What controls marine boundary layer aerosols over the Atlantic Ocean?

• Applicant: Dr. Birgit Wehner
• Subject Area: Atmospheric Science
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 326874835

Aerosol particles play an important role for the regional and global climate. Therefore, a network of measurement sites has been established worldwide, but only a small fraction of them is capturing the marine boundary layer (MBL) while approximately 70% of the Earth´s surface is covered with water. The main focus of this project is to improve the knowledge of sources and exchange processes of aerosol particles in the MBL joining a measurement campaign over the Azores in the northeastern Atlantic Ocean where the influence of local anthropogenic sources is negligible. The main hypothesis of the project is that long-range transport from North America as well as new particle formation in the free troposphere (FT) and at cloud edges followed by vertical transport contribute significantly to the aerosol budget in the MBL. The knowledge of sources and sinks of aerosol particles in combination with vertical exchange between FT and MBL is a prerequisite to predict aerosol particle number concentrations in the lowest regions of MBL and its influence on the formation of clouds. These processes are not sufficiently quantified over the ocean up to now. To verify the hypothesis stated above vertical exchange processes and particle sources over the Azores will be quantified with high spatial resolution using a helicopter-borne platform developed at TROPOS. Here, aerosol particle number concentration and vertical wind speed will be measured with a temporal resolution allowing the direct estimate of the vertical turbulent flux of aerosol particles in different heights for the first time. The required fast measurements of particle number concentrations with at least 10 Hz are possible by utilizing a Fast Condensation Particle Counter developed at TROPOS, which has been applied successfully within the framework of an earlier DFG project. Using this instrument it will also be possible to check whether new particle formation at cloud edges occurs regularly in this region as observed at trade wind cumuli before on scales of a few decameters. In addition, aerosol particle number size distributions and particle absorption at three different wavelengths will be determined. These data will be used to conclude sources and origin of the investigated aerosol particles. Measurement flights performed by the helicopter are limited in time and provide therefore a "snapshot" of the atmosphere. Thus, additional continuous measurements of aerosol particle number size distributions will be installed at two ground-based sites. One of them is located few meters above sea level and the second one in 2200 m asl in the FT. These continuous measurements will be used to study connections and exchange processes between MBL and FT over the whole measurement period of one month. The data set will help to estimate possible influences of global warming on the local climate and potential feedback effects on the influence of aerosol particles on cloud formation and properties in this region.

• DFG Programme: Research Grants