Dean R. Cutten

Complex refractive index of ammonium nitrate in the 2–20-μm spectral range

Using high-resolution Fourier-transform infrared absorbance and transmittance spectral data for ammonium sulfate (AMS), calcium carbonate (CAC), and ammonium nitrate (AMN), we made comparisons with previously published complex refractive-index data for AMS and CAC to infer experimental parameters to determine the imaginary refractive index for AMN in the infrared wavelength range from 2 to 20 μm. Subtractive Kramers-Kronig mathematical relations were applied to calculate the real refractive index for the three compositions. Excellent agreement for AMS and CAC with the published values was found, validating the complex refractive index obtained for AMN. We performed backscatter calculations using a log-normal size distribution for AMS, AMN, and CAC aerosols to show differences in their backscattered spectra.

Wavelength dependence of backscatter by use of aerosol microphysics and lidar data sets: application to 2.1-µm wavelength for space-based and airborne lidars

An aerosol microphysics dataset was used to model backscatter in the 0.35-11-mum wavelength range, with the results validated by comparison with measured cw and pulsed lidar backscatter obtained during two NASA-sponsored airborne field experiments. Different atmospheric features were encountered, with aerosol backscatter ranging over 4 orders of magnitude. Modeled conversion functions were used to convert existing lidar backscatter datasets to 2.1 mum. Resulting statistical distribution shows the midtropospheric aerosol backscatter background mode of beta(2.1) to be between ~3.0 x 10(-10) and ~1.3 x 10(-9) m(-1) sr(-1), ~10-20 times higher than that for beta(9.1); and a beta(2.1) boundary layer mode of ~1.0 x 10(-7) to ~1.3 x 10(-6) m(-1) sr(-1), ~3-5 times higher than beta(9.1).

Lidar-derived variations in the backscatter-to-extinction ratio in Southern Hemisphere coastal maritime aerosols

Abstract Lidar measurements were made of the optical properties of maritime aerosols from a tropical coastal site in north Queensland, Australia. From horizontal firings approximately 2 m over the ocean, data obtained during homogeneous conditions were used to derive the atmospheric extinction coefficient. During periods when the extinction and backscatter coefficients varied, but their ratio remained constant, it was possible to calculate this ratio and also to provide an independent calibration of the lidar. This calibration was then used to determine the backscatter-to-extinction ratio in periods when it was quite variable. This variability is related to variations in the meteorological and airmass data which were measured concurrently on the shore. Data obtained during a summer and a winter study are analysed.

Comparison of performance of 3–5- and 8–12-μm infrared systems

On the basis of a modified version of the LOWTRAN 6 model, an absolute comparison is made of typical and background-limited IR sensors operating in the 3-5-microm and 8-12-microm wavebands in a tropical maritime environment. Allowance is made for slant paths and a variety of targets and backgrounds including hot targets and backgrounds spectrally different from the targets. It is found that with current detector technology the 8-12-microm waveband is superior for all except very hot targets at long ranges. The validity of various approximations is also investigated, and in particular it is found that the blackbody composite background approximation should not be used for slant paths.

Airborne Doppler lidar investigation of the wind-modulated sea-surface angular retroreflectance signature

Concurrent measurements of sea-surface retroreflectance and associated wind velocity acquired with an airborne CO2 Doppler lidar are described. These observations provide further insight into thermal infrared optical phenomenology of air-sea interface processes, contribute to a greater understanding of radiation transfer between the atmosphere and the hydrosphere, and enable improved models of wind-driven ocean-surface stress applicable to other remote sensing applications. In particular, we present lidar measurements of azimuthally anisotropic reflectance behavior and discuss the implications to current understanding of sea-surface optical properties.

Atmospheric transmission measurements and predictions in the 2100-2300-cm(-1) region: comparison of LOWTRAN 6 and FASCOD models.

In recent years computation of atmospheric transmission has been important for several visible and IR windows. One such window is the 2000-2800-cm spectral region which spans the 2325-cm absorption band of CO2 and is important with respect to the spectral shape of the two windows on either side of 2325 cm for broadband transmission calculations. CO2 absorption in this region arises from strong and weak line absorption as well as a N2-CO2 collision-broadening continuum absorption and absorption arising from local water H2O and N2 continuum. In the CO2 high-frequency wing, absorption is dominated by an underlying continuum which is the accumulated effect from the wings of strong lines occurring at a lower frequency superimposed on the weaker CO2 line spectrum. This has been the subject of recent study into the line shape factors of the CO2 wing for self-broadening and foreign-broadening. For the low-frequency wing (2100-2300 cm) there is no clear separation between strong and weak lines, which makes it much more difficult to investigate line shape factors, and consequently predictions for this region are less certain. In view of the uncertainty in the 2100-2300-cm CO2 farwing absorption region, broadband transmission calculations for the 2100-2300-cm region using both the LOWTRAN 6 and FASCOD (version 1C) atmospheric transmission computer programsare compared in this Letter to experimental data. Furthermore, the shape of the predicted spectral window is compared to some high-resolution field spectra (degraded to a resolution of 20 cm) obtained by the Naval Research Laboratory (NRL). These comparisons have revealed significant discrepancies between the predictions of LOWTRAN 6 and degraded FASCOD 1C with the latter providing much better agreement with the experimental data. In the first case LOWTRAN 6 and FASCOD 1C transmission data were convolved with the source, filter, and detector response curves from the transmissometer. (The filter half-power points were 2180 and 2295 cm, and for all FASCOD calculations used in this work the monochromatic spectral data were degraded with a triangular slit function whose halfwidth at half-maximum was 10 cm.) The predicted data were then compared with experimental absolute transmission data gathered over a horizontal path at two coastal environments. The experimental and analysis details have been described elsewhere. Again, the Navy aerosol model was employed. However, since FASCOD 1C does not contain aerosol models, the values predicted by the Navy aerosol model in LOWTRAN 6 were used and easily incorporated as the data were almost constant over the spectral region considered (<1% variation). Figures 1(a), (b), and (c) reproduce the experimental data for ranges of 5, 7, and 9 km, respectively. In each figure the LOWTRAN 6 and FASCOD 1C predictions are shown as dashed and solid curves, respectively. Clearly in all three cases the FASCOD 1C prediction agrees better with the experimental data. (A very similar result was obtained for those data not

Radiometric calibration of an airborne CO2 pulsed Doppler lidar with a natural earth surface.

Radiometric calibration of an airborne CO2 pulsed Doppler lidar has been accomplished with surface retroreflection signals from the White Sands National Monument, New Mexico. Two circular passes were made at altitudes of 6.3 and 9.3 km. The computed calibration factors for both altitudes are in excellent agreement with the value derived from standard ground-based measurements involving a fixed sandpaper target of known reflectance. This finding corroborates a previous study that successfully calibrated an airborne cw Doppler lidar with a variety of natural Earth surfaces. The present results indicate that relatively uniform Earth surface targets can be used for in-flight calibration of CO2 pulsed airborne and, in principal, other infrared lidars.