Modelling Aerosol Effects on Liquid Clouds in the Summertime Arctic
Journal of Geophysical Research - Atmospheres
Climate change is more severe in the Arctic than the lower latitudes, and the rapid warming has the potential to influence the ocean, land and atmospheric interactions (IPCC, 2013) . Given the Arctic sensitivity to climate change, aerosol particles are among the important climate forcing agents in this region, with a net cooling effect and offsetting around 60% of the greenhouse gas warming (Najafi et al., 2015; Stuecker et al., 2018) . Aerosols can influence climate directly, by absorbing and
... cattering sunlight, and indirectly, by modifying cloud properties (Haywood & Boucher, 2000) . The formation of clouds depends on the presence and availability of aerosol particles to uptake or condense water vapor at a given supersaturation. These particles are known as cloud condensation nuclei (CCN). Indirect effects of aerosols include their impact on cloud microphysical processes (first indirect effect-, cloud-albedo or Twomey effect) (Twomey, 1977) , and cloud lifetime (second indirect effect-Albrecht effect) (Albrecht, 1989) . Despite the importance of aerosol indirect effects, key uncertainties remain in the cloud radiative processes, and a better understanding of the Arctic aerosol-clouds interaction and its effect on the surface radiative flux is needed (Boucher et al., 2013; Kay et al., 2011; Shupe and Intrieri, 2004) . Specifically, thermodynamic state, cloud base height and cloud microphysics, such as number concentrations/size/shapes/ phases of aerosol/droplets, are the factors that strongly influence the cloud radiative effects (Chen et al., 2006; Coopman et al., 2018; Rosenfeld et al., 2019) . The impact of atmospheric chemistry connecting emissions to aerosols and hence to these radiative effects are thus of interest. Aerosols observed in the Arctic during winter and spring, including particulate organic matter, nitrate, sulfate and black carbon, mostly originate from North America and Eurasia (Sirois and Barrie, 1999; Stone et al., 2014) . Using four years of ground-based aerosol and radiation measurements based on Barrow and Alsaka, Garrett and Zhao (2006) showed an increase of the longwave emissivity and the presence of thin water clouds, due to the influence of pollution from mid-latitudes, especially during winter and spring when the long-range transport from lower latitudes to the Arctic is more prevalent. Coopman et al. (2016) examined solely anthropogenic pollution impacts on the low-level Arctic liquid clouds for a period between 2008 and 2010, and found the net aerosol-cloud interaction (ACI net ) values are sensitive to the pollution plumes originating from long-range transport.