IGACtivities No. 21
September 2000

Tropospheric OH: In the beginning

Contributed by
Hiram Levy, Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, New Jersey, USA.

Introduction

Prior to 1971, most of the troposphere, with the exception of a few uniquely polluted regions, was thought to have a relatively benign photochemistry with the only exceptions being some possible ozone reactions. The oxidizing of trace gases, such as CO and CH4, was thought to occur only in the stratosphere. For reviews from this period, see Bates and Witherspoon [1952], Junge [1963], and Cadle and Allen [1970]. Our theoretical understanding of the role of hydroxyl radicals (OH) in tropospheric chemistry evolved from earlier (1950s and 1960s) work in both the stratosphere and the polluted boundary layer.

Although an H2O–O3 chemical mechanism was developed in the 1950s for the mesosphere [e.g., Bates and Nicolet, 1950], it was not until the 1960s that Engleman [1965] identified the reaction that became essential to OH production in the troposphere:

O(1D) + H2O ® 2 OH
(1)

This reaction and the earlier H2O-O3 reactions from the mesosphere were used to construct a H2O-based catalytic destruction mechanism for stratospheric O3 [Hampson 1964; Hunt, 1966], but revised O(1D) quenching rates greatly reduced its stratospheric significance. The work of this period, which just precedes Crutzen's development of the NOX catalytic destruction scheme [1970], is nicely summarized by Nicolet [1970].

At the same time that stratospheric research was identifying the key source of OH in the troposphere, studies of smog chemistry were beginning to focus on O3 formation from hydrocarbon oxidation in polluted environments [for a detailed discussion of the early research, see P.A. Leighton, Photochemistry of Air Pollution, 1961]. In the 1960s, smog chemistry research began to consider OH. Westberg and Cohen [1969] and Heicklen, Westberg and Cohen [1971] then proposed a catalytic mechanism to explain the O3 formation observed in smog chambers, which would also provide an important component of the photochemical mechanism for the troposphere as a whole.

OH + CO + O2 ® HO2 + CO2
(2)

or

OH + RH ® RO2 + H2O
(3)

followed by

HO2 + NO ® OH + NO2
(4)

or

RO2 + NO ® RO + NO2
(5)

However, the rate coefficients for R4 and R5, which were the key, had not yet been measured. While estimates ranged from 1 x 10—11 (cm3 molecule—1 sec—1) to 2 x 10—15, they needed at least 5 x 10—13 to match the smog chamber data. At about the same time Weinstock [1969] argued, based on carbon isotope studies, that tropospheric CO had a short lifetime (~0.2 year) and that most of its destruction must occur in the troposphere. One of the suggested destruction paths was OH oxidation (R2), but no general tropospheric source of OH was known.

Motivated by Weinstock's 1969 paper, Levy [1971] constructed a O3-H2O-CO-CH4-NOX photochemical mechanism for the background or clean troposphere which combined: 1. The O(1D) production of OH (R1) from the stratosphere with tropospheric H2O levels; 2. The Heicklen, Westberg and Cohen catalytic OH - HO2 cycle, the known NOX oxidation chemistry, and the CH4 oxidation cycle; and 3. Existing "measurements" of background surface NOX levels of a few ppbv. This mechanism predicted for the summertime mid-latitude boundary layer, noon OH concentrations of ~ 3 x 106 molecules cm—3, a CO tropospheric lifetime of ~0.2 year, and daytime H2CO mixing ratios of 2 ppbv. McConnell et al. [1971] then extended the mechanism to include the oxidation of H2CO and predicted a large natural source for CO from the completed CH4 oxidation path. Chameides and Walker [1973] and Crutzen [1974] next employed the peroxy radical oxidation of NO to NO2, first devised to explain O3 formation in smog chambers, to produce a global tropospheric source of O3 that was fueled by CH4 and CO oxidation and driven by a NO-NO2 catalytic cycle. By 1974 the basic tropospheric photochemistry, which predicted OH concentrations of ~106 molecules cm—3, large chemical production of tropospheric O3, and OH control of CH4 and CO destruction as well as NOX conversion to HNO3, was qualitatively well developed (see a review by Levy [1974] for a summary up to that point). While those early calculations remain remarkably reasonable, the estimates of background NOX were much too high and the estimated rate coefficients for R4 and R5 were much too low. Fortunately these errors compensated.

For the rest of the 1970s there was a rapid development and quantification of the chemical mechanism and OH's reactive role in the troposphere. Kineticists began to measure many of the key rate coefficients that had previously only been estimated. Most important of these was the measurement by Howard and Evenson [1977], which increased the rate coefficient for R4 from an estimated 5 x 10—13 to a measured 8 x 10—12. At the same time, sophisticated measurement devices were being developed for the trace gases in the troposphere, the first of which was a chemiluminescence NO measurement that lowered the detection limit to the pptv range and measured background NOX levels of 0.1 ppbv or less (see McFarland et al. [1979], as an example of this development). Attempts to make direct measurements of tropospheric OH also began. Theoretical calculations improved quantitatively with the inclusion of more accurate rate coefficients and increased in complexity with the addition of non-methane hydrocarbon oxidation. A more realistic global picture of OH and its impact on other trace gases was also developing through the use of approximate 2-D global models. It is not possible to accurately capture the explosion of theoretical, laboratory and field studies of tropospheric chemistry during the 1970s and early 1980s in this short introduction. Rather, I have selected a few examples that provide an idea of the evolution of our theoretical and observational understanding during this period [Fishman et al., 1979; Oltmans, 1981; Logan et al., 1981; Gregory et al., 1985; Beck et al., 1987; Liu et al., 1987; Fehsenfeld et al., 1988; Crutzen, 1988].

The following articles examine the current state of our understanding of tropospheric OH and its role in tropospheric chemistry. William Brune discusses our current understanding of OH chemistry and its interactions with NOX and O3. Hans-Peter Dorn and Andreas Hofzumahaus review the current state of local OH measurements and their intercomparison with photochemical models. Ivar Isaksen focuses on the important relationship between OH and CH4 and the impact of this relationship on our current understanding of CH4 sources and lifetime. Leonid Yurganov examines the role of OH in the current inter-annual variations and trends in CO. And Yuhang Wang reports on recent measurement and modeling studies that attempt to constrain global tropospheric OH concentration and its past and future trends.

References

  1. Bates, D.R. and M. Nicolet, The photochemistry of the atmospheric water vapor, J. Geophys. Res., 55, 301, 1950.
  2. Bates, D.R. and A.E. Witherspoon, The photochemistry of some minor constituents of the earth's atmosphere (CO2, CO, CH4, N2O), Mon. Not. R. Astr. Soc., 112, 101, 1952.
  3. Beck, S.M. et al., Operational overview of NASA GTE/CITE 1 airborne instrument intercomparisons: Carbon monoxide, nitric oxide, and hydroxyl instrumentation, J. Geophys. Res., 92, 1977-1985, 1987.
  4. Cadle, R.D. and E.R. Allen, Atmospheric photochemistry, Science, 167, 243-249, 1970.
  5. Chameides, W. L., and J. C. G. Walker, A photochemical theory of tropospheric ozone, J. Geophys. Res., 78, 8751-8760, 1973.
  6. Crutzen, P.J., The influence of nitrogen oxide on the atmospheric ozone content, Quart. J. R. Met. Soc., 96, 320-325, 1970.
  7. Crutzen, P. J., Photochemical reaction initiated by and influencing ozone in unpolluted tropospheric air, Tellus, 26, 45 - 55, 1974.
  8. Crutzen, P. J., Tropospheric ozone: an overview, in Tropospheric Ozone, Regional and Global Scale Interactions, edited by I.S.A. Isaksen, pp. 185-216, D.Reidel Publ. Comp., Dordrecht, 1988.
  9. Engleman, R., The vibrational state of hydroxyl radicals produced by flash photolysis of a water-ozone-argon mixture, J. Amer. Chem. Soc., 87, 4193, 1965.
  10. Fehsenfeld, F.C., D.D. Parrish, and D.W. Fahey, The measurement of NOX in the non-urban environment, in Tropospheric Ozone, Regional and Global Scale Interactions, edited by I.S.A. Isaksen, pp. 3-32, D.Reidel Publ. Comp., Dordrecht, 1988.
  11. Fishman, J., S. Solomon, and P. J. Crutzen, Observational and theoretical evidence in support of a significant in-situ photochemical source of tropospheric ozone, Tellus, 31, 432-446, 1979.
  12. Gregory, G.L. et al., Operational overview of Wallops Island instrument intercomparison: Carbon monoxide, nitric oxide, and hydroxyl instrumentation, J. Geophys. Res., 90, 12,808-12,818, 1985.
  13. Hampson, J., Photochemical behavior of the ozone layer, Tech. Note 1627/64, CARDE, Valcartier, Quebec, 1964.
  14. Heicklen, J., K. Westberg, and N. Cohen, Chemical Reactions in Urban Atmospheres, C.S. Tuesday, ed., American Elsivier Pub. Co., NY, 55, 1971.
  15. Howard, C.J. and K.M. Evenson, Kinetics of the reaction of HO2 and NO, Geophys. Res. Lett., 4, 437-440, 1977.
  16. Hunt, B.G., Photochemistry of ozone in a moist atmosphere, J. Geophys. Res., 71, 1385-1398, 1966.
  17. Junge, C.E., Air Chemistry and Radioactivity, Academic Press, New York, 1963.
  18. Levy II, H., Normal atmosphere: Large radical and formaldehyde concentrations predicted, Science, 173, 141 - 143, 1971.
  19. Levy, H. II. Photochemistry of the Troposphere, Advances in Photochemistry, 9, John Wiley & Sons: New York, 369-524, 1974.
  20. Liu, S. C., M. Trainer, F. C. Fehsenfeld, D. D. Parrish, E. J. Williams, D. W. Fahey, G. Hubler, and P. C. Murphy, Ozone production in the rural troposphere and the implications for regional and global ozone distributions, J. Geophys. Res., 92, 4191-4207, 1987.
  21. Logan, J., M.J. Prather, S.C. Wofsy, and M.B. McElroy, Tropospheric chemistry: A global perspective, J. Geophys. Res., 86, 7210-7254, 1981.
  22. McConnell, J.C., M.B. McElroy, and S.C. Wofsy, Natural sources of atmospheric CO, Nature, 233, 187, 1971.
  23. Oltmans, S. J., Surface ozone measurements in clean air, J. Geophys. Res., 86, 11,974-11,180, 1981.
  24. Nicolet, M., Ozone and hydrogen reactions, Ann. Geophys., 26, 531-546, 1970.
  25. Weinstock, B., Carbon monoxide residence time in the atmosphere, Science, 166, 224, 1969.
  26. Westberg, K. and N. Cohen, The chemical kinetics of photochemical smog as analyzed by computer, Aerospace Corp. Rep. ATR-70-(8107)-1, El Segundo, CA, 1969.
top ^