Vertical profiles of trace gas and aerosol properties over the Eastern North Atlantic: Variations with season and synoptic condition
- 1Center for Aerosol Science and Engineering, Washington University in St. Louis, St. Louis, Missouri, USA
- 2Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, Missouri, USA
- 3Environmental and Climate Sciences Department, Brookhaven National Laboratory, Upton, New York, USA
- 4School of Marine and Atmospheric Sciences, Stony Brook University, State University of New York, Stony Brook, New York, USA
- 5Department of Chemistry, Purdue University, West Lafayette, Indiana, USA
- 6Atmospheric Sciences & Global Change, Pacific Northwest National Laboratory, Richland, Washington, USA
- 7Department of Geography & Atmospheric Science, University of Kansas, Lawrence, Kansas, USA
- 8Sonoma Technology, San Francisco, CA, USA
- 9Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA
- 10Department of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, Georgia, USA
- 11Department of Atmospheric Science, Washington University, Seattle, Washington, USA
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 at ENA. The impact of synoptic condition 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 leads to much 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.
Yang Wang et al.
Yang Wang et al.
Yang Wang et al.
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