Grantee Research Project Results
Final Report: The First Robust RO2 Analyzer for Urban Air
EPA Grant Number: R826176Title: The First Robust RO2 Analyzer for Urban Air
Investigators: Hard, Thomas M. , George, Linda A. , O'Brien, Robert J.
Institution: Portland State University
EPA Project Officer: Hahn, Intaek
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 , Safer Chemicals
Objective:
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.
We developed a new RO2 analyzer, based on Fluorescence Assay with Gas Expansion (FAGE), which already measures the related radicals OH and HO2. The new RO2 analyzer is suitable for use at urban NO2 concentrations, and free of interference by peroxynitrates.
Summary/Accomplishments (Outputs/Outcomes):
Our work in this research project shows that FAGE, with suitable modifications, can measure RO2 in urban air. The FAGE instrument's response to RO2 is calibrated relative to HO2 and OH, and absolute calibrations have been achieved for the latter two species. All three species can be measured simultaneously, or in close sequence, which is highly desirable for measurements of interrelated chemically reactive substances. We found that the most efficient RO2 detection path is conversion of RO2 to HO2 at atmospheric pressure, followed by conversion of HO2 to OH at low pressure. Urban air poses strict requirements for prevention of RO2 interferences, due to high ambient NO2 and the resulting high ratio of peroxyacetylnitrate (PAN) to acetylperoxy radicals, and these strict requirements are met by the present instrument.
The following are our major accomplishments for this project:
We developed an effective, efficient, and reproducible converter of HO2 and RO2 to OH, with negligible interference from thermal dissociation of peroxynitrates, by experimentation with atmospheric pressure flowtubes and injectors.
We combined the latter converter with an existing FAGE instrument, enabling detection of the important atmospheric radicals RO2, HO2, and OH in a single analyzer.
We generated HO2 in a separate atmospheric pressure flowtube by photolysis of Cl2 at 366 nm in the presence of H2. We chose this method because it allows generation of RO2 radicals by substitution of appropriate hydrocarbons or aldehydes for H2, and because it does not generate OH. If present, OH also would react with H2, hydrocarbons, and aldehydes; the absence of OH greatly simplifies the interpretation of our measurements. With the 366 nm wavelength, direct radical production by photolysis of aldehydes is negligible. Hence, this flowtube serves as a reproducible generator of HO2 and of specific RO2 radicals, limited by the Cl2 photolysis rate.
Using the latter radical generator, with a total flow in excess of that required by the analyzer, we characterized the analyzer's responses to HO2 and three important RO2 species as functions of the concentration of the added reagent, NO. The three species are ethylperoxyl radical (EtO2), methylperoxyl radical (MeO2), and acetylperoxyl radical (MeCO3).
We characterized the RO2 analyzer with the FAGE low-pressure channel operating in two modes. In the FAGE OH mode, NO is injected only in the atmospheric pressure converter. In the FAGE HOx mode, NO also is injected in the low-pressure channel, at a rate near the optimum for conversion of HO2 to OH. In each mode, we have measured the OH signal versus NO injection rate in the atmospheric-pressure converter. This enabled us to determine the NO injection rate that gives nearly equal responses for HO2 and the above three RO2 species. As expected, the required NO injection rate in the 1-atm converter is lower in the HOx mode than in the OH mode, and this leads to significant advantages described below.
We found that the most efficient detection of RO2 occurs with its conversion to HO2 at atmospheric pressure, with subsequent conversion of most of the HO2 to OH at low pressure. In this configuration, the FAGE low-pressure channel is operated in the HOx mode, with reagents injected immediately after the expansion nozzle: constant NO injection at a rate near the optimum for HO2 conversion to OH, plus chemical modulation of the resulting OH signal by periodic injection of isobutane. At twice the period of the latter modulation, NO also is injected in the atmospheric pressure converter, at a rate that gives nearly equal signals from HO2, and each of the three RO2 species studied. Compared with the alternative method, the OH mode, in which all HO2 and RO2 conversion is performed at atmospheric pressure, gives greater responses for both RO2 and HO2. This occurs because fewer radicals are lost by conversion to HONO, due to lower NO concentration in the atmospheric pressure section. Thus, the HOx mode gives higher signal-to-noise ratios at ambient radical concentrations. Moreover, it allows a simpler chemical modulation scheme, in which the 1-atm NO alternates between one value and zero. In contrast, the OH mode requires switching among two NO settings and zero. The preferred HOx mode yields total ambient RO2 via alternating measurements of HOx (OH + HO2) and HOx + total RO2, considering the signal efficiencies we measured previously.
We calibrated the analyzer's responses to OH, HO2, EtO2, MeO2, and MeCO3, using methods suitable for field measurements.
We measured OH, HO2, and total RO2 at an urban site in downtown Portland, OR, using FAGE. Our results indicated a lower ratio of RO2 to HO2 than we would have predicted from a standard photochemical model using the available concurrent measurements of other important species. This result may indicate major HO2 production by photolysis of formaldehyde, which was not measured.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 6 publications | 2 publications in selected types | All 2 journal articles |
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George LA, Hard TM, O'Brien RJ. Measurement of free radicals OH and HO2 in Los Angeles smog. Journal of Geophysical Research 1999;104(D9):11643-11655. |
R826176 (1999) R826176 (2000) R826176 (Final) R823319 (Final) |
Exit Exit |
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Hard TM, George LA, O'Brien RJ. An absolute calibration for gas-phase hydroxyl measurements. Environmental Science & Technology 2002;36(8):1783-1790. |
R826176 (Final) R823319 (Final) |
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Supplemental Keywords:
RFA, Scientific Discipline, Air, Toxics, Ecology, particulate matter, air toxics, Environmental Chemistry, Chemistry, VOCs, tropospheric ozone, Engineering, Chemistry, & Physics, monitoring, hydroxyl radical, particulates, urban air, organic peroxy radicals, fine particles, ozone, peroxy radical analyzers, fine particulates, particles, ambient nitrogen dioxide, flourescence assay, urban air , atmospheric OH, peroxynitratesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.