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PTR-TOFMS Polluting Particles: Direct Analysis of Organic Aerosol via PTR-TOFMS

| Author / Editor: Markus Müller*, Philipp Sulzer*, Jens Herbig* & Lukas Märk* / Dr. Ilka Ottleben

From morning rush-hour urban air-pollution to ship diesel emissions — organic particulate matter directly affects air quality and health while its analysis is still challenging. A new technological solution for PTR-TOFMS shall now find a remedy.

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Fig. 1 To improve air quality, sulfur emission control areas were recently established in declared coastal areas.
Fig. 1 To improve air quality, sulfur emission control areas were recently established in declared coastal areas.
(Source: ©krungchingpixs - stock.adobe.com)

Organic aerosol is either emitted directly e.g. by biomass burning and by traffic or formed secondarily in the atmosphere by photo-oxidation of volatile precursors like aromatic compounds from traffic and terpenoids emitted from vegetation. Particulate organic matter accounts for a significant fraction of atmospheric sub-micrometer aerosol. It contains reactive and toxic species and therefore directly affects air quality and health. Nanometer-sized particles can enter the alveolar respiratory system, big fractions get deposited and toxic species may enter our blood stream. A fine-grained knowledge about the chemical composition and sources of organic aerosol is vital to counteract air pollution in urban environments.

Current situation & solution

Despite the ongoing progress in analytical instrumentation, the analysis of the chemical composition of organic particulate matter is still challenging. Current methods with high chemical resolution are often off-line, rely on sample pre-concentration on a filter substrate and are prone to sampling artifacts, or they rely on direct sampling methods but only detect limited species (e.g. total organics) [1].

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On the other hand, Ionicon’s proton-transfer-reaction mass spectrometry (PTR-MS) is a well-established method for monitoring organic trace gases in the atmosphere [2]. Soft chemical ionization with hydronium ions (H3O+) allows for a quantitative real-time detection of volatile organic compounds (VOC) down to single digit parts per trillion (pptV) volume mixing ratios. With the novel CHARON (CHemical Analysis of aeRsol ONline) inlet system for PTR-MS now all analytical benefits from the gas-phase are extended to a direct chemical analysis of sub-micrometer particulate organic matter [3].

The Charon particle inlet consists of a honeycomb activated charcoal denuder that efficiently adsorbs organic gases and transmits particles, a high-pressure aerodynamic lens system that collimates and extracts sub-micrometer particles, and a thermo-desorber that evaporates non-refractory organic particulate matter at moderate temperatures of 100 – 150 °C and reduced pressures of a few mbar. These organics are subsequently analyzed as gas-phase analytes with one of Ionicon’s high-resolution PTR-TOFMS instruments. Figure 1 displays Ionicon’s Charon PTR-TOF 6000 X2 flagship instrument. By coupling the Charon inlet to a PTR-TOFMS, the VOC inlet remains fully operational. An automated valve system allows for scheduled switching between gas- and particle-phase measurements as well as zeroing of the particle inlet.

Analytical Benefits

Charon PTR-TOFMS is a designated on-line and real-time particle analyzer. There is no need for off-line particle pre-concentration, e.g. by collection/desorption on surfaces. Analytical artifacts, which may result by reactions on such collection surfaces or thermal degradation at high desorption temperatures and residence times, are efficiently reduced.

In addition, the Charon particle inlet significantly extends the range of by PTR-MS measurable compounds from gas-phase volatile and intermediate volatile organics (VOC and IVOC) to particle-phase intermediate, semi and low volatile organic compounds (IVOC, SVOC and LVOC, respectively) in sub-ng/m3 concentrations. Therefore, the employed technology allows for the detection of almost the full range of atmospheric organic carbon with one single instrument. The controlled chemical ionization at reduced pressures and defined reaction energies of a PTR-MS drift tube impede the formation of ionic artifacts (e.g. clusters of organics) that might be falsely attributed. Ionization typically proceeds at collision rates that are well predictable (+/- 10%). Fragmentation due to ionization is typically low. 60% – 100% of the organic mass concentrations can be directly calculated without the need of any additional corrections.

With its high temporal resolution and the high degree of conserved chemical composition information, Charon PTR-TOFMS is thus the perfect analytical technique to identify and quantitatively follow atmospheric particulate tracer compounds like levoglucosan from biomass burning and polycyclic aromatic hydrocarbons from traffic. Beyond that one-minute resolved data of hundreds of identified chemical compositions boost the quality of source apportionment to a so far unseen level.

Over the past three years, Charon PTR-TOFMS was extensively tested and applied to a series of measurement campaigns to explore the capabilities of the system and improve our knowledge on organic aerosol. These studies include ambient air measurements in three European cities [4], measurements in large environmental simulation chambers as well as laboratory-based aerosol flow tubes [5–7], various emission studies from engine and vehicular emissions [8] and quantification and comparison studies with current state of the art instrumentation. In the following we want to introduce two applications in more detail: the detailed characterization of ship diesel engine emissions and new insights into the chemical composition and source of urban organic aerosol in the city of Innsbruck, Austria.

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