Exploring the Feasibility of an Air Sensor Array for the Real-Time Detection and Characterization of VOCs

Abstract. Volatile organic compounds (VOCs) are an important class of atmospheric chemical species that can be directly harmful to human health and contribute to the formation of hazardous secondary products. Measurements of ambient VOCs are typically made using “offline” techniques, which are well-suited for distributed measurements but have low time resolution, or real-time measurements using state-of-the-art in situ instruments, which have high precision and time resolution, but tend to be expensive and so cannot be deployed in a widespread manner. An alternative VOC measurement approach that is both real-time and lower in cost would open the possibility of widespread, spatially distributed measurements of VOCs in air quality and atmospheric chemistry contexts. While there are several commercially available air sensors that are sensitive to environmental VOCs, these sensors are “broadband,” meaning that each can only output a single scalar value that reflects the sensitivity of the sensor toward a wide and poorly defined range of VOCs. As a result, VOC air sensors have, to date, seen little use in research. Here, we investigate the feasibility of a novel method for measuring environmental VOCs that uses an array of such broadband sensors. This array includes VOC air sensors representing three fundamentally different sensor types, and takes advantage of operational parameters that achieve a diversity of responses amongst sensors with the same type. Within a controlled laboratory setting, we obtained calibration curves for ten typical atmospheric VOCs between 5 and 100 ppb and explored the effects of varying RH and introducing binary mixtures on sensor responses. Overall, we found that all observed sensor responses can be parameterized with linear or power-law models, consistent with results of prior studies and expectations based on physical sensing principles. Our results show that each of the 12 sensors in our array appear to have their own unique sensitivities to various VOCs, resulting in distinctive “fingerprints” of array responses for each compound tested. However, we also show that interferences by water vapor and other gases pose substantial challenges that likely cannot be fully addressed in the laboratory. Thus, co-location with a reference instrument in the field may first be required if this measurement approach is to yield quantitative, chemically specific information about ambient VOCs in indoor or outdoor environments.