| dc.description.abstract | Sentinel-5 Precursor (S5P) is a new generation environmental satellite and provides products regarding trace gases concentrations along with cloud and aerosol information in global scale. This study focuses on nitrogen oxide (NOx), the main air pollutant in urban areas and the precursor of secondary aerosols and ozone, under the regional scale. As NOx is primary in the form of Nitrogen dioxide (NO2), this study aims to understand the spatiotemporal feature of NO2 in Taiwan. Tropospheric NO2 data retrieved from the TROPOMI onboard S5P and surface NO2 data collected by surface instruments from the Environmental Protection Administration (EPA) of Taiwan are used for analysis. The analyzed temporal period spans from June 2018 to May 2019, and surface observation is collocated with a daily revisit of S5P spatiotemporally. Those data are analyzed in 15 sub-regions in Taiwan to understand the relationship between tropospheric vertical column and surface concentration of NO2.
The preliminary results reveal high correlations between surface and tropospheric NO2 concentration in North (Taipei/ New Taipei City/ Keelung), Yunlin, Chiayi, Tainan, and Kaohsiung in S5P’s daily revisit rate, while Yilan, Taoyuan, and Hsinchu shows relatively low correlations. The mean concentration of NO2 on the monthly scale is analyzed to avoid possible impacts due to high-frequency variation in environmental factors. Results indicate there are high correlations in most counties between ground-based and space-borne observations. In other words, the tendency of surface NO2 is similar to the tropospheric NO2 in most countries in Taiwan, particularly on the monthly scale. Therefore, to monitor and analyze NO2 concentration, spaceborne TROPOMI on S5P might be an alternative option for conducting related research in Taiwan.
From the aspect of spatiotemporal characteristic analysis, Eastern Taiwan often has the lowest concentration; there is a higher concentration in the urban area because of the abundant emission sources. Moreover, during winter the concentration tends to be higher than summer, indicating certain connections between the environment and air pollutants. The NCEP reanalysis data, which represents the meteorological factors, is also applied in this study for investigating the uncertainty sources, revealing a strong correlation between wind direction and pollution concentration. The result shows that higher NO2 concentrations are usually under lower planetary boundary height (PBL), lower wind speed, lower tropospheric stability (LTS), lower 975 hPa temperature, and higher 975 hPa Relative humidity (RH), and this feature is enhanced in summer and spring.
An analysis of the collected data demonstrates that ground-based observation represents the local effect; satellite observation can represent the state of the environmental pollution, offering certain characteristics that cannot be identified by surface observation because of its ability to provide coverage of large area (urban, suburban and ocean) and observe total column density (near-surface and higher-level NO2). This study also proves that pollutants at higher altitudes play an important role in extreme air pollution events. | en_US |