Airborne system for fast measurements of upwelling and downwelling spectral actinic flux densities

An airborne system for fast measurements of spectral actinic flux densities in the wavelength range 305-700 nm is introduced. The system is called the Actinic Flux Density Meter (AFDM). The AFDM utilizes the diode array technique and measures downwelling and upwelling spectral actinic flux densities separately with a time resolution of less than 1 s. For airborne measurements this means a spatial resolution of approximately 60 m, assuming an average aircraft velocity of 60 m/s. Thus the AFDM resolves fast changes in the actinic radiation field, which are of special importance for conditions of inhomogeneous clouds or surface reflection. Laboratory characterization measurements of the AFDM are presented, and a method to correct the nonideal angular response of the optical inlets is introduced. Furthermore, exemplar field data sampled simultaneously with spectral irradiance measurements are shown. The horizontal variability of the measured spectra of actinic flux density is quantified, and profile measurements for overcast situations are presented. Finally, the effects of clouds on the spectral actinic flux density are discussed.     

All-weather ultraviolet solar spectra retrieved at a 0.5-Hz sampling rate

A measurement scheme and an algorithm have been developed to retrieve global irradiance ultraviolet solar spectra (290-400 nm) at a sampling rate of 0.5 Hz. The algorithm combines spectral irradiance measurements performed with a slow (a few minutes) scanning spectroradiometer (Optronic Model OL752) and a moderate bandwidth multichannel radiometer (Biospherical ground-based ultraviolet radiometer Model 541). The filter radiometer instrument allows for continuous observations of global UV radiation at five channels (approximately 10-nm bandwidth), performed simultaneously with spectral measurements. Information about changing cloud conditions during a spectral scan was retrieved from filter measurements and applied to spectral data, hence estimated spectra without cloud variations could be constructed. The quality of the estimated spectra depends on data quality from both instruments. The method works well in all kinds of weather conditions, as long as the Sun is above the horizon and none of the instruments are hampered by measurement errors.     

Direct solar spectral irradiance and transmittance measurements from 350 to 2500 nm

A radiometrically stable, commercially available spectroradiometer was used in conjunction with a simple, custom-designed telescope to make spectrally continuous measurements of solar spectral transmittance and directly transmitted solar spectral irradiance. The wavelength range of the instrument is 350-2500 nm and the resolution is 3-11.7 nm. Laboratory radiometric calibrations show the instrument to be stable to better than 1.0% over a nine-month period. The instrument and telescope are highly portable, can be set up in a matter of minutes, and can be operated by one person. A method of absolute radiometric calibration that can be tied to published top-of-the-atmosphere (TOA) solar spectra in valid Langley channels as well as regions of strong molecular absorption is also presented. High-altitude Langley plot calibration experiments indicate that this technique is limited ultimately by the current uncertainties in the TOA solar spectra, approximately 2-3%. Example comparisons of measured and modtran-modeled direct solar irradiance show that the model can be parameterized to agree with measurements over the large majority of the wavelength range to the 3% level for the two example cases shown. Side-by-side comparisons with a filter-based solar radiometer are in excellent agreement, with a mean absolute difference of tau = 0.0036 for eight overlapping wavelengths over three experiment days.     

Measurements of biologically effective UV doses, total ozone abundances, and cloud effects with multichannel, moderate bandwidth filter instruments

I describe a method to derive biologically effective UV dose rates, total ozone abundances, and cloud optical depths from irradiance measurements with moderate bandwidth filter instruments that have only a few channels in the UV region. These quantities are determined when the measured irradiances are combined with radiative transfer calculations. The method was applied to a four-channel filter instrument with center wavelengths at 305, 320, 340, and 380 nm and bandwidths of 10 nm. I compared the instrument with a high-wavelength-resolution spectroradiometer during a 1-week period in San Diego, California, with variable cloudiness. The relative difference in Commission Internationale de l'éclairage (CIE)-weighted UV dose rates for solar zenith angle's (SZA's) < 80° was 1.4 ± 3.2%. The relative difference for clear sky was 0.6 ± 1.5% for SZA's < 80°. The total ozone inferred from the irradiance measurements with the filter instrument is insensitive to clouds. The instrument was compared with a Dobson and a Brewer instrument in Oslo, Norway, 60°N, for more than 1 year. The relative difference in derived ozone abundance for the entire period, including cloudy days, was 0.3 ± 2.9%. The standard deviation was reduced to 1.9% when only clear sky and SZA's < 60° were included. By using the total ozone and the cloud optical depth derived from the filter instrument as input to a radiative transfer model, one can compute a complete spectrum from 290 to 400 nm with 1-nm resolution. Such calculated spectra are in good agreement with spectra measured simultaneously with a high-wavelength-resolution spectroradiometer for clear as well as cloudy sky conditions and can be used to determine dose rates for any desired action spectrum. Only one UV-B channel and one UV-A channel are required to compute the spectra.     

Off-axis measurements of atmospheric trace gases by use of an airborne ultraviolet-visible spectrometer

An airborne UV-visible spectrometer, the Gas Analyzer Spectrometer Correlating Optical Differences, airborne version (GASCOD/A4pi) was successfully operated during the Airborne Polar Experiment, Geophysica Aircraft in Antarctica airborne campaign from Ushuaia (54 degrees 49' S, 68 degrees 18' W), Argentina in southern spring 1999. The instrument measured scattered solar radiation through three optical windows with a narrow field of view (FOV), one from the zenith, two from the horizontal, as well as actinic fluxes through 2pi FOV radiometric heads. Only a few airborne measurements of scattered solar radiation at different angles from the zenith are available in the literature. With our configuration we attempted to obtain the average line-of-sight concentrations of detectable trace gases. The retrieval method, based on differential optical absorption spectroscopy, is described and results for ozone are shown and compared with measurements from an in situ instrument as the first method of validation.     

Ground-based zenith sky abundances and in situ gas cross sections for ozone and nitrogen dioxide with the Earth Observing System Aura Ozone Monitoring Instrument

High-accuracy spectral-slit-function calibration measurements, in situ ambient absorption gas cell measurements for ozone and nitrogen dioxide, and ground-based zenith sky measurements with the Earth Observing System Aura Ozone Monitoring Instrument (OMI) flight instrument are reported and the results discussed. For use of high-spectral-resolution gas absorption cross sections from the literature in trace gas retrieval algorithms, accurate determination of the instrument's spectral slit function is essential. Ground-based measurements of the zenith sky provide a geophysical determination of atmospheric trace gas abundances. When compared with other measurements, they can be used to verify the performance of the OMI flight instrument. We show that the approach of using published high-resolution absolute absorption cross sections convolved with accurately calibrated spectral slit functions for OMI compares well with in situ gas absorption cell measurements made with the flight instrument and that use of these convolved cross sections works well for reduction of zenith sky data taken with the OMI flight instrument for ozone and nitrogen dioxide that are retrieved from measured spectra of the zenith sky with the differential optical absorption spectroscopy technique, the same method to be used for the generation of in-flight data products. Finally, it is demonstrated that the spectral stability and signal-to-noise ratio performance of the OMI flight instrument, as determined from preflight component and full instrument tests, are sufficient to meet OMI mission objectives.     

Differential absorption lidar system for routine monitoring of tropospheric ozone

A differential absorption lidar system for routine profiling of tropospheric ozone for daytime and nighttime operation is described. The system uses stimulated Raman scattering in hydrogen and deuterium of 266-nm radiation from a quadrupled Nd:YAG laser. Ozone profiles from altitudes of 600 m to approximately 5 km have been obtained with analog detection. Implementing corrections for differential Rayleigh scattering, differential absorption from oxygen, sulphur dioxide, and nitrogen dioxide, and differential aerosol extinction and backscatter can reduce the total system inaccuracy to 5-15% for a clear day and 20-30% for a hazy day, except at the top of the mixed layer. Photon counting must be installed to increase the measurement range from 5 to 15 km. An example of an application of routine measurements of tropospheric ozone profiles is given.     

Monitoring O3 with solar-blind Raman lidars

The benefits of retrieving ozone concentration profiles by a use of a single Raman signal rather than the Raman differential absorption lidar (DIAL) technique are investigated by numerical simulations applied either to KrF- (248 nm) or to quadrupled Nd:YAG- (266 nm) based Raman lidars, which are used for both daytime and nighttime monitoring of the tropospheric water-vapor mixing ratio. It is demonstrated that ozone concentration profiles of adequate accuracy and spatial and temporal resolution can be retrieved under low aerosol loading by a single Raman lidar because of the large value of the ozone absorption cross section both at 248 nm and at 266 nm. Then experimental measurements of Raman signals provided by the KrF-based lidar operating at the University of Lecce (40 degrees 20'N, 18 degrees 6'E) are used to retrieve ozone concentration profiles by use of the Raman DIAL technique and the nitrogen Raman signal.     

Compact airborne lidar for tropospheric ozone: description and field measurements

An airborne lidar has been developed for tropospheric ozone monitoring. The transmitter module is based on a solid-state Nd:YAG laser and stimulated Raman scattering in deuterium to generate three wavelengths (266, 289, and 316 nm) that are used for differential ozone measurements. Both analog and photon-counting detection methods are used to produce a measurement range up to 8 km. The system has been flown on the French Fokker 27 aircraft to perform both lower tropospheric (0.5-4-km) and upper tropospheric (4-12-km) measurements, with a 1-min temporal resolution corresponding to a 5-km spatial resolution. The vertical resolution of the ozone profile can vary from 300 to 1000 m to accommodate either a large-altitude range or optimum ozone accuracy. Comparisons with in situ ozone measurements performed by an aircraft UV photometer or ozone sondes and with ozone vertical profiles obtained by a ground-based lidar are presented. The accuracy of the tropospheric ozone measurements is generally better than 10-15%, except when aerosol interferences cannot be corrected. Examples of ozone profiles for different atmospheric conditions demonstrate the utility of the airborne lidar in the study of dynamic or photochemical mesoscale processes that control tropospheric ozone.     

Multichannel moderate-bandwidth filter instrument for measurement of the ozone-column amount,cloud transmittance,and ultraviolet dose rates

A six-channel moderate-bandwidth filter instrument for measurement of UV and visible radiation has been developed. The characteristic of the instrument are described, including the spectral and the angular responses. Furthermore the calibration procedure is outlined. Combining information from several channels, one may determine the total ozone-column amount, various biological dose rates, a cloud transmission factor, and the effective cloud optical depth. The methods used to determine these parameters are presented, and the measured parameters are compared with similar ones obtained from other instruments. The total ozone as measured by the instrument agrees with measurements from a standard Brewer to -0.05% +/- 2.04% over a two-year period. Two weeks of cloudless Commission Internationale de L'Eclairage dose rates agree with those from a Bentham double monochromator spectroradiometer to 0.99 +/- 0.03.     

Atmospheric CO2 measurements with a 2 μm airborne laser absorption spectrometer employing coherent detection

We report airborne measurements of CO(2) column abundance conducted during two 2009 campaigns using a 2.05 μm laser absorption spectrometer. The two flight campaigns took place in the California Mojave desert and in Oklahoma. The integrated path differential absorption (IPDA) method is used for the CO(2) column mixing ratio retrievals. This instrument and the data analysis methodology provide insight into the capabilities of the IPDA method for both airborne measurements and future global-scale CO(2) measurements from low Earth orbit pertinent to the NASA Active Sensing of CO(2) Emissions over Nights, Days, and Seasons mission. The use of a favorable absorption line in the CO(2) 2 μm band allows the on-line frequency to be displaced two (surface pressure) half-widths from line center, providing high sensitivity to the lower tropospheric CO(2). The measurement repeatability and measurement precision are in good agreement with predicted estimates. We also report comparisons with airborne in situ measurements conducted during the Oklahoma campaign.     

Cloud Detection and Clearing for the Earth Observing System Terra Satellite Measurements of Pollution in the Troposphere (MOPITT) Experiment

The Measurements of Pollution in the Troposphere (MOPITT) instrument, which was launched aboard the Earth Observing System (EOS) Terra spacecraft on 18 December 1999, is designed to measure tropospheric CO and CH(4) by use of a nadir-viewing geometry. The measurements are taken at 4.7 mum in the thermal emission and absorption for the CO mixing ratio profile retrieval and at 2.3 and 2.2 mum in the reflected solar region for the total CO column amount and CH(4) column amount retrieval, respectively. To achieve the required measurement accuracy, it is critical to identify and remove cloud contamination in the radiometric signals. We describe an algorithm to detect cloudy pixels, to reconstruct clear column radiance for pixels with partial cloud covers, and to estimate equivalent cloud top height for overcast conditions to allow CO profile retrievals above clouds. The MOPITT channel radiances, as well as the first-guess calculations, are simulated with a fast forward model with input atmospheric profiles from ancillary data sets. The precision of the retrieved CO profiles and total column amounts in cloudy atmospheres is within the expected ?10% range. Validations of the cloud-detecting thresholds with the moderate-resolution imaging spectroradiometer airborne simulator data and MOPITT airborne test radiometer measurements were performed. The validation results showed that the MOPITT cloud detection thresholds work well for scenes covered with more than 5-10% cloud cover if the uncertainties in the model input profiles are less than 2 K for temperature, 10% for water vapor, and 5% for CO and CH(4).     

Estimation of ozone with total ozone portable spectroradiometer instruments. II. Practical operation and comparisons

We used a microcomputer-controlled total ozone portable spectroradiometer instrument #21 (MTOPS21) to measure solar radiation at 298, 304 and 310 nm in Greenbelt, Md., during 1995. One day's ozone measurements from a Brewer instrument (B105) were used to calibrate the 304- and 310-nm channel ratios to a theoretical model. Total ozone estimates were then determined for the entire MTOPS21 data set. Differences between individual B105 and MTOPS21 ozone estimates show a 1% drop as solar zenith angles increase and depend on atmospheric attenuation and SO(2) variation at the ±2% level. Daily average values agree well (<0.5% average offset, 2% standard deviation).     

OH absorption spectroscopy in a flame using spatial heterodyne spectroscopy

We demonstrate measurements of OH absorption spectra in the post-flame zone of a McKenna burner using spatial heterodyne spectroscopy (SHS). SHS permits high-resolution, high-throughput measurements. In this case the spectra span approximately 308-310 nm with a resolution of 0.03 nm, even though an extended source (extent of approximately 2x10(-7) m(2) rad(2)) was used. The high spectral resolution is important for interpreting spectra when multiple absorbers are present for inferring accurate gas temperatures from measured spectra and for monitoring weak absorbers. The present measurement paves the way for absorption spectroscopy by SHS in practical combustion devices, such as reciprocating and gas-turbine engines.     

Traveling reference spectroradiometer for routine quality assurance of spectral solar ultraviolet irradiance measurements

A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelength and the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visits to two European UV monitoring sites as well as by regular comparisons with the reference spectroradiometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectral solar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from 2002 to 2004. If the differences in irradiance scales of as much as 2% are taken into account, the agreement is of the order of 4% over the wavelength range of 300-400 nm.     

Spectrophotometer for noninvasive measurement of intrinsic fluorescence and reflectance of the ocular fundus

A spectrophotometer for noninvasively measuring the intrinsic fluorescence and the reflectance of the ocular fundus is described. The instrument uses multichannel spectral analysis for recording fluorescence emission spectra (500-800 nm) with seven excitation wavelengths between 430 and 550 nm and for the determination of fundus reflectance spectra (400-800 nm). Measurements are performed from a discrete fundus area, with a spatial resolution of a 1-2° visual angle. Calibration procedures are detailed. Representative fluorescence and reflectance spectra obtained from five normal subjects indicate that the fluorescence originates from within the fundus layers. Although the absolute fundus fluorescence measurement is affected by lens absorption and ocular refraction, it is minimally influenced by the strong fluorescence of the crystalline lens.     

High-Resolution Fourier-Transform Ultraviolet-Visible Spectrometer for the Measurement of Atmospheric Trace Species: Application to OH

A compact, high-resolution Fourier-transform spectrometer for atmospheric near-ultraviolet spectroscopy has been installed at the Jet Propulsion Laboratory's Table Mountain Facility (34.4 degrees N, 117.7 degrees W, elevation 2290 m). This instrument is designed with an unapodized resolving power near 500,000 at 300 nm to provide high-resolution spectra from 290 to 675 nm for the quantification of column abundances of trace atmospheric species. The measurement technique used is spectral analysis of molecular absorptions of solar radiation. The instrument, accompanying systems designs, and results of the atmospheric hydroxyl column observations are described.     

Sun and aureole spectrometer for airborne measurements to derive aerosol optical properties

We have designed an airborne spectrometer system for the simultaneous measurement of the direct Sun irradiance and aureole radiance. The instrument is based on diffraction grating spectrometers with linear image sensors. It is robust, lightweight, compact, and reliable, characteristics that are important for airborne applications. The multispectral radiation measurements are used to derive optical properties of tropospheric aerosols. We extract the altitude dependence of the aerosol volume scattering function and of the aerosol optical depth by using flight patterns with descents and ascents ranging from the surface level to the top of the boundary layer. The extinction coefficient and the product of single scattering albedo and phase function of separate layers can be derived from the airborne measurements.     

A spectrometer for double-differential neutron-emission cross section measurements in the energy range 1.6 to 16 MeV

A neutron spectrometer for the measurement of double-differential neutron-emission cross sections has been set up.An electron linac (GELINA) is used as a pulsed white neutron source. The energy of the incident neutrons is determined by the time-of-flight method. The secondary neutron spectra are determined by unfolding the pulse-height distributions observed in eight NE213-scintillators surrounding the sample.The measured spectra are normalised to the shape of the incident neutron flux measured with a ²³⁵U-fission chamber, and afterwards converted to absolute cross sections using as standard the carbon differential elastic scattering cross section below 2 MeV.