Analysis of AERONET Remote Sensing and LARGE In-Situ Measurements of Aerosol Properties During the KORUS-AQ Campaign with Focus on Pollution Transport Events and the Influence of Cloud/Fog and High RH

The focus of our investigation is of major fine mode aerosol pollution events in South Korea during the KORUS-AQ campaign, particularly when cloud fraction was high. We analyze the newly developed AERONET Version 3 data that have significant changes to cloud screening algorithms, allowing many more fine-mode observations in the near vicinity of clouds. Particular case studies include May 25-26, 2016 when cloud fraction was very high over much of the peninsula, associated with a frontal passage and advection of pollution from China. This is compared to the May 31 case of pollution transport from China with less cloud cover over the sites, yet with likely cloud influence upwind. These cloud influenced Chinese transport dates had the highest AOD and highest surface PM concentrations of the entire campaign. Another cloud/high relative humidity (RH) case is June 9, 2016 when there was fog over the West Sea, and this seems to have affected aerosol properties well downwind over the Korean peninsula. We investigate the meteorological factors involved in influencing the aerosol transport to Korea and possible cloud and/or high RH influences on aerosol properties that are also conducive to gas-to-particle conversion. In comparison the aerosol properties on air stagnation days with very low cloud cover and relatively low RH (May 17 & 18), when local Korean emissions dominated, were also investigated. Volume size distributions show marked differences between the Chinese transport days with cloud influences and the local pollution stagnation days. All of these days had KORUS-AQ research aircraft flights that provided observations of aerosol absorption, particle chemistry, particle size distributions and vertical profiles of extinction. AERONET retrievals and aircraft in situ measurements both showed high single scattering albedo (weak absorption) on these cloudy or cloud influenced days, plus in situ measurements showed large AOD enhancements at ambient relative humidity versus dry AOD on the China transport days. We also investigate the relationship between aerosol fine mode radius and AOD and the relationship between aerosol single scattering albedo and fine mode particle radius from the AERONET almucantar retrievals for the interval of April through June 2016 for 17 AERONET sites in South Korea. Strongly increasing fine mode radius (leading to greater scattering efficiency) as fine mode AOD increased is one factor contributing to a trend of increasing single scattering albedo as fine AOD increased. Additionally, the new AERONET Hybrid sky radiance scan retrievals that allow for inversions to be made at smaller solar zenith angles are analyzed and compared to almucantar retrievals.