Comparison of Nonmethane Organic Compound Concentration Data Collected by Two Methods in Atlanta

Abstract

Title I of the Clean Air Act Amendments of 1990 calls for “enhanced monitoring” of ozone, which is planned to include measurements of atmospheric non-methane organic compounds (NMOCs). NMOC concentration data gathered by two methods in Atlanta, Georgia during July and August 1990 are compared in order to assess the reliability of such measurements in an operational setting. During that period, automated gas chromatography (GO) systems (Field systems) were used to collect NMOC continuously as one-hour averages. In addition, canister samples of ambient air were collected on an intermittent schedule for quality control purposes and analyzed by laboratory GC (the Lab system). Data from the six-site network included concentrations of nitrogen oxides (NOX), carbon monoxide (CO), ozone, total NMOC (TNMOC), and 47 identified NMOCs. Regression analysis indicates that the average TNMOC concentration from the Lab system is about 50 percent higher than that from the Field system, and that the bulk of the difference is due to unidentified NMOCs recorded by the Lab system. Also, there are substantial uncertainties in predicting a single Field TNMOC concentration from a measured Lab concentration. Considering individual identified NMOCs, agreement between the systems is poor for many olefins that occur at low concentrations but may be photochemically important. Regressions of TNMOC against CO and NOX lead to the conclusion that the larger unidentified component being reported by the Lab system is not closely related to local combustion or automotive sources.