Air Monitoring
Simple Off-The-Shelf Ceilometers Find Increasing Applications in Air Quality
In recent decades meteorological sensors based on the scattering of light have moved from specialised research applications to routine operational use. For example roadside weather stations often have forward scatter sensors measuring visibility or identifying weather and most people reading this will have seen them. Similar sensors are widely used at airports and for a variety of other applications. Ceilometers using LIDAR (LIght Detection And Ranging) techniques are now used routinely by meteorologists and at many airports to determine cloud base by measuring the time it takes for light scattered from clouds to return to the sensor. These sensors are becoming increasingly sophisticated and are able to derive a lot more information about the atmosphere than just cloud height. In particular they can derive boundary layer structure of great interest for air quality forecasts more cost- effectively than Doppler LIDAR or other techniques.
Reliable values for backscatter coeffi cients are important for air quality and research applications but checking or calibrating an instrument in the fi eld has been, until now, very diffi cult.
Basic LIDAR techniques are now widely used to determine cloud base by measuring the time it takes for light scattered from clouds to return to the sensor. In practice this is not as straightforward as it fi rst sounds. Clouds do not have an abrupt, clearly defi ned base returning a sharp ‘echo’. In practice a ceilometer measures a profi le of scattering (in units of sr-1
m-1 , in effect the proportion
of scatter over a unit solid angle over unit distance). It then uses specialised algorithms to identify a cloud base. Typically this is based on a combination of identifying a rate of change of scattering and some criteria based on the calculated horizontal visibility. For the Campbell Scientifi c CS135 ceilometer (fi gure 1) this criterion, at lower levels, is a calculated horizontal visibility of 1,000m. This is the visibility criteria for fog so is a familiar threshold for pilots. Interestingly, it might be that the pilot of an aircraft fl ying in cloud a hundred meters above its base could see the ground below if he looked straight down (an observer on the ground would also see the aircraft as it passed directly overhead) but as regards aircraft around him or obstacles ahead he would be fl ying well within fog limits.
Author/Contact Details: Mike Brettle
Product Manager, Optical Sensors Campbell Scientifi c Ldt. 80 Hathern Road, Shepshed, Lecis., LE12 9GX United Kingdom
Email:
mike.brettle@
campbellsci.co.uk Figure 1: A modern ceilometer.
A ceilometer such as the Campbell Scientifi c CS135 (fi gure 1) may look simple but contains sophisticated optics and signal processing. The low noise signal processing allows measurements of small backscatter signals from aerosols. The optics in this model are based on a relatively large lens allowing a large collection area. This lens is cut and divided by an opaque barrier to allow the laser emitter and photodiode detector to be very close (fi gure 2). This is important because it means there will be
signifi cant overlap between their fi elds of view at relatively low levels allowing useful measurements within the lower levels of the atmospheric boundary layer, while still maintaining good optical isolation of the channels.
Figure 2: Schematic of the optical system of the CS135 ceilometer showing how a lens split by an opaque barrier allows the laser and detector to be close together.
A ceilometer measures the profi le of scattering in the atmosphere and this raises the obvious question as to whether more can be made of this information, especially since it is available from relatively cheap ‘off the shelf’ instruments. Air quality is an obvious candidate application. It is of great importance but some of the key parameters required for forecasting the development of the atmospheric boundary layer are diffi cult to measure.
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Figure 3: Idealised diurnal evolution of the atmospheric boundary layer.
Figure 3 gives a very simplifi ed model of the typical evolution of the atmospheric boundary layer. Note the growth of the mixing layer during the day and its collapse around sunset. This is, as it’s name implies, the layer through which air, and by implication pollution released at the surface, is mixed.
www.envirotech-online.com IET Annual Buyers’ Guide 2014/15
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