Robert J. O'Brien

Measurement of free radicals OH and HO2 in Los Angeles smog

Atmospheric free radicals hydroxyl and hydroperoxyl (OH and HO2, collectively HOx) 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 models HOx sources are too large, (2) there are unaccounted HOx 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.

Diurnal HO2 cycles at clean air and urban sites in the troposphere

We have determined HO2 concentrations at two Oregon sites for continuous periods of 36 to 48 hours, using fluorescence assay with gas expansion. At the sea level coastal site (45°N 124°W), 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 1–2 × 108 cm−3 under clear-sky conditions, with an estimated overall uncertainty of 40%. HO2 was detected by continuous low-pressure sampling with flowing chemical conversion to HO, which was detected by laser-excited fluorescence. The instrumental response to HO2 was calibrated by the self-decay of HO2 at atmospheric pressure. Interference in the measured daytime HO2 concentrations by RO2 was estimated at less than 20%.

FAGE determination of tropospheric HO and HO

Abstract 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 excita...

Intercomparison of Local Hydroxyl Measurements by Radiocarbon and FAGE Techniques

Abstract 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.

Interference suppression in HO fluorescence detection.

Let us know how access to this document benefits you.

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 daytime ambient HO concentrations are in the range (2.5 to 8)×106 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 ppb and 10 torr H2O, with excitation and detection at a total pressure of 4 torr, the net interference is equal to [HO] = −1.3 × 106 molecule cm−3. Production of HO by the reaction of isobutane with O(1D) accounts for the negative interference. Quenching of HO fluorescence by the modulating reagent contributes a smaller positive term to the interference; kinetic measurements of the quenching rate coefficient yield kQ0i(HO A (v′=0) + isobutane) = (1.0±0.15) ×10−9 cm3 molecule−1 s−1. The experimental interference results are compared with a detailed kinetic model of HO production, excitation, relaxation, and detection; reasonable agreement is found despite uncertainties in beam spatial and temporal profiles and in the rate coefficients necessary to the model. The model also computes the interference due to H2O2 photolysis. Net interference and signal-to-noise ratio are computed as functions of photon flux for 1,0 (YAG/dye 282 nm) and 0,0 (Cu/dye 308 nm) excitation.

Comment on "Laboratory evaluation of an airbone ozone instrument that compensates for altitude/sensitivity effects".

SIR: The article (1) by G. L. Gregory, C. H. Hudgins, and R. A. Edahl, Jr. (GHE), addresses a commonly encountered difficulty with chemiluminescence (CL) analyzers: their sensitivity to sample pressure. We read this article with interest because we have recently studied the general behavior of CL analyzers (2). We would like to point out some inconsistencies in the paper, as well as make some suggestions. Figure 1 of GHE depicts an “altitude correction factor” for the CL instrument as well as for the reference UV absorption instrument. This correction factor converts the instrument reading to relative concentration in mole fraction, parts per million, etc. Referring to Figure 1, GHE state “In all cases, the uncertainties in obtaining the true relationship between pressure and sensitivity ... result in additional error in the measured parameter as well as require additional data taking and reduction”. Figure l b applies to the UV instrument which, according to Beer’s law, responds to absolute concentration. Thus, the “true relationship” to which GHE refer is simply the ideal gas law, P V = nRT, which does not really require the verification shown in Figure lb , nor is its accuracy or applicability limited by the experimental measurements which Figure l b portrays. Since the appropriate correction factor is easily found from the ideal gas law and the ambient temperature and pressure [for Figure l b it is Pref/P where Pref is the pressure corresponding to unit relative response (about 705 torr)], and since both temperature and pressure must be known or measured in any atmospheric situation where the data are to be useful, we question GHE’s rationale in defining an altitude correction factor in the first place. More interesting in this light, however, is Figure l a for the CL instrument, whose points differ from those of Figure l b by less than the error bars. Thus, the CL analyzer with the manufacturer’s operating parameters also responds linearly with absolute concentration, and its output can be converted to mole fraction (if desired) by using the ideal gas law. However, since chemical reaction rates and many biological processes are proportional to absolute concentration rather than mole fraction, even if mole fraction is measured by the instrument, use of the data for many quantitative applications will require conversion from mole fraction back to absolute concentration anyway. Indeed, the National Air Quality Standards are written in terms of absolute concentration. In modifying the CL instrument for constant molefraction response with altitude, GHE base their approach on eq 1 which states that the “output response of an airquality instrument is governed by an equation of the type output cx cmx”. This assumption, which is clearly of no general applicability, has led to several errors in the paper. On the basis of the paper’s context, when GHE say “x = species concentration”, they refer to relative concentration (mole fraction, or mixing ratio), not to concentration per se in absolute units such as molecules, moles, or weight per unit volume. The UV reference instrument is itself an example of an air-quality instrument to which this equation does not apply, since UV absorption measures absolute rather than relative concentration, and its steady-state response is independent of mass flow into the instrument. We discourage the use of this equation either as a definition or as a statement of fact, particularly with regard to CL analyzers. Since our own work (2) has not been published, we will base our remarks largely upon equations in the paper ”Optimization of the Operating Parameters of Chemiluminescent Nitric Oxide Detectors” (3). Equation 9 of this paper gives the correct response equation of a CL analyzer operating in a plug-flow mode. This equation gives the total light emission rate (photons/s) as