Volatile organic compounds (VOCs) are released into the atmosphere from numerous anthropogenic and biogenic sources. Traditionally VOCs have been measured using offline techniques such as Gas Chromatography-Mass Spectrometry (GC-MS). The development of the proton transfer reaction mass spectrometer (PTR-MS) has enabled the online analysis of VOC’s from both biogenic and anthropogenic sources. This instrument, however, provides little structural information making it impossible to distinguish between isomeric compounds. Here a range of New-PsychoactiveSubstances (NPS) are analysed using the recently developed Selective Reagent IonTime of Flight-Mass Spectrometer (SRI-ToF-MS) demonstrating its ability to distinguish between isomeric compounds. This instrument is then applied to the analysis of biogenic VOCs (bVOCs). Plants emit a wide variety of VOCs into that atmosphere. These compounds play an important role in plant communication and defence, with predatory insects making use of VOC emissions from plants following biotic stress to identify and locate their prey. This process is termed tritrophic signalling. Ozone will readily react with any bVOCs containing an alkene functional group, and as many alkenes (primarily monoterpenes and sesquiterpenes) have been shown to play a significant role in tritrophic signalling it was hypothesised that ozone may disrupt this signalling. This thesis investigates the effect of ozone on tritrophic signalling using a Brassica napus – Myzus persicae – Adalia bipunctata larvae (rapeseed – green peach aphid – two-spotted ladybird larvae) model system. Plant volatile emission was monitored using a PTR-MS and SRI-ToF-MS which enabled the better detection and identification of bVOCs than is possible using a traditional PTR-MS. Following ozone fumigation of B. napus it was shown that a large number of oxygenated compounds are emitted by the plant and that the emission of monoterpenes and sesquiterpenes from a plant chamber is reduced. However, ozone fumigation of the plant leaves was shown to have no impact on the emission of bVOCs below ground. Using a Y-tube olfactometer it was shown that ozone at environmentally-realistic mixing ratios (ca. 100 ppbv) disrupts the ability of M. persicae to locate a host plant. Ozone was also shown to disrupt tritrophic signalling by inhibiting the location of prey by A. bipunctata larvae. This disruption in tritrophic signalling was shown to be caused by degradation of bVOCs via ozonolysis and not changes to bVOC emission from the plant. Finally fluxes of VOCs above a temperate forest canopy were recorded using PTRMS and a Proton Transfer Reaction-Time of Flight-Mass Spectrometer (PTR-ToFMS) enabling a direct comparison to be made between these instruments during field scale measurements.