Wen Tan, Abigail Koss, Liang Zhu
TOFWERK, Boulder, CO, USA and Thun, Switzerland
The Vocus PTR-TOF is a robust, low-power, mobile instrument that measures VOCs in air in real time, addressing environmental and industrial problems not accessible by traditional mass spectrometry. In this application note, patterns of VOC emissions from a Vocus PTR-TOF S mobile laboratory are shown around a Chinese megacity, including near a petrochemical facility.
Characterization of VOC Emissions in Urban Areas
In dense urban and industrial areas, people live and work in an atmosphere rich with volatile organic compounds (VOCs). Certain VOCs (so-called “air toxics”) such as single-ring aromatics, can harm human health and cause complaints by nearby residents due to odors. Strict regulations also control compounds that contribute to ozone and fine particulate formation via photochemical reactions. Large amounts of VOCs can originate from point sources in either industrial sites or residential areas, and from non-point anthropogenic activities in populated areas. As an indicator of leaks or other inefficiencies, discovering and locating the VOCs’ sources may help industrial operators to tighten manufacturing systems and reduce waste, as well as ensure regulatory compliance. This is especially important in eastern China, considering the high density and vicinity of industrial zones to residential districts. For instance, the air quality inside Shanghai city is often heavily affected by its suburban industrial activities and transport of pollutants from neighboring cities.
Proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF) is an analytical technique uniquely situated to mobile monitoring and point-source chemical fingerprinting. The Vocus PTR-TOF is a robust instrument platform that measures ambient air directly at a rate of several times per second, providing an essentially real-time measurement of the VOC environment. A high-resolution time-of-flight mass analyzer allows simultaneous measurement of tens to thousands of VOC species, identifiable by their exact mass-to-charge ratios.
In this study, a Vocus S PTR-TOF operating in H3O+ mode was mobilized in a van with a height of 3.8 meters. A 1/8′ sampling line drew ambient air ~0.5 meter above the van top, at a flow rate of ~2 L/min. The total length of the sampling line was approximately 3 meters in front of the Vocus PTR instrument inlet. Both zero air and calibration measurements were performed before and after daily drives. The acquisition rate of the instrument was set to 1 second and the mass range 1-500 Th.
Detection of VOC Hotspots and Point Sources
The mobile Vocus S was driven in a search pattern in Shanghai during a large multi-institution campaign. The drive path included urban, rural, and industrial areas. Figure 1 shows detected hotspots for selected VOCs in an area of 57.000 km2. Download data set (.kmz) to explore aromatic VOCs in Shanghai using Google Earth or LocaSpace Viewer.
Mobile PTR-TOF can also be used to target point sources in a much smaller area. The Vocus S was driven in a search pattern through a petrochemical plant in Eastern China. Hundreds of VOCs were detected, in mixing ratios ranging from a few parts-per-trillion (ppt) to more than a part-per-million (ppm) (Figure 2 A). In Figure 2A only a subset of the detected VOCs is shown, demonstrating the need for a large dynamic range and simultaneous acquisition of a large number of VOC masses. Several isobaric masses are shown. These can only be measured separately by an instrument with high mass resolving power.
The composition of the VOC mixture also varied dramatically from point to point. Correlation analysis of VOCs determined the top ten sources of VOC mixtures present at the facility, and was used to quantify the contribution of each source to total VOC (Figure 2). The VOCs shown in Figure 2A were used for the source apportionment analysis, and were selected because they have a signal-to-noise ratio higher than 2. Even VOCs with smaller mixing ratios are important to include in a source apportionment analysis, because they may provide the necessary distinction between the chemical fingerprints of two sources.
The top ten sources of mixed VOCs account for approximately 92% of the total detected VOC emissions at the site. The remainder is almost entirely pure benzene. The various sources are related to refinement and storage equipment located in different areas of the facility. The location of each VOC source can be determined by plotting the measured VOCs on a map (Figure 3). The characterization of this site using a mobile laboratory took less than an hour.