Long-range transported continental aerosol in the Eastern North Atlantic: three multiday event regimes influence cloud condensation nuclei
Authors: Gallo Francesca , Uin Janek , Sanchez Kevin  J. , Moore Richard H. , Wang Jian , Wood Robert , Mei Fan , Flynn Connor , Springston Stephen , Azevedo Eduardo B. , Kuang Chongai , Aiken Allison C. 
Published: September 2022

Abstract. The Eastern North Atlantic (ENA) is a region dominated by pristine marine environment and subtropical marine boundary layer clouds. Under unperturbed atmospheric conditions, the regional aerosol regime at ENA varies seasonally due to different seasonal surface-ocean biogenic emissions, removal processes, and meteorological regimes. However, during periods when the marine boundary layer aerosol at ENA is impacted by particles transported from continental sources, aerosol properties within the marine boundary layer change significantly, affecting the concentration of cloud condensation nuclei. Here, we investigate the impact of long-range transported continental aerosol on the regional aerosol regime at ENA using data collected at the U.S. Department of Energy’s (DOE) Atmospheric Radiation Measurement (ARM) User Facility on Graciosa Island in 2017 during the Aerosol and Cloud Experiments (ACE-ENA) campaign. We develop an algorithm that integrates number concentrations of particles with optical particle dry diameter (Dp) between 100 and 1000 nm, single scattering albedo, and black carbon concentration to identify multiday events (with duration > 24 consecutive hours) of long-range continental aerosol transport at ENA. In 2017, we detected nine multiday events of long-range transported particles that correspond to ~7.5 % of the year. For each event, we perform HYSPLIT 10-day backward trajectories analysis, and we evaluate CALIPSO aerosol products to assess respectively origins and compositions of aerosol particles arriving at ENA. Subsequently, we group the events into three categories 1) mixture of dust and marine aerosols from North Africa, 2) mixture of marine and polluted continental aerosols from industrialized areas, and 3) biomass burning aerosol from North America and Canada, and we evaluate their influence on aerosol population and cloud condensation nuclei in terms of potential activation fraction and concentrations at supersaturation of 0.1 % and 0.2 %. The arrival of dust and marine aerosol mixture plumes at ENA in the winter caused significant increases in Ntot. Simultaneously, the particle size modes and CCN potential activation fraction remained almost unvaried, while cloud condensation nuclei concentrations increased proportionally to Ntot. Events dominated by mixture of marine and polluted continental aerosols in spring, fall, and winter led to statistically significant increase in Ntot, shift towards larger particular sizes, higher CCN potential activation fractions, and cloud condensation nuclei concentrations > 170 % and up to 240 % higher than during baseline regime. Finally, the transported aerosol plumes characterized by elevated concentration of biomass burning aerosol from continental wildfires detected in the summertime did not statistically contribute to increase aerosol particle concentrations at ENA. However, particles diameters were larger than under baseline conditions and CCN potential activation fractions was > 75 % higher. Consequentially, cloud concentration nuclei concentrations increased ~115 % during the period affected by the events. Our results suggest that, through the year, multiday events of long-range continental aerosol transport periodically affect ENA and represent a significant source of CCN in the marine boundary layer. Based on our analysis, in 2017, the multiday aerosol plume transport events at ENA caused a total NCCN increase at SS 0.1 % of ~22 % (23 % at SS 0.2 %) being 6.6 % (6.5 % at SS 0.2 %), 8 % (8.2 % at SS 0.2 %), and 7.4 % (7.3 % at SS 0.2 %) respectively the contribution attributable to plumes dominated by mixture of dust and marine aerosols, mixture of marine and polluted continental aerosols, and biomass burning aerosols. Changes in baseline Ntot and particle size modes during the events might be used as a proxy to estimate the contribution to NCCN.