2007 Progress Report: Effects of Clouds and Tropospheric Air Quality on Surface UV at 6 UV Research SitesEPA Grant Number: R833224
Title: Effects of Clouds and Tropospheric Air Quality on Surface UV at 6 UV Research Sites
Investigators: Lantz, Kathleen O. , Kiedron, Peter , Petropavlovskikh, Irina
Institution: Cooperative Institute for Research in Environmental Sciences
EPA Project Officer: Chung, Serena
Project Period: October 1, 2006 through September 30, 2010
Project Period Covered by this Report: October 1, 2006 through September 30, 2007
Project Amount: $299,988
RFA: Implications of Tropospheric Air Pollution for Surface UV Exposures (2005) RFA Text | Recipients Lists
Research Category: Global Climate Change , Air Quality and Air Toxics , Climate Change , Air
Six locations across the continental United States will be used to evaluate the impacts of tropospheric air quality on surface UV irradiance measurements. These six sites are the re-established EPA UV Network and include Table Mountain near Boulder, CO, Rocky Mountain Research Station at Niwot Ridge, CO, Bondville, IL, Fort Peck, MT, Raleigh, NC, and Houston, TX. Our goals will be to evaluate UV-B irradiance from the six locations under a variety of atmospheric conditions.
The specific goals include (1) developing and implementing QA/QC procedures on the UV measurements and ancillary data. Procedures include comparing solar irradiance from the Brewer spectrophotometer to measurements from other co-located instruments (i.e. UV broadband radiometers and UV filter radiometers). (2) An algorithm will be developed for determining ozone profiles from Brewer Umkehr measurements and inferring tropospheric ozone column. (3) Cloud and aerosol properties will be collected from a suite of co-located instruments as part of the SURFRAD Network within the Surface Radiation Research Branch (SRRB), Earth System Research Laboratory, NOAA, and the USDA UV monitoring program. (4) Using the direct-to-diffuse ratio from the UV-MFRSR combined with a UV radiative transfer code, the aerosol single scattering albedo will be estimated for clear-sky conditions at the six sites. (5) The atmospheric conditions will be characterized for total ozone, cloud properties, and atmospheric pollutants. (6) We will provide a comprehensive evaluation of the impact of total ozone, cloud cover, and air quality on the tropospheric UV-B transmission. The database provided by the Central UV Calibration Facility, the Surface Radiation and Research Branch of ESRL/NOAA, and the USDA UV Monitoring Program provides a unique and valuable data-set that will help identify the impacts of variable conditions of cloud cover and tropospheric air quality on UV irradiances.
This document describes results and progress during the first year under the objectives of the EPA STAR funded proposal “Effects of Clouds and Tropospheric Pollution on Surface UV at six EPA UV Research Sites”. The project’s focus is the study of changes in surface UV levels caused by total ozone, clouds and atmospheric pollutants, including aerosols and tropospheric ozone. UV radiation drives photochemical reactions rates that are essential in pollution formation, and exposure to UV radiation can be a human health hazard. The re-established NOAA/EPA UV Network at six locations across the United States is currently measuring UV radiation and automated direct sun and zenith sky ozone measurements from which daily ozone profiles can be derived.
Ozone profile algorithm development. One of the goals of the three-year project is to improve the quality of ozone profiles retrieved from the Brewer Umkehr measurements and to infer an improved tropospheric ozone column. In accordance with the project objectives, a working version of the Brewer Umkehr ozone profile retrieval algorithm was developed. Each instrument in the NEUBrew network has specific spectral settings that were measured at the calibration facility in Boulder, CO. Parameters such as each channels’ wavelength settings, band-pass profile, filter polarization characteristics, and out-of-band signal were assessed for several instruments located at the Table Mountain Site, Boulder, CO. The ozone profile retrieval algorithm (originally developed for the Dobson Umkehr measurements [Petropavlovskikh et al., 2005 a, b and c]) was optimized by incorporating information specific to optical and spectro-radiometric characteristics of each Brewer Mark IV instrument in the NEUBrew network.
Umkehr measurements are performed by the Brewer spectrophotometer during sunrise and sunset elevations for solar zenith angles between 70 to 90 degrees. Eight specific wavelengths are scanned, i.e. 306 nm, 310 nm, 313 nm, 317 nm, 319 nm, 323 nm, 326 nm, and 329 nm. The photon counts are converted to the log of the zenith-sky radiances and are plotted as a function of solar zenith angle for both morning and afternoon measurements. These automated plots are available and viewed daily for quality control checks at the NEUBrew web page at http://esrl.noaa.gov/gmd/grad/neubrew/ Exit . Using the improved algorithm, vertical ozone profiles are routinely derived from Umkehr scans and subsequently uploaded to the NEUBrew web-pages. High and poor quality retrieved profiles have the sub-title text colored in green and red respectively. Retrievals are considered of high quality if their iterations are less than four, and final residuals of the retrieval are less than one. The work will continue in the second year to characterize all instruments in the NEUBrew network. The algorithm now has an option to modify instrumental settings for data processing from each individual instrument in the network. The total ozone column and instrument temperature variability across the day are also plotted for data quality control purposes.
Cloud interference in the automated Brewer Umkehr measurements is addressed. To assure quality of the Brewer ozone profile retrievals, zenith sky radiation measurements are screened for interference of clouds in the zenith angle view. The method uses a semi-empirical relationship between observed cloudiness in the zenith angle view and variability in zenith radiance recorded by two sets of measurements at the same wavelength channel taken within a short period of time (~80 secs). The preliminary results suggest that this method can be prone to errors due to the wavelength instability of the compared channels, and therefore needs to be closely monitored throughout the year. A newer algorithm that corrects potential wavelength shifts is under the development. This work needs further refinement and will be continued in the second year.
Database and QA/QC procedures and development: Several diagnostic plots of Brewer spectrophotometer measurements have been made available on the NEUBrew web-site including Brewer event timing, Brewer summary scans, temperature, humidity, wavelength stability through Hg line scans, dark counts, B-File comments and summary plots, and Umkehr summary scans as described above. The diagnostic plots allow assessment of the data on a daily basis. In the latter part of the year, several level 101 data products have been made available. These include calculation of the UV irradiance, UV index, erythema, and total ozone. The latter is labeled as Level 1 ozone in our nomenclature. Level 101 data have been corrected for dark counts and dead time. Responsivities from in field calibrations with CUCF lamps were used. Level 201 and 301 irradiances and their derivative products (UV index and erythema) that detect and remove spikes and correct stray light effects, respectively, are currently being developed. The level 311 data will be corrected for angular response. The routine to calculate angular corrections for the Brewer spectrophotometer solar irradiance measurements is in progress [Lantz et al., 2006]. The angular correction routine uses the direct-to-diffuse irradiance ratio (DDR) from co-located UV-MFRSR measurements and uses an isotropic assumption for overcast days and a radiative transfer model radiance distribution for clear skies with and without aerosols. Lastly, total (level 1) ozone is routinely compared to OMI total ozone with plots available on the web-site.Further corrections and refinements are planned. We want to implement temperature correction (Level 312) and responsivity drift correction (Level 313) for UV irradiance and its derivative products. Higher level products for ozone column will be obtained after solar blind filter stability is corrected from “SL”: routine (Level 2) and after instrument throughput is corrected with Langley regressions (Level 3).
Aerosol property algorithm development: For our studies of air quality on UV radiation, aerosol optical properties are required which include aerosol optical depth and aerosol single scattering albedo. Direct-to-diffuse solar irradiance ratios (DDR) from a co-located UV Rotating Shadowband Spectrograph (UV-RSS) and a UV-MFRSR have been compared as part of the development of an algorithm for the calculation of aerosol single scattering albedo. Initial results are promising for six of the seven filter channels (i.e. excluding the nominal 300 nm channel). The six longer wavelength channels will be used for the aerosol single scattering retrievals and for the angular response corrections to the Brewer spectrophotometer measurements as described above. In addition, sensitivity studies were conducted of the DDR to several atmospheric parameters (i.e. AOD, aerosol single scattering albedo, asymmetry parameter, ground albedo, and tropospheric and stratospheric O3 and NO2) using the TUV radiative transfer model. The sensitivity studies provide guidelines on the uncertainty of the input parameters to the DDR and derived aerosol single scattering albedo. Apart from the collocated UV-MFRSR the aerosol optical depth (AOD) can be derived from data obtained by Brewer spectrophotometer during its routine ozone measurements. We expect that AOD from Brewer will be more reliable at shortest wavelengths than AOD from UV-MFRSR.
Tropospheric ozone column: The work will continue to finalize the cloud-clearing procedures for Umkehr data processing. The range of the expected “clear-sky” variability for each instrument will be established and used as a limit for an automated processing. The sensitivity studies will be done to assess the effect of the channel spectral instability for the range of solar zenith angles and total ozone columns that are station-representative. The algorithm will be expanded to include out-of-band and filter transition corrections. All “high quality” retrieved ozone profiles will be integrated to separate tropospheric and stratospheric ozone columns information. The tropospheric ozone columns will be validated against ozone-sonde data collected in Boulder, CO.
UV radiation and air quality. This work will continue with quality control and assurance procedures before studying the effect of air quality on UV solar radiation. This includes applying angular corrections based on UV-MFRSR measurements and angular response measurements to the Brewer solar irradiance data and application of spike identification and removal. The next steps will include retrieving UV-MFRSR and broadband data from the SURFRAD and USDA Networks to the NEUBrew web site, then automating comparisons between filter radiometer solar irradiance measurements, broadband erythema irradiance, and filter-weighted and erythema-weighted Brewer solar irradiance measurements. The automated plots will provide a means for quickly identifying problems with the instrumentation before analysis of the data. In the second year there will be an emphasis on deriving aerosol properties from co-incident instruments at the sites. Specifically, the Table Mountain site will be the first site investigated because of the ability to cross-validate direct-to-diffuse irradiance ratios (DDR) between the UV-RSS instrument and the UV-MFRSR. The aerosol single scattering albedo algorithm will be further refined and uncertainties identified. In the final year of the project, the effect of tropospheric ozone and air quality will be accessed after the aerosol properties have been determined, the tropospheric ozone algorithm finished, and a troposopheric ozone product for the sites calculated.
Petropavlovskikh, I., C. Ahn, P. K. Bhartia, and L. E. Flynn (2005a), Comparison and co-validation of ozone anomalies and variability observed in SBUV(/2) and Umkehr northern mid-latitude ozone profile estimates, Geophys. Res. Lett., 32, L06805, doi:10.1029/2004GL022002, 2005a.
Petropavlovskikh, I., P. K. Bhartia, and J. DeLuisi, New Umkehr ozone profile retrieval algorithm optimized for climatological studies, Geophys. Res. Lett., 32, L16808, doi:10.1029/2005GL023323, 2005b.
Petropavlovskikh, I., S. Kireev, E. Maillard, R. Stube, and P. K. Bhartia (2005c), New Brewer algorithm for a single pair, 9th Biennial WMO/GAW Brewer Users Group Meeting, Delft, Netherlands, June 2005c.