Analysis of Atmospheric Hydrocarbons.
Personnel
Volatile organic compounds (VOCs) are emitted into the troposphere by a vast number of sources. These emissions can be divided into 2 categories: anthropogenic and biogenic. Major anthropogenic sources include combustion of fossil fuels and solvent evaporation and many trees and plants emit a wide variety of biogenically emitted volatile organics compounds. A recent report stated: "VOCs contribute significantly to the formation of ground-level ozone (smog), the nation's most pervasive air pollutant. Smog exposure can damage lung tissue and cause serious respiratory illness in humans, and also harm farm crops" (US EPA press release 26-4-96).
The rates of emission, and degradation mechanisms of these biogenic emissions are still poorly understood, and if accurate and true models of the troposphere are to be constructed these biogenic VOCs must be included. Possibly the most important biogenic species, is 2-Methyl-1,3-butadiene (isoprene), emitted by many forest species such as oak, poplar, cotton wood and eucalytus, and it this species, and its degradation products along with a group of compounds known as monoterpenes that form a major part of our in-situ field measurements. We also measure anthropogenic emissions and DMS (dimethyl sulphide) which is thought to be a very important species in the oceanic environment.
Methodology.
A technique has recently been developed to allow for rapid in-situ field measurements of VOCs using programmed temperature vaporization injection. This produces a single step desorption, and thus removes the need for intermediate cryogenic coolants which previously made sustained on-site analysis impractical. Samples of air are collected, trapping occurs on a activated charcoal or mixed bed sorbent trap (depending on volatility) prior to analysis. Simultaneous analysis is be carried out using 2 instruments; C1-C6 (including methane) - (Carlo Erba GC8000) and C6-C12 range - Ai GC94. Both instruments are equipped with an OPTIC programmed temperature vapourisation injector (Ai Cambridge) used for trapping and injection and a CO2 oven cooling unit. Detection is done using flame ionisation using nitrogen as a make-up gas. The sampling system which is used, has recently been automated allowing unattended sampling, for extended periods.
One of the major drawbacks with fieldwork in remote environments is the transport and availability of cylinders of compressed gas required for gas chromatography, and liquid nitrogen for cryogenic trapping. We have removed the need for cryogenics by using PTV methods for sample aquisition, as well as some of the cylinders through the use of gas generators (hydrogen and compressed air).
Recent Work.
Helium Ionisation Detector
The importance of oxygenated species in atmospheric process has long been known, as they are both primary pollutants and products of oxidation reactions. Unfortunately, as they have greatly reduced response to normal detection methods such as FID we have recently begun working using a Helium Ionisation Detector (HID) which has a much greater response to these compounds. The most important of these oxgenated species is formaldehyde, which is produced from primary sources as well as by many atmospheric reactions including the oxidation of methane. Formaldehyde has practically no response to FID - however, we have recently shown that HID has a greatly enhanced response, by as much as 400 times (a research paper has been submitted to The Journal of High Resolution Chromatography). Work is currently being undertaken to investigate the range of HID applications, analysing other oxygenated atmospheric species using different chromatographic conditions.The system has been developed most recently using liquid nitrogen trapping of the analytes, subsequent desorption and separation
by gas chromatography and detection with the HID (a research paper has been published recently in Analytical Communications). This system has been successfully deployed as part of the NERC funded SOAPEX II field campaign
(January/February 1999) at the Cape Grim Baseline Air Pollution Station in Tasmania, Australia. The main species that were
monitored were formaldehyde and acetone with detection limits less than 20 parts per trillion by volume.
Here for some of our recent publications.
This page was constructed and is maintained by Jim McQuaid - [e-mail]
Tel: + 44 (0) 113 233 6724
Last modified on 08-08-01