The First Robust RO2 Analyzer for Urban AirEPA Grant Number: R826176
Title: The First Robust RO2 Analyzer for Urban Air
Investigators: Hard, Thomas M.
Current Investigators: Hard, Thomas M. , George, Linda A. , O'Brien, Robert J.
Institution: Portland State University
EPA Project Officer: Shapiro, Paul
Project Period: January 1, 1998 through December 31, 2000
Project Amount: $355,290
RFA: Exploratory Research - Environmental Chemistry (1997) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Engineering and Environmental Chemistry
Organic peroxy radicals (RO2) are important sources of both ozone and fine particulates in urban and regional air. Few direct measurements of urban total RO2 concentrations are available, because existing RO2 instruments are subject to major positive and negative interferences when used at the NO2 concentrations that prevail in urban air. These interferences arise from rapid chemical equilibrium between RO2, NO2, and peroxynitrates (RNO4). A total RO2 analyzer that is insensitive to ambient NO2 is urgently needed.
This project will develop the first total RO2 analyzer that is free of NO2-related interferences at urban pollution levels. The analyzer employs the conversion of RO2 to hydroxyl radicals (OH), which are detected by FAGE (fluorescence assay with gas expansion). The conversion takes place at atmospheric temperature and pressure with a very short residence time, thus preventing formation or dissociation of peroxynitrates. Moreover, the high sensitivity of FAGE obviates the need for either pre-concentration or chemical amplification, which are used by the two existing RO2 methods. Unlike those methods, FAGE can distinguish OH and HO2 (hydro-peroxyl radical) from RO2.
The proposed analyzer requires modification of one of the four sampling channels of the FAGE instrument, used in the determination of atmospheric OH and HO2 for more than a decade. First, the air-sampling configuration will be optimized for response to a single RO2 species, generated by adaptation of an existing method. Second, the response of the analyzer to each of the significant classes of atmospheric RO2 radicals will be calibrated, using individual species. Third, the analyzer will be tested for RNO4 interference. It is important to characterize the analyzer thoroughly before taking it into the field. Fourth, the analyzer will be used to determine total ambient RO2 in urban air at both downtown and downwind sites near Portland OR, with simultaneous measurements of OH, HO2, ozone, NO, NO2, total non-methane hydrocarbons, and ultraviolet radiation, yielding data suitable for atmospheric chemical modeling. Finally, an inter-comparison with other total peroxy radical analyzers will be undertaken.
The proposed work will yield a well-characterized total RO2 analyzer, whose response to individual RO2 species is known from direct, calibrated measurements. The RO2 analyzer will be suitable for use at any atmospheric NO2 level without interference from per-oxynitrates, and thus will be reliable in urban air. This analyzer will be available for inter-comparison experiments with other RO2 analyzers (which have shown poor reproducibility in past inter-comparisons). The proposed work includes simultaneous, continuous ambient RO2, HO2, and OH determinations in urban air, with the supporting measurements needed for atmospheric chemical modeling. All three of these free radicals are important intermediates in the production of ozone and fine particulates in polluted air.