Knowledge of an atmospheric particle’s chemical composition is of importance as it determines the optical properties of particles and affects atmospheric chemistry in the gas as well as in the particulate phase. Moreover – and that is in the main focus of our research interests – the aerosol chemical composition influences the ability of particles to act as cloud condensation nuclei (CCN) or ice nuclei (IN). For instance, while some particles (such as minerals) do not make very good CCN, they act as very good ice nuclei (IN) in colder parts of the atmosphere. In order to better understand these aerosol-cloud interactions our group carries out measurements of the aerosol chemical composition using mass spectrometry.
An ideal aerosol mass spectrometer should be capable of determining the size of an individual aerosol particle in situ, and provide a quantitative measure of each of its molecular constituents in real time. This is a difficult task because atmospheric particles range in size from less than 10 nm to greater than 10 μm. In addition, the molecular constituents are often mixtures that can include sea salt, soot, heavy metals, mineral dust, and a large number of different organic molecules.
The ability to detect individual particles is important in atmospheric studies where it is essential to know whether particles are uniform mixtures of many constituents (internally mixed), or whether the aerosol is a heterogeneous mixture of various types of particles (externally mixed). Finally, great benefits accrue if such instruments are portable so that they can be transported to various locations for field experiments.
The main instrument in our group used for chemical analysis is an Aerosol Time-of-Flight Mass Spectrometer (ATOFMS, TSI Model 3800). The instrument determines aerodynamic size and chemical composition of single particles in near real-time. It uses an aerodynamic sizing technique to measure particle size, and it uses time-of-flight mass spectrometry to determine the chemical composition of particles. Particles are drawn into the instrument from ambient air, sized, and - due to the bipolar design of the mass spectrometer - a positive and a negative ion spectrum are acquired from each particle. The main differences compared to the Aerodyne aerosol mass spectrometer which is widely used in the field of aerosol mass spectrometry, are the capability of the ATOFMS to analyse individual particles and refractory materials such as sodium chloride, elemental carbon and mineral dust constituents can be obtained. This fits into our major group research interests as particle types containing those components often show distinct ice and cloud condensation nuclei characteristics.
The instrument has been extensively deployed in the laboratory and in the field, e.g. during the Arctic Summer Cloud Ocean Study ASCOS, at the high Alpine research station Jungfraujoch, or at the Leipzig Aerosol Cloud Interaction Chamber LACIS.
In addition, our group shares ownership of two thermal desorption, High-resolution Time-of-Flight Aerosol Mass Spectrometer (Hr-ToF-AMS) from Aerodyne together with the Laboratory of Atmospheric Chemistry at the Paul Scherrer Institute.
Animated sketch of ATOFMS working principle
Aerosol-cloud interactions are governed by the ability of a particle to act as CCN and IN which depends on various parameters. This includes particle size, shape, structure, mixing state, its chemical composition, and the supersaturation in the cloud. In the context of black carbon (BC) particles, the knowledge of the mixing state and the chemical composition is of major importance. Fresh BC aerosols age during their residence time in the atmosphere by developing a coating of secondary species and become internally mixed with other chemical components. The properties of the coating material determine whether the particle is activated to a cloud droplet or an ice crystal. However, detailed studies of the chemical composition and the mixing state in connection with CCN- and IN-activity of BC particles are still scarce.
An ongoing project involves field and laboratory studies of the chemical composition and mixing state of BC particles and their ability to act as cloud condensation and ice nuclei. Herein, CCN-activity, IN-activity, and the chemical composition of atmospheric and laboratory generated aerosols will be measured simultaneously and in situ, deploying an exclusive set of instrumentation especially suitable for the chemical characterization of BC particles and their mixing state. The instrumental approach involves the utilization of the ATOFMS and the new Aerodyne Soot Particle Aerosol Mass Spectrometer (SP-AMS). The SP-AMS utilizes laser induced heating of absorbing soot particles to vaporize both the coatings and elemental carbon cores within the ionization region of a mass spectrometer. This provides a unique and selective method for measuring the mass and size of the refractory carbon cores (i.e., black carbon mass), and the mass, size, and chemical composition of any coating material (e.g., organics and inorganics) via standard electron impact time-of-flight mass spectrometry. It is also capable of distinguishing different types of black carbon.
The measurement features of the ATOFMS and the SP-AMS complement each other which make them ideal tools to characterize BC in large detail.
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