10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Effective Density Determination For Workplace Real-Time Monitoring of Ultrafine Aerosols Using An ELPI+
LOÏC WINGERT, Yves Cloutier, IRSST/ETS
Abstract Number: 975 Working Group: Workplace Aerosol
Abstract Ultrafine particles aerosols (UFPs) can be found in many workplaces depending on workers activity or industrial processes (combustion, mining, grinding…). Because of health issues linked to the UFPs, workplace air monitoring is performed by Occupational Health and Safety agent. Many aerosols properties are often needed to assess their toxicity or to fully describe them such as particle size distributions, total concentrations or the size-resolved particles chemical composition. Furthermore, time-resolved and time-weighted averages values are also of interest. Therefore, it is often necessary to use simultaneously several devices (SMPS, filter samplings, cascade impactor...) leading sometimes to issues of space and weight-carrying.
The ELPI+ (Electrical Low Pressure Impactor, Dekati®) could solve this issue but to be operated properly the particles effective density of the particles need to be known. The ELPI+ has the advantages to be able to collect aerosols on removable substrates for further gravimetric and chemical analysis. The ELPI+ is equipped with an electrometer for each impaction stage that gives a time-resolved number particle size distributions (NPSD). As it has been said, for these NPSD to be correctly calculated, one needs to know the particle effective density. In the ideal case of spherical and non-porous particles, this density is equal to the raw material density. Unfortunately, almost all of the time workplace aerosols are unsteady mixtures of irregularly shaped and porous UFPs whose effective density is decreasing with particle diameter. To determine this size-dependent effective density, tandem analysis described in the literature (DMA-ELPI+, DMA-APM...) can be used. Although these methods are precise and adapted to laboratory experiments, they present the disadvantages to be time-consuming and unable to monitor quick changes in aerosols composition that occur frequently in workplaces. Furthermore, it would mean to carry another heavy and expensive device with the already cumbersome ELPI+. Because of these difficulties, people don’t use ELPI+ in the workplaces for real-time measurements or assume a unit density leading to significant errors.
To overcome these issues, we developed a method based on: (1) the synchronous use of an ELPI+ and a hand-held condensation particles counter (CPC) and (2) a custom made code reproducing, inter alia, the ELPI+ software calculations. The first step is to synchronously save the currents measured by the ELPI+ electrometers and the real total number concentration measured with the CPC. Then, by using the custom-made code for a given measurements interval, an averaged density is adjusted to minimize the gap between the total number concentrations given by the two devices. The NPSD corresponding to this fitted average density is then considered as the closest to the real distribution. This method was validated in laboratory exposure chamber experiments by using a SMPS as a reference for the NPSD. This validation was performed using four UFPs aerosols. In addition to the NPSD, the adequacy of the fitted average effective density with the real size-dependent one was verified. A validation of this method was also successfully performed in electronic waste recycling factories using a Nanoscan SMPS as a reference instead of a SMPS. This method is easy to use in the field or in a laboratory by hygienists or OHS researchers interested by UFPs. Because it allows for the determination of the real average effective density, ELPI+ can be used to collect UFPs particles on its collection substrates for further analysis while recording real-time data of aerosol concentrations that can be rightfully corrected. This method offers also an easy alternative to evaluate particle effective density of any UFP aerosols.