Extending the Analysis of Ozone Precursors – Continuous, Unattended, Cryogen-Free On-Line Monitoring of PAMS Hydrocarbons and Polar VOCs in Ambient Air by TD–GC–MS

Air Methods & Monitoring Part 1
Oral Presentation

Prepared by , D. Wevill

Contact Information: nwatson@markes.com; 866-483-5684


The presence of volatile hydrocarbons in urban atmospheres is believed to contribute to the formation of ground-level ozone, one of the main constituents of urban smog. The compounds of interest range in volatility from acetylene to trimethylbenzene, and are generally referred to as ‘ozone precursors’.
There is currently growing interest in carrying out continuous time-resolved measurement (‘on-line monitoring’) of ozone precursors and other compounds, for source apportionment and in order to better understand the factors that influence their concentrations. This is particularly the case in Asia, with recent strong growth in on-line monitoring of industrial emissions in China, following the mandating of a nationwide real-time air monitoring system in the 13th Five-Year Plan, and numerous regional controls on industrial VOC emissions. On-line industrial air monitoring is also experiencing a high level of interest in South Korea, while other areas also receiving attention include emissions from road vehicles, and long-range transport of pollutants.
The proposed list of target compounds for the re-engineered PAMS program comprises 28 ‘priority compounds’ and 35 ‘optional compounds’. These are all aliphatic or aromatic hydrocarbons except for formaldehyde, acetaldehyde and acetone (priority) and ethanol, benzaldehyde, carbon tetrachloride, tetrachloroethene, α-pinene and β-pinene (optional). However, it is widely recognised that numerous polar compounds (including oxygenated species such as alcohols, aldehydes, ketones and esters) and terpenoids play an important role in atmospheric chemistry, including in the formation of ozone and secondary organic aerosols.
As a result, there is a growing desire to ‘future-proof’ on-line GC systems for VOC monitoring, by ensuring that polar compounds as well as hydrocarbons can be monitored simultaneously. This has historically presented a challenge to analysts because of the tendency for volatile polar species and monoterpenes to be lost when conventional Nafion™ dryers are used to remove humidity from the air stream.
This presentation describes an on-line thermal desorption (TD)–GC–MS system that overcomes this difficulty by using an innovative, cryogen-free water-removal device upstream of the thermal desorption focusing trap. The result is improved performance for an extended range of analytes that includes hydrocarbons, polar species and monoterpenes.