Real-Time Measurements of Atmospheric Aerosol Particles with Extractive Electrospray Ionization Source (EESI)

Atmospheric Aerosol Particles

On the Fate of Oxygenated Organic Molecules in Atmospheric Aerosol Particles

V. Pospisilova et. al
Science Advances, 2020
DOI: 10.1126/sciadv.aax8922

For the first time, aerosol particles can be measured in real time with very limited fragmentation at levels that are relevant for the atmospheric conditions. Thanks to the collaboration of researchers at the Laboratory of Atmospheric chemistry at the Paul Scherrer Institute and TOFWERK, new extractive electrospray ionization source (EESI) coupled to API-TOF was developed and deployed for a study examining the fate of the organic molecules present in biogenic aerosol particles. Results of this study show direct evidence to contradict the existing assumption that the molecules do not further chemically evolve in the particulate matter.  

The research was conducted in so called ‘smog chamber’, 27 m3 Teflon reservoir where atmospheric chemical reactions can be studied under simplified and well controlled conditions. The aerosol taken under the loop was of biogenic origin, created by oxidation of α-pinene, compound emitted mainly via coniferous trees responsible for the pleasant smell in the forest. This type of biogenic aerosol was found to be one of the most prevalent during the summer in Zurich as reported previously with the same measurement technique.

During the experiment a tiny drop of α-pinene was injected into the chamber pre-filled with ozone. This leads to a chain of oxidation reactions and cascade of products with eventually low enough volatility to create particles, similarly as in the pristine regions of the atmosphere. Newly developed EESI-TOF enabled continuous measurements of the composition of these particles for more than ten hours. The combination of soft ionization and high time resolution of this instrument provides an unprecedented look into the evolution and kinetics of the molecules directly in the sampled aerosol. No existing technique is able to do so.

The findings of this study show that some of the highly oxidized organic molecules, partially responsible for the formation and growth of the aerosol in the first place, further evolve and react away in the particulate matter. During this process simple gaseous molecules, such as formic acid, are released into the atmosphere. This information can improve existing simulation models regarding cloud formation and suggests that intra-particle reactions contribute to formic acid emissions, which are currently underestimated by global models.

Encouraged by the results, the research team deployed the EESI-TOF to a boreal forest, specifically to the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II) in Hyytiälä, Finland. Here, the organic aerosol is typically dominated by monoterpene oxidation products with concentrations bellow ~10 μg m−3. The team observed similar patterns of molecular formulas as in the chamber experiments with the main difference being the presence of nitrogen-containing species in the ambient spectrum coming from anthropogenic NOx emissions in the atmosphere.

Currently, the EESI technology is commercially available by TOFWERK and atmospheric groups are taking the advantage of this new technique. Numerous field campaigns have been ongoing since the initial study targeting mainly better understanding of the source of air pollution in countries such as China or India.

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