top cooling induced a potential temperature jump (i.e., sharp increase of potential temperature) with much warmer temperatures above the cloud top cooling and that this particular thermal structure was maintained until the end of the fog period, which significantly suppressed the daytime development of the planetary boundary layer after fog dissipation. The fog-induced reduction of the CBLH further increased the PM2.5 mass concentration. We also found that the wet deposition of fog on PM2.5 was negligible. The scavenging effect of fog on aerosols only acts during a fog period. When the fog dissipates, the aerosols are liberated from the fog droplets to the atmosphere.

Meteorological Characteristics during Periods of Greatly Reduced PM2.5 Concentrations in March 2020 in Seoul

“…Local winds are dominant under clear skies and when a weak synoptic system is present (Park, 2018). The influence of factors such as wind, temperature, and relative humidity on PM2.5 concentrations is similar to findings from previous studies (Park et al, 2019;Meng et al, 2020;Zou et al, 2020).…”

“…Figure 10 shows the vertical profile of virtual potential temperature at the Osan WMO monitoring station at 09 LST and 21 LST on days with the highest PM2.5 concentrations in March 2019 and the lowest PM2.5 concentrations in March 2020. The formation and evolution of high PM2.5 concentration events are closely related to temperature inversion layers (Li et al, 2018;Zou et al, 2020). The inversion strength below 2 km at 9 LST and 21 LST was 7.5 K km -1 and 5.7 K km -1 on March 5, 2019 on the day with the highest concentration, and 6.7 K km -1 and 6.9 K km -1 on March 5, 2020 on the day with the lowest concentration (Table 3).…”

“…Mixing height ranged from 430 m to 1,986 m in March and from 444 m to 1,987 m in March 2020, with the maximum occurring in the afternoon and the minimum occurring in the early morning. Mixing height is generally higher during periods with lower PM2.5 concentrations than in periods with higher PM2.5 concentrations(Park et al, 2018;Park et al, 2019;Zou et al, 2020) Monin-Obukhov length ranged from 62 m to 999 m in March 2019 and from 97 m to 999 m in March 2020. Monin-Obukhov length is the height at which…”