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• 1. [Article] Interference suppression in HO fluorescence detection

  In this Letter we report preliminary results on a sampling method that greatly reduces the above interferences relative to hydroxyl fluorescence.

  Citation × Citation Citation Abstract Copy Title: Interference suppression in HO fluorescence detection Author: Hard, Thomas M., O'Brien, Robert J., Cook, T. B., Tsongas, G. A. Year: 1979 In this Letter we report preliminary results on a sampling method that greatly reduces the above interferences relative to hydroxyl fluorescence. Title: Interference suppression in HO fluorescence detection Author: Hard, Thomas M., O'Brien, Robert J., Cook, T. B., Tsongas, G. A. Year: 1979 In this Letter we report preliminary results on a sampling method that greatly reduces the above interferences relative to hydroxyl fluorescence. Close

• 2. [Article] FAGE measurements of tropospheric HO with measurements and model of interferences

  Ambient HO measurements by low-pressure laser-excited fluorescence with chemical modulation, and supporting ozone and water-vapor data, are presented for periods in May and August 1987. The observed peak ...

  Citation × Citation Citation Abstract Copy Title: FAGE measurements of tropospheric HO with measurements and model of interferences Author: Hard, Thomas M., Mehrabzadeh, A. A., Chan, C. Y., O'Brien, Robert J. Year: 1992 Ambient HO measurements by low-pressure laser-excited fluorescence with chemical modulation, and supporting ozone and water-vapor data, are presented for periods in May and August 1987. The observed peak daytime ambient HO concentrations in Oregon are in the range (2.5 to 8) times106 molecules cm⁻³ and show small negative offsets due to photochemical interference. Direct measurements of the interference at fixed [O₃] give the dependence on ambient [H₂O] and on the modulating reagent [isobutane]. At ambient [O₃]=30 p.p.b. and 10 torr H₂O, with excitation and detection at a total pressure of 4 torr, the net interference is equal to [HO]=-1.3times10⁶ molecule cm⁻³. Title: FAGE measurements of tropospheric HO with measurements and model of interferences Author: Hard, Thomas M., Mehrabzadeh, A. A., Chan, C. Y., O'Brien, Robert J. Year: 1992 Ambient HO measurements by low-pressure laser-excited fluorescence with chemical modulation, and supporting ozone and water-vapor data, are presented for periods in May and August 1987. The observed peak daytime ambient HO concentrations in Oregon are in the range (2.5 to 8) times106 molecules cm⁻³ and show small negative offsets due to photochemical interference. Direct measurements of the interference at fixed [O₃] give the dependence on ambient [H₂O] and on the modulating reagent [isobutane]. At ambient [O₃]=30 p.p.b. and 10 torr H₂O, with excitation and detection at a total pressure of 4 torr, the net interference is equal to [HO]=-1.3times10⁶ molecule cm⁻³. Close

• 3. [Article] Third-generation FAGE instrument for tropospheric hydroxyl radical measurement

  The authors have constructed a single-stage, frequency-doubled, copper vapor laser-pumped dye laser to be used in the measurement of atmospheric hydroxyl radical concentrations. This laser can be tuned ...

  Citation × Citation Citation Abstract Copy Title: Third-generation FAGE instrument for tropospheric hydroxyl radical measurement Author: Chan, C. Y., Hard, Thomas M., Mehrabzadeh, A. A., O'Brien, Robert J. Year: 1990 The authors have constructed a single-stage, frequency-doubled, copper vapor laser-pumped dye laser to be used in the measurement of atmospheric hydroxyl radical concentrations. This laser can be tuned to either the 1← 0 or 0← 0 vibrational transition in the electronic transition ²Sigmalarr²Pi. A new photon counting instrument is used for HO fluorescence detection. Theoretical and experimental studies of instrument performance show better sensitivities and reduced photolytic interferences than have been possible with previous systems based upon Nd:YAG pumping. Title: Third-generation FAGE instrument for tropospheric hydroxyl radical measurement Author: Chan, C. Y., Hard, Thomas M., Mehrabzadeh, A. A., O'Brien, Robert J. Year: 1990 The authors have constructed a single-stage, frequency-doubled, copper vapor laser-pumped dye laser to be used in the measurement of atmospheric hydroxyl radical concentrations. This laser can be tuned to either the 1← 0 or 0← 0 vibrational transition in the electronic transition ²Sigmalarr²Pi. A new photon counting instrument is used for HO fluorescence detection. Theoretical and experimental studies of instrument performance show better sensitivities and reduced photolytic interferences than have been possible with previous systems based upon Nd:YAG pumping. Close

• 4. [Article] Diurnal HO₂ cycles at clean air and urban sites in the troposphere

  HO₂ concentrations have been determined at two Oregon sites for continuous periods of 36 to 48 hours, using fluorescence assay with gas expansion. At the sea level coastal site (45degN 124degW), NNW winds ...

  Citation × Citation Citation Abstract Copy Title: Diurnal HO₂ cycles at clean air and urban sites in the troposphere Author: Hard, Thomas M., Chan, C. Y., Mehrabzadeh, A. A., O'Brien, Robert J. Year: 1992 HO₂ concentrations have been determined at two Oregon sites for continuous periods of 36 to 48 hours, using fluorescence assay with gas expansion. At the sea level coastal site (45degN 124degW), NNW winds prevailed during daytime, and a point measurement of very low total nonmethane hydrocarbon concentration indicated the presence of remote tropospheric air of oceanic origin. At the urban site, HO₂ was determined during moderately low ozone pollution levels. At both sites, maximum daily [HO₂] was in the range of 1-2times10⁸ cm⁻³ under clear-sky conditions, with an estimated overall uncertainty of 40%. Title: Diurnal HO₂ cycles at clean air and urban sites in the troposphere Author: Hard, Thomas M., Chan, C. Y., Mehrabzadeh, A. A., O'Brien, Robert J. Year: 1992 HO₂ concentrations have been determined at two Oregon sites for continuous periods of 36 to 48 hours, using fluorescence assay with gas expansion. At the sea level coastal site (45degN 124degW), NNW winds prevailed during daytime, and a point measurement of very low total nonmethane hydrocarbon concentration indicated the presence of remote tropospheric air of oceanic origin. At the urban site, HO₂ was determined during moderately low ozone pollution levels. At both sites, maximum daily [HO₂] was in the range of 1-2times10⁸ cm⁻³ under clear-sky conditions, with an estimated overall uncertainty of 40%. Close

• 5. [Article] Intercomparison of Local Hydroxyl Measurements by Radiocarbon and FAGE Techniques

  A direct intercomparison of near-surface tropospheric HO concentration measurements by two different techniques was made in October–November 1992 at a rural site near Pullman, Washington. The atmosphere ...

  Citation × Citation Citation Abstract Copy Title: Intercomparison of Local Hydroxyl Measurements by Radiocarbon and FAGE Techniques Author: Campbell, Malcolm J., Hall, Brad D., Sheppard, John C., Utley, Philip L., O'Brien, Robert J., Hard, Thomas M., George, Linda Acha Year: 1995 A direct intercomparison of near-surface tropospheric HO concentration measurements by two different techniques was made in October–November 1992 at a rural site near Pullman, Washington. The atmosphere at the site is believed to contain low levels of anthropogenic pollution. The instruments' inlets were located at the same height (3.5 m) above the ground and were separated by 10 m along a line normal to the prevailing wind. Readings of the FAGE3 and radiocarbon instruments showed a high correlation (r² = 0.74) despite HO concentrations that were frequently near the detection limit of the instruments. An unweighted least squares regression shows a slope significantly different from unity, indicating different calibration scales of the two instruments Title: Intercomparison of Local Hydroxyl Measurements by Radiocarbon and FAGE Techniques Author: Campbell, Malcolm J., Hall, Brad D., Sheppard, John C., Utley, Philip L., O'Brien, Robert J., Hard, Thomas M., George, Linda Acha Year: 1995 A direct intercomparison of near-surface tropospheric HO concentration measurements by two different techniques was made in October–November 1992 at a rural site near Pullman, Washington. The atmosphere at the site is believed to contain low levels of anthropogenic pollution. The instruments' inlets were located at the same height (3.5 m) above the ground and were separated by 10 m along a line normal to the prevailing wind. Readings of the FAGE3 and radiocarbon instruments showed a high correlation (r² = 0.74) despite HO concentrations that were frequently near the detection limit of the instruments. An unweighted least squares regression shows a slope significantly different from unity, indicating different calibration scales of the two instruments Close

• 6. [Article] FAGE determination of tropospheric HO and HO₂

  FAGE (fluorescence assay with gas expansion) was developed as a sensitive technique for the detection of low-concentration free radicals in the atmosphere. The application of FAGE to tropospheric hydroxyl ...

  Citation × Citation Citation Abstract Copy Title: FAGE determination of tropospheric HO and HO₂ Author: George, Linda Acha, Hard, Thomas M., O'Brien, Robert J. Year: 1995 FAGE (fluorescence assay with gas expansion) was developed as a sensitive technique for the detection of low-concentration free radicals in the atmosphere. The application of FAGE to tropospheric hydroxyl (H0) and hydroperoxyl (H0₂) radicals has yielded calibrated measurements of both species in both clean air and highly polluted urban air. For HO calibration, a continuously stirred tank reactor provides a uniform external HO concentration, which can be measured by gas chromatography of an HO-reactive hydrocarbon. The aerodynamics of the air-sampling process has been modeled computationally, with results that agree with empirical observations of the effects of nozzle diameter on HO loss during sampling. The authors have also modeled airborne fluid dynamics of a FAGE probe. They have recently obtained FAGE sensitivity as high as ± 1 × 10⁶ cm-³ for a 6-minute averaging period, during field studies in highly polluted Los Angeles air, yielding a 7:1 signal-to-noise ratio near midday. Multipass excitation can further improve this sensitivity. The authors summarize their recent field studies of HO and HO², current work on improved calibration methods, other improvements, and future plans. Title: FAGE determination of tropospheric HO and HO₂ Author: George, Linda Acha, Hard, Thomas M., O'Brien, Robert J. Year: 1995 FAGE (fluorescence assay with gas expansion) was developed as a sensitive technique for the detection of low-concentration free radicals in the atmosphere. The application of FAGE to tropospheric hydroxyl (H0) and hydroperoxyl (H0₂) radicals has yielded calibrated measurements of both species in both clean air and highly polluted urban air. For HO calibration, a continuously stirred tank reactor provides a uniform external HO concentration, which can be measured by gas chromatography of an HO-reactive hydrocarbon. The aerodynamics of the air-sampling process has been modeled computationally, with results that agree with empirical observations of the effects of nozzle diameter on HO loss during sampling. The authors have also modeled airborne fluid dynamics of a FAGE probe. They have recently obtained FAGE sensitivity as high as ± 1 × 10⁶ cm-³ for a 6-minute averaging period, during field studies in highly polluted Los Angeles air, yielding a 7:1 signal-to-noise ratio near midday. Multipass excitation can further improve this sensitivity. The authors summarize their recent field studies of HO and HO², current work on improved calibration methods, other improvements, and future plans. Close

• 7. [Article] Measurement of free radicals OH and HO₂ in Los Angeles smog

  Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO₂, collectively HOᵪ ) are the catalysts that cause secondary or photochemical air pollution. Chemical mechanisms for oxidant and acid formation, ...

  Citation × Citation Citation Abstract Copy Title: Measurement of free radicals OH and HO₂ in Los Angeles smog Author: George, Linda Acha, Hard, Thomas M., O'Brien, Robert J. Year: 1999 Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO₂, collectively HOᵪ ) are the catalysts that cause secondary or photochemical air pollution. Chemical mechanisms for oxidant and acid formation, on which expensive air pollution control strategies are based, must accurately predict these radical concentrations. We have used the fluorescence assay with gas expansion (FAGE) technique to carry out the first simultaneous, in situ measurements of these two radicals in highly polluted air during the Los Angeles Free Radical Experiment. A complete suite of ancillary measurements was also made, including speciated hydrocarbons, carbon monoxide, aldehydes, nitric oxide, nitrogen dioxide, and ozone along with meteorological parameters. Using this suite of measurements, we tested the ability of a lumped chemical mechanism to accurately predict radical concentrations in polluted air. Comparison of model predictions with measured radical concentrations revealed generally good agreement for OH early and late in the day, including the early evening hours, when OH persisted at low concentrations after dark. During midday, however, modeled [OH] was high by about 50%. Agreement for HO₂ was quite good in the early morning hours, but model-calculated HO₂ concentrations were significantly too high during midday. When we used our measured HO₂ concentrations as model input, agreement between calculated and measured OH concentrations was improved. It seems likely that (1) the model's HOᵪ sources are too large, (2) there are unaccounted HOᵪ loss processes in Los Angeles air, and/or (3) the complex parameterization of RO₂/HO₂ radical chemistry in the reaction mechanism does not adequately describe the behavior of these radicals in the Los Angeles atmosphere. Title: Measurement of free radicals OH and HO₂ in Los Angeles smog Author: George, Linda Acha, Hard, Thomas M., O'Brien, Robert J. Year: 1999 Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO₂, collectively HOᵪ ) are the catalysts that cause secondary or photochemical air pollution. Chemical mechanisms for oxidant and acid formation, on which expensive air pollution control strategies are based, must accurately predict these radical concentrations. We have used the fluorescence assay with gas expansion (FAGE) technique to carry out the first simultaneous, in situ measurements of these two radicals in highly polluted air during the Los Angeles Free Radical Experiment. A complete suite of ancillary measurements was also made, including speciated hydrocarbons, carbon monoxide, aldehydes, nitric oxide, nitrogen dioxide, and ozone along with meteorological parameters. Using this suite of measurements, we tested the ability of a lumped chemical mechanism to accurately predict radical concentrations in polluted air. Comparison of model predictions with measured radical concentrations revealed generally good agreement for OH early and late in the day, including the early evening hours, when OH persisted at low concentrations after dark. During midday, however, modeled [OH] was high by about 50%. Agreement for HO₂ was quite good in the early morning hours, but model-calculated HO₂ concentrations were significantly too high during midday. When we used our measured HO₂ concentrations as model input, agreement between calculated and measured OH concentrations was improved. It seems likely that (1) the model's HOᵪ sources are too large, (2) there are unaccounted HOᵪ loss processes in Los Angeles air, and/or (3) the complex parameterization of RO₂/HO₂ radical chemistry in the reaction mechanism does not adequately describe the behavior of these radicals in the Los Angeles atmosphere. Close