LAUSR

The meteorological surface variables and boundary-layer structures in the Seoul Metropolitan Area (SMA) were examined during the MAPS-Seoul (Megacity Air Pollution Studies in Seoul) intensive field experiment (18 May–13 June 2015). Data were obtained from a surface energy balance system and a ceilometer installed at a UMS-Seoul (urban meteorological observation system in the SMA) station located in the eastern part of the city of Seoul. A series of migratory anticyclone passages under a strong El Nino event meant that the experimental period was much warmer and drier than the climatological ones. The mean air temperature during this period was 2.6°C higher than the recent 30-year (1981–2010) climatological value, while only one-eighth of the climatological precipitation was recorded. Twelve mist event days were recorded, as were four of haze and six of rainfall events. The SMA was found to be affected by the sea-land breeze: Westerly winds (sea-breezes) were dominant from the afternoon till the early evening, while easterlies (land-breezes) were dominant before the morning. Finally, the vertical profile of the attenuated backscatter obtained by the ceilometer indicated that the maximum daily mixing-layer heights (MLHs) on days with mist/haze and much cloud cover were lower than on days with no-mist/haze and less cloud cover, respectively, mainly due to a decrease in the downward solar radiation. However, the effect of mist/haze on the daytime MLH evolution was larger than that of cloud cover. The MLH also exhibited an altitude similar to that of the steepest vertical gradient of sulfate and organic aerosol concentration, obtained from the airborne measurement on 13 June 2015.