Atmospheric broadband transmission measure­ments and predictions in the 8-13-μm window: influence of water continuum absorption errors D. R. Cutten

Department of Defence, Electronics Research Laborato­ry, GPO Box 2151, Adelaide 5001, Australia. Received 10 November 1984. 0003-6935/85/081085-03$02.00/0. © 1985 Optical Society of America. In assessing the impact of the environment on various

electrooptic systems, computer models such as the AFGL LOWTRAN model1 are used to calculate the transmission and emission effects of the intervening atmosphere. Propagation studies at Electronics Research Laboratory have included a measurement program to validate predictions of LOWTRAN in different Australian environments. The tropical envi­ronment is of particular interest where the model has not been extensively tested for long paths with water vapor contents around 25 g/m3. An aim of this program was to provide a transmission data base for this environment.

In the 8-13-μm window the predominant absorber is the water continuum whose absorption is a function of a wave­length- and temperature-dependent absorption coefficient and the water vapor pressure. The water continuum model used in LOWTRAN 6 over the 0-1200-cm-1 region uses ab­sorption coefficient values for 296 K derived from the modi­fied water vapor line shape based on the impact approxima­tion theory.2 This modification to the line shape theory in­cludes an empirical function which has parameters that are adjusted to fit the self-broadening C0

s(γ) and foreign broad­ening C0

ƒ(γ) coefficients measured by Burch3 in the late 1960s. The experimental accuracy quoted for the C0

s(γ) data in the 8-13-μm region is ±10%. If one examines the effect of these

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Fig. 1. Comparison of measured and calculated atmospheric transmittance for a 7-km path and three temperature intervals vs absolute humidity in the 8.2-11.8-μm spectral region: experimental (I); LOWTRAN 6 Navy (—); LOWTRAN 6 Maritime (-·-); and modi­

fied LOWTRAN 6 Navy (- - ) .

Fig. 2. Comparison of measured and calculated atmospheric transmittance for a 7-km path and three temperature intervals vs absolute humidity in the 8.33-9.80-μm spectral region: experimental (I); LOWTRAN 6 Navy (—); LOWTRAN 6 Maritime (-·-); and modi­

fied LOWTRAN 6 Navy (--) . ,

Fig. 3. Comparison of measured and calculated atmospheric transmittance for 5- and 9-km paths vs absolute humidity in (a) 8.2-11.8-μm and (b) 8.33-9.80-μm spectral regions: experimental (I); LOWTRAN 6 Navy (—); and modified LOWTRAN

6 Navy ( - - ) .

errors on the water continuum transmission (TH2O) for long ranges and high water vapor contents (ρ, it becomes apparent that TH 2O is very sensitive to errors in C0

s(γ) coefficients. This comes about because the water continuum extinction

exhibits a quadratic dependence2-4 on ρ and any errors from C0

s(γ) affect the product C0s(γ) · 2.ρ For example, a -10%

error in C0s(γ) reduces TH2O by ~1.4 for a horizontal 7-km path

with ρ = 24 g/m3, while at 10 km it approaches 1.65.

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An indication that such a discrepancy could possibly exist in this window was revealed by Ben-Shalom et al.5 in their long-path low-resolution transmission measurement, however, that discrepancy appeared to be excessive. Clearly for a model which has been developed to provide low-resolution atmospheric transmission to ~10%, such errors in C0

s(γ) are not acceptable. On the other hand, for path lengths near 5 km with ρ ≤ 15 g/m3 the error in C0

s(γ) generally results in an error in TH2O being much less than that in C0

s(γ). Since LOWTRAN predictions have generally been compared (for 8-13 -μm window) with transmission data measured for such conditions, any discrepancy arising would be much harder to observe (see, for example, Fig. 3), particularly as it is difficult to minimize errors in the calibration of the transmissometer and from predicting the aerosol extinction in low visibility situations. However, if broadband transmissometer mea­surements are conducted over long paths where ρ exceeds, say, 15 g/m3, such discrepancies if present would become more apparent. Hence, such measurements would reveal the sen­sitivity of predicted transmission to errors in water continuum absorption for high absolute humidities if the influence of the aerosol extinction can be minimized by using data collected in conditions of high visibility and within a restricted tem­perature range.

The purpose of this Letter is to present a comparison of LOWTRAN 6 predictions with absolute transmission mea­surements made in an environment having high water vapor content. The experimental data, which represent two spectral regions in the 8-13-μm window, were obtained for a 7-km horizontal path.6 Figures 1 and 2 reproduce the data mea­sured on 68 different days when open-ocean maritime con­ditions existed at the site. The data were sorted into groups that correspond to intervals of 0.5 g/m3 for ρ and averaged. (The bars represent the actual data limits for that interval.) These data represent all visibilities, which generally fell within the range of 40-150 km. To directly compare the measured data with LOWTRAN 6 for similar meteorological conditions, the LOWTRAN transmission has been convolved with the source, filter, and detector response curves.

In each figure predicted curves are shown by a solid curve for the Navy aerosol model and the dot-dash curve for the Standard Maritime aerosol model. Clearly a large discrep­ancy can be seen with the predicted transmission lower than the measured data, and the discrepancy becomes greater with an increase in ρ. The transmissometer calibration error7 is estimated to be ±8% and cannot account for all the difference as it would result in a constant discrepancy with wavelength. The Navy aerosol model was chosen with an air mass character number of 2 as this represents the open-ocean air mass present. (A radon counter was used to monitor the air mass type.) It is noted that the results shown in Figs. 1 and 2 from using either the Maritime or Navy aerosol are not significantly different, although the difference does increase as ρ decreases. Hence, it is felt that with the moderately high visibilities prevailing, errors from predicting the aerosol extinction will have little influence on the result. Local water line absorption for the 8-12-μm region is represented in LOWTRAN by a single parameter band model and has been found to compare well with the FASCOD1C predictions. Furthermore, it does not predominate as an absorber in the way water continuum does and exhibits a linear dependence on p. The results in Figs. 1 and 2 must then point to a possible error in the C0

s(γ) coef­ficients.

Neglecting for the time being any errors that would arise in the transmissometer calibration and aerosol extinction, one can obtain an estimate of the error in C0

s(γ). The dashed curve represents a LOWTRAN 6 prediction where the C0

s(γ)

coefficients for 296 K in the 8-13-μm region were multiplied by 0.60. The comparison is shown to be considerably better. This method of measurement clearly indicates the sensitivity of the transmission to errors in C0

s(γ) coefficients. [Errors in C0

ƒ(γ) coefficients are much less significant as these values are ~ 3 orders of magnitude lower than C0

s(γ) values.] To see how the same modified LOWTRAN 6 predictions compare with measured data when ρ < 15 g/m3 a comparison was done with measurements made using the same transmissometer at a different site.7 Figure 3 reproduces the comparison and re­veals that in spite of a much smaller discrepancy it is generally an improvement over using the standard LOWTRAN 6 pre­dictions. No radon measurements were made with these transmission measurements although it could be assumed that the air mass was still representative of an open ocean as only maritime conditions were selected for the data analysis. The comparisons in Figs. 1-3 indicate how transmission mea­surements made for ρ < 15 g/m3 do not always clearly resolve the error in the C0

s(γ) coefficients. A review6 has been done to assess all the measurements made to date to evaluate C0

s(γ) using the multipass White cell and spectrophone techniques. It reveals that there is a large scatter present (in excess of ±10%) in these values, especially those determined with the spectrophone method.

In conclusion, the above comparisons indicate that the present C0

s(γ) absorption coefficients are too high. This result is supported by Burch and Alt's most recent work8 where C0

s(γ) values were revised downward by as much as 25-30%. However, further work must still be done to refine the values. Measurement accuracies for these coefficients will need to be achieved such that the LOWTRAN model can predict low-resolution transmission to within 10% when long paths (>10 km) and high water vapor content (>15 g/m3) are encoun­tered. It should be noted that the correction factor of 0.6 derived here is only an estimate and should be used with caution as the errors from transmissometer calibration, aerosol extinction, and temperature dependence of the absorption coefficients have not been taken into account.

References 1. F. X. Kneizys et al., "Atmospheric Transmittance/Radiance:

Computer code LOWTRAN 6," AFGL Report TR-83-0187 (Air Force Geophysics Laboratory, Hanscom AFB, Mass. 1983).

2. S. A. Clough et al., "Theoretical Line Shape for H2O Vapor: Application to the Continuum," AFGL Report TR-81-0283 (Air Force Geophysics Laboratory, Hanscom AFB, Mass., 1981).

3. D. E. Burch, "Investigation of the Absorption of Infrared Radiation by Atmospheric Gases," Semi-Annual Technical Report, Aer-onutronic Report No. U-4784 (Jan. 1970).

4. R. E. Roberts, J. E. Selby, and L. M. Biberman, "Infrared Con­tinuum Absorption by Atmospheric Water Vapor in the 8-12-μm Window," Appl. Opt. 15, 2085 (1976).

5. A. Ben-Shalom et al., Spectral Characteristics of Infrared Transmittance of the Atmosphere in the Region 2.8-14 micron— Preliminary Results," Infrared Phys 20, 165 (1980).

6. D. R. Cutten, "Atmospheric IR Transmission Measurements in a Tropical Maritime Environment: Comparison with LOWTRAN 6," ERL Technical Memorandum Report (Electronics Research Laboratory, Salisbury, South Australia, 1984).

7. D. R. Cutten, "Atmospheric IR Transmission Data for a Temperate Maritime Environment," ERL-0265-TR (Electronics Research Laboratory, Salisbury, South Australia, 1983).

8. D. Burch and R. Alt, "Continuum Absorption by Water in the 700-1200-cm1 and 2400-2800-cm-1 Windows," AFGL Report TR-84-0128 (Air Force Geophysics Laboratory, Hanscom AFB, Mass., 1984).

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