VOC Interference with Standard Diesel Particulate Analysis for Mine Samples: Exploring Sources and Possible Solutions

Abstract

Exposure to diesel engine exhaust is linked to chronic and acute illness. In underground mines, workers can be exposed to high concentrations for extended periods of time. Therefore, Mine Safety and Health Administration (MSHA) enforces personal exposure and engine emission limits. These regulations target just the solid portion of diesel exhaust, known as diesel particulate matter (DPM). The majority of DPM mass is attributed to particulate organic carbon (POC) and elemental carbon (EC). Total carbon (TC) is the sum of POC and EC and currently used as the surrogate to represent DPM as a whole. The NIOSH Method 5040 is the standard sample collection and analysis procedure. It outlines collection of submicron particulate matter samples on a quartz filter then measurement of POC and EC using a thermal-optical analysis. Error in DPM measurement occurs when volatile organic carbon (VOC) sorbs onto the particulate matter deposit and filter resulting in a positive sampling artifact. To correct for this, a dynamic blank method with two quartz filters (i.e., primary and secondary) in tandem is used. However, the accuracy of the dynamic blank correction method is dependent on equal sorption of VOC onto each filter. Observed instances of higher VOC on the secondary filter result in underestimated POC measurements and in some cases negative POC. The work presented in this thesis investigates the sources of VOC interference in particulate matter sampling and possible solutions. Three existing datasets containing information from blank samples and laboratory and field DPM samples were analyzed to look into instances of higher VOC sorption onto the secondary filter. Negative total POC results were limited to blank samples, but negative results for the POC of individual isotherms were observed in blank and DPM samples. A follow-up study looked into the possibility of sampling materials as a source of VOC that preferentially sorbs onto the secondary filter. Blank samples were assembled to test five sampling materials (i.e., two types of sample cassette, cellulose support pads, impactor cassettes, and impactors). In addition, sample storage conditions (i.e., temperature and duration) were tested for their impact on VOC sorption. It was discovered that all of the sample materials tested contributed VOC and, as expected, higher storage temperatures and longer storage durations increase the amount of VOC. Preferential sorption onto the secondary filter was observed in most conditions as well. A field study explored thermal separation of VOC and POC as a possible alternative to the dynamic blank correction method. Two sets of DPM samples were collected from two locations in an underground stone mine and one set of ambient particulate matter samples was collected from a highly trafficked truck stop. The temperature of 175°C was used for this preliminary investigation. The effectiveness of a temperature separation may depend on sample location. To better understand VOC and POC evolution characteristic, further testing with a wide range of sample mass and composition as well as different temperatures is suggested. It seems unlikely that a correction method using a separation temperature would be more effective than the standard dynamic blank in occupational DPM monitoring. The work presented in this thesis highlights the difficulty in accurately measuring POC.