An efficient and stable computational scheme for parameterization of atmospheric chemistry is described. The 24-hour-average concentration of OH is represented as a set of high-order polynomials in variables such as temperature, densities of H₂O, CO, O₃, and NO_t (defined as NO + NO₂ + NO₃ + 2N₂O₅ + HNO₂ + HNO₄) as well as variables determining solar irradiance: cloud cover, density of the overhead ozone column, surface albedo, latitude, and solar declination. This parameterization of OH chemistry was used in the three-dimensional study of global distribution of CH₃CCl₃ (Spivakovsky et al., this issue). The proposed computational scheme can be used for parameterization of rates of chemical production and loss or of any other output of a full chemical model. Coefficients for the polynomials are computed to provide the least squares fit to results of the full chemical model. Highly overdetermined systems are used with the sets of independent variables selected randomly in accordance with the distributions expected in the atmosphere. The least squares problem is solved using the Householder method of triangularization (by orthogonal transformations). The method allows detection and rectification of ill-defined conditions (i.e., linear dependence among terms), as well as evaluation of the individual contribution of each term of the polynomial in reducing the residual vector. On the basis of that information the terms that have little bearing on the residual norm are discarded. Once the domain and the statistical distributions of independent variables are chosen, the entire parameterization procedure is implemented as a complete sequence of computer programs requiring no subjective analysis. The output of the procedure includes estimates of accuracy of the approximation against an independent sample of points, and computer written FORTRAN subroutines to compute the polynomials.

Measurements of the emission of ²²²Rn from Amazon forest soils, and profiles of ²²²Rn in air were used to study the ventilation of the soil atmosphere and the nocturnal forest canopy. The emission of ²²²Rn from the yellow clay soils dominant in the study area averaged 0.38±0.07 atom cm⁻² s⁻¹. Nearby sand soils had similar fluxes, averaging 0.30 ± 0.07 atom cm⁻² s⁻¹. The effective diffusivity in the clay soil (0.008±0.004 cm² s⁻¹), was lower than that for the sand soil (0.033±0.030 cm² s⁻¹). Profiles of ²²²Rn and CO₂ in air showed steepest concentration gradients in the layer between 0 and 3 m above the soil surface. Aerodynamic resistances calculated for this layer from ²²²Rn and CO₂ varied from 1.6 to 18 s cm⁻¹, with greater resistance during the afternoon than at night. Time averaged profiles of²²²Rn in the forest canopy measured during the evening and night were combined with the soil flux measurements to compute the resistance of the subcanopy to exchange with overlying air. The integrated nocturnal rate of gas exchange between the canopy layer (0 to 41 m) and overlying atmosphere based on ²²²Rn averaged 0.33±0.15 cm s⁻¹. An independent estimate of gas exchange, based on 13 nights of CO₂ profiles, averaged 0.21±0.40 cm s⁻¹. These exchange rates correspond to flushing times for the 41 m canopy layer of 3.4 and 5.5 hours, respectively. Comparison of ²²²Rn and CO₂ profiles show that the nocturnal production of CO₂ by above-ground vegetation was about 20% of the soil emission source, consistent with data from eddy-correlation experiments (Fan et al., this issue).

It is shown that the equations describing chemical partitioning among Cl_x (HCl, Cl, ClO, ClNO₃), NO_t (NO, NO₂, NO₃, N₂O₅, HNO₂, ClNO₃) and HO_x (OH, HO₂) may admit multiple solutions. These solutions apply to the high latitude winter stratosphere where abrupt spatial variations may be expected for NO₂, ClO and ClNO₃.

The equations which determine partitioning of Cl_x in steady state have multiple (three) solutions under conditions which might arise in the high-latitude winter stratosphere. Two of these solutions are stable, one is unstable, to infinitesimal perturbations. The relative stability of solutions is examined by subjecting the system to finite perturbations. The more stable solution is found to eliminate the less stable when semi-infinite volumes of the two solutions are placed in contact. The high-ClO, low-NO₂ solution is more stable under most conditions. Transitions from less to more stable states are slow in winter but may occur more rapidly when the seasonal variation of insolation is taken into account.

The three-dimensional global distribution of OH over a year is calculated as a function of temperature, ultraviolet irradiance, and densities of H₂O, CO, O₃, CH₄, and NO_t, (defined as NO + NO₂ + NO₃ + 2N₂O₅ + HNO₂ + HNO₄). The concentration of OH is computed within a chemical tracer model (CTM) with an accuracy comparable to that of a detailed photochemical model. Distributions of CO, NO_t, O₃, CH₄, and the density of O₃ column were specified on the basis of observations. Meteorological fields were derived from the general circulation model developed at the Goddard Institute for Space Studies. The numerical method for parametrization of chemistry is described inSpivakovsky et al. (this issue). The CTM is used to simulate the global distribution of CH₃CCl₃. The computed distribution of OH implies a lifetime of 5.5 years for CH₃CCl₃ (obtained by relating the global burden of CH₃CCl₃ to the global loss, integrated using simulated three-dimensional distributions). Analysis of the long-term trend in CH₃CCl₃ as defined by observations suggests a lifetime of 6.2 years (consistent with Prinn et al. (1987)), indicating that model levels of OH may be too high by about 13%. This estimate for the lifetime depends on industry data for global emissions and on the absolute calibration of observations. It is argued that seasonal variations of CH₃CCl₃ provide an independent test for computed OH fields that is insensitive to the uncertainties in the budget of CH₃CCl₃. The annual cycle of CH₃CCl₃ from about 25°S to the South Pole is dominated by seasonal changes in OH. Observed seasonal variations of CH₃CCl₃indicate that the OH field south of 20°S±4° should be scaled by 0.75±0.25 from computed values, consistent with the result based on long-term trends. Reactions involving non-methane hydrocarbons were not included in the current model. These reactions could account for lower concentrations of OH than computed. Seasonal variations of CH₃CCl₃ in the tropics and in the northern mid-latitudes are dominated by effects of transport. If use of CH₃CCl₃ is phased out (as envisioned by the Montreal protocol), the dynamically driven seasonal variations of CH₃CCl₃ will decrease dramatically, whereas the chemically driven variations will remain proportional to the concentration of CH₃CCl₃; then the annual cycle of CH₃CCl₃ in northern mid-latitudes will provide a measure of OH as does at present the annual cycle in southern mid-latitudes. The influence of chemistry on the latitudinal distribution of CH₃CCl₃ is small and at present does not provide a constraint for the globally averaged OH or for the latitudinal distribution of OH. However, if emissions of CH₃CCl₃ were to cease, the tropical depression in the concentration of CH₃CCl₃ caused by high levels of OH in the tropics may provide an additional means to test OH models.