Volatile Organic Compounds (VOCs) encompass a wide range of compounds. There are the hydrocarbons that contain Carbon and Hydrogen only, which include the alkanes, alkenes, alkynes and the aromatics. Methane, because of its long lifetime, is often considered separately, and the remaining non-methane hydrocarbons (NMHCs) are a very important class of compound. It is these NMHC compounds in particular that we are concerned with the accurate and precise measurement of.

Instrumentation

The instrumentation used in the measurement of C2-C8 NMHCs is based around the same principle as many of the other systems discussed, i.e. preconcentration on peltier cooled adsorbent microtraps. The basic principles of the adsorption technique are shown below.

Separation is achieved by means of a KCl modified alumina PLOT (porous layer open tubular) column. Detection is achieved by means of a flame ionisation detector (FID). A detection limit of 10pptv is achieved for the compounds of interest.

Schematic of Valves used in the NMHC Instrument

Deployment

Mace Head Research Station

The instrument has been deployed at Mace Head Atmospheric Research Station since January 2001. Situated on the West Coast of Ireland it offers a unique opportunity to study atmospheric compositions under Northern Hemispheric background conditions as well as European continental emissions. The site location, at 530^o20' N, 90^o54' W, is in the path of the mid latitude cyclones which frequently traverse the North Atlantic. It is 88km west of Galway, which is the nearest urban conurbation.

Sample hydrocarbon data from Mace Head.

All data for C2-C5 alkanes measured at Mace Head from January 2005 until December 2009

Monthly averaged data measured at Mace Head from 2005-2009 separated into baseline (black), European (red) and southerly transported (blue) air masses with error bars as 1σ.