Abstract
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.
Highlights
Remote marine low cloud systems have a large spatial coverage and are susceptible to aerosol perturbations because of their relatively low optical thickness and low background cloud condensation nuclei (CCN) concentrations
The vertical profiles of potential temperature and liquid water content (LWC) show that the marine boundary layer (MBL) heights during the summer and winter intense operation periods (IOPs) are 1220 ± 450 and 1640 ± 480 m, respectively, indicating a strong seasonal variation
To understand the air mass origin and its impact on the vertical profiles of trace gases and aerosols, we calculated the 10 d back trajectories of air masses arriving at three altitudes (500, 1500, and 3000 m) above the eastern North Atlantic (ENA) site during the G-1 flight days using the Hybrid SingleParticle Lagrangian Integrated Trajectory (HYSPLIT) version 4 model (Stein et al, 2015)
Summary
Remote marine low cloud systems have a large spatial coverage and are susceptible to aerosol perturbations because of their relatively low optical thickness and low background cloud condensation nuclei (CCN) concentrations. The properties of aerosols from the ENA were studied using long-term observations at the Mace Head Atmospheric Research Station located on the west coast of Ireland (O’Dowd et al, 2004; Ovadnevaite et al, 2014; Yoon et al, 2007) These studies show that a major driver of the seasonal variation of midlatitude marine aerosol is biological activity because the majority of the aerosol mass, both inorganic sea salt and organic matter, was linked to bubble-mediated aerosol production. During ACE-2, the variation of aerosol size distribution and chemical composition was examined during three Lagrangian experiments over periods of ∼ 30 h as air masses advected over the North Atlantic (Johnson et al, 2000) These experiments show that the production of sea salt particles at elevated wind speeds leads to an increase in accumulation-mode particle concentration (Hoell et al, 2000). The impact of synoptic conditions on MBL structure and the vertical profiles of the aerosol populations are examined, and the implications for studying aerosol–cloud interactions using ground-based aerosol measurements are discussed
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