Organic (OC) and Elemental (EC) Carbon Measurements using a Thermal Optical Method

Studies have been conducted at UC Davis to evaluate carbon artifacts and to improve the reproducibility of carbon measurements using a thermal optical method.

Organic Carbon Sampling Artifacts

PI: Ann M. Dillner, Graduate Student: Ashley Mefferd

This project focuses on evaluating some of methods IMPROVE uses to measure and corrects for the organic carbon sampling artifact. One module in the IMPROVE sampler is loaded with a quartz filter for thermal optical analysis to obtain organic (OC) and elemental carbon (EC) concentrations. Quartz filters collect both particle and gas phase organic material. To correct for the gas phase material, i.e., artifact due to the gas phase material, a back filter is added to the sampler to collect only gas phase material (see figure below). The material collected on the back quartz filter is subtracted from front filter data. It is assumed that the adsorption of gases onto the upstream filter is the dominate artifact and that volitilization of particles off the upstream filter is negligible.

In the IMPROVE network, six sites have both front and back filters and the two filters are loaded into the filter cartiridge such that they are in contact with each other. The objectives of this project are (1) to evalute if there is material transfer between the filters when two are loaded into the same cartridge and (2) to determine if the artifact value obtained from the back quartz filter is a good representation of the artifact. Studies have shown that the back filters of a Teflon-quartz setup are a more accurate measurement of positive artifacts (i.e. ambient gas-phase organic carbon adsorbed onto a filter) than quartz back filtersbehind a quartz filter. Since the quartz filter has more surface area, it takes longer for the front filter to reach equilibrium, thus reducing the concentration on the quartz back filter.

To investigate these questions, parallel samples were collected using IMPROVE samplers at UCDavis during the fall of 2008 and winter of 2009. The sampler set up is below.

To investigate objective 1, front and back filters from the left and center module in the figure above are compared. Careful attention is given to to measurement uncertainty so that we can to determine if the concentrations on the front filters are different from each other. Significant differences in the front and back filters would indicate that material is being transfered. To investigate objective 2, a comparison is made between filter 2 in the center module and filter 1 in right hand module. If the quartz behind teflon filter in the right module is higher than the quartz behind quartz in the center module, that suggests that the current method of artifact correcting used by IMPROVE may underestimate the gas phase carbon adsorbed on the front filter.

Organic Carbon Post-Sampling Artifact

PI: Ann Dillner, Graduate Student: Sunny Phuah

Organic carbon may be lost from filter samples after sampling is complete but prior to analysis in the labortory. We have recently published a paper describing experiments that show that organic carbon can be lost from filters in the field or during shipping and handling. For more information, see reference below.

Dillner, A. M., Phuah, C. H., Turner, J. R., Effects of Post-Sampling Conditions on Ambient Carbon Aerosol Filter Measurements, Atmospheric Environment, 43 (37) 5937–5943, 2009.

Calibrating the Filter Temperature in a Thermal Optical Instrument

PI: Ann Dillner, Graduate Student: Sunny Phuah

Thermal Optical methods heat PM samples to specified temperatures and report the carbon that evolves at each temperature. However, the temperature of the oven, which has a large temperature gradient across it, is measured at a single point and the sample temperature is not measured. We have shown that differences between the measured oven temperature and true sample temperature produce different reported data. To solve this problem, we developed a temperature probe to directly measure the filter temperature. By utilizing the probe, PM researchers and monitoring networks worldwide can analyze samples using the same filter temperature (not oven temperature as is currently done) and produce more robust, reproducible and comparable measurements.

Phuah, C. H., Peterson, M. R., Richards, M. H., Turner, J. R., Dillner, A.M., A Temperature Calibration Procedure for the Sunset Laboratory Carbon Aerosol Analysis Lab Instrument, Aerosol Science and Technology, 43 (10) 1013-1021, 2009.