10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Method for Chemical Analysis of Nano-Aerosol Particles – Gas-to-Particle Transitions of Highly Oxygenated Organic Molecules
ANDREA C. WAGNER, Andreas Kürten, Martin Heinritzi, Mario Simon, Joachim Curtius, Goethe University Frankfurt
Abstract Number: 470 Working Group: Instrumentation
Abstract Atmospheric aerosols are key players for cloud formation and properties. They influence our weather and climate. A large fraction of global cloud condensation nuclei (CCN) originates from nucleation. It is therefore desirable to understand the processes and substances involved in atmospheric new particle formation.
The Cosmics Leaving OUtdoor Droplets (CLOUD) experiment at CERN investigates the formation and growth of aerosol particles in an ultraclean stainless steel chamber under well-controlled conditions. Experiments simulating an urban environment were conducted. Naphthalene, trimethylbenzene and toluene were introduced as examples of anthropogenically emitted volatile organic compounds (VOCs). The oxidation of these compounds, also under the influence of nitrogen oxides, ammonia and UV light, produces highly oxygenated organic molecules (HOMs). Some of these form new particles or partition into the particle phase.
To understand the process of new particle formation, the condensing vapors need to be speciated in both gas and particle phase. For gas phase measurements of the HOMS, we are using an chemical ionization atmospheric pressure interface long time-of-flight mass spectrometer (CI-APi-LTOF) with nitrate primary ions. The measurement of the small particles resulting from nucleation is experimentally very challenging and to date, only a few instruments are capable of such an analysis. We developed a new instrument for the size-selected chemical analysis of aerosol particles from 5 to 40 nm with a lower detection limit of 10pg. The Thermal Desorption Differential Mobility Analyzer (TD-DMA) uses a semi-online principle by collecting particles on a filament and evaporating them in front of the ion reaction zone of the CI-APi-LTOF mass spectrometer. Like this, it is possible to measure both particle and gas phase HOMs with the same mass spectrometer which allows us to observe the gas-to-particle conversion or to identify species which can only be detected in one of the phases.
The particles are charged before entering the DMA-unit, which allows selecting a defined particle size. The selected particles are then collected on a filament inside the central electrode of the DMA using electrostatic precipitation. Once enough material is collected, the filament is moved outside the DMA and in front of the mass spectrometer, where it is slowly heated by an electric current. The material evaporates into ultrapure nitrogen and is detected by the mass spectrometer. Like this, the aerosol particles are separated from the gas phase to make sure only the particulate matter is analyzed and the concentration is enhanced to fit the detection limit of the CI-APi-LTOF. The LTOF by TOFWERK reaches a mass resolving power of more than 10 000 Th/Th, allowing the separation and identification of many different substances even in the complex spectra of anthropogenic HOMs.
Here, we present the instrument system of the TD-DMA regarding its setup, measurement procedures and characterization. Results comparing gas and particle phase measurements of HOMs from anthropogenically emitted precursor VOCs at the CLOUD experiment are discussed.