We present first results from the newly developed remote sensing instrument CLEO (CLear Sky Observatory). CLEO
consists of a commercially available CCD miniature spectrometer (Hamamatsu C10082CAH) and foreoptics to measure
the global and diffuse solar irradiance. The irradiance is measured through a teflon diffuser. The diffuse irradiance is
obtained moving a 180° metal band in the optical path to block the solar direct beam. CLEO measures simultaneously
UV and Visible radiation from 163nm to 845nm, in steps of 0.3nm with a resolution of 1nm. The spectrometer is
temperature controlled to 10°C to stabilize its optical properties. The dark count is frequently measured using a
motorized four positions filterwheel with an opaque disc at one position that acts as a shutter used to block the light
input. The system automatically adjusts the integration time to optimize the signal-to-noise. Another difference to
previous shadowband instruments is that CLEO moves the shadowband over the whole hemisphere instead of just a few
positions in and around the sun's direction. This has the advantage of simplifying the installation procedure and solves
the problem with the shadow only partially covering the diffuser due to instrument misalignment.
O3, SO2 and NO2 vertical column amounts and aerosol optical depths at 18 wavelengths from 303 to 363nm were measured daily for the past two years by using a Brewer MK3 double spectrometer in direct-sun mode. The algorithms used are described and compared to the standard algorithm. For O3 and SO2 the standard algorithm was modified by using all 6 measured wavelengths instead of only 4 or 5 and by including a correction for the diffuse irradiance entering the instrument's field of view. This reduces the statistical error of the retrievals by 40% and 50%, respectively, for O3 and SO2. The NO2 retrievals are based on a spectral fitting technique using wavelengths between 349 and 363nm. A 'Bootstrap method' has been developed to calibrate the Brewer for NO2 measurements. This method selects data with lowest NO2-amounts and uses them to derive the needed extraterrestrial solar spectrum. We discuss possible reasons why an intent to apply to same technique for deriving total HCHO columns failed. The main advantage of direct sun methods compared to Differential Optical Absorption Spectroscopy DOAS is that the uncertainty in the air mass factor is significantly smaller. We think that a better correction for the diffuse irradiance entering the instrument's field of view will further improve the retrievals, especially in the low wavelength range (303 to 320nm) used for O3 and SO2.
A significant database of simultaneous measurements of NO2 column amounts and aerosol optical properties has recently become available that permits partitioning between aerosol and gaseous absorption. The aerosol column absorption optical thickness, (AAOT) was inferred from the measurements of global and diffuse atmospheric transmittances by a UV-Multifilter Rotating Shadowband radiometer (UV-MFRSR), calibrated using AERONET CIMEL sun-sky radiometers. The NO2 column amounts were measured using a double-Brewer MK III spectrometer (#171) operated in direct-sun mode using a new 6-wavelength retrieval algorithm. Ancillary measurements of column particle size distribution and refractive index in the visible wavelengths (by AERONET sun-sky almucantar inversions), ozone (by Brewer) and surface pressure constrained the forward radiative transfer model input, so that a unique solution for AAOT was obtained in each UV-MFRSR spectral channel. In fall-winter months with typically dry conditions and low aerosol loadings, the NO2 absorption represented a significant source of error in aerosol absorption measurements. This was confirmed by UV-MFRSR AAOT retrievals at 325nm, where the NO2 absorption cross-section is only half the value at 368nm. Thus, the NO2 correction not only reduces AAOTs obtained from traditional aerosol remote sensing techniques (shadowband or Cimel sunphotometer), but also is capable of changing the spectral dependence of aerosol absorption, which could result in an incorrect interpretation of aerosol composition. To further confirm these findings, a new UV-MFRSR instrument was modified by adding a 440 nm channel to provide spectral overlap with AERONET AAOT inversions in the visible wavelengths.
The polarization sensitivity of a Brewer MKIII double spectrophotometer was measured in the laboratory. We found two major sources of polarization sensitivity. 1) The flat quartz plate as the first optical element alters the polarization state of the transmitted light by Fresnel reflection at oblique incident angles. 2) The internal grating produces almost 100% polarization of the incident light perpendicular to the direction of the ruled grating. The combination of both effects results in a zenith angle (ZA) dependence of the instrument’s sensitivity for unpolarized input such as from Direct Sun measurements. The Brewer is 2% more sensitive at ZA=0° and 10% less sensitive at ZA=80° with respect to normal incidence (ZA=35°). Since the ZA-dependence is independent of wavelength this effect cancels out when calculating wavelength-ratios as used for total ozone retrieval. However the ZA-dependence causes errors when absolute signals at single wavelengths are needed as for aerosol optical depth (AOD) retrievals. Based on our laboratory measurements an overestimation of the Langley extrapolation between 3% and 5% is estimated even at best atmospheric conditions. The ZA-dependence causes 0.025-0.045 overestimation of AOD and an underestimation of the Angstrom exponent. We believe that this effect has not been detected from Brewer AOD-measurements since it is masked by larger uncertainty sources of other nature and AOD-comparisons to other instruments in the short UV-region are rare. Knowing the ZA-dependence it is possible to correct for the ZA-effect. We modified our Brewer by incorporating a depolarizer in its optical path and replacing the flat quartz window by a curved one, so that the input is always at normal incidence, which reduces the ZA-effect.
A Brewer MKIII double spectrophotometer has been modified to measure direct sun and sky radiance from 303nm to 363nm for the purpose of measuring aerosol optical depth, Angstrom parameter, and single scattering albedo. Results from a detailed instrument calibration showed that there is a temperature dependence of -0.3% per degree Celsius, the field of view was 2.6° full width half maximum, and the wavelength calibration was accurately determined using a dye-LASER. Using both integrating sphere and lamp-diffuser plate combinations, absolute diffuse radiometric calibration was performed and converted into direct calibration using the measured field of view. Aerosol optical depth and Angstrom parameter were measured on 4 clear sky days in June 2003 at Greenbelt, Maryland and compared to AERONET-data at the same location. The average difference in the aerosol optical depth at 340nm was smaller than 0.02. A depolarizing element was inserted in the Brewer's optical path to reduce the very pronounced polarization sensitivity, and additional polarized filters were added to explore the possibility to obtain additional aerosol information. Because of a defect in the depolarizer, the current residual polarization is 5%, which has to be reduced to less than 1% to derive additional aerosol parameters from sky radiance measurements.