10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Impact of Mixing State and Size-resolved Hygroscopicity of Urban Aerosols on CCN Activity in Seoul, Korea
NAJIN KIM, Minsu Park, Seong Soo Yum, Hye Jung Shin, Jong Sung Park, Joon Young Ahn, Yonsei University
Abstract Number: 661 Working Group: Air Quality in Megacities: from Sources to Control
Abstract Studies for atmospheric aerosol properties and their impacts on global climate change and air quality has been conducted steadily. Especially, hygroscopic properties of aerosols, which describe how well aerosols interact with water vapor in sub- and supersaturated conditions, are considered to be particularly important as they are closely related to cloud condensation nuclei (CCN) activity that is connected to aerosol-cloud interaction problems. Although many field campaigns for examining aerosol properties have performed around the world, observational data in various regions is still needed as aerosols have diverse sources and temporal-spatial distribution.
As an effort to add such observational data, field campaigns with a comprehensive set of measurements were made during May-June 2015 (Megacity Air Pollution Studies, MAPS-Seoul) and 2016 (KORea-United States Air Quality study, KORUS-AQ) in Seoul, highly urbanized area where various anthropogenic sources exist. Ground-based measurements for aerosols included aerosol (condensation nuclei, CN) and CCN number concentrations (NCN and NCCN), aerosol size distribution and aerosol chemical composition. Especially, size-resolved aerosol hygroscopicity with four dry diameters were measured by hygroscopic tandem differential mobility analyzer (HTDMA).
In Seoul, average NCN (>10nm) was about 11000 cm-3 for KORUS-AQ which was slightly lower than those of MAPS-Seoul and showed diurnal pattern peaking at morning and midday. Unlike NCN , both of NCCN at 0.6% supersaturation had little diurnal variation. κ values which are obtained from HTDMA results and represent aerosol hygroscopicity, ranged 0.17-0.27 (MAPS-Seoul) and 0.11-0.24 (KORUS-AQ). Aerosols were classified into three types based on mixing state and hygroscopic growth factor (GF): Type 1 (externally mixed aerosol), Type 2 (Internally mixed and growth aerosol) and Type 3 (Internally mixed and non-growth aerosol). During both of campaign periods, small (30nm and 50nm) and large (100nm and 150nm) particles have different diurnal pattern of aerosol types as well as aerosol hygroscopicity. Significant portion of externally mixed particles and their distinct diurnal pattern implies that mixing state of aerosols has to be considered when prediction of NCCN is performed. Predicted NCCN with two assumptions (internal and external mixture) were calculated based on size-resolved hygroscopicity data and then compared with measured NCCN . In practice, better results of predicted NCCN with externally mixture assumption were obtained rather than simple internal mixture assumption. Comparison of campaign results with long-term observational data and detailed analysis will be shown at the conference.