The PTR-MS (Proton Transfer Reaction Mass Spectrometer) is an aircraft-certified instrument for the real time measurement of a selected range of volatile organic compounds (VOCs) in the atmosphere at concentrations down to approximately 50 parts per trillion (ppt). The types of compound measured include:

• Oxygenated hydrocarbons (e.g. methanol, acetone, acetaldehyde, acetic acid, methyl vinyl ketone and methacrolein)
• Aromatic hydrocarbons (benzene, toluene, C8 and C9 aromatics)
• Biogenic hydrocarbons (isoprene, total monoterpenes)
• Acetonitrile; an excellent tracer of biomass burning

The PTR-MS is ideally suited for use on an aircraft since it produces a near real-time signal. In this way it has a distinct advantage over (and acts as a valuable complement to) traditional chromatographic or bottle sampling techniques. Sampling cycle times of down to ~1 second can be achieved, dependant on the number of compounds analysed at any one time, enabling small scale events such as pollution or biomass burning plumes to be sampled effectively. The instrument samples from the main aircraft inlet manifold via a short length of PFA tubing.

Potential applications include:
• Measurement of oxygenates in the upper troposphere in a study of the free radical chemistry and its sources in this region
• Studies of oxygenate and isoprene emissions from vegetation in the tropics and at mid latitudes
• Studies of oxygenate formation in urban plumes in the boundary layer for improvement of models of regional photochemistry
• Studies of biomass burning in the tropics and over Northern Hemisphere forests to assess their contribution to the global carbon budget and to ozone formation