Radiative transfer. II. Impact of meteorological variables and surface albedo on atmospheric optical properties retrieved from ground-based multispectral measurements

In a companion paper we describe a radiative transfer model and a consequent algorithm for retrieving atmospheric variables from ground-based multispectral measurements of direct solar irradiance. The accuracy of retrieved data depends on measured spectral irradiance as well as surface meteorological variables. Here we analyze the impact of the surface albedo on diffuse scattered solar irradiance in the Sun-sensor direction. We also investigate the impact of visibility on the retrieved spectral transmission function and optical thickness. We discuss the application of a spectrometric system, the passive pyrheliometric scanner (PPS), for the estimation of atmospheric turbidity and visibility. The spectral transmission of the atmosphere derived with the PPS for the Athens atmosphere and for different zenith angles is given. We present results of retrieved aerosol optical properties using as atmospheric turbidity those values estimated from the ground-based measurements of direct solar radiation with the aid of the PPS. It is shown that another application of the PPS may be the estimation of horizontal visibility.     

Celestial body irradiance determination from an underfilled satellite radiometer: application to albedo and thermal emission measurements of the Moon using CERES

The Clouds and the Earth's Radiant Energy System (CERES) is a program that measures the Earth radiation budget (ERB) from two polar orbiting satellite platforms. CERES radiometers are designed to make stable broadband measurements of scattered solar and emitted thermal radiative flux leaving Earth with an accuracy of 1% or better. Using versatile and programmable scan modes, it is also possible for every CERES instrument to view the Moon on each orbit. However, until now, it has not been possible to derive absolute measurements of lunar irradiance using CERES because the Moon's disk fills only 10% of the telescope field of view. This work presents a method of integrating CERES raster-scan data in order to obtain a measurement of the average scattered solar and emitted thermal radiance from the entire lunar disk. The technique results in excellent agreement between CERES instruments on different satellites as to lunar albedo and emitted thermal flux. The average broadband Moon albedo is measured by CERES at a value of 0.1362 (+/-2-3%) when normalized to a static lunar phase angle of 7 degrees using the U.S. Geological Survey lunar irradiance Robotic Lunar Observatory model. The method for the first time also yields very accurate measurements of the thermal irradiance emitted from the Moon. These suggest an average long-wave flux of 977 Wm(-2) (+/-2-3% at 7 degrees phase), implying an approximate mean surface temperature of around 92 degrees C. Statistical analysis on available data suggests that a CERES instrument performing monthly lunar measurements could utilize the Moon as a stability target and reduce calibration drifts to 0.3% per decade or less within an instrument's lifetime. Given the success of the technique, a solar calibration system is proposed that will allow precise tracking of an ERB instrument's optical degradation using the Sun.     

Absolute spectral measurements of direct solar ultraviolet irradiance with a Brewer spectrophotometer

A methodology for the absolute calibration of spectral measurements of direct solar ultraviolet radiation, performed with a Brewer spectrophotometer is presented. The method uses absolute measurements of global and diffuse solar irradiance obtained practically simultaneously at each wavelength with the direct-Sun component. On the basis of this calibration, direct-Sun spectra, measured over a wide range of solar zenith angles at a high altitude site, were used to determine the extraterrestrial solar spectrum by applying the Langley extrapolation method. Finally this spectrum is compared with a solar spectrum derived from the airborne tunable laser absorption spectrometer 3 Space Shuttle mission, showing an agreement of better than +/-3%.     

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.     

Separation and analysis of forward scattered power in laboratory measurements of light beam transmittance through a turbid medium

In laboratory measurements of the transmittance of a light beam through a diffusing medium (water plus latex spheres), a distinction between the attenuated beam power and the received forward scattered power was made possible by the use of a transmissometer whose receiver has a variable field of view. The dependence of the received scattered power on the FOV angle and on the medium optical depth was analyzed. The deduced separated contributions of first- and second-order scattering, as well as the total received scattered power, were compared to the results of calculations.     

Concurrent multiaxis differential optical absorption spectroscopy system for the measurement of tropospheric nitrogen dioxide

The development of a new concurrent multiaxis (CMAX) sky viewing spectrometer to monitor rapidly changing urban concentrations of nitrogen dioxide is detailed. The CMAX differential optical absorption spectroscopy (DOAS) technique involves simultaneous spectral imaging of the zenith and off-axis measurements of spatially resolved scattered sunlight. Trace-gas amounts are retrieved from the measured spectra using the established DOAS technique. The potential of the CMAX DOAS technique to derive information on rapidly changing concentrations and the spatial distribution of NO2 in an urban environment is demonstrated. Three example data sets are presented from measurements during 2004 of tropospheric NO2 over Leicester, UK (52.62 degrees N, 1.12 degrees W). The data demonstrate the current capabilities and future potential of the CMAX DOAS method in terms of the ability to measure real-time spatially disaggregated urban NO2.     

Effect of topography on average surface albedo in the ultraviolet wavelength range

The reflectivity of the 22 km x 24 km region surrounding Sonnblick Observatory near Salzburg, Austria (3104-m altitude, 47.05 degrees N, 12.95 degrees E), was calculated with a three-dimensional albedo model. The average albedo of the region was calculated at 305 and 380 nm for different solar zenith angles, ground reflectances, and solar azimuth angles. To determine geometrical effects, we first carried out the simulations without taking account of the effects of the atmosphere. The ratio to the reflectivity of a corresponding flat surface area (area with the same ground characteristics) was always less than 1 and showed a decrease with increasing solar zenith angle and with diminishing ground reflectance. Even when the ground reflectance was 100%, the average albedo was less than 1. The effect of the atmosphere was then taken into consideration in these calculations and was found to diminish the reflected components. This diminishing effect was compensated for, however, by the scattered irradiance. Finally, simulations of real conditions (nonhomogeneous ground reflectivities) were performed for different snow lines in the Sonnblick region. The average albedos obtained when all the surroundings were covered with snow were 0.32-0.63 with low solar zenith angles and 0.38-0.77 with a 40 degrees solar zenith angle.     

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).     

Reduction of skylight reflection effects in the above-water measurement of diffuse marine reflectance: comment

Use of a vertical polarizer has been suggested to reduce the effects of surface reflection in the above-water measurements of marine reflectance. We suggest using a similar technique for airborne or spaceborne sensors when atmospheric scattering adds its own polarization signature to the upwelling radiance. Our own theoretical sensitivity study supports the recommendation of Fougnie et al. [Appl. Opt. 38, 3844 (1999)] (40-50 degrees vertical angle and azimuth angle near 135 degrees , polarizer parallel to the viewing plane) for above-water measurements. However, the optimal viewing directions (and the optimal orientation of the polarizer) change with altitude above the sea surface, solar angle, and atmospheric vertical optical structure. A polarization efficiency function is introduced, which shows the maximal possible polarization discrimination of the background radiation for an arbitrary altitude above the sea surface, viewing direction, and solar angle. Our comment is meant to encourage broader application of airborne and spaceborne polarization sensors in remote sensing of water and sea surface properties.     

Influence of crystal tilt on solar irradiance of cirrus clouds

The single and multiple scattering and absorption properties of hexagonal ice columns with different degrees of particle orientation are modeled in the solar spectral range by means of a ray-tracing single-scattering code and a Monte Carlo radiative-transfer code. The scattering properties are most sensitive to particle orientation for the solar zenith angles of 50 degrees (asymmetry parameter) and 90 degrees (single-scattering albedo). Provided that the ice columns are horizontally oriented, the usual assumption of random orientation leads to an overestimation (underestimation) of the reflected (transmitted) solar broadband radiation at high Sun elevation and to an underestimation (overestimation) at medium solar zenith angles. The orientation effect is more (less) pronounced in scattering and transmission (absorption) for smaller ice crystals.     

Rotational Raman scattering (Ring effect) in satellite backscatter ultraviolet measurements

A detailed radiative transfer calculation has been carried out to estimate the effects of rotational Raman scattering (RRS) on satellite measurements of backscattered ultraviolet radiation. Raman-scattered light is shifted in frequency from the incident light, which causes filling in of solar Fraunhofer lines in the observed backscattered spectrum (also known as the Ring effect). The magnitude of the rotational Raman scattering filling in is a function of wavelength, solar zenith angle, surface reflectance, surface pressure, and instrument spectral resolution. The filling in predicted by our model is found to be in agreement with observations from the Shuttle Solar Backscatter Ultraviolet Radiometer and the Nimbus-7 Solar Backscatter Ultraviolet Radiometer.     

Twilight polarization and optical depth of stratospheric aerosols over Beijing after the Pinatubo volcanic eruption

After the volcanic eruption of Mt. Pinatubo the degree of polarization of skylight during twilight over Beijing was monitored with a polarimeter aimed at the local zenith. We analyze the effect of changes in the scattering coefficient of atmospheric aerosols for the case of multiple scattering on skylight polarization at the zenith and then discuss the evolution of skylight polarization over Beijing during the posteruption period. As a reference and for comparison we also discuss the evolution of the aerosol optical depth retrieved from the combination of skylight polarization and backscattering ratio measured by the polarimeter and a lidar for the period beginning with the eruption of Mt. Pinatubo through the end of 1993. The contributions of atmospheric aerosols at different altitudes to the ground-observed twilight polarization depend on the solar zenith angle. For larger solar zenith angles, the skylight polarization is mostly sensitive to aerosol variations in the upper layer that range from 15 to 30 km. The twilight polarization at the zenith from June 1991 to mid-1994 shows different features for three periods: (1) From October 1991 to February 1992, volcanic dust traveled to mid-latitudes, and the degree of polarization decreased substantially. (2) From February 1992 to November 1993, volcanic dust was dispersed the minimum degree of polarization at the solar zenith angle of 93.5 degrees disappeared and the maximum increased. In addition, polarization for solar zenith angles less than 90 degrees also increased. (3) From November 1993 to May 1994, most of the volcanic dust had fallen off, the atmosphere was restored to the background state, and the skylight polarization approached the preeruption condition.     

Variability of the downwelling diffuse attenuation coefficient with consideration of inelastic scattering

In situ time-series measurements of spectral diffuse downwelling irradiance from the Bermuda Testbed Mooring are presented. Averaged diffuse attenuation coefficients of downwelling irradiance, Kd,and their elastic and inelastic components are investigated at seven wavelengths. At shorter wavelengths (<510 nm), Kd is weakly dependent on the solar zenith angle owing to the prevailing scattering effect and therefore can be considered a quasi-inherent optical property. At longer wavelengths (>510 nm), Kd shows a strong dependence on the solar zenith angle. As depth increases, inelastic scattering plays a greater role for the underwater light field at red wavelengths.     

Retrieval of profile information from airborne multiaxis UV-visible skylight absorption measurements

A recent development in ground-based remote sensing of atmospheric constituents by UV-visible absorption measurements of scattered light is the simultaneous use of several horizon viewing directions in addition to the traditional zenith-sky pointing. The different light paths through the atmosphere enable the vertical distribution of some atmospheric absorbers, such as NO2, BrO, or O3, to be retrieved. This approach has recently been implemented on an airborne platform. This novel instrument, the airborne multiaxis differential optical absorption spectrometer (AMAXDOAS), has been flown for the first time. In this study, the amount of profile information that can be retrieved from such measurements is investigated for the trace gas NO2. Sensitivity studies on synthetic data are performed for a variety of representative measurement conditions including two wavelengths, one in the UV and one in the visible, two different surface spectral reflectances, various lines of sight (LOSs), and for two different flight altitudes. The results demonstrate that the AMAXDOAS measurements contain useful profile information, mainly at flight altitude and below the aircraft. Depending on wavelength and LOS used, the vertical resolution of the retrieved profiles is as good as 2 km near flight altitude. Above 14 km the profile information content of AMAXDOAS measurements is sparse. Airborne multiaxis measurements are thus a promising tool for atmospheric studies in the troposphere and the upper troposphere and lower stratosphere region.     

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.     

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.     

Charge-coupled device spectrograph for direct solar irradiance and sky radiance measurements

The characterization of a charged-coupled device (CCD) spectrograph developed at the Laboratory of Atmospheric Physics, Thessaloniki is presented. The absolute sensitivity of the instrument for direct irradiance and sky radiance measurements was determined, respectively, with an uncertainty of 4.4% and 6.6% in the UV-B, and 3% and 6% in the UV-A, visible and near-infrared (NIR) wavelength ranges. The overall uncertainty associated with the direct irradiance and the sky radiance measurements is, respectively, of the order of 5% and 7% in the UV-B, increasing to 10% for low signals [e.g., at solar zenith angles (SZAs) larger than 70 degrees ], and 4% and 6% in the UV-A, visible, and NIR. Direct solar spectral irradiance measurements from an independently calibrated spectroradiometer (Bentham DTM 300) were compared with the corresponding CCD measurements. Their agreement in the wavelength range of 310-500nm is within 0.5% +/- 1.1% (for SZA between 20 degrees and 70 degrees ). Aerosol optical depth (AOD) derived by the two instruments using direct Sun spectra and by a collocated Cimel sunphotometer [Aerosol Robotic network (AERONET)] agree to within 0.02 +/- 0.02 in the range of 315-870 nm. Significant correlation coefficients with a maximum of 0.99 in the range of 340-360 nm and a minimum of 0.90 at 870 nm were found between synchronous AOD measurements with the Bentham and the Cimel instruments.     

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.     

Underwater light polarization and radiance fluctuations induced by surface waves

The underwater light field is an ever-changing environment. Surface waves induce variability in the radiance and the light's polarization. We examined the dependence of the polarization fluctuations associated with diffuse light (not including contribution from direct skylight) on the viewing zenith angle (30 degrees, 70 degrees, and 90 degrees), solar zenith angle (23 degrees -72 degrees), depth of 0.5-3 m, and light wavelength (380-650 nm) while observing within the azimuthal plane in the wind-wave direction. Polarization and radiance fluctuated with time. Light variability (presented by the coefficient of variation calculated over a series of fluctuations in the radiance and percent polarization, and by the standard deviation calculated over a series of fluctuations in the e-vector orientation) was highest at a viewing zenith angle of 70 degrees , depended positively on the solar zenith angle, and decreased with depth at viewing zenith angles of 30 degrees and 70 degrees . Additionally, the variability of the percent polarization was significantly higher than that of the radiance. The temporal light fluctuations offer possibilities, such as enhancing the detection of transparent and reflecting objects; however, they set constraints on the optimal underwater polarization vision by both animals and by the use of instruments.     

Solar actinic flux spectroradiometry: a technique for measuring photolysis frequencies in the atmosphere.

A spectroradiometer has been developed for direct measurement of the solar actinic UV flux (scalar intensity) and determination of photolysis frequencies in the atmosphere. The instrument is based on a scanning double monochromator with an entrance optic that exhibits an isotropic angular response over a solid angle of 2pi sr. Actinic flux spectra are measured at a resolution of 1 nm across a range of 280-420 nm, which is relevant for most tropospheric photolysis processes. The photolysis frequencies are derived from the measured radiation spectra by use of published absorption cross sections and quantum yields. The advantage of this technique compared with the traditional chemical actinometry is its versatility. It is possible to determine the photolysis frequency for any photochemical reaction of interest provided that the respective molecular photodissociation parameters are known and the absorption cross section falls within a wavelength range that is accessible by the spectroradiometer. The instrument and the calibration procedures are described in detail, and problems specific to measurement of the actinic radiation are discussed. An error analysis is presented together with a discussion of the spectral requirements of the instrument for accurate measurements of important tropospheric photolysis frequencies (J(O(1))(D), J(NO(2)), J(HCHO)). An example of measurements from previous atmospheric chemistry field campaigns are presented and discussed.