Analysis of PM2.5 Speciation Network Carbon Blank Data
Flanagan, J. B., Peterson, M. R., Jayanty, R. K., & Rickman, E. E. (2002). Analysis of PM2.5 Speciation Network Carbon Blank Data. In .
This presentation describes results of an analysis of field and laboratory carbon blank data for the PM2.5 Chemical Speciation Trends Network (STN), which is managed and funded by EPA/OAQPS, and for which Research Triangle Institute is the contract laboratory. Data from over two years of network operation were used for this analysis.
PM2.5 chemical speciation samplers are multi-filter units which include a quartz fiber filter for collecting PM2.5 aerosol samples. These filters are analyzed for carbon constituents using an adaptation of NIOSH Method 5040, a thermal-optical transmittance method. This method, which relies on thermal evolution of carbon species followed by quantitation by FID, does not identify specific compounds, but reports method-defined aggregate filter loadings of organic carbon (OC), elemental carbon (EC), and total carbon (which is the sum of OC and ED).
Almost 1500 laboratory QC filters; 2727 trip and field blank filters; Data span February 2000 through March 2002; Data were managed using Microsoft Access®, and were analyzed using both MS Excel ® and SAS®. The data analyses performed for this report resulted in several conclusions that have direct bearing the blank correction issue. The most significant of these are as follows:
1. Laboratory blank values are smaller (by an order of magnitude) than field and trip blank values. Furthermore, any contribution to the overall blank levels is already implicitly included in the field and trip blanks. Therefore, the laboratory blank levels are not necessary for blank-correcting the routine exposure data.
2. Trip Blanks and Field Blanks have very similar statistical characteristics (e.g., mean and standard deviation). Therefore, the Trip and Field Blank data will be combined when calculating the blank correction factors.
3. Average blank levels differ by sampler type. Therefore any blank corrections must be computed using the appropriate sampler-specific blank value.
4. Average Field and Trip Blank masses were found to be between 10 and 30% of average routine (24-hour) levels. Data users should consider the impact of this finding on their data applications.5. It was also found that the analytical "split time" used to separate the OC and EC fractions by the analyzer's software is usually early (before oxygen is added) for most blanks and other low-level samples. Because of this systematic pre-oxygen split, refractory OC that volatilizes from the filter without forming char can be mistakenly categorized as EC in some blanks. Pre-oxygen splits are much less likely in more heavily loaded filters. This finding has implications for how blank data might be used to correct the OC/EC data for 24-hour environmental samples.