Statistical Characteristics of Air Quality Index DAQx*-Specific Air Pollutants Differentiated by Types of Air Quality Monitoring Stations: A Case Study of Seoul, Republic of Korea

Seoul has a high density of air quality monitoring stations (AQMSs) grouped into roadside, urban, and background types. Using the extensive data from 42 AQMSs in the period 2018 to 2021, the statistical characteristics of air pollutants required to calculate the daily air quality index DAQx* (daily maximum 1 h O-3 and NO2 means and daily 24 h PM10 and PM2.5 means) are determined, depending on station types and three temporal periods (individual years, winters, and summers). The results for (i) annual cycles, which include peak concentrations of PM10 (up to 517 mu g/m(3) in May 2021) and PM2.5 (up to 153 mu g/m(3) in March 2019) owing to transboundary transport, (ii) annual medians, (iii) annual scattering ranges, (iv) partitioning of frequencies into DAQx*-related concentration ranges, and (v) maximum daily variations within individual station types indicate clear statistical air pollutant characteristics depending on the station types. They were primarily caused by different emission and atmospheric exchange conditions in a circular buffer around each AQMS, which are often approximated by urban form variables. The maximum daily variations were highest in the middle NO2 concentration range of the "satisfying" class for the roadside type (between 53% in summer 2019 and 90% in winter 2020).