Impact of Dry Intrusion Events on Composition and Mixing State of Particles During Winter ACE-ENA Study

Abstract. Long-range transport of continental emission has far reaching influence over remote regions resulting in substantial change in the size, morphology, and composition of the local aerosol population and cloud condensation nuclei (CCN) budget. Here, we investigate the physiochemical properties of atmospheric particles collected onboard a research aircraft flown over the Azores during the winter 2018 Aerosol and Cloud Experiment in the Eastern North Atlantic (ACE-ENA) campaign. Particles were collected within the marine boundary layer (MBL) and free troposphere (FT), after long-range atmospheric transport episodes facilitated by dry intrusion (DI) events. Chemical and physical properties of individual particles were investigated using complementary capabilities of computer-controlled scanning electron microscopy and X-ray spectro-microscopy to probe particle external and internal mixing state characteristics in the context of real-time measurements of aerosol size distribution, cloud condensation nuclei (CCN) concentration, and back trajectory calculations. While carbonaceous particles were found to be the dominant particle-type in the region, changes in the percent contribution of organics across the particle population (i.e., external mixing) shifted from 68 % to 43 % in the MBL and from 92 % to 46 % in FT samples during DI events. This change in carbonaceous contribution is counterbalanced by the increase of inorganics from 32 % to 57 % in the MBL and 8 % to 55 % in FT. The quantification of organic volume fraction (OVF) of individual particles derived from X-ray spectro-microscopy, which relates to the multi-component internal composition of individual particles, showed a factor of 2.06 ± 0.16 and 1.11 ± 0.04 increase in the MBL and FT, respectively, among DI samples. We show that supplying particle OVF into the κ-Kohler equation can be used as a good approximation of field measured in-situ CCN concentrations. We also report changes in the κ values between κMBL, non-DI = 0.48 to κMBL, DI = 0.41 and κFT, non-DI = 0.36 to κFT, DI = 0.33, which is consistent with enhancements in OVF followed by the DI episodes. Our observations suggest that entrainment of particles from long-range continental sources alters the mixing state population and CCN properties of aerosol in the region. The work presented here provides field observation data that can inform atmospheric models that simulate sources and particle composition in the Eastern North Atlantic.