Performance Evaluation and Field Deployment of a Novel Vocus PTR-TOF for Quantification, Identification, and Apportionment of Gaseous Air Pollutants

Volatile and semi-volatile organic compounds (S/VOCs) are emitted from a wide variety of human and non-human sources. Some S/VOCs play important roles in secondary particulate and ozone formation, while others are directly hazardous to human health. Due to the sheer number and complexity of S/VOCs, there is an ever-present need for greater sensitivity and specificity in S/VOCs measurement techniques. Here, we present laboratory and ambient measurements of S/VOCs from a novel drift tube proton transfer reaction source, the Vocus, coupled to a high-resolution time of flight mass analyzer (Vocus PTR-TOF). A quadrupole ion guide surrounding the drift tube uses a radio frequency (RF) voltage to effectively focus ions for transmission into the differential pumping stages and the mass analyzer. The new instrument achieves sensitivities for VOCs of 5,000 - 18,000 counts per second per parts per billion volume (counts ppbv^-1) with a mass resolving power up to 14,000 Th/Th. The Vocus was deployed to the 2017 ACTRIS PTR-MS intercomparison campaign at Cabauw (PICAB), Netherlands and in multiple mobile laboratory field studies in the continental United States in areas with large anthropogenic influence. The high-resolution measurement allowed identification of additional elemental ion signals at many unit masses that would only be resolved with a resolving power > 8000 Th/Th. Several mass spectral factors were identified from ambient time series using a positive matrix factorization (PMF), indicating different anthropogenic, biogenic, and background emissions. We use the field data to evaluate the stability and precision of the calibration, background, and measurement.