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Deliquescence in suspended particles
Suspended particles can be categorised into two basic groups:-
- Hygroscopic particles – These grow in size during periods of higher humidity due to the introduction of available moisture - a process called deliquescence
- Hydrophobic particles – These do not grow during periods of higher humidity
The point at which hygroscopic particles start to grow, and their rate of growth, depends on the specific particle type; for example, pollen, mould or dust. Most particles start to absorb moisture which has an effect on their measured size when experiencing conditions of 50-55% relative humidity.
Optical particle counters, such as those found in AQMesh, are unable to identify each individual particle species, if they are hygroscopic or hydrophobic, or detect the amount of moisture held by the particle. The worst case is therefore assumed i.e. that all particles measured are hygroscopic.
Our "deliquescence" flag is triggered based on individual pod measurements and a combination of criteria which ALL need to be met before the flag is triggered. This means during periods of RH >50-55%, coupled with our other specified criteria, we flag the data point as potentially erroneous, suggesting the data points should be reviewed as part of a robust QA/QC process before using or redacting these data points.
While our deliquescence flag offers a reliable solution for identifying potentially erroneous data points- which then can or cannot be used based on user discretion - a more robust method of correcting for the effects of humidity was incorporated into AQMesh pods a few years ago in the form of the heated inlet. This dries the sample, removing moisture from the hygroscopic particles so they are sized correctly.
