Hydroxyl Radical Oh Processes Controlling OH Concentrations

A detailed and still fairly current discussion of OH chemistry in the troposphere is given by Logan et al. (1981). The primary source of OH is the photolysis of 03 to produce an excited state of atomic oxygen, O('D), which then reacts with water vapor:

Here M is an inert molecule (N2 or 02). Only ~ 1 % of the 0( D) atoms produced by (Rl) react with H20; most are deactivated to the ground-state 0(3P) and recombine with 02 to return 03. Photolysis of 03 to O('D) in the troposphere is determined by a narrow band of radiation in the 290- to 330-nm range, reflecting the combined wavelength dependences of the actinic flux, 03 absorption cross section, and 0(' D) quantum yield (Fig. 1). Radiation in this wavelength range is strongly absorbed by overhead 03, and hence the production of 0(*D) is strongly dependent on the thickness of the stratospheric 03 layer (Madronich and Granier, 1992).

The OH radical is consumed on a time scale of ~ 1 s by oxidation of a large number of reduced atmospheric species. Its main sinks in the troposphere are CO and CH4. Nonmethane hydrocarbons (NMHCs) are also important sinks in the lower troposphere over continents. Oxidation of CO or hydrocarbons by OH propagates a

280 300 320 340

280 300 320 340

280 300 320 340 Wavelength, nm

Figure 1 Computation of the rate constant of reaction (Rl) as the integral over all wavelengths of the actinic flux of solar radiation (1) times the absorption cross-section ffQ3 of ozone (2) and times the O(ID) quantum yield (3). From Jacob (1999).

280 300 320 340 Wavelength, nm

Figure 1 Computation of the rate constant of reaction (Rl) as the integral over all wavelengths of the actinic flux of solar radiation (1) times the absorption cross-section ffQ3 of ozone (2) and times the O(ID) quantum yield (3). From Jacob (1999).

radical reaction chain initiated by the generation of OH radicals from (R4). The simplest case is oxidation of CO:

The H02 radicals may self-react to produce H202 (hydrogen peroxide):

or they may regenerate OH by reaction with NO or 03:

Hydrogen peroxide produced by (R7) is removed from the atmosphere by deposition. It may also photolyze, regenerating OH,

or react itself with OH:

The same type of chain mechanism applies to the oxidation of hydrocarbons, but the complexity increases rapidly as the size of the hydrocarbon molecule increases. The mechanism for CH4 is described here. It begins by

CH4 + OH -> CH3 + H20 CH3 + 02 + M —► CH302 + M

The CH302 molecule (methylperoxy radical) is analogous to H02. Its dominant sinks in the atmosphere are reactions with H02 and no:

ch3o2 + hoz -> ch3ooh + o2 ch302 + no ch30 + n02

Similarly to H202, methylhydroperoxide (CH3OOH) may either react with OH or photolyze:

CH3OOH + OH CH20 + OH + H20 CH3OOH + OH CH302 + H20 CH3OOH + hv CH30 + OH

The methoxy radical CH30 produced by (R15) and (R18) reacts rapidly with 02:

Formaldehyde produced by (R16) and (R19) may either react with OH or photolyze (two photolysis branches):

previously, these peroxides may photolyze to recycle HOr; alternatively, they may deposit or react with OH, providing a terminal sink for HOr. Sources of NOr in the troposphere include combustion, microbial activity in soils, and lightning. Sources of CO and hydrocarbons include combustion, industrial processes, soils, and vegetation.

An analytical expression for the dependence of OH concentrations on chemical variables can be obtained from the simplified O3-HOj.-NOj.-CO system by assuming chemical steady state for the short-lived species O('D), H, OH, and also for the chemical family HOr The lifetime of HOr against formation of peroxides is of the order of minutes, so that the steady-state assumption is appropriate. The production rate PH0 of HOr from reaction (R4) is given by

where ki is the rate constant for reaction i. In writing Eq. (1) we have used the approximation (R2)»(R4) to simplify the denominator. Steady state for OH is defined by

Loss of HOr in this system is by (R7). Steady state for HOr is therefore defined by

from which we derive the following expression for the OH concentration:

We see from Eq. (4) together with Eq. (1) that OH concentrations depend negatively on CO and positively on water vapor, 03, and NO. The dependence on hydrocarbons is more complicated (as hydrocarbons provide both sinks of OH and sources of HO.,.) but is generally negative, similar to CO.

One important caveat to this simplified representation of OH chemistry must be made for high-NOr environments. When NOr concentrations exceed a few parts per billion by volume (ppbv), as in urban air, oxidation of N02 by OH can become the dominant sink for HOr:

Under these conditions, OH concentrations decrease with increasing NOr (as may be derived by repeating the steady-state calculation above) and increase with increasing hydrocarbons. This situation is commonly denoted the NOx-saturated (or hydrocarbon-limited) regime, as opposed to the NOx-limited regime normally encountered in the troposphere.

A second caveat applies to the upper troposphere where water vapor concentrations are low (~100ppmv). Under these conditions, reaction (R4) may be less important as a primary source of HOx than photolysis of acetone originating from the biosphere (Singh et al., 1995) or convective injection of peroxides and aldehydes produced in the lower troposphere (Jaegle et al., 1997; Prather and Jacob, 1997; Miiller and Brasseur, 1999). Reaction of OH with H02 provides in general the dominant HOx sink in the upper troposphere, which yields a square root rather than linear dependence of OH concentrations on NO.

Figure 3 shows zonal mean global distributions of OH concentrations computed with a global three-dimensional model of tropospheric 03 -NOx-hydrocarbon chemistry (Wang et al., 1998b). The highest concentrations (averaging over 2 x 106 molecules/cm3) are in the tropical middle troposphere, reflecting a combination of high ultraviolet (UV) and high humidity. The large seasonal variation at mid-latitudes follows UV radiation. Concentrations tend to be higher in the Northern than in the Southern Hemisphere, reflecting higher NOx concentrations.

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