Coarse Dust Monitor


General
Sampling


General

To be able to measure on line and semi real time coarse aerosol concentrations the Coarse Dust Monitor is being developed.The instrument consists of a cylindrical tube (tunnel) with a cross section of 30 cm, and a length of 50 cm with a large fan at the rear end		 A Dust Monitor
of the tube, which sucks the air into the tunnel at a constant speed of 10 ms-1. The inlet of the tunnel is pointed towards the wind by means of servomechanism coupled to a wind vane. Aerosol concentrations are measured by a secondary probe inside the tube being an inertial impactor strip with a width of 5 mm, which is analyzed using image analysis. This measurement system will be fully automated in the future.
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Sampling

The sampling efficiency of the measurement is described on the basis of existing theory. The parts of the measurement that could not be described on the basis of theory are investigated experimentally, both in the field and in a wind tunnel. Though no absolute reference measurement exists, the rotorod is used as a reference instrument. The rotorod cannot be used in every field situation, together with difficulties using the image analysis system it can easily be subject to sampling errors.
Sampling efficiency can be divided into several different factors; the mean sampling efficiency in the tube entrance and the collection efficiency of the inertial impactor can be well described on the basis of literature. The relation between the sampling efficiency in the entrance plane and the sampling efficiency in the sampling plane as well as the spatial distribution of the aerosol in the sampling plane have been determined experimentally. Further the difference between the axis of the tube and wind direction and the actual sampling speed as a function of wind speed are determined, both under standard sampling conditions.
Not all possible field situations have been investigated and high variations of the concentration in the sampling plane under field conditions exist that cannot fully be explained. These uncertainties remain unknown and need to be investigated further. It is however found that when considering the total sampled aerosol mass, the sampling efficiency of the CDM is:
1.0 +/- 10% for 20£ dae<50 µm and
1.0 +/- 20% for 50£ dae<70 µm if the following conditions are satisfied:

- difference between wind direction and the axis of the tube £ 15°
- actual sampling speed is 10 ms-1
- wind speed at sampling height is between is between 1.7 and 12 ms-1
- the impactor strip is located at (x,y)=(20,10) cm

The first three conditions are found to be satisfied during regular sampling in 98% of an average Dutch year. The last condition can be easily realized in the future version of the CDM.
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