Line‐by‐line radiative excitation model for the non‐equilibrium atmosphere: Application to CO₂ 15‐μm emission

Abstract

We describe a new llne-by-llne (LBL) algorithm for radiative excitation in infrared bands in a non local thermodynamic equilibrium (non-LTE) planetary atmosphere. As a specific application, we present a predictive model for the terrestrial CO 2 15-ttm emission that incorporates this generic algorithm, and validate the model by comparing its results with emission spectra obtained in a limb-scannlng rocket experiment. The radiative-excitation algorithm has certain unique features. Being a completely monochromatic calculation, it includes the detailed layer-by-layer variation of the shapes of the individual lines in its evaluation of atmospheric transmittivity; and, being an iterative algorithm, it avoids the need to construct and invert large matrices, so that a fine layering scheme can be implemented. It also includes a simple correction procedure to minimize the most serious error due to having discrete layers. These features contribute to accurate radiative transfer results and reliable atmospheric cooling rates. For altitudes above 40 kin, we present results of mode] calculations of CO2(v2) vibrational temperatures, 15-ttm limb spectral radiances, and cooling rates, for the main band as well as for weaker hot and isotopic bands. We calculate the excitation and deexcitation rates due to different processes, including the radiative pumping due to individual atmospheric layers. We compare the predicted limb radiance with earthlimb spectral scans obtained in the SPIRE rocket experiment over Poker Flat, Alaska, and get excellent agreement as a function of both wavelength and tangent height. This constitutes the first validation of a long-wavelength CO 2 non-LTE emission model using an actual atmospheric data set, and it verifies the existence of certain aeronomic features that have only been predicted by models. It also constrains the previously unknown value of the very important rate constant for deactivation of the CO 2 bending mode by atomic oxygen to the range of 5-6 x 10 -12 cm3/(mol s) at mesospheric and lower thermospheric temperatures. We discuss the significance of this large value for the terrestrial and VenusJan thermospheres. We also discuss the convergence rate of the iterative scheme, the model's sensitivity to the background atmosphere, the importance of the lower boundary surface contribution, and the effects of the choice of the layer thickness and the neglect of line overlap.

Retrieval of Kinetic Temperature and Carbon Dioxide Abundance From Non-Local Thermodynamic Equilibrium Limb Emission Measurements Made by the SABER Experiment on the TIMED Satellite

“…13 The same is also true for CO 2 4.3 urm limb radiance. The onion-peel technique ensures that the modeled emission matches the measured radiance from the upper altitude layers, even though the retrieved temperature-pressure-CO 2 combination may be incorrect at intermediate steps in the relaxation process.…”

Kinetic temperature and carbon dioxide from broadband infrared limb emission measurements taken from the TIMED/SABER instrument

“…The lidar measurements will be used to assess the quality of the SABER-CO 2 data, as described below. Figure 10 is a comparison of the SABER-CO 2 daytime mean profile, the TIME-GCM profile, and a rocket mass spectrometer measurement taken at mid-latitudes in the 1970s (see Wintersteiner et al, 1992) Interestingly, the SABER-CO 2 mean profile is consistent with the rocket measurement made several decades ago (note: the rocket CO 2 profile has been scaled to reflect the current uniformly mixed value of ∼ 365 ppmv). Figure 12).…”

Development of Atmospheric Infrared Emission Models

“…18 The species number density and temperature profiles were obtained using the MSISE-90 model 19 and an altitude-dependent C0 2 mixing ratio representative of rocket and satellite observations. 8,20 Up to 10,000 Monte Carlo "photons" per band were used in the NEMESIS runs, which was sufficient to essentially converge the vibrational temperatures.…”

“…As in other non-LTE atmospheric models (e.g., Wintersteiner et al, 8 Lopez-Puertas et al 9 ' 10 ), steady-state kinetics are assumed for the quiescent atmospheric processes of collisional excitation, de-excitation, energy transfer, radiative decay, illumination by the sun, earth and atmosphere, and chemical production. In SHARC, additional production and excitation mechanisms resulting from electron deposition are included using a time-dependent kinetic model.…”