Monday, 12 January 2009
An investigation of positive and negative sampling artifacts for organic carbon in IMPROVE samplers
Hall 5 (Phoenix Convention Center)
The Interagency Monitoring of Protected Visual Environments (IMPROVE) network was established in 1985 as a long term monitoring program to measure current visibility and aerosol conditions, track long term trends, and identify chemical species and emission sources responsible for anthropogenic visibility impairment in Class 1 areas including 156 national parks and wilderness areas (http://vista.cira.colostate.edu/improve/). The IMPROVE network uses air samplers with four modules to measure PM 2.5 and PM 10 particles on filters of different material in order to measure mass as well as different species of elements. Organic and elemental carbon is measured from samples collected on quartz filters using a Thermal Optical Reflectance (TOR) method. Carbon is present in the atmosphere as both solid particles and gaseous vapors, both of which are collected on quartz filters. The gas phase organic carbon adsorbs onto the quartz filters causing positive artifacts while negative artifacts are caused by collected particulate organic carbon volatilizing during sampling. Typically, a second quartz filter or back-up quartz is located downstream of the first quartz filter and is used to collect organic carbon vapors. The carbon concentrations measured on the back-up quartz are subtracted from the front quartz to obtain a more accurate particulate organic carbon value. The implicit assumption in this method is that the positive artifact is much larger than the negative artifact. However, this assumption is not always true and challenging this assumption for IMPROVE samplers is the motivation for this study. Using an IMPROVE sampler on the campus of the University of California, Davis, a study is being conducted to investigate organic carbon artifacts that occur during field sampling throughout the IMPROVE network.
In this study, five parallel IMPROVE modules are used to obtain five distinct methods of collection for particles and/or vapors to help determine the extent of positive or negative artifacts that occur out in the field. Twenty-four hour samples are collected every day during two separate periods. The first sampling campaign occurred during September 2008 while the second will take place in the winter of 2009 in order to compare summer and winter results. From the five modules, one channel measures the total artifact with a front and back-up filter. A second channel measures the negative artifact by using a denuder to remove organic vapor from the atmosphere and a back-up filter to capture organic gas from particulate organic carbon volatilizing off the front filter. Vapor phase organics are measured in a third channel to determine the positive artifact while a fourth channel measures denuder breakthrough. The final channel addresses issues of material transferring between front and back filters. Select results from the summer 2008 sampling campaign will be discussed, including the magnitudes and percentages of positive and negative artifacts measured.