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INVESTIGATION OF A SYSTEMATIC OFFSET IN THE MEASUREMENT OF ORGANIC CARBON WITH A SEMI-CONTINUOUS ANALYZER
Citation:
OFFENBERG, J. H., M. LEWANDOWSKI, MOHAMMAD JAOUI, E. O. EDNEY, AND T. E. KLEINDIENST. INVESTIGATION OF A SYSTEMATIC OFFSET IN THE MEASUREMENT OF ORGANIC CARBON WITH A SEMI-CONTINUOUS ANALYZER. JOURNAL OF AIR AND WASTE MANAGEMENT. Air & Waste Management Association, Pittsburgh, PA, 57(5):596-599, (2007).
Impact/Purpose:
PM Chemistry models for predicting chemical compositions of PM2.5 are needed to assess PM2.5 control strategies. Because of the chemical complexity of ambient PM2.5, models must be developed to predict ambient concentrations of water, inorganics, and organics in the aerosols. Under this research task, the current treatments of secondary organic aerosol formation and the chemistry of N2O5 in CMAQ will be improved by HEASD and integrated into CMAQ by AMD. CMAQ will be used by AMD to predict ambient concentrations and compositions of PM2.5 for OAQPS.
Description:
Organic carbon was measured semi-continuously in laboratory experiments of steady-state secondary organic aerosol formed by hydrocarbon + NOx irradiations. Examination of the mass of carbon measured on the filter for various sample volumes reveals a systematic offset that is not observed when performing an instrumental blank. These findings suggest that simple subtraction of instrumental blanks determined as the standard analysis without sample collection (i.e. by cycling the pump and valves yet filtering zero liters of air followed by routine chemical analysis) from measured concentrations may be inadequate. This may be especially true for samples collected through the filtration of small air volumes wherein the influence of the systematic offset is greatest. All experiments show that filtering a larger volume of air minimizes the influence of contributions from the systematic offset. Application of these results to measurements of ambient concentrations of carbonaceous aerosol suggests a need for collection of sufficient carbon mass to minimize the relative influence of the offset signal.