Development of total non-methane organic carbon instrument and its applications to the analysis of volatile organic compounds in a smog chamber and in the Los Angeles air basin (California)

Here we describe development of a new instrument to measure the airborne total nonmethane organic carbon concentration (TNMOC), and the ratio of this value to the sum of speciated volatile organic carbon (VOCs) measured by standard gas chromatography. The approach is to make a measurement that minimizes sample contact, cryo-trapping whole air samples with minimal trapping of carbon dioxide, carbon monoxide and methane. Samples are processed by an oxidation catalyst to generate carbon dioxide that is measured as TNMOC. Simultaneously, a standard speciated VOCs measurement is made with the same instrument. The ratio of the TNMOC and speciated VOCs measurements provides the “excess VOCs”, or that not detected by routine monitoring techniques. Measurements of ambient air were made in West Los Angeles, Burbank and Azusa. Fresh pollution sources including gasoline vapor and diesel exhaust were tested as well. These sources are, not surprisingly, measured well by the standard analysis approach, and had 5–20% “excess” VOCs. The Burbank site, which is surrounded by freeways and light industrial sources had only 10% excess VOCs, as did UCLA during winter when there is minimal photochemistry. In contrast, between 29 and 42% of the VOCs were unmeasured by the standard technique at the UCLA and Azusa sites during summer. The dominant source of the unmeasured VOCs appears to be air that has been aged for more than 12 hours, possibly for a day or more.