Variations of Aerosol Concentrations due to Summer Afternoon Thunderstorm Activities in the Taipei Basin

The Taipei Basin, located in northern Taiwan, is surrounded by hills and mountains with two river valleys funneling the surface airflow to open sea. Its orographic structure, along with other synoptic and thermodynamic features, creates a favorable environment for afternoon thunderstorm activity. Due to the local southwest monsoon, summer is a very dry season in northern Taiwan, but is the season of maximum rainfall in the Taipei Basin. This is mainly induced by afternoon cumulus convection due to the transport of moist from ocean to land by sea breeze, and the orographic lifting of air. Because the Taipei Basin is a very urbanized region, the surface heat budget is another reason for enhanced convection, and thus rainfall. Urbanization also enhances the contribution of aerosols, having different effects in precipitation. Most aerosols are eventually cleansed from the atmosphere by different sink processes. The wet deposition is the main aerosol removal process and it involves the mechanisms of rainout (in-cloud scavenging), and washout (below-cloud scavenging). The aerosols over the Taipei Basin can be washed out into rivers by precipitation and transported into the open sea by streamflow. This means that rainfall produced by the afternoon thunderstorms, could possibly be a mechanism of pollution in the Taipei basin. In an effort to understand the variations of aerosol number concentration due to washout, a computational analysis was conducted to find the below-cloud scavenging efficiency over three locations within the Taipei Basin. Aerosol concentration and rainfall measurements were retrieved from the pollution monitoring system of the Taiwan Environmental Protection Administration (EPA), for the stations of Banqiao, Yonghe, and Guting. In a period of 10 years (2009-2018), three afternoon thunderstorm cases per year were selected, in which the maximum rainfall exceeded the 1.0 mm/hr. According to the Continuity equation, the aerosol concentration change rate will depend on the aerosol advection, the source and the sink processes. However, because most afternoon thunderstorm cases are associated with heavy precipitation, it was assumed that there is no advection and no source of aerosol during the thunderstorm periods. This assumption will leave the scavenging efficiency to be only a function of the concentration change rate and the rainfall rate, with the latter being inversely proportional to the efficiency. Results for the three different locations showed similar scavenging coefficients, but outlier cases proved that other factors, besides the rainfall rate, contribute to the variations in aerosol concentrations. Further research needs to be done taking into consideration the rain drop size and to identify signals of precipitation effects on aerosol concentrations.