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The use of wind sensors for air quality applications This article will give an overview of the types of wind sensors available for air quality applications. The focus is on the use of ‘in-situ’ sensors rather than remote sensing techniques such as wind radars (often called wind profilers).
The article will cover sensor location, a much neglected but critical aspect of wind measurement. This is often as important as the sensors themselves.
Wind is obviously the most important factor in the spread of pollution. It has critical effects on both the area of spread of pollution and concentrations of material within the plume. There are two different aspects to ‘air quality’. One is issues of levels of pollutant produced over long periods usually as a result of continuous or intermittent releases not in themselves an immediate danger.
The other is large scale sudden releases due to a major accident or deliberate act. Of course there is considerable overlap but at the extremes the information required, and possible, is very different. Emergency response applications require information very fast but detailed knowledge of pollutant concentrations is not only less important but also usually impossible to obtain anyway as source terms are unlikely to be available in the first tens of minutes. Both these applications will however require good quality wind data and the appropriate sensors will usually be the same. That is a wind sensor used as part of a routine emissions monitoring system can also be used to provide information to the emergency services in the event of a major release.
‘In – Situ’ Wind Sensor Types
The simplest wind sensor is the humble wind sock. This is not to be underestimated, especially for emergency response applications. It is reliable and easy to interpret. However it has serious limitations. There is no logging of data and no defined averaging period. Wind speed is poorly indicated, if at all and of course data transfer is ‘line of sight’. Although, in my opinion, every site involved with materials that, in an emergency, could form an airborne hazard should have a wind sock I will not say any more about them.
Mechanical cup and vane instruments, such as in Fig 1, are in widespread use. They do have some problems though. Firstly they are poor at low wind speeds because they will stall or stop moving below a threshold speed. This is quite serious for air quality applications since it is at low wind speeds that pollutant concentrations are highest.
In addition the gust response of a cup anemometer is less than ideal. The cups will speed up in response to a sudden increase in speed faster than they slow down in a sudden reduction. This results in an over reading of wind speed in gusty conditions. Research on the ideal characteristics of cup anemometers has shown that conical cups are superior to hemispherical, three cups are best for a steady rotation and there is an optimum ratio of cup diameter to the sensor radius.
This has led to a kind of evolutionary convergence between the designs of different manufacturers. Cup anemometers also suffer from poor ‘cosine response’. This means that as the wind direction changes to hit the sensor from below or above the horizontal the response of the sensor deviates from the ideal response, which is spinning at a rate proportional to the cosine of the angle of the wind above or below horizontal.
For most designs the cups will actually spin faster for a wind a few degrees above or below horizontal. Again this results in an over reading in turbulent conditions.
Propeller anemometers are also popular and overcome many of the problems of cup and vane types. Firstly they are less sensitive to gusty conditions. The design is usually a perfect helical shape which has a good cosine response close to its axis overcoming the cosine response problems of cup and vane designs.
Unfortunately propeller sensors suffer very poor cosine response when the wind is nearly at right angles to the sensor axis which causes problems with designs incorporating fixed orthogonal propeller sensors. A popular configuration combines a propeller wind speed sensor attached to a wind vane. This ensures that the propeller is always operating with the wind along its axis.
Orthogonal propeller anemometers can be used to measure in three dimensions and give useful turbulence data. This is described in the section on ultrasonic anemometers below but because of their slower response there will be a minimum height for these sensors to be useful in this application.
Mechanical wind sensors respond faster to change in strong winds than in lighter ones. For this reason their response characteristics are usually given as a length or distance constant, typically around a metre, rather than a time constant. This is usually defined as the length of the run of wind that will pass for a 63%, 1/e, response to a step change. The response of mechanical sensors is largely independent of the air density, and therefore of temperature and pressure, at least for winds well above the threshold velocity.
Mechanical sensors are now being replaced by ‘sonic anemometers’, sometimes called ‘ultrasonic anemometers’ such as that in Fig 2. These operate by sending pulses of ultrasonic sound between transducers and measuring the time of flight. This allows a calculation of the component of wind velocity along the line between the transducers. They all have the advantage of working down to very low wind speeds, in principle down to zero wind speed.
This is very important in air quality applications since it is in low wind speeds that pollutant concentrations are highest. The problems of gust response and cosine response that plague mechanical sensors don’t exist with sonic anemometers. Issues of response time and averaging are much more manageable. Sonic anemometers can provide measurements with short averaging periods, typically measured in tenths of a second, over the whole wind speed range.
This allows useful real measurements of turbulence statistics, again useful for air quality applications. Some designs allow measurement of the wind in three dimensions. These are at present mostly confined to research applications but offer the prospect of direct measurements of turbulence in three dimensions. Combined with fast temperature measurements this can allow measurements of the rate of flow of heat from the surface resulting from turbulence, the so-called heat flux.
This is based on a technique called ‘eddy correlation’ whereby the fast variations in temperature are combined with the vertical wind speed, also measured at fast time intervals. In a subtle variation on this theme some 3D sonic anemometers can make temperature measurements themselves based on variations in the speed of sound with temperature. By combining this with the simultaneous wind measurement the heat flux can be measured directly by the one instrument. This can be extremely useful input to dispersion models.
There are various other techniques of ‘in-situ’ wind measurement but space does not allow for a full description. These include the use of pressure changes along the lines of a pitot-static tube, wind force sensors and heated sensors. The latter use the heat loss of an array of heated elements to measure wind. Most of them fall into two categories, heated fine wires used in research, and not suited to an industrial application, and more robust, but power hungry, designs often used for military applications on fighting vehicles or warships.
Remote sensing of wind
Wind can be measured with radar wind profilers, lidar and sodar based instruments. These are a lot more expensive but can give wind information over a depth of the atmosphere which can be very useful for accurate estimates of plume depth and therefore the concentration of pollutants. Some versions can also give direct measurements of turbulence in the atmosphere which is again very useful for assessing how a plume of material will spread.
The most sophisticated arrangement of all is possibly the combination of a wind profiler and powerful sound projectors to form a radio acoustic sounding system (RASS). The speed of sound is dependent on the air temperature (or more correctly on the virtual temperature which is the temperature at which dry air would have the same density as the actual air). By projecting sound up through the volume scanned by the radar it can measure the speed of sound at different levels. In this way the system can give an estimate of both wind and a rough temperature profile allowing the characteristics of the atmosphere to be very well defined.
Wind profilers with or without RASS are widely used to assess pollution from major sources. Fig 3 shows such an installation. Typically such a system will measure wind speed and direction up to a few kilometres with a resolution of about a hundred metres and will provide virtual temperature profiles up to about one km with one hundred metre resolution. Since these systems are measuring up to these levels location is often less critical than for in-situ sensors. However data from levels influenced by local effects, which is usually the lower levels, will still be suspect.
Installation – the importance of appropriate exposure for wind sensors
For good quality data the quality of the installation and its location is even more important than the sensors themselves. A mediocre instrument intelligently used will often give better results than a state of the art sensor in a poor location. This is an important point as most users of wind monitoring equipment are not meteorologists themselves and may not appreciate the scale of local variation or possible errors. A wind sensor incorrectly mounted on a building could indicate a wind direction literally 180° in error.
“Meteorologists have agreed standards of sensor exposure that define the height of the sensors and the acceptable distance to obstacles of various sizes”
The starting point for any installation must be to consider exactly what the data is being collected for. If the information will be compared to existing reference data, for example if maps of expected frequency of gales is to be used with climate records as part of an environmental impact assessment then the installation should match as closely as possible those used to collect the original statistics.
Meteorologists have agreed standards of sensor exposure that define the height of the sensors and the acceptable distance to obstacles of various sizes. The standards are defined by the World Meteorological Organisation but local variations and interpretations are often produced by weather services and other organisations and manufacturers. For the most part these do not vary much but they are often difficult to achieve, especially in an industrial environment.
For example a wind sensor should ideally be exposed at 10 metres above open terrain. This is the basis of most of the observations used in producing climatological statistics as used in building standards, risk assessments for air quality and so on (and also the basis of forecasts). This quality of sensor exposure is something of a luxury for most users. If local data is needed to compare with climatological data then some careful compromises are likely to be necessary.
However, for many applications where there is no need to compare local data with other data then sensor location should be based on the application. For example if there is a need for wind speed at the top of a chimney or other structure then this is the height that the wind sensor should ideally be installed
If in doubt users should consult an accredited meteorologist through their suppliers or an organisation such as the Royal Meteorological Society, www.rmets.org . The RMetS web page also has links to other organisations and sources of information, including the WMO.
Note that even wind socks are affected by distortions caused by buildings or complex terrain. A wind sock low down at a site entrance, in disturbed wind flow will be just as misleading as any other sensor.
Data processing
The way raw data is processed can have a major effect on the measured wind values.
First of all if ‘gusts’ or maximum wind speeds are of interest then it is important to be aware of the gust definition used. For most UK applications, including aviation and climatological datasets, the ‘maximum gust’ is defined as the highest running mean averaged over 3 seconds. Obviously different definitions would give different values. Mean winds are usually defined as either 2 minute or 10 minute averages.
Secondly, wind averages can be scalar (separately calculated mean values for recorded speed and direction, allowing for the discontinuity at north) or vector. A vector average is based on the distance and direction a particle blown by the wind would have travelled over the relevant time period. In light and variable winds in particular this can be different to the scalar average and is much more relevant to air quality applications.
Display of data
The traditional format for displaying wind measurement is dials, sometimes supplemented by a chart recorder. Alternatively wind data can be displayed as numbers on a digital display. There is one major pitfall however. It is easy to forget that wind direction is the direction from, not the direction to. Passing on, for example, a wind direction of 270°, or ‘westerly’ could lead someone unfamiliar with the terminology to think the hazardous area resulting from a release is actually east of the site. The implications are obvious.
Recording instruments such as chart recorders have problems resulting from the fact that wind direction is not continuous and has a discontinuity from 360° to 0°. Some systems reduce this problem by having scales up to 540°.
Possibly the best format for displaying wind information is on a PC screen. This can be in the form of a virtual wind dial and/or digital display and can include an archive as a strip chart or wind rose. A PC display can incorporate a map of the site and can include a certain amount of processing applied before the information is displayed such as an area at risk from emissions. Fig 4 shows such a display. This approach also eliminates the risk of wind direction being misunderstood and taken 180° degrees in error.eference WMO Guide to Meteorological Instruments and Methods of Observation, seventh edition, WMO-no.8; Secretariat of the World Meteorological Organisation -Geneva-Switzerland 2006.
For more details on air quality please visit http://www.osedirectory.com/environmental.php
Published: 01st Mar 2010 in AWE International
