The Sky's The Limit

Mike Bannister, Managing Director of Michell Instruments, explains how cooled-mirror dew-point technology is being used to great effect in a UK study examining pollution of the urban Midlands atmosphere.

Many urban centres in the developed world have experienced over 150 years of industrialisation and rapid urbanisation, leaving a legacy of contamination and dereliction that presents risks to the health of urban inhabitants and eco-systems, inhibits safe and cost-effective redevelopment, and diminishes the quality of life. Air pollution in particular is widely recognised as a threat to health and in the long term leads to man-made climate change.

Funded by the Natural Environment Research Council (NERC), the Urban Regeneration and the Environment (URGENT) Programme of Thematic Research, aims to stimulate the regeneration of the urban environment through understanding and managing the interaction of natural and man-made processes.

Focused on a range of urban conurbations in the UK, all of which are now underway, URGENT hopes to 'clean up' the legacy of the past and institute sustainable development. It is expected that these research projects will develop generic models and solutions that can be applied widely in the UK and internationally.

One of these projects is Pollution of the Urban Midlands Atmosphere (PUMA). The objectives of PUMA include:
· applying the first high spatial resolution meso-scale meteorological model to the West Midlands;
· adding a coupled dispersion and atmospheric chemistry model, capable of predicting both primary and secondary air pollutant concentrations at urban background locations across the conurbation with a horizontal resolution of about 2km and a vertical resolution as low as 25km;
· making concentrations of a wide range of both long-lived and transient chemical species including hydrocarbons, carbonyl compounds, oxyacids and free radicals which play a role in atmospheric chemistry;
· validating the atmospheric chemical reaction mechanisms within the model in a depth not previously attempted;
· gaining insights into the chemical processes controlling the composition of the urban environment at a fundamental level;
· producing a management model applicable for national and local government to predict the impact of air quality for specific control strategies for a wide range of criteria pollutants on a range of timescales.

The approach adopted by PUMA promises to advance urban air quality modelling considerably (being subject to extensive validation at a level not previously attempted), to deliver an urban air quality management model which, although developed for the West Midlands, will be generic in nature and therefore applicable elsewhere. It is hoped the model will prove particularly useful to organisations such as local government authorities, automotive companies and pollution control companies.

The University of Manchester Institute of Science and Technology (UMIST) is supporting the project using an instrumented Cessna light aircraft to deliver a database of boundary layer characteristics including profiles of temperature, dew point, total aerosol, aerosol distribution and trace gases; and provide layer structure information under a variety of atmospheric thermal conditions within the PUMA model domain. In order to verify water vapour levels in the atmosphere, the Cessna is equipped with a dew-point hygrometer featuring a cooled-mirror sensor.

Ideal for scientific measurement applications, top-level humidity calibration work and a wide variety of on-line industrial applications, cooled-mirror dew-point hygrometers are capable of providing higher levels of accuracy, repeatability and stability than any other humidity measuring instruments.

Developed by Michell Instruments, the cooled-mirror technology employed in the study is capable of operating within an extremely broad temperature range; ideal for the wide range of altitudes at which the Cessna flies. The technology also allows for an extremely fast response time and ensures a high level of accuracy, which is crucial to the integrity of the project.

Much of the success of Michell Instruments's cooled-mirror dew-point hygrometers can be attributed to the cooled-mirror sensors, integral to their method of operation.

The technology uses a solid-plate Peltier thermoelectric heat pump to cool a miniature polished stainless-steel mirror until it reaches the dew point of the gas under test. When the correct temperature has been reached, condensation forms on the mirror surface. An electro-optical loop will then detect that condensation is forming, by a reduction in the intensity of light reflected from the mirror surface and through the control electronics of the cooled-mirror instrument. This ensures that the cooling power applied to the Peltier is modulated correctly.

The surface of the mirror is then controlled in an equilibrium state with evaporation and condensation occurring at the same rate. At this stage, the temperature of the mirror (measured by a platinum resistance thermometer) is equal to the dew-point temperature of the gas.

A problem common to many cooled-mirror systems is contamination of the mirror, which can significantly compromise the accuracy of readings. While most commercially available instruments cannot distinguish between moisture and contamination on the mirror surface, all Michell Instruments's cooled-mirror dew-point hygrometers feature a unique Automatic Balance Compensation (ABC) system which ensures optimum performance in contaminated environments. The system operates periodically to drive condensation off the mirror, while at the same time providing automatic electronic compensation for any reduction in signal, to ensure readings are consistently accurate.

While the Cessna component of the PUMA project will be completed in March 2002, Michell Instruments will continue to play an important role in other studies carried out by UMIST. In 2002, the plane and Michell Instruments's cooled-mirror dew-point technology will be used to measure UV and aerosol profiles at other sites in Europe, enabling boundary layer depth and characteristic stability to be determined; while a joint proposal from UMIST and the University of Birmingham was recently submitted concerning the use of the instrumented Cessna to investigate power station plumes.

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