The Effect of Sampling Inlet Direction and Distance on Particle Source Measurements for Dispersion Modelling

The source rate is the single most critical input parameter in dispersion models. Determining accurate source rates from workplace processes can be challenging due to interference with work operation and poorly known dilution between the outlet of the particle generator and the measurement point. In this work, we measured the aerosol source rate in a chamber with a steady release of TiO2 particles generated by an aerosol brush generator.

The number concentrations measured directly from the particle generator and in the source position near the source spanned three orders of magnitude depending on the relative location and orientation to the source. Moreover, a dispersion factor was calculated based on a single mode fit of the obtained source rates.

The dispersion factor takes into account the dispersion and dilution occurring between the measurement point and the source outlet for subsequent modelling. The particle emission rates were implemented in a previously published multi-box aerosol dispersion model using a one-box layout. The modelled concentrations were compared with concentrations measured in three locations in the chamber.

We found that using a dispersion factor of one, meaning that at-source dilution or dispersion was not accounted for, the modelled concentrations were 1 to 3 orders of magnitude lower than measured concentrations for all source rates except the source rates measured directly from the aerosol generator.

When applying the calculated dispersion factor, thereby correcting the source rate for initial dilution and dispersion, the concentrations were within 0.5 to 2 times the measured concentrations suggesting the use of such a factor to correctly estimate the source rate, and hence the occupational exposure.