Development and Types of Passive Samplers for Monitoring Atmospheric NO2 and NH3 Concentrations
Numerous passive samplers based on the ‘Palmes-tube’ have been developed for ambient air monitoring. In each case, the diffusion path length and/or crosssectional area are modified to achieve the desired sampling rate. ‘Tube-type’ samplers are low sensitivity samplers suitable for long-term monitori...
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| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Wiley
2001-01-01
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| Series: | The Scientific World Journal |
| Online Access: | http://dx.doi.org/10.1100/tsw.2001.82 |
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| Summary: | Numerous passive samplers based on the ‘Palmes-tube’ have been developed for
ambient air monitoring. In each case, the diffusion path length and/or crosssectional
area are modified to achieve the desired sampling rate. ‘Tube-type’
samplers are low sensitivity samplers suitable for long-term monitoring, whereas
the ‘badge-type’ samplers have faster sampling rates suited to short-term
monitoring.
In the U.K., diffusion tubes are widely used for monitoring nitrogen dioxide
(NO2) and ammonia (NH3). The open-ended diffusion tubes are prone to positive
bias caused by incursion of wind eddies, leading to a shortening of the diffusion
path. By using a porous membrane at the inlet, wind incursion is prevented, but
an additional diffusion resistance is imposed and it is necessary to calibrate the
tubes against a reference method to obtain an effective sampling rate.
For NO2 sampling, positive bias also arises from the reaction of NO with O3
within the sampler. The interference from the chemical reaction is severe close to
NO sources, with errors up to 30% for curbside locations when using the ‘tubetype’
sampler. In rural areas, where NO concentrations are small relative to NO2,
these errors are small. In some implementations, there is also a negative bias over
long sampling periods caused by the degradation of trapped NO2.
The low sampling rates of diffusion tubes make them too uncertain for use at
background NH3 concentrations (<1 μg NH3 m-3) where they significantly
overestimate concentrations. Badge-type samplers such as the ‘Willems badge’
samplers permit accurate sampling at low ambient NH3 concentrations, but suffer
from saturation at high concentrations and sensitivity to wind speed. A passive
sampler optimised for monthly measurements of NH3 is reported here, together
with its application in the U.K. National Ammonia Monitoring Network. |
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| ISSN: | 1537-744X |