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A new method for the real-time measurement of diesel aerosol.

Kittelson DB; Watts WF Jr.; Johnson JP; Ragatz AC
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-008676, 2008 Oct; :1-60
Diesel exhaust is a complex mixture of gases and Diesel Particulate Matter (DPM), and in metal and non-metal underground mines the Mine Safety and Health Administration (MSHA) regulates DPM concentrations using a time-weighted-averaged, 8 hr, full-shift, permissible exposure limit. The limit is 160 microg/m3 of total carbon (TC) and National Institute for Occupational Safety and Health (NIOSH) standard method 5040 is used to determine the amount of elemental (EC), organic carbon (OC) and TC in the sample. This sampling and analysis method does not provide real-time data, nor does it provide information on particle volatility, particle size, or aerosol number concentration. These factors are known to be affected by emission control devices, engine duty cycle, fuel and lubrication oil composition and other factors. The goals of the project were: 1. Evaluate the DPM control efficiency of selected catalyzed emission control devices in the laboratory using traditional and non-traditional measures. Non-traditional measures include particle size, particle volatility, aerosol number and volume concentrations. Traditional measures include mass, elemental carbon (EC) and organic carbon (OC). 2. Evaluate and recommend procedures for the use of a low cost instrument package for the routine test cell evaluation of Diesel engines equipped with and without emission control devices. 3. Develop a portable CS for use with portable aerosol instruments used in underground, non-gassy mines to obtain real-time data on the physical and chemical characteristics of DPM to which miners are exposed. Control of temperature, residence time, flow, and dilution air quality are critical for repeatable, real-time measurement of nanoparticles, and the minimization of artifacts on filters collected for gravimetric analysis. This is particularly important for evaluating DPM emissions from 2007 compliant engines conforming to the 0.01 g/bhp-hr U.S. Environmental Protection Agency (EPA) standard with or without aftertreatment devices. This standard is roughly a 90% reduction from the previous level corresponding to an approximate 100 microg mass gain on the filter during an EPA certification test. A catalyzed Diesel particulate filter (CDPF) consisting of a catalyzed metallic "prefilter" and a catalyzed filter was evaluated. The CDPF particulate matter removal efficiency as measured by number, surface area or volume was > 99.9 %, and filters used to collect mass and EC/OC had no visible deposits. We compared the gravimetric based filter measurements used to determine the mass concentration to estimates of mass derived from the scanning mobility particle sizer (SMPS) to determine the amount of filter artifact. Filter artifact is defined as the weight gain not attributable to the collection of suspended particles. Our estimates show that in this case the standard TX40 filter overestimated mass by roughly a factor of 45, which is in general agreement with what is reported in the literature. The significance of this finding is that reliance upon filter based measurements to determine mass emissions from a low emitting engine with CDPF can lead to significant mass measurement error and misinterpretation of results. Extensive tests were conducted using a Diesel oxidation catalyst (DOC) and the findings were mixed. These devices have little impact on EC, reduce OC and may increase or decrease nitrogen dioxide (N02) and sulfate concentration, depending upon operating conditions. The DOC reduced the OC concentration when the catalyst light off temperature was achieved allowing oxidation of volatile organic material. On the other hand, oxidation of sulfur dioxide (S02) at higher operating temperatures to sulfates increased the mass concentration. The same was true for the formation of N02 under some circumstances. The nuances of catalyst storage and release are not well understood and prediction of what a specific catalyst will do under specific conditions is problematical without laboratory testing. Use of DOCs on Diesel equipment used in occupational settings such as an underground mine is advisable only when sufficient information is available to evaluate the potential consequences of using these devices on mine air quality. Laboratory measurements from a suite of portable, real-time aerosol instruments correlated well (correlation coefficient (R2) > 0.95) with time weighted average DPM mass and EC concentrations. The relationship between the instruments and the time weighted averaged measurements (and each other) was affected primarily by the amount of volatile material available to form nanoparticles < 30 nm in diameter). The amount of volatile material and the nanoparticle concentrations were affected by the engine condition (load and speed), fuel type, dilution condition, and presence of a catalyzed aftertreatrnent device. Real-time instruments can estimate the solid particulate matter mass attributable to EC in the laboratory. However, the user must be aware of how the physical and chemical aerosol characteristics can impact the estimate. In field tests conducted at an underground limestone mine the TSI AIM 510 portable photometer (equipped with a Dorr Oliver cyclone and 1.0 microm impactor) qualitatively tracked time weighted average mass and EC/OC measurements. The correlation coefficient (R2) between the underground EC and photometer measurements was 0.83. The main issue holding back the use of a photometer for real-time estimation of DPM is the removal of non-DPM associated particulate matter (PM) from the aerosol stream and calibration of the photometer to mine aerosol. Work remains to reduce the interfering dust to improve the specificity of the measurement. The mini-catalytic stripper (CS) designed and built for this project is battery operated and was used in combination with the photoelectric aerosol sensor (PAS) to provide information on the volatile and non-volatile fractions. The CS could also be used with other portable instruments such as the diffusion charger (DC) or photometer. The prototype was operated for over 4 hrs using two lithium ion batteries typical of those used in laptop computers. Further design modifications will package the mini-catalytic stripper with one or more portable instruments or sensors in an instrument that is smaller than the size of a shoebox. The EC to OC ratio is altered by the type of fuel, engine operating condition, and by the use of a catalyzed aftertreatrnent device such as a DOC. The DOC operates just like a CS in that it removes volatile organic material that is available to nucleate and form nanoparticles. The fact that the EC to OC ratio varies makes it problematical to attempt to establish a consistent relationship between TC and EC.
Aerosols; Aerosol-particles; Air-contamination; Airborne-particles; Airborne-dusts; Filtration; Mining-industry; Underground-mining; Diesel-emissions; Diesel-engines; Diesel-exhausts; Nanotechnology; Analytical-processes; Analytical-methods
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Final Grant Report
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National Institute for Occupational Safety and Health
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University of Minnesota Twin Cities, Minnesota
Page last reviewed: March 25, 2022
Content source: National Institute for Occupational Safety and Health Education and Information Division