Showing papers by "Nickolay A. Krotkov published in 2005"

Aerosol ultraviolet absorption experiment (2002 to 2004), part 2: absorption optical thickness, refractive index, and single scattering albedo

Abstract: Compared to the visible spectral region, very little is known about aerosol absorption in the UV. Without such information it is impos- sible to quantify the causes of the observed discrepancy between mod- eled and measured UV irradiances and photolysis rates. We report re- sults of a 17-month aerosol column absorption monitoring experiment conducted in Greenbelt, Maryland, where the imaginary part of effective refractive index k was inferred from the measurements of direct and diffuse atmospheric transmittances by a UV-multifilter rotating shadow- band radiometer (UV-MFRSR, U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network). Colocated ancillary measure- ments of aerosol effective particle size distribution and refractive index in the visible wavelengths (by CIMEL sun-sky radiometers, National Aero- nautics and Space Administration (NASA) Aerosol Robotic Network (AERONET)), column ozone, surface pressure, and albedo constrain the forward radiative transfer model input, so that a unique solution for k is obtained independently in each UV-MFRSR spectral channel. Inferred values of k are systematically larger in the UV than in the visible wave- lengths. The inferred k values enable calculation of the single scattering albedo v, which is compared with AERONET inversions in the visible

Assessment of TOMS UV bias due to absorbing aerosols

Abstract: [1] We compared NASA Total Ozone Mapping Spectrometer (TOMS) overpass irradiances against ground-based Brewer measurements at Ispra, Italy, and Thessaloniki, Greece, with the main objective of evaluating the effect of absorbing aerosols on TOMS UV bias using direct measurements of aerosol optical properties. Dependence of the TOMS/Brewer bias on aerosol absorption optical depth is significant. Our study demonstrates that the bias between TOMS and measured Brewer 324 nm irradiances increases with increasing aerosol absorption optical depth τabs. The correlation coefficients between the ratio TOMS/Brewer and τabs at Ispra and Thessaloniki are ∼0.7 or more and ∼0.8 or more, respectively, depending on the range of solar zenith angle values selected for the analysis. We found that the correlation with single-scattering albedo ω or aerosol optical depth is significantly smaller than with τabs. If measurements or climatology of τabs are available in the UVB range, the TOMS UV product can be postcorrected for absorbing aerosols using similar site-dependent regressions as established in our study. If no correction is applied, the mean positive biases between TOMS and Brewer 324 nm irradiances in Thessaloniki and Ispra are 19.2% and 29.7%, respectively, while the standard deviations are 8.9% and 16.5%. Depending on the level of correction, the mean positive bias can be essentially removed, and the standard deviations can be reduced down to ∼6% and ∼10%, respectively.

Aerosol ultraviolet absorption experiment (2002 to 2004), part 1: ultraviolet multifilter rotating shadowband radiometer calibration and intercomparison with CIMEL sunphotometers

Abstract: Radiative transfer calculations of UV irradiance from total ozone mapping spectrometer (TOMS) satellite data are frequently over- estimated compared to ground-based measurements because of the presence of undetected absorbing aerosols in the planetary boundary layer. To reduce these uncertainties, an aerosol UV absorption closure experiment has been conducted at the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) site in Greenbelt, Maryland, using 17 months of data from a shadowband radi- ometer (UV-multifilter rotating shadowband radiometer (UV-MFRSR), U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network) colocated with a group of three sun-sky CIMEL radiometers (rotating reference instruments of the NASA Aerosol Robotic Network (AERONET)). We describe an improved UV-MFRSR on-site calibration method augmented by AERONET-CIMEL measurements of aerosol ex- tinction optical thickness (t a) interpolated or extrapolated to the UV- MFRSR wavelengths and measurement intervals. The estimated t a is used as input to a UV-MFRSR spectral-band model, along with indepen- dent column ozone and surface pressure measurements, to estimate zero air mass voltages V0 in three longer wavelength UV-MFRSR chan- nels (325, 332, 368 nm). Daily mean ^V0&, estimates and standard de- viations are obtained for cloud-free conditions and compared with the on-site UV-MFRSR Langley plot calibration method. By repeating the calibrations on clear days, relatively good stability (62% in ^V0& )i s found in summer, with larger relative changes in fall-winter seasons. The changes include systematic day-to-day ^V0& decline for extended peri- ods along with step jump changes after major precipitation periods (rain or snow) that affected the diffuser transmission. When daily ^V0& values are used to calculate t a for individual 3-min UV-MFRSR measurements on the same days, the results compare well with interpolated AERONET t a measurements (at 368 nm most daily 1s root mean square (rms) differences were within 0.01). When intercalibrated against an AERO- NET sunphotometer, the UV-MFRSR is proven reliable to retrieve t a , and hence can be used to retrieve aerosol column absorption in the UV. The advantage of the shadowband technique is that the calibration ob- tained for direct-sun voltage can then be applied to diffuse-radiance volt- age to obtain total and diffuse atmospheric transmittances. These trans- mittances, in combination with accurate t a data, provide the basis for estimating aerosol column absorption at many locations of the USDA UV-B Monitoring and Research network and for correction of satellite estimations of surface UV irradiance. © 2005 Society of Photo-Optical Instru-

Long-term UV irradiance changes over Moscow and comparisons with UV estimates from TOMS and METEOSAT

Abstract: We analyzed long-term variations of UV irradiance 300-380 nm over Moscow 55.7N, 37.5E since 1968 using a complex dataset that includes ground-based UV measurements, UV retrievals from two satellites, and the results of a previously developed empirical model. Long-term interannual changes of UV irradiance, 300-380nm, during 1968-2003 show the absence of any linear trends although an increase is detected in the late 90-s due to cloud amount and aerosol content decrease. The ground-based data are compared with UV satellite retrievals from two independent methods as well as with the results of an empirical model that accounts for the physical dependence of UV on cloud parameters (amount and optical thickness), surface albedo, total ozone, and aerosol properties of the atmosphere. UV datasets over Moscow obtained from different satellite instruments: from the Total Ozone Mapping Spectrometer (TOMS) data (version 8) since 1979 and from METEOSAT/MVIRI since 1984. The original METEOSAT processor, using visibility observations at a nearby meteorological station to quantify the aerosol load, leads to a significant underestimation of the UV daily doses (-23% in warm period and -31% in cold period). Substituting the visibility observations by in situ monthly mean aerosol optical depth improves significantly the agreement in both warm and cold periods (respectively, -9% and -10%) but the bias still remains. The difference between TOMS UV retrievals and ground-based data has different signs in warm (+6%) and cold (-15%) periods. Applying off-line absorbing aerosol correction in TOMS UV retrievals eliminates the positive bias in warm period. The negative bias during the cold period can be due to the application of minimum Lambertian effective reflectivity (MLER) approach to determine the surface albedo especially in conditions with non stable snow cover (end of February- March, and November-December). Model reconstruction of UV variability demonstrates high correlation with aerosol corrected satellite UV retrievals (0.83-0.94) as well as with ground data (0.82) during warm period. During cold months the correlation between satellite UV retrievals and ground-based measurements is much worse.

Partitioning between aerosol and NO 2 absorption in the UV spectral region

Abstract: A significant database of simultaneous measurements of NO 2 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 NO 2 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 NO 2 absorption represented a significant source of error in aerosol absorption measurements. This was confirmed by UV-MFRSR AAOT retrievals at 325nm, where the NO 2 absorption cross-section is only half the value at 368nm. Thus, the NO 2 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.

UV aerosol optical properties at three US sites

Abstract: Aerosol amount (AOD) and optical properties single scattering albedo (SSA) and asymmetry parameter play a crucial role in determining the amount of UVB radiation reaching the Earth's surface as well as the energetic driver of tropospheric ozone pollution chemistry. Using a Baynesian optimal estimation technique pioneered by Rodgers approach that constructs a sensitivity type Jacobian resposne matrix from a forward model RT model, we obtain AOD and SSA at 7 UV wavelengths (300- 305-, 311-, 317-, 325-, 332-, and 368-nm). The basis of the technique rests on the direct to diffuse ratio, which is readily obtained by the UV Multi-filter Shadowband spectrometer (UV-MFRSR). We present a one month time series of aerosol optical properties from 3 sites close to 40° N, each with a different aerosol climate characteristics.