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: April 2023

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 in the 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 in the 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 (CCN). Here, we investigate the impact of long-range
transported continental aerosol on the regional aerosol regime 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 in the Eastern North Atlantic (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 in the 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 d backward trajectories
analysis, and we evaluate CALIPSO aerosol products to assess, respectively,
the origins and compositions of aerosol particles arriving at the ENA site. Subsequently,
we group the events into three categories, (1) mixture of dust and marine
aerosols, (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
plumes dominated by the mixture of dust and marine aerosol in the ENA in the
winter caused significant increases in baseline Ntot. Simultaneously,
the baseline particle size modes and CCN potential activation fraction
remained almost unvaried, while cloud condensation nuclei concentrations
increased proportionally to Ntot. Events dominated by a mixture of marine
and polluted continental aerosols in spring, fall, and winter led to a
statistically significant increase in baseline Ntot, a shift towards
larger particular sizes, a higher CCN potential activation fractions, and
cloud condensation nuclei concentrations of >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 baseline aerosol particle concentrations in the ENA.
However, particle diameters were larger than under baseline conditions, and
CCN potential activation fractions were >75 % higher.
Consequentially, cloud concentration nuclei concentrations increased by
∼ 115 % during the period affected by the biomass burning
events. Our results suggest that, through the year, multiday events of
long-range continental aerosol transport periodically affect the ENA and
represent a significant source of CCN in the marine boundary layer. Based on
our analysis, in 2017, the multiday aerosol plume transport dominated by
a mixture of dust and marine aerosol, a mixture of marine and polluted
continental aerosols, and biomass burning aerosols caused increases in the
NCCN baseline regime of, respectively, 6.6 %, 8 %, and 7.4 % at SS
0.1 % (and, respectively, 6.5 %, 8.2 %, and 7.3 % at SS 0.2 %) in the
ENA.