Mining Project: Feasibility Study For a Novel Field-Portable DPM Monitor
To investigate the feasibility of using alternative measurement approaches to quantify both elemental carbon and organic carbon in filter samples of mine air.
Diesel engines are one of the primary contributors to the presence of ultrafine and nano aerosols in ambient air and occupational environments. Exposure to diesel emissions has been shown to contribute to various adverse health outcomes of the pulmonary system and cardiovascular system. New technologies are being developed to reduce engine emissions and thereby airborne diesel particulate matter (DPM) levels, yet high exposures persist, especially in underground mines.
In order to reduce worker exposures, it is critical to measure the levels of DPM in active work settings. In mine settings, exposures to DPM are measured by collecting air samples onto filters (usually full-shift samples), and sending them to a lab for analysis by the NIOSH 5040 method. The NIOSH 5040 method is based on a thermal-optical technique and is aerosol-type- and size-independent, but has the significant limitation of being a laboratory method. The time frame to collect samples, analyze them, and get quantitative results is usually days to weeks. This limitation makes it very difficult for mine safety personnel to utilize the results for effectively planning DPM reduction strategies. For this reason, this pilot research effort was focused on investigating and developing alternative methods for measuring DPM in workplaces.
In the past, several research efforts have focused on investigating and developing alternative methods for measuring DPM in workplaces. Although particulate mass can be estimated from data using assumptions for particle shape and density, this approach does not differentiate the fraction of DPM that is elemental carbon (EC) versus the fraction that is organic carbon (OC). Among the variety of instruments available to estimate carbon concentration in aerosols, two types have been shown to give good correlations with the EC in DPM: light-extinguishing devices and light-scattering devices. Light-extinguishing instruments (such as an aethalometer and the AirTec diesel particulate monitor) estimate the EC in DPM by measuring the mass concentration of carbon in sampled aerosols, employing the optical light absorption properties of black carbon (BC) at specified wavelengths. These results correlate very well with EC concentrations evaluated by the NIOSH 5040 Method. Although these instruments require aerosol-dependent calibration, they are convenient to use and can provide a near real-time measurement.
Light-scattering instruments (namely direct-reading photometers), of which there are many, are simple and inexpensive devices that give readings shown to correlate well with total particulate mass concentration measurements; however, as with most of the other real-time methods, they do not differentiate between EC and OC. While the above-mentioned methods have both shown potential for quantitating EC in mine atmospheres, that approach (using EC as a surrogate for DPM/TC) has a significant limitation. A variety of environmental and equipment-related factors lead to a wide variance in EC/OC ratios in the mine air, in both time and space, making it difficult to compare real-time indications of EC with time-weighted average measurements of DPM/TC over a shift, which is how the levels of DPM are measured for regulatory purposes. Therefore, the aim of this pilot research was to investigate the feasibility of using alternative measurement approaches that may be capable of quantifying both EC and OC in mine air, in order to set the stage for a larger full research project.
The results of this pilot study were as follows:
Part 1. Research focused on evaluating the use of Fourier transform infrared spectroscopy (FTIR) to quantify EC and OC in filter samples derived from diesel tailpipe emissions and from mine air samples. The results helped NIOSH researchers understand the efficacy of using an existing spectrometry method that is well-developed and understood (FTIR), along with multi-variate analysis of the spectrometry data, to estimate the EC and OC in airborne diesel emissions. Preliminary test data showed that FTIR may be a useful tool for DPM quantification, and led to the development of a full research proposal.
Part 2. A full project proposal was developed to further the pilot results and investigate additional spectrometry methods, including Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) and laser-induced breakdown spectrometry (LIBS) for measuring EC and OC in the field. Based on the results of the pilot work, the full proposal will entail a four-year study in which the FTIR method is further developed and other spectrometry methods are investigated with the same goal. That work has been funded by NIOSH, and more information can be found at the project webpage, "Develop Portable DPM Monitor."
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