Use In Atmospheric Models Research Articles

Collision-Induced Absorption in the a1Δg(v′=0)←X3Σg−(v=0) Transition in O2–CO2, O2–N2, and O2–H2O Mixtures

When O2 is perturbed by collisions with other molecules, the weak spin-forbidden magnetic dipole transition a1Δg←X3Σg− shows a broad continuum absorption underlying the sharp lines. This collision-induced enhancement absorption plays a role in the Earth's atmosphere and much experimental work has been carried out to measure the binary absorption coefficient with different perturber gases. Recent work on the v′=0←v=0 band in O2–CO2 mixtures yielded a value for the coefficient that was approximately three times that of earlier measurements on O2–N2 mixtures. In the present note, we calculate the absorption theoretically assuming that the long-range quadrupole-induced dipole mechanism is dominant. Using experimental polarizability matrix elements of CO2 and ab initio results in the literature for the quadrupolar transition matrix element for O2, we find good agreement for O2–CO2 mixtures without any adjustable parameters. The agreement for O2–N2 is less good, and because of the much smaller polarizability of N2 than of CO2, we suggest that one has to include a short-range component in addition to the long-range one treated here. We also calculate the binary absorption coefficient for O2–H2O, for which no experimental data are available, and we synthesize the corresponding spectrum for use in atmospheric modeling.

Carbon and hydrogen kinetic isotope effects for the reaction of Cl with CH4: Consolidating chemical kinetics and molecular dynamics measurements

In order to estimate carbon and hydrogen kinetic isotope effects for the reaction Cl+ CH4 at atmospheric temperatures, measured values of the rates of Cl reacting with 13CH4, 12CH3D, 12CH2D2, 12CHD3, and 12CD4 relative to those of Cl+12CH4 were fit using a model that accounts for quantum mechanical tunneling. The model also includes vibrational enhancement of the reaction rate by excitation of the bending and stretching modes of CH4. These fits allow extrapolation of measured values to temperatures outside the range in which they were measured. In addition, the kinetic isotope effect for Cl+14CH4 was predicted using this model based on the results for Cl+ 13CH4. The model results for Cl+14CH4, Cl+13CH4, and Cl+12CH3D were parameterized for use in atmospheric models.

Atmospheric chemistry of small organic peroxy radicals

Global atmospheric models play a key role in international assessments of the human impact on global climate and air pollution. To increase the accuracy and facilitate comparison of results from such models, it is essential they contain up‐to‐date chemical mechanisms. To this end, we present an evaluation of the atmospheric chemistry of the four most abundant organic peroxy radicals: CH3O2, C2H5O2, CH3C(O)O2, and CH3C(O)CH2O2. The literature data for the atmospheric reactions of these radicals are evaluated. In addition, the ultraviolet absorption cross sections for the above radicals and for HO2 have been evaluated. The absorption spectra were fitted to an analytical formula, which enabled published spectra to be screened objectively. Published kinetic and product data were reinterpreted, or in some case reanalyzed, using the new cross sections, leading to a self‐consistent set of kinetic, mechanistic, and spectroscopic data. Product studies were also evaluated. A set of peroxy radical reaction rate coefficients and products are recommended for use in atmospheric modeling. A three‐dimensional global chemical transport model (the Intermediate Model for the Global Evolution of Species, IMAGES) was run using both previously recommended rate coefficients and the current set to highlight the sensitivity of key atmospheric trace species to the peroxy radical chemistry used in the model.

Time-Resolved O3 Chemical Chain Reaction Kinetics via High-Resolution IR Laser Absorption Methods

Excimer laser photolysis in combination with time-resolved IR laser absorption detection of OH radicals has been used to study O3/OH(v = 0)/HO2 chain reaction kinetics at 298 K, (i.e.,(k(sub 1) is OH + 03 yields H02 + 02 and (k(sub 2) is H02 + 03 yields OH + 202). From time-resolved detection of OH radicals with high-resolution near IR laser absorption methods, the chain induction kinetics have been measured at up to an order of magnitude higher ozone concentrations ([03] less than or equal to 10(exp 17) molecules/cu cm) than accessible in previous studies. This greater dynamic range permits the full evolution of the chain induction, propagation, and termination process to be temporally isolated and measured in real time. An exact solution for time-dependent OH evolution under pseudo- first-order chain reaction conditions is presented, which correctly predicts new kinetic signatures not included in previous OH + 03 kinetic analyses. Specifically, the solutions predict an initial exponential loss (chain induction) of the OH radical to a steady-state level ([OH](sub ss)), with this fast initial decay determined by the sum of both chain rate constants, k(sub ind) = k(sub 1) + k(sub 2). By monitoring the chain induction feature, this sum of the rate constants is determined to be k(sub ind) = 8.4(8) x 10(exp -14) cu cm/molecule/s for room temperature reagents. This is significantly higher than the values currently recommended for use in atmospheric models, but in excellent agreement with previous results from Ravishankara et al.

Parameterizations for Cloud Overlapping and Shortwave Single-Scattering Properties for Use in General Circulation and Cloud Ensemble Models

Abstract Parameterizations for cloud single-scattering properties and the scaling of optical thickness in a partial cloudiness condition have been developed for use in atmospheric models. Cloud optical properties are parameterized for four broad bands in the solar (or shortwave) spectrum; one in the ultraviolet and visible region and three in the infrared region. The extinction coefficient, single-scattering albedo, and asymmetry factor are parameterized separately for ice and water clouds. Based on high spectral-resolution calculations, the effective single-scattering coalbedo of a broad band is determined such that errors in the fluxes at the top of the atmosphere and at the surface are minimized. This parameterization introduces errors of a few percent in the absorption of shortwave radiation in the atmosphere and at the surface. Scaling of the optical thickness is based on the maximum-random cloud-overlapping approximation. The atmosphere is divided into three height groups separated approximately by ...

PARAMETERISING SUB-GRID SCALE VERTICAL TRANSPORT IN ATMOSPHERIC MODELS UNDER STATICALLY STABLE CONDITIONS

Five different formulations of the stability functions used forvertical turbulent transfer in atmospheric models are compared in a 1-Dmodel of the nocturnal boundary layer. One of them has a critical valueof the Richardson number around 0.2 and leads to the traditional log-linear profile, while other more empirical formulations maintain sometransfer at values of Ri around 1.0. Although no new observationalevidence is presented, it is suggested that the latter formulations aremore appropriate for use in atmospheric models because the unresolvedvariability inside a model grid box induces some turbulent transfer evenat super-critical values of the mean Ri. The study shows that themagnitude of the stability functions is principally important in theeffective range of Ri values found in the stable boundary layer and thattheir slopes near the origin are less important. This permits the use inatmospheric models of a simple explicit function of Ri containing asingle parameter, with results similar to those obtained with earlier morecomplex formulations. The results of the simulation with the 1-D model are used toexamine the errors introduced by the relatively thick surface layers of most atmospheric models, in which, for the stable case, the traditionalassumption of constancy of the fluxes with height is often clearlyviolated. When a height variation of the fluxes is introduced in surface-layer formulations, the error in the magnitude of the surface fluxes isdecreased with some of the formulations but not all of them. This lackof sensitivity is explained by a compensation mechanism in which theassumed decrease of the fluxes with height implies a correspondingdecrease of the Obukhov length which acts in the oppositedirection, reducing, and sometimes eliminating, the adverse effect of theunrealistic specification of the fluxes. It may be argued that thiscompensation mechanism also explains the wide range of validity of theMoini-Obukhov similarity theory.

Atmospheric reactions of alkoxy and ?-hydroxyalkoxy radicals

Alkoxy and β-hydroxyalkoxy radicals are key intermediates formed in the atmospheric degradations of alkanes and alkenes, respectively. In the troposphere, these alkoxy radicals can decompose, isomerize, and react with O2. The literature data concerning the rates of these reactions are evaluated, and predictive schemes allowing the calculation of rate constants for these alkoxy radical reactions for atmospheric purposes are proposed. Good agreement between calculated reaction rates and experimental data concerning the absolute and relative importance of these reaction pathways is obtained, and alkoxy and β-hydroxyalkoxy radical reaction rates for radicals for which experimental data are not presently available can now be calculated for use in atmospheric modeling. © 1997 John Wiley & Sons, Inc.

Effects of neutral temperature on meridional winds estimated from ionospheric data

Indirect determination of meridional winds using ground-based ionosonde data from low latitude regions, under the assumption that the thermosphere and the F-region of the ionosphere behave as a closely coupled system, has been critically examined. The significance of neutral temperature and its variations in the above estimates has been demonstrated through individual case studies after duly validating the procedure adopted. Since the measured neutral temperatures have shown large deviations from the existing atmospheric models on many occasions and more so during high solar activity periods, it has been shown that the neutral temperature effects on the F-region heights should be properly accounted for before one attempts to estimate meridional winds. However, it has also been shown that during low solar activity periods, use of atmospheric models may still provide a fairly reasonable average picture. Examples of these effects are presented and discussed.

Temperature dependence of the HNO3 UV absorption cross sections

The temperature dependence of the HNO3 absorption cross sections between 240 and 360 K over the wavelength range 195 to 350 nm has been measured using a diode array spectrometer. Absorption cross sections were determined using both (1) absolute pressure measurements at 298 K and (2) a dual absorption cell arrangement, in which the absorption spectrum at various temperatures is measured relative to the room temperature absorption spectrum. The HNO3 absorption spectrum showed a temperature dependence which is weak at short wavelengths but stronger at longer wavelengths which are important for photolysis in the lower stratosphere. The 298 K absorption cross sections were found to be larger than the values currently recommended for atmospheric modeling (DeMore et al., 1992). Our absorption cross section data are critically compared with the previous measurements of both room temperature and temperature‐dependent absorption cross sections. Temperature‐dependent absorption cross sections of HNO3 are recommended for use in atmospheric modeling. These temperature dependent HNO3 absorption cross sections were used in a two‐dimensional dynamical‐photochemical model to demonstrate the effects of the revised absorption cross sections on loss rate of HNO3 and the abundance of NO2 in the stratosphere.

A Resistance Representation of Schemes for Evaporation from Bare and Partly Plant-covered Surfaces for Use in Atmospheric Models

In the parameterization of land surface processes, attention must be devoted to surface evaporation, one of the main processes in the air-land energy exchange. One of the most used approaches is the resistance representation which requires the calculation of aerodynamic resistances. These resistances have been calculated using K theory for different morphologies of plant communities, then the performance of the evaporation schemes within the “α”, “β”, and their combination approaches that parameterize evaporation from bare and partly plant-covered soil surfaces are discussed. Additionally, a new “α” scheme is proposed based on an assumed power dependence α on volumetric soil moisture content and its saturated value. Finally, the performance of the considered and the proposed schemes is tasted based on time integrations using real data. The first set was for 4 June 1982, and the second for 3 June 1981 at the experimental site in Rimski Šanševi, Yugoslavia, on chernozem soil, as representative for a bare, and partly plant-covered surface, respectively. The accuracy of the schemes was compared with the observations using a root-mean-square error and the factor-of-deviation analyses.

Towards a quantitative understanding of atmospheric ozone

This review deals with the historical development of a chemical kinetics and photochemical data base for use in atmospheric modeling. Initiated by the suggestion that the release of nitric oxide into the stratosphere from the exhaust gases of supersonic aircraft could seriously deplete stratospheric ozone, the quest for such a data base began in 1971. The present article traces the development of the detailed chemistry of the homogeneous gas-phase reactions which control the levels of ozone in the stratosphere. It also includes coverage of the more recent heterogeneous chemistry which is believed to be involved in the Antarctic “ozone hole,” as well as the background to the on-going establishment of a reaction rate data base for application to tropospheric chemistry and modeling.

Kinetics of the OH + CO reaction under atmospheric conditions

The pulsed laser photolysis‐pulsed laser‐induced fluorescence technique has been employed to study the kinetics of the reaction, OH + CO → k1 products, as a function of temperature and pressure in N2, air, and O2 buffer gases. At pressures of 1 atm and below, k1 is found to increase linearly with increasing pressure. Within experimental uncertainty, both the rate constant in the low‐pressure limit and the slope of the k1 versus pressure dependence are found to be the same at 262 K as at 299 K. From our data we obtain the expression k1 = (1.47 ± 0.08) × 10−13 [1 + (0.59 ± 0.10)P] independent of temperature, where P is in units of atmospheres and k1 is in units of cubic centimeters per molecule per second. Within experimental uncertainty, the pressure dependence of k1 at 299 K is the same in N2 buffer gas as in O2 buffer gas. In 700‐torr air at 299 K, k1 is found to be independent of water vapor concentration over the range 0.01–20 torr. Our results will have little effect on current panel recommendations of k1 values for use in atmospheric models. However, uncertainties in the temperature dependence under atmospheric conditions, the third body efficiency of O2, and the effect of water vapor on k1 are now greatly reduced.

Fall-Off Curves for the Reaction ClO + NO2( + N2) ClONO2 (+N2)

Abstract Fall-off curves for the reaction ClO+NO2(+N2)→ClONO2(+N2) at 300 and 220 K have been calculated from Kassel integrals by use of a theoretically derived high pressure limit and the experimentally determined low pressure limit as reference points. The reaction is found to deviate considerably from the low pressure limit at pressures larger than 10 Torr. Effective rate constants for the use in atmospheric models of chlorine nitrate formation are derived.