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Final Report: Atmospheric Free Radical Measurements Related to Photochemical Oxidants in Urban Air

EPA Grant Number: R823319
Title: Atmospheric Free Radical Measurements Related to Photochemical Oxidants in Urban Air
Investigators: Hard, Thomas M. , George, L. A. , O'Brien, Robert J.
Institution: Portland State University
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1995 through September 30, 1997 (Extended to September 30, 1999)
Project Amount: $276,323
RFA: Exploratory Research - Chemistry and Physics of Air (1995)
Research Category: Engineering and Environmental Chemistry

Description:

Objective:

Polluted air chemistry leads to oxidant and acid production in the presence of sunlight, hydrocarbons, and oxides of sulfur and nitrogen. The indispensable agent of this transformation is HO, the hydroxyl free radical, which is generated by sunlight wherever kaces of ozone, nitrous acid, or aldehydes are present. HO attacks hydrocarbons (from natural or human-made sources), and NOx transforms the intermediate products into larger concentrations of ozone and other oxidants, which are the undesirable irritants of urban smog. The hydroperoxyl radical, HO2, is closely linked with HO by photochemical reactions, and is an essential intermediate in ozone production, through its reaction with nitric oxide. Together, HO and HO2 are commonly called HOx. Hydroxyl combines directly with nitrogen dioxide and sulfur dioxide to produce their corresponding acids which are the chief agents in acid precipitation.

Although extensive regions of the United States are in exceedance of the Federal Air Quality Standard for ozone, none approaches the Los Angeles area in the severity and duration of population exposure to this harmful pollutant. In 1993, with support from a prior USEPA grant, we carried out the first direct measurements of HO and HO2 in the Los Angeles atmosphere. Both HO and HO2 were measured by FAGE (fluorescence assay with gas expansion), a technique which we originated, and whose advantages have led to its use by several researchers in the US and abroad. Our 1993 Los Angeles HOx measurements were performed in concert with other research groups to provide the largest suite of supporting measurements yet obtained for the study of urban air photochemistry and the verification of oxidant control chemical models.

Under the titled grant, we have continued these ambient radical concentration measurements, as well as experiments designed to improve HOx calibration methods, accuracy, and sensitivity. Our laboratory studies have shown that ozone-alkene reactions in flow tubes can prepare known HO steady-state concentrations, suitable for calibration of atmospheric HO analyzers. The results of these flow tube calibrations agree with those we obtain by hydrocarbon consumption in continuously-stirred tank reactors (CSTR). Compared with the CSTR method, the flow tube HO calibration requires less equipment and less time, and is thus more cost -effective. Moreover, the flow tube method can be implemented more frequently during field measurements, thus lowering uncertainties in measured radical concentrations.

The ozone-alkene flow tube method provides an absolute primary calibration for the HO response of the FAGE instrument. For the HOx mode, the flow tube method provides only a secondary calibration. We achieve primary calibration of the FAGE instrument's HO: response by the CSTR method, using NO oxidation by HO2, generated by formaldehyde photolysis. We have analyzed systematic errors due to other NO sources and sinks, and have found conditions under which these errors are negligible.

We have developed a new method of determining wall loss rates in the FAGE probes, using a pulsed HO (or HOx) source at the entrance nozzle. As the air flow conveys the radicals downstream, they diffuse radially and axially, and some of them are removed by chemical reaction wi~ the inner surface of the tube. At the fluorescence detection zone, the detector sees a distribution of fluorescence intensity vs. radical arrival time, or waveform, with a duration of many pulses of the excitation laser. Radicals which arrive later have been exposed to the tube wall for a longer period, so the intensity of the trailing part of the waveform is most sensitive to wall reactivity. We have confirmed this by comparing observed waveforms in tubes with and without halocarbon wax coatings. Moreover, the arrival time of the leading edge of the waveform demonstrates that the prevalent flow regime in these FAGE probes is laminar.

In the fall of 1996, we measured HO and HO2 at a rural site downwind from Portland. That field campaign resulted in improvements in detection techniques, which we applied during the 1997 HOx campaign in Riverside CA.

The principal goal of this project was improved HOx measurements in the Los Angeles atmosphere, this time in cooperation with the 1997 Southern California Oxidant Study (SCOS97), which included a much larger set of other atmospheric chemical and meteorological measurements. SCOS97 was organized by the California Air Resources Board (ARB) and the South Coast Air Quality Management District (SCAQMD), and took place in Riverside.

We called our pan of SCOS97 the Radical Balance Experiment (RBE), emphasizing our stated aim: to obtain supporting data on all significant HOx sources and sinks, within the limits of available equipment. The RBE site was at the UCR CEICERT facility in Riverside, California, about 90 miles downwind from Los Angeles. We measured HO and HO2 in Riverside air, in concert with other species, for use in testing models of urban and regional air photochemistry.

Air pollution chemical research over several decades has revealed a vast amount of information about the reactivity of atmospheric HOx radicals. Nevertheless, since HO and HO2 are important atmospheric catalysts, it is important that their roles be quantified in the open atmosphere in real-world smog. The Radical Balance Experiment seeks to do just that. Since both HO and HO2 are so reactive, their atmospheric chemical lifetimes are quite short. Thus their concentrations are not transport-dependent. Therefore, if we can measure the concentrations of all 'stable' atmospheric species involved in HOx catalysis, we can check chemical mechanisms of air pollution models for accuracy and applicability to the South Coast Air Basin. To succeed, we must measure the concentrations of all important chemical species in the HO and HO2 radical balance equations on as short a time scale as possible.

Due to the short photochemical lifetime of HOx (less than 10 seconds), this test of radical balance in urban air depends directly and quantitatively on the measured parents and reaction partners of the radicals, and only indirectly on the upstream history of the sample.

At the CE/CERT site, from mid-August to late September 1997, we measured HO, HO2, 03, NO, NO2, H2O, CO, speciated hydrocarbons, speciated carbonyls, aerosol number and size distribution, and wind speed and direction. We obtained several weeks of useful data, with nearly continuous 24-hour data for the period 13-24 September. We are grateful to Dr. Annmarie Eldering of UCLA for use of an aerosol spectrometer, to UCR CE/CERT for solar spectral irradiance data, to CE/CERT staff for practical assistance, and to ARB and SCAQMD for organizational assistance.

Summary/Accomplishments (Outputs/Outcomes):

A steady-state hydroxyl concentration, obtained with ozone-alkene reactions in an atmospheric-pressure flowtube, provides a rapid and reliable absolute external calibration for FAGE response.

A CSTR method, using NO oxidation by HO2, provides an absolute external calibration for FAClE response to HO2.

Wall losses and flow regimes in FAGE probes have been characterized with a pulsed hydroxyl source.

Analysis of radical-balance data, obtained at Riverside, California in 1997, with model comparisons, is still in progress. When completed, the results of the measurements and analysis will be reported in scientific journals.


Journal Articles on this Report: 1 Displayed | Download in RIS Format

Other project views: All 3 publications 1 publications in selected types All 1 journal articles

Type Citation Project Document Sources
Journal Article George LA, Hard TM, O?Brien RJ. Measurement of free radicals OH and HO2 in Los Angeles smog. Journal of Geophysical Research 1999;104:11643-11655. R823319 (Final)
R826176 (1999)
R826176 (2000)
R826176 (Final)
 not available
Supplemental Keywords:

Toxics, Air, Geographic Area, Scientific Discipline, RFA, Engineering, Chemistry, & Physics, indoor air, HAPS, Physics, Chemistry, EPA Region, State, tropospheric ozone, California (CA), photochemical processes, indoor air quality, sulfur, chemical composition, ozone, hydrocarbon, nitrogen removal, atmospheric deposition, urban air , acid rain, Oregon, meterology, hydroxyl radical, OR, atmospheric free radicals, acid deposition, ambient aerosol, smog
Relevant Websites:

http://horizons.sb2.pdx.edu/~fage/ Exit EPA icon
http://www.agmdgov/scos97/member/readv.html Exit EPA icon

Progress and Final Reports:
Original Abstract

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The 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.

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