Regional Characteristics of NO_2 Column Densities from Pandora Observations during the MAPS-Seoul Campaign

Vertical column density (VCD) of nitrogen dioxide (NO_2) was measured using Pandora spectrometers at six sites on the Korean Peninsula during the Megacity Air Pollution Studies-Seoul (MAPS-Seoul) campaign from May to June 2015. To estimate the tropospheric NO_2 VCD, the stratospheric NO_2 VCD from the Ozone Monitoring Instrument (OMI) was subtracted from the total NO_2 VCD from Pandora. European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis wind data was used to analyze variations in tropospheric NO_2 VCD caused by wind patterns at each site. The Yonsei/SEO site was found to have the largest tropospheric NO_2 VCD (1.49 DU on average) from a statistical analysis of hourly tropospheric NO_2 VCD measurements. At rural sites, remarkably low NO_2 VCDs were observed. However, a wind field analysis showed that trans-boundary transport and emissions from domestic sources lead to an increase in tropospheric NO_2 VCD at NIER/BYI and KMA/AMY, respectively. At urban sites, high NO_2 VCD values were observed under conditions of low wind speed, which were influenced by local urban emissions. Tropospheric NO_2 VCD at HUFS/Yongin increases under conditions of significant transport from urban area of Seoul according to a correlation analysis that considers the transport time lag. Significant diurnal variations were found at urban sites during the MAPS-Seoul campaign, but not at rural sites, indicating that it is associated with diurnal patterns of NO_2 emissions from dense traffic.