Atmospheric Chemistry
The laboratory data provide the only sound basis for the interpretation of atmospheric behaviour and for the construction of diagnostic and prognostic models. We pursue research in three areas. The first is the development of methods to directly observe the kinetics of radical reactions in the laboratory using chemical ionization mass spectrometry in conjunction with a turbulent flow system (TF - CIMS). The second area of work focuses on the development of novel analytical techniques to quantify trace species in the atmosphere and the third area is on the study of the interaction of gases with aerosols.
Turbulent Flow Technique Chemical Ionisation Mass spectrometer
Pictures showing mixing in the flow tube.
The Turbulent flow technique is suitable for measuring elementary reactions over a wide range of pressures (70 - 600 Torr) and Temperatures (180 - 300K). Unlike the conventional flow technique, the method operates under turbulent flow conditions. Turbulent flow is achieved by operating under fluid flow conditions with Reynolds numbers greater than 3000.
Working under turbulent flow conditions the plug flow approximation still holds and turbulent mixing by eddy diffusion allows the traditional versatility of flow tube methods to be regained when operating at higher pressures. Furthermore the wall collision frequency for the reactants is greatly reduced in comparison with the conventional low pressure flow techniques which facilitates the study of gas phase kinetics at lower temperatures (180 – 300 K).
The turbulent flow system is coupled to a chemical ionisation mass spectrometer (CIMS) for the detection of the radical species. The Chemical Ionisation Mass Spectrometer (CIMS) is a highly versatile and sensitive detector that has been used to monitor both stable and radical species
The TF-CIMS in action.
in the laboratory such as HO2NO2, ClONO2, O3, HNO3, C2H5O2, C3H7O2, CH3O2 and HO2 at the ppt level. Unlike conventional electron impact mass spectrometers CIMS produces ions at high pressures (20 - 760 Torr). The main benefit of creating ions at high pressures is that ions can be focused by electrostatic lenses at each pumping stage and thus minimise loss of ion signal and enhancing sensitivity.
The highly sensitive CIMS detector allows kinetic studies to be undertaken at very low radical concentrations, typically (0.1- 20) x 1010 molecule cm-3. Such low radical concentrations ensures that numerical modelling is not required to deconvolute the rate coefficient from the observed concentration-time profiles. Furthermore, the versatility of the CIMS detector enables the quantification of products as a function of temperature and pressure, thus allowing the mass balance of reactions to be verified.
Development of QCM sensors
Quartz crystals have the unusual property of piezoelectric resonance. When any molecules stick to the surface of the QCM the resonant frequency will change and hence the mass can be detected, but the molecules can't be identified. The aim of the work is to develop artificial recognition materials to coat onto the surface of a QCM to produce a selective detector. Among the most promising examples of artificially generated recognition materials are the molecularly imprinted polymers (MIPs). MIPs are highly cross-linked polymers, which are inherently stable and capable of high selectivities, approaching those of their natural counterparts, indeed, the selectivity and binding affinities of MIPs are comparable to nature's antibody-antigen interactions.
Selective detectors have been developed for a range of analytes ranging from anabolic steroids to PAHs (polycyclic aromatic hydrocarbons), to terpenes, to atmospheric aerosols and amino acids. More recently in collaboration with Nottingham Trent University to develop Acoustic wave sensors to assess MHC-peptide interactions. (http://www.ntu.ac.uk/research/school_research/bns/31028gp.html)
Reactive uptake on aersols
In collaboration with School of Chemistry we have developed an apparatus to simultaneously probe the kinetic and mechanistic details of the heterogeneous conversion of atmospheric trace species upon solid surfaces representative of the materials present in parts of the atmosphere. Flow-tube apparatus with an in situ surface sensitive spectroscopic probe is used to establish the kinetics and mechanism of reactive uptake on a range of proxies (NaCl, mineral dust etc.) from the analysis of vibrational spectra during variation of controllable parameters such as relative humidity, temperature and reactive gas partial pressure. The information obtained will complement existing and ongoing kinetic studies of heterogeneous chemistry on these substrates and further assist our fundamental understanding of the dependency of local, global and short-lived heterogeneous processing upon temperature, relative humidity and atmospheric composition.