Abstract
It is widely reported that the peripheral circulation of typhoon favors for the formation of sustained ozone episodes. However, the process how it impact on the day-to-day ozone pollution levels during the episodes have not been clearly studies, which is crucial for better prediction of the daily ozone variation. In this study, the analysis of ground observation, wind profile data, and model simulation are integrated. By analyzing the wind profile radar observations, we found a weak winds depth deepening (WWDD; vertical depth of the weak winds increased), which is more correlated to the ground-level ozone variation than surface weak wind. Long-term statistical analyses show that the WWDD is a common weather phenomenon that occurs in the peripheral subsidence region of typhoons and was generally accompanied by ozone pollution episodes. WRF-Chem with process analysis simulation show that under the impact of the peripheral subsidence chemical formation (CHEM) and vertical mixing (VMIX) effects are two major contributors to the enhancement of ozone levels, while the advection (ADV) are always negative values. But regarding the daily variability of the daytime ozone levels during the episode, it do not determined by the daily variation of daytime CHEM and VMIX, but that of the ADV term. A detail day-to-day analysis show that weak subsidence associated with typhoon periphery provide the premise for the clear sky and warmer air, which is conducive for the ozone photolysis formation (CHEM) above the ground in planetary boundary layer (PBL) and compensate the ozone through the positive VMIX effects on the ground. The WWDD induced by the peripheral circulation of typhoon system provide the premise for the day-to-day positive contribution of ADV term to ozone enhancement, which play an important role in determining the day-to-day daytime ozone variation. These results indicate the important role of the WWDD in the lower troposphere for the formation of sustained ozone episodes due to the peripheral circulation of the typhoon, which helps to better predict the daily changes of daytime ozone levels.
Highlights
The Pearl River Delta (PRD) located in the coastal region of South China, which is often affected by typhoon systems, has experienced major economic development and urbanization in the past two decades, and has been accompanied by large increases in air pollution and decreases in visibility (Wang et al, 1998, 2001; Lai and Sequeira, 2001)
After Typhoon Nepartak made landfall at Shishi City on 9 July, the peripheral subsidence had moved to the western area of the PRD region (Fig. 2g–h) and the PRD region was influenced by weak vertical motion and a weak horizontal wind field
By calculating a sum of chemical formation (CHEM)+vertical mixing (VMIX)+convective processes (CONV) ( TOTAL_SUM_CAC in the Table 4), we found this three processes changed to negative values during the episode period, while the advective transport (ADV) term changed to positive values and determined the sustained increase of daytime ozone
Summary
The Pearl River Delta (PRD) located in the coastal region of South China, which is often affected by typhoon systems, has experienced major economic development and urbanization in the past two decades, and has been accompanied by large increases in air pollution and decreases in visibility (Wang et al, 1998, 2001; Lai and Sequeira, 2001). Ozone pollution episodes are mainly triggered by weather conditions rather than by sudden increases from emission sources(Ziomas et al, 1995; Giorgi and Meleux, 2007; Lin et al, 2019). Several studies have reported the impacts of TC activity on meteorological factors that are favourable for air pollution over the PRD region(Feng et al, 2007; Chen et al, 2008; Wu et al, 2013). TCs are typical weather systems that are responsible for both high ozone and PM2.5 pollution over the PRD(Chen et al, 2008; Deng et al, 2019)
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