Vertical profiles of trace gas and aerosol properties over the eastern North Atlantic: variations with season and synoptic condition
Authors: Wang Yang , Zheng Guangjie , Jensen Michael P. , Knopf Daniel A. , Laskin Alexander , Matthews Alyssa A. , Mechem David , Mei Fan , Moffet Ryan , Sedlacek Arthur J. , Shilling John E. , Springston Stephen , Sullivan Amy , Tomlinson Jason , Veghte Daniel , Weber Rodney , Wood Robert , Zawadowicz Maria A. , Wang Jian 
Published: July 2021

Abstract. Because of their extensive coverage, marine low clouds greatly impact the
global climate. Presently, the response of marine low clouds to the changes
in atmospheric aerosols remains a major source of uncertainty in climate
simulations. One key contribution to this large uncertainty derives from the
poor understanding of the properties and processes of marine aerosols under
natural conditions and the perturbation by anthropogenic emissions. The
eastern North Atlantic (ENA) is a region of persistent but diverse
subtropical marine boundary layer (MBL) clouds, where cloud albedo and
precipitation are highly susceptible to perturbations in aerosol properties.
Here we examine the key processes that drive the cloud condensation nuclei
(CCN) population in the MBL using comprehensive characterizations of aerosol
and trace gas vertical profiles during the Aerosol and Cloud Experiments in
the Eastern North Atlantic (ACE-ENA) field campaign. During ACE-ENA, a total
of 39 research flights were conducted in the Azores: 20 during summer 2017
and 19 during winter 2018. During summer, long-range-transported aerosol
layers were periodically observed in the lower free troposphere (FT),
leading to elevated FT CCN concentrations (NCCN). Both biomass burning
and pollution from North America contribute to submicron aerosol mass in
these layers, with pollution likely the dominant contributor. In contrast,
long-range transported continental emissions have a much weaker influence on
the aerosol properties in the ENA during the winter season. While the
entrainment of FT air is a major source of particle number in the MBL for
both seasons, on average it does not serve as a direct source of CCN in the
MBL because the average FT NCCN is the same or even lower than that in
the MBL. The particle number flux due to FT entrainment is dominated by
pre-CCN (particles that are too small to form cloud droplets under typical
conditions, i.e., particles with sizes below the Hoppel minimum) due to the
elevated Npre-CCN in the lower FT. Once these pre-CCN are entrained into
the MBL, they can grow and reach CCN size range through condensational
growth, representing an indirect and major source of MBL CCN in the ENA. The
impact of synoptic conditions on the aerosol properties is examined. Under
pre-front and post-front conditions, shallow convective activity often leads
to a deep and decoupled boundary layer. Coalescence scavenging and
evaporation of drizzle below clouds lead to reduced NCCN and
larger accumulation-mode particle sizes in the upper cloud-containing
decoupled layer, indicating that surface measurements overestimate the
NCCN relevant to the formation of MBL clouds under decoupled conditions.