HOx Research Group
The HOx group at Harvard University studies the chemistry of the hydroxyl (OH) and hydroperoxyl (HO2) radicals in the lower stratosphere and upper troposphere. The current research continues over two decades of in situ measurements of OH and HO2 by the Anderson group, drawing on experience from the pioneering rocket-borne measurements of OH in the upper stratosphere and the balloon-borne laser experiment, the progenitor of the present day aircraft-borne instrumentation.
OH and HO2, collectively known as HOx, are produced in the stratosphere primarily from the reaction of O(1D) with H2O and CH4, and from the photolysis of HNO3. In the troposphere the sources of HOx are not well understood. In both the lower stratosphere and upper troposphere the concentrations of HOx are small, OH ~1 part per trillion (ppt) and HO2 ~5 ppt, but its high reactivity makes HOx a dominant player in atmospheric photochemistry.
HOx either directly or indirectly participates in the key radical controlled catalytic ozone loss processes in the stratosphere. HOx directly removes ozone from the stratosphere through the following catalytic cycle:
||OH + O3 ®
|| HO2 + O2|
||HO2 + O3 ®
|| OH + O2 + O2|
||2 O3 ®
|| 3 O2|
and indirectly through reactions with ClO and BrO, where Z=Cl or Br:
||HO2 + ZO ®
|| HOZ + O2|
||HOZ + hv ®
|| OH + Z|
||Z + O3 ®
|| ZO + O|
||OH + O3 ®
|| HO2 + O2|
||2 O3 ®
|| 3 O2|
The measurement of HOx, along with the halogen radicals ClO and BrO, and the nitrogen radicals NO and NO2 is crucial to our understanding of these and other catalytic pathways that control ozone loss. With this in mind, the Harvard HOx instrument was designed (Wennberg, et al. 1994) to measure OH and HO2 concentrations in the lower stratosphere aboard the NASA ER-2.
In order to fully take advantage of the measurement capabilities of the NASA ER-2, the Harvard HOx instrument has several important capabilities, including:
- Fast time response - better than 125 ms
- Sensitivity - better than 0.01 ppt
- Accuracy - ±25% for OH and ±30% for HO2
- Precision - ±0.010 ppt in 10 s
- Free from measurement artifacts
- Lightweight - < 300 lbs
- Fully autonomous operation
The instrument resides in the nose of the ER-2. Air flow is introduced to the detection region of the instrument in a boundary layer-free manner using a two-ducted system. OH radicals are measured by laser induced fluorescence (LIF) at the center of the sampling duct, avoiding the edges where ambient OH is lost to the walls. HO2 is measured as OH after chemical conversion to OH with nitric oxide:
HO2 + NO ® OH + NO2.
LIF is used for detection because of its sensitivity and selectivity. Tunable ultraviolet light is produced by a narrow bandwidth tunable dye laser pumped by a high repetition rate, solid state Nd:YAG laser. The UV light is used to excite ground state OH molecules to the first electronically excited state via the A2S+ (n'=1)¬ C2P3/2 (n"=0) transition at 282 nm. Some of the vibrationally excited molecules relax to n'=0 via vibrational energy transfer and subsequently fluoresce near 309 nm.
Approximately 1% of the UV light from the dye laser is sent through optical fiber to an OH reference cell. This cell is used as the fundamental frequency reference for feedback to the laser-wavelength control system. During operation, the frequency of the dye laser is scanned, and the computer determines the location of the OH line-center using the frequency reference cell. The dye laser frequency is dithered on and off-resonance throughout the experiment. The signal due to OH flourescence is determined by subtracting the laser-scatter background (obtained with the laser off-resonance) from the on-resonance signal.
To convert the retrieved flourescence signal to absolute OH concentration requires a combination of laboratory and in-flight calibration. During flight, in addition to OH flourescence, the N2-Raman scatter is also collected. In the laboratory, the absolute OH sensitivity is measured relative to the sensitivity for N2 Raman scattering by producing a known concentration of hydroxyl radicals in a low pressure flow tube. In-flight measurement of pressure and temperature allows accurate measurement of the sensitivity as a function of time. The calibration in the laboratory is performed under stratospheric temperatures and pressures. A significant amount of effort is spent in the laboratory determining the excitation rate, fluorescence efficiency and collection efficiency. For a more detailed discussion of the laboratory calibration please see Review of Scientific Instruments Vol. 65 (6),1858-1876, 1994.
The Harvard HOx instrument has participated in numerous NASA supported field missions since its completion in 1992. These missions include SPADE, ASHOE/MAESA, STRAT, and POLARIS.
The first mission, SPADE, deployed out of NASA AMES in Mountain View, California. The flights of the ER-2 during April/May of 1993 set a milestone for stratospheric photochemistry. For the first time, simultaneous in situ measurements of all the key radical species in the lower stratosphere were obtained. Measurements of OH, HO2, NO, NO2, ClO, and BrO were used to determine ozone loss rates in the lower stratosphere in the mid-latitude region near California. Also, for the first time, measurements of OH and HO2 were used to show empirically that HOx dominates ozone loss rates in the mid-latitude lower stratosphere. [Wennberg et al., 1994, Cohen et al., 1994].
The second mission, ASHOE/MAESA, covered two hemispheres in 9 months. The mission began in Febrary, 1994, in Mountain View, California and ended in November after travels to Barber's Point, Hawaii; Nandi, Fiji; and Christchurch, New Zealand. The bulk of the measurements were obtained from flights out of Christchurch during four month-long deployments in March/April, May/June, July/August and October. These flights provided the first measurements of HOx in the Antarctic region during winter. Combined with the transit flights between California and New Zealand, the ER-2 flights during ASHOE/MAESA set another milestone for HOx as measurements were obtained from nearly pole-to-pole, over latitudes from -70°S to 70°N. Another major accomplishment was the first and only in situ measurement of the exhaust of an Air France Concorde. This encounter provides information on the pollutants that we can expect from super-sonic aircraft flying in the stratosphere. [Hanisco et al., 1997].
During the third mission, STRAT, the HOx instrument participated in three deployments to Mountain View, California and Barber's Point, Hawaii between October 1995 and August 1996. These deployments featured a new flight strategy of the ER-2 allowing for longer flying time in the troposphere. These flights provided the first extensive measurements of HOx in the upper troposphere. For the first time simultaneous measurements of HOx, NO, O3, CO, H2O and CH4 were used to determine ozone production rates in the upper troposphere. These measurements also showed that the concentration of HOx is much higher than expected, and spurred an ongoing investigation into the possible sources of HOx in the upper troposphere. [Wennberg et al., 1998].
The fourth mission, POLARIS, consisted of three month-long deployments in Fairbanks, Alaska, and a transit through Barber's Point, Hawaii. The deployments in Fairbanks during April/May, July/August, and September 1997 extended the range of HOx measurements to the North pole, 90° N latitude. The measurements of HOx, NOx, and ClO above the Arctic circle in the summertime provide a unique opportunity to examine photochemistry under the condition of constant solar illumination.
- P. O. Wennberg et al., Removal of Stratospheric O3 by Radicals: In Situ measurements of OH, HO2, NO, NO2, ClO, and BrO, Science, 266, 398-404, 1994.
Summary: Simultaneous in situ measurements of the concentrations of OH, HO2, ClO, BrO, NO, and NO2 demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO2 with ozone, accounted for nearly one-half of the total O3 removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O3; reactions involving BrO account for one-half of this loss. Catalytic destruction by NO2, which for two decades was considered to be the predominant loss process, accounted for less than 20% of the O3 removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO2, and ClO are inversely correlated with those of NO and NO2. The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O3 removal was inversely correlated with total NOx loading.
- R. C. Cohen et al., Are Models of Catalytic Removal of O3 by HOx Accurate? Constraints From In Situ Measurements of the OH to HO2 Ratio, Geophysical Research Letters, 21, 2539-42,1994.
Summary: Measurements of the ratio OH/HO2, NO, O3, ClO, and BrO were obtained at altitudes from 15-20 km and latitudes from 15-60 degrees N. A method is presented for interpreting these simultaneous in situ observations that constrains the rates of chemical transformations that (1) are responsible for over half the ozone removal rate in the lower stratosphere via reactions of HO2 and (2) control the abundance of HO2 through coupling to nitrogen and halogen radicals. The results show our understanding of the chemical reactions controlling the partitioning of OH and HO2 is complete and accurate and that the potential effects of 'missing chemistry' are strictly constrained in the region of the atmosphere encompassed by the observations. The analysis demonstrates that the sensitivity of the ratio OH/HO2 to changes in NO is described to within 12% by current models. This reduces by more than a factor of 2 the effect of uncertainty in the coupling of hydrogen and nitrogen radicals on the analysis of the potential effects of perturbations to odd nitrogen in the lower stratosphere.
- M.K. Dubey, T.F. Hanisco , P. O. Wennberg, and J. G. Anderson, Monitoring Potential Photochemical Interference in Laser-Induced Fluorescence Measurements of Atmospheric OH, Geophysical Research Letters, 23, 3215-18, 1996.
Summary: In situ laser-induced fluorescence measurements of atmospheric OH are susceptible to interference from laser generated OH, particularly in the troposphere. To quantify this interference we implement the addition of perfluoropropene, C3F6, for the chemical removal of OH from the ambient air. The removal rate of OH by C3F6 is determined in the laboratory using the discharge flow technique. Over the temperature range 249 to 296 K the rate constant is (6.0±0.8)x10-13exp[(370±40)/T] cm3 molecule-1, independent of pressure. In situ measurements using C3F6 addition are performed in both aircraft-borne and ground-based experiments. These studies show that laser excitation of the A2S+ (n' =1) ¬ C2P3/2 (n" =0) transition (282 nm) at high pulse repetition rates and low peak power can provide reliable and sensitive measurements of tropospheric OH.
- T. F. Hanisco et al., The Role of HOx in Super- and Subsonic Aircraft Exhaust Plumes, Geophysical Research Letters, 24, 65-8, 1997.
Summary: The generation of sulfuric acid aerosols in aircraft exhaust has emerged as a critical issue in determining the impact of supersonic aircraft on stratospheric ozone. It has long been held that the first step in the mechanism of aerosol formation is the oxidation of SO2 emitted from the engine by OH in the exhaust plume. The authors report in situ measurements of OH and HO2 in the exhaust plumes of a supersonic (Air France Concorde) and a subsonic (NASA ER-2) aircraft in the lower stratosphere. These measurements imply that reactions with OH are responsible for oxidizing only a small fraction of SO2 (2%), and thus cannot explain the large number of particles observed in the exhaust wake of the Concorde.
- E. J. Lanzendorf, T. F. Hanisco, N. M. Donahue, and P. O. Wennberg, Comment on: "The measurement of tropospheric OH radicals by laser-induced fluorescence spectroscopy during the POPCORN field campaign" by Hofzumahaus et al. and "Intercomparison of tropospheric OH radical measurements by multiple folded long-path laser absorption and laser induced fluorescence" by Brauers et al., Geophysical Research Letters, 24, 3037-3038, 1997.
Summary: Recently, a novel and seemingly simple technique to calibrate a laser induced fluorescence (LIF) instrument that measures OH was described. Trace quantities of water vapor were dissociated in air with a low pressure mercury (Hg) lamp to produce low concentrations (105-109 cm-3) of OH. The lamp fluence was calibrated by ozone actinometry and the OH concentration inferred from the ambient water vapor and O2 mixing ratios, the measured O3 concentration, and the relevant cross sections. Accurate absorption cross sections for O2 and H2O at the 1849.5Å Hg emission wavelength are required to infer the OH density from the measured O3 and H2O concentration. This comment shows that commercially available Mercury pen-ray lamps are broadened by self-reversal. In addition, the O2 absorption does not show Beer's Law behavior because the cross section is a function of the O2 column. The effective O2 cross section for the lamp was determined from the O2 absorption spectrum weighted by the Hg emission spectrum. This cross section depends upon both the lamp conditions and the O2 concentration between the lamp and the photolysis region. Therefore, direct measurement of the effective O2 cross section under field conditions is essential.
- P. O. Wennberg et al., Hydrogen Radicals, Nitrogen Radicals, and the Production of O3 in the Upper Troposphere, Science, 279, 49-53, 1998.
Summary: The concentrations of the hydrogen radicals OH and HO2 in the middle and upper troposphere were measured simultaneously with those of NO, O3, CO, H2O, CH4, non-methane hydrocarbons, and with the ultraviolet and visible radiation field. The data allow a direct examination of the processes that produce O3 in this region of the atmosphere. Comparison of the measured concentrations of OH and HO2 with calculations based on their production from water vapor, ozone, and methane demonstrate that these sources are insufficient to explain the observed radical concentrations in the upper troposphere. The photolysis of carbonyl and peroxide compounds transported to this region from the lower troposphere may provide the source of HOx required to sustain the measured abundances of these radical species. The mechanism by which NO affects the production of O3 is also illustrated by the measurements. In the upper tropospheric air masses sampled, the production rate for ozone (determined from the measured concentrations of HO2 and NO) is calculated to be about 1 part per billion by volume each day. This production rate is faster than previously thought and implies that anthropogenic activities that add NO to the upper troposphere, such as biomass burning and aviation, will lead to production of more O3 than expected.
- P. O. Wennberg, R. C. Cohen, N. L. Hazen, L. B. Lapson, N. T. Allen, T. F. Hanisco, J. F. Oliver, N. W. Lanham, J. N. Demusz, and J. G. Anderson, Aircraft-Borne, Laser-Induced Fluorescence Instrument for the In Situ Detection of Hydroxyl and Hydroperoxyl Radicals, Review of Scientific Instruments, 65, 1858-1876, 1994.
- R. J. Salwitch et al.,The Distribution of Hydrogen, Nitrogen, and Chlorine Radicals in the Lower Stratosphere: Implications for Changes in O3 due to Emission of NOy from Supersonic Aircraft, Geophysical Research Letters, 21, 2547-50, 1994.
- R. J. Salawitch et al. , The Diurnal Variation of Hydrogen, Nitrogen, and Chlorine Radicals: Implications for the Heterogeneous Production of HNO2, Geophysical Research Letters, 21, 2551-4, 1994.
- H. A. Michelsen, R. J. Salawitch, P. O. Wennberg, and J. G. Anderson, Production of O(1D) from photolysis of O3, Geophysical Research Letters, 21, 2227-2230, 1994.
- D. W. Fahey et al., Emission Measurements of the Concorde Supersonic Aircraft in the Lower Stratosphere, Science, 270, 70-4, 1995.
- L. Jaegle et al., Evolution and Stoichiometry of Heterogeneous Processing in the Antarctic Stratosphere, Journal of Geophysical Research, 102, 13235-53, 1997.
- P. O. Wennberg, J. W. Brault, T. F. Hanisco, R. J. Salawitch, and G. H. Mount, The Atmospheric Column Abundance of IO: Implications for Stratospheric Ozone, Journal of Geophysical Research, 102, 8887-98, 1997.
- I. Folkins, P. O. Wennberg, T. F. Hanisco, J. G. Anderson, and R. J. Salawitch, OH, HO2, and NO in two Biomass Burning Plumes: Sources of HOx and Implications for Ozone Production, Geophysical Research Letters, 24, 3185-8, 1997.
- L. Jaegle et al., Observed OH and HO2 in the Upper Troposphere Suggest a Major Source from Convective Injection of Peroxides, Geophysical Research Letters, 24, 3181-4, 1997.
- S. A. McKeen, et al., The Photochemistry of Acetone in the Upper Troposphere: a Source of Odd-Hydrogen Radicals, Geophysical Research Letters, 24, 3177-80, 1997.
These people are currently working on the HOx system:
- Thomas F. Hanisco
- Jessica Smith