Spatial Characterization of VOC Emissions with a Mobile Vocus Elf

Abigail Koss, Luca Cappellin, Christoph Gasser
TOFWERK, Boulder, CO, USA and Thun, Switzerland

The small footprint and ruggedness of Vocus Elf PTR-TOF are ideal for real-time mobile monitoring of VOCs.

Characterization of VOC Emissions Using Mobile Laboratories

Mobile laboratories equipped with fast, sensitive instrumentation allow researchers and regulatory authorities to directly map chemical emissions and dispersal in the environment. Mobile laboratories can be used to locate and “fingerprint” point sources, understand ambient concentrations, and determine the real-world life cycle of VOC emissions. This information provides crucial input for air quality modeling, environmental policy, and public health studies.

Proton-transfer-reaction mass spectrometry (PTR-MS) has a long history of mobile lab application, because it is capable of direct measurement of ambient air and is able to distinguish individual VOCs in a complex sample matrix. Mobile laboratories equipped with conventional quadrupole PTR-MS (PTR-qMS) have contributed to dozens of studies, and measurements made with these instruments are a foundation of modern atmospheric science. Vocus Elf PTR-TOF offers the advantages of Vocus PTR time-of-flight mass spectrometry (multiple-Hz measurement speed, simultaneous measurement of all masses, and high sensitivity from the Vocus PTR reactor) at the size, price-point, and power requirements of PTR-qMS.

*Feature Image: A Vocus Elf PTR-TOF mounted in an electric car was driven through the streets of Thun, Switzerland. The color and height of the markers shown along the drive path indicate the measured ambient concentration of toluene (C7H9+) at a 4-Hz measurement speed.

Methods

In this study, a Vocus Elf was installed in the rear passenger seat of an electric car with a ratchet strap (Figure 1). No special engineering modifications to the car were made. The instrument sampled external air via a 1/4” PFA line run through the roof window and attached to a longitudinal sampling probe extending over the hood of the car. To eliminate the possibility of sampling the vehicle’s own emissions, the drive time of 90 minutes was accomplished on full electric power, with the Vocus Elf powered by the car battery.

Figure 1. Vocus Elf PTR-TOF installed in a vehicle. A PFA sampling tube attached to a longitudinal roof sampling probe (visible in the upper right corner of the image) was connected to the instrument inlet and small external pump.
Figure 1. Vocus Elf PTR-TOF installed in a vehicle. A PFA sampling tube attached to a longitudinal roof sampling probe (visible in the upper right corner of the image) was connected to the instrument inlet and small external pump.

The drive path was chosen to include a number of major arterial streets, the official air quality monitoring station, and point sources such as gas stations, a water treatment plant, and the train station.

Spatial Characterization of VOC Emissions

A number of VOC hotspots were immediately visible to the operators during the drive. An overlay of the measured toluene on a map reveals hotspots near intersections, the train station, arterial roads, and gas stations, sometimes with ambient concentration more than 100 ppbv (Feature image). Exceptionally high concentrations of BTX were detected near a large construction site on a major roadway (Figure 2). Single stationary monitoring stations are often not sufficient to detect such large, transient emission sources.

Enhanced signal was observed on more than 100 unit-mass-integrated ions.  Correlations between different compounds can be used to determine the chemical “fingerprints”, or VOC profile composition, of different sources. A number of distinct sources were observed, including possibly two different combustion traffic-related sources, a toluene-rich source, an acetone-rich source, and a source containing especially m/Q 73, 91, 101, and 119. Combustion sources are recognizable by their distinct profile of aromatics. The toluene source is possibly associated with the construction site. The VOC detected at m/Q 101 is possibly hexanal or pentanedione from a nearby medical clinic.

Figure 2. High concentrations of BTX observed near a construction site along the Allmendstrasse. A time series of mixing ratios is shown in the upper panel, and the location of the hotspot is shown in the map in the bottom panel.
Figure 2. High concentrations of BTX observed near a construction site along the Allmendstrasse. A time series of mixing ratios is shown in the upper panel, and the location of the hotspot is shown in the map in the bottom panel.
Figure 3. Distinct sources detected in Thun. The map shows the drive track of the mobile laboratory through the city, with color and intensity indicating the identity and relative abundance of each source. The smaller panels show the mass spectral VOC fingerprint of each source.
Figure 3. Distinct sources detected in Thun. The map shows the drive track of the mobile laboratory through the city, with color and intensity indicating the identity and relative abundance of each source. The smaller panels show the mass spectral VOC fingerprint of each source.
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