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DEVELOPMENT OF AN RH -DENUDED MIE ACTIVE SAMPLING SYSTEM AND TARGETED AEROSOL CALIBRATION
Citation:
RHODES, C., J. THORNBURG, J. SEAGRAVES, A. F. VETTE, AND R. W. WILLIAMS. DEVELOPMENT OF AN RH -DENUDED MIE ACTIVE SAMPLING SYSTEM AND TARGETED AEROSOL CALIBRATION. Presented at 2005 Detroit/Windsor Research Studies Meeting, Detroit, MI, October 21, 2005.
Impact/Purpose:
Six objectives have been defined for this study.
(1) To determine the associations between concentrations measured at central site monitors and outdoor residential, indoor residential and personal exposures for selected air toxics, PM constituents, and PM from specific sources.
(2) To describe the physical and chemical factors that affect the relationship between central site monitors and outdoor residential and indoor residential concentrations, including those that affect ambient source impacts.
(3) To identify the human activity factors that influence personal exposures to selected PM constituents and air toxics.
(4) To improve and evaluate models used to characterize and estimate residential concentrations of and human exposures to selected air toxics, PM constituents, and PM from specific sources.
(5) To investigate and apply source apportionment models to evaluate the relationships for PM from specific sources and to determine the contribution of specific ambient sources to residential concentrations and personal exposures to PM constituents and air toxics.
(6) To determine the associations between ambient concentrations of criteria gases (O3, NO2, and SO2) and personal exposures for these gases as well as personal exposures to air toxics, PM constituents, and PM from specific sources.
Description:
The MIE pDR 1200 nephelometer provides time resolved aerosol concentrations during personal and fixed-site sampling. Active (pumped) operation allows defining an upper PM2.5 particle size, however, this dramatically increases the aerosol mass passing through the photometer optics. An examination of the potential calibration bias from internal soiling was needed. The factory MIE aerosol calibration relating photometer response to concentration is derived using poly-dispersed Arizona Test Dust (ATD). A more robust, targeted calibration using NH4NO3 at 5g/m3 levels instead of ATD in the mg/m3range was needed to better represent Detroit metro PM during the Detroit Exposure and Aerosol Research Study (DEARS) sampling. Another concern was limiting the operation of the MIE photometry to relative humidities RH's less than ~65%. An inlet humidity denuder was needed to limit the RH levels reaching the photometer, while minimally affecting aerosol transport. Addressing these issues was critical to the reduction in uncertainty when comparing MIE-obtained concentrations with DEARS gravimetric measurements.
A new NH4NO3 laboratory calibration proved simple and cost-effective, and targeted a known PM2.5 constituent of Detroit air, generated in much more realistic concentrations and size ranges relative to ambient particle loadings. Limited comparison of the MIE data against collocated gravimetric PM2.5 sampling will be presented. It was determined that the ability to precisely set the internal MIE calibration potentiometers limited the attainable agreement against a referee measurement to 䔳%. A low transport loss silica gel inlet denuder was developed for both personal and fixed-location sampling. Humidity data from a HOBO downstream of the photometer, showed that the humidity denuder effectively maintained the RH between 40%-60% under field conditions. Silica gel dusting into the aerosol stream required a design change to stabilize the bed packing. Post-season MIE re-calibration showed no detectible influence from internal aerosol losses to the optics during 7 weeks of sampling. This was partially attributed to the PM2.5scalping inlet.
Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.