PNNL

Abstract

The Eastern North Atlantic (ENA) is a region dominated by the pristine marine environment and subtropical marine boundary layer clouds. 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, impacting cloud condensation nuclei (CCN). Here, we investigate the impact of long-range transported continental aerosol in the 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 submicron aerosol number concentration in (Ntot), single scattering albedo (SSA) and black carbon concentration to identify multiday events (> 24 consecutive hours) of long-range continental transport at ENA. In 2017, we identified nine multiday events of long-range transported particles that correspond to ~7.5% of the year. We group the events in three clusters 1) Elevated background, 2) High Accumulation mode, and 3) High CCN activation fraction, based on how they affect the regional baseline regime in terms of aerosol population and cloud condensation nuclei (CCN) budget. Furthermore, for each event, we perform HYSPLIT 10-day backward trajectories analysis, and we analyse CALIPSO aerosol products to assess respectively origins and compositions of aerosol particles arriving at ENA. Elevated background events are typically dominated by a mixture of dust and marine aerosols and cause significant increase in Ntot while the size mode is constant. The CCN activation fraction remain similar to baseline conditions and the CCN concentration (NCCN) increase proportionally to Ntot. High Accumulation mode events provoke statistically significant increase in Ntot, a shift to larger size distributions and higher CCN activation fractions. These events are dominated by a mixture of marine and continental polluted aerosols and lead to NCCN > +170% and up to 240% higher during the event than under baseline aerosol regime. Aerosol plumes arriving at ENA during high CCN activation fraction events are characterized by elevated concentration of biomass burning aerosol from continental wildfires. Despite not causing statistically significant increase in Ntot, the size of the particles is shifted towards larger diameters and CCN activation fractions are > 75% higher than under baseline conditions. As consequence NCCN increase ~115% during the period affected by the event. 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. In 2017, these events contributed to ~16% increase in NCCN at ENA. Changes in baseline Ntot and particle size modes during the events might be used as a proxy to estimate the contribution to NCCN.