Grantee Research Project Results
2000 Progress Report: Surface Levels of Ultraviolet-B Radiation Under Variable Conditions of Tropospheric Air Quality And Cloudiness
EPA Grant Number: R825248Title: Surface Levels of Ultraviolet-B Radiation Under Variable Conditions of Tropospheric Air Quality And Cloudiness
Investigators: Saxena, Vinod K. , Frederick, John
Institution: North Carolina State University , University of Chicago
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 1999
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $374,702
RFA: Exploratory Research - Air Chemistry & Physics (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Safer Chemicals
Objective:
Surface levels of ultraviolet-B (UV-B) radiation determine the safe duration of human exposure to direct solar radiation-the basis for estimating the UV-B index. The former is affected by tropospheric air quality and the cloud coverage conditions of the locality. The overall objective of the project is to provide a pilot database for interpreting the measurements of surface UV-B radiation under variable conditions of tropospheric air quality and cloud cover. The accomplishment of this objective will help answer the following question: How successfully can we detect a downward trend in stratospheric ozone by monitoring the surface UV-B irradiance from a network of ground-based stations in the United States? The database also will help refine the forecast of the UV-B Index.
Progress Summary:
During the past year, project research has focused on developing methods and deriving the total ozone column and aerosol optical depth using the measurements of an Ultraviolet Multi-Filter Rotating Shadowband Radiometer (UVMFR). The impact of these derived quantities on surface erythemal UV levels and the National Weather Service UV Index (UVI) was assessed. Surface measurements of total and diffuse UV irradiance at the 7 narrowband wavelength channels of the Ultraviolet Multi-Filter Rotating Shadowband Radiometer (UVMFR) were used to determine total column ozone and aerosol optical depth for two 6-month periods in 1997 and 1999. The derived total ozone column displayed a seasonal pattern of higher column amounts during the summer and lower amounts during the fall/winter as expected for the northern mid-latitudes. A comparison with the ozone column derived from the Total Ozone Mapping Spectrometer (TOMS) showed a mean ratio, UVMFR(O3)/TOMS(O3), of 0.98 (standard deviation = 0.02) for 1997 and 0.98 (standard deviation = 0.01) for 1999 (TOMS data obtained from http://jwocky.gsfc.nasa.gov ). The repeatability in the ozone retrieval for clear periods demonstrates that the UVMFR is an adequate field research tool for obtaining accurate ground-based total ozone column measurements. The automated UVMFR takes measurements continuously and does not require frequent operator attention, thus providing a greater ozone retrieval rate than other ground-based instruments such as the Dobson or Brewer Spectrophotometer. It must be stressed, however, that regular external lamp calibrations are necessary for the UVMFR to detect and correct any drift in the cosine response and spectral response functions of the filters.
The aerosol optical depth (AOD) was derived from the UVMFR for all seven-wavelength channels during significant clear sky times throughout the 6-month measurement period in 1999. The aerosol optical depth at UV wavelengths is difficult to retrieve with great accuracy due to the complicating factors of strong ozone absorption and Rayleigh scattering in the UV region, as well as the relatively large inherent uncertainty in the UV irradiance measurement. A total optical depth was computed using the Langley method. The ozone optical depth and Rayleigh optical depth were calculated based on the total ozone column and altitude, respectively. Subtraction of these two components yields the aerosol optical depth. For a clear day it can be shown that the fractional uncertainty of total optical depth ranges from 0.1 percent at 300 nm to 1.9 percent at 368 nm, which demonstrates the stability of the retrieval method.
A pattern of higher aerosol optical depths during summer and lower aerosol optical depths during the winter was observed for all wavelengths. For example, at 332 nm the summer mean AOD was 0.663 - 0.26, the fall mean AOD was 0.217 - .18, and the winter mean AOD was 0.076 - 0.03. The higher optical depth values during the summer can in part be attributed to the persistent haze (of both anthropogenic and natural origin) that exists in the region throughout the summer. This haze significantly decreases the visibility in the nearby Smoky Mountain National Park and is a topic of great concern to the National Park Service. The large degree of variability in the aerosol optical depth is due to the complex interaction of UV-B and aerosols. The aerosol optical properties include both scattering and absorption components that have a strong wavelength dependence as well as a dependence on the physico-chemical properties of the aerosol.
The aerosol optical depth increases as wavelength decreases. Ozone absorption becomes the dominant factor shaping the total optical depth at shorter wavelengths, whereas at the longer wavelengths the aerosol and Rayleigh terms are the significant contributors. The increasingly strong absorption by ozone at the shorter wavelengths accounts for approximately 70 percent, 50 percent, and 30 percent of the total optical depth at 300, 305, and 311 nm, respectively. The Rayleigh optical depth increases from approximately 26 percent of the total optical depth at 300 nm to 70 percent at 368 nm. Aerosol optical depth also increasingly contributes to the total optical depth from approximately 7 percent at 300 nm up to 34 percent at 368 nm. However, it must be noted that the uncertainty in the aerosol optical depth measurements at the shorter three wavelength channels is relatively large, and combined with the fact that ozone is the dominant attenuating factor at these wavelengths decreases the confidence in the retrieved aerosol optical depths at the lowest three wavelength channels.
The UV Index (UVI) is a forecast tool developed by the National Weather Service to inform the general public about the health hazards of exposure to UV radiation. The UVI is the predicted noontime erythemally-weighted UV irradiance in W/m2 converted to a unitless index scale. The Tropospheric Ultraviolet-Visible Radiation Model (TUV) is used to calculate UVI for cloud-free conditions at the field research site. Currently, the U.S. UVI uses a standard aerosol optical depth input of 0.2, independent of wavelength, with no aerosol absorption (single scatter albedo, v = 1.0) for all 58 U.S. cities for which the UVI is generated. For each day that ozone and aerosol optical depth were derived from the UVMFR over the 6-month period, the UVI was calculated using both the standard aerosol inputs and the actual retrieved aerosol optical depth. Since the UVI standard aerosol optical depth is referenced to 340 nm, Angstrom's formula was used for each day to extrapolate the measured spectral extinction to an aerosol optical depth at 340 nm. The resulting difference between the two scenarios is computed as the UVI calculated using the measured AOD minus the UVI calculated using the standard AOD. Three scenarios of aerosol absorption are considered in conjunction with the measured AODs, no absorption (v = 1.0), moderate absorption (v = 0.90), and strong absorption (v = 0.75). The percentage decrease in erythemally-weighted UV for the three different absorption scenarios and each retrieved aerosol optical depth was calculated. The percentage change is relative to the standard aerosol inputs (AOD = 0.2, v = 1.0). As the AOD increases, the decrease in surface erythemally-weighted UV becomes substantial, up to 17 percent for the highest measured AOD for the no absorption case. Inclusion of aerosol absorption in the calculation shifts to a larger decrease for each AOD. The extreme case of high absorption and high AOD indicated a nearly 50 percent decrease in surface erythemally-weighted UV. The lesser aerosol attenuation during the low AOD cases can lead to an increase of up to 4 percent in the surface erythemally-weighted UV.
Deviations in the UVI were calculated for 74 days, comparing the standard to the measured aerosol inputs. For the no aerosol absorption scenario the majority of the days (63.5 percent) had no change in the resulting UVI. However, there was a deviation of -1 index unit for 28.4 percent of the days where using the actual measured AOD results in a lower UVI prediction versus the standard aerosol inputs. There were also a few days (4.1 percent), where the UVI deviation was either -2 or +1 index units. Including a moderate amount of aerosol absorption increases the frequency of negative UVI deviations with no change for 59.5 percent of the days, -1 index unit was 20.3 percent, -2 index units was 13.5 percent, and -3 index units was 5.4 percent. Strong aerosol absorption shifted the UVI deviations further negative, with 44.6 percent of the days having no change, 23 percent were -1 index unit, 12.1 percent were -2 index units, 13.5 percent were -3 index units, 5.4 percent were -4 index units, and 1.4 percent were -5 index units. Grouping the deviations by month revealed a seasonal pattern, with a majority of the -2 and -1 UVI deviations occurring during the summer months of July and August and the late fall and winter months showing 0 or +1 deviations. The moderate and strong absorption cases showed similar seasonal patterns. Deviations grouped by measured AOD demonstrated that the higher the optical depth the greater the chance of a UVI deviation. These results indicate that the aerosol optical properties can have an impact on the UVI calculation, that during the summer months when aerosol optical depths are typically higher, particularly in urban areas, the use of the standard AOD input of 0.2 can lead to an overestimation of the surface UV and thus a higher UVI forecast value. In terms of improving the accuracy of the UVI forecast by including the effect of haze, it is suggested that during the summer months, a larger value of AOD and a more realistic value of aerosol absorption (v = 0.90) should be used. Based on the values retrieved in this study, an AOD in the range of 0.6 to 0.75 is recommended. The actual summertime AOD will naturally vary, dependent upon each forecast city location, however, it can be expected that in most urban locations the AOD in the UV will be greater than the standard input of 0.2.
Future Activities:
The analysis of the collected database continues. The ongoing nature of the field campaign will further enlarge and add to this database. The analysis of this large data set will be carried out through the combined efforts of North Carolina State University, the University of Chicago, and NOAA-SRRB. Some of the anticipated directions of future work are as follows: (1) continued derivation of aerosol optical depth in the UV from the UVMFR instrument; (2) retrieve aerosol optical properties in the UV using spectrally resolved Brewer measurements; (3) study the inter-annual variability in ground-level UV spectral irradiance based on an analysis of a multiyear data set from Brewer spectrophotometers; (4) analyze diffuse-direct irradiance ratio measurements of the UVMFR to derive aerosol optical parameters; (5) analyze the long-term database for seasonal trends; (6) incorporate aerosol physico-chemical measurements into radiative transfer model calculations; and (7) conduct an error analysis on selected Brewer instruments to determine feasibility of long term UV trends.
Journal Articles on this Report : 7 Displayed | Download in RIS Format
Other project views: | All 32 publications | 13 publications in selected types | All 13 journal articles |
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Erlick C, Frederick JE. Effects of aerosols on the wavelength dependence of atmospheric transmission in the ultraviolet and visible. 1. A "single-scattering-separate" delta-Eddington model. Journal of Geophysical Research-Atmospheres 1998;103(D10):11465-11472. |
R825248 (1999) R825248 (2000) R825248 (2001) R825248 (Final) |
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Erlick C, Frederick JE, Saxena VK, Wenny BN. Atmospheric transmission in the ultraviolet and visible: Aerosols in cloudy atmospheres. Journal of Geophysical Research - Atmospheres 1998;103(D24):31541-31555. |
R825248 (1999) R825248 (2000) R825248 (2001) |
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Erlick C, Frederick JE. Effects of aerosols on the wavelength dependence of atmospheric transmission in the ultraviolet and visible. 2. Continental and urban aerosols in clear skies. Journal of Geophysical Research - Atmospheres 1998;103(D18):23275-23285. |
R825248 (1999) R825248 (2000) R825248 (2001) |
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Im JS, Saxena VK, Wenny BN. An assessment of hygroscopic growth factors for aerosols in the surface boundary layer for computing direct radiative forcing. Journal of Geophysical Research-Atmospheres 2001;106(D17):20,213-20,224. |
R825248 (1999) R825248 (2000) R825248 (2001) R825248 (Final) |
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Schafer JS, Saxena VK, Wenny BN, Barnard W, DeLuisi JJ. Observed influence of clouds on ultraviolet-B radiation. Geophysical Research Letters 1996;23(19):2625-2628. |
R825248 (1999) R825248 (2000) |
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Wenny BN, Schafer JS, DeLuisi JJ, Saxena VK, Barnard WF, Petropavlovskikh IV, Vergamini AJ. A study of regional aerosol radiative properties and effects on ultraviolet-B radiation. Journal of Geophysical Research - Atmospheres, 1998, Volume: 103, Number: D14 (JUL 27), Page: 17083-17097. |
R825248 (2000) |
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Wenny BN, Saxena VK, Frederick JE. Aerosol optical depth measurements and their impact on surface levels of ultraviolet-B radiation. Journal of Geophysical Research-Atmospheres 2001;106(D15):17,311-17,319. |
R825248 (2000) R825248 (2001) R825248 (Final) |
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Supplemental Keywords:
atmosphere, stratospheric ozone, modeling, southeast US., RFA, Scientific Discipline, Air, Environmental Chemistry, climate change, tropospheric ozone, Atmospheric Sciences, urban air, environmental monitoring, ambient ozone data, boundry layer processes, climate variations, ozone, weather factors, aerosol/ cloud interactions, air pollution models, air quality data, aerosol sampling, atmospheric monitoring, ambient aerosol particles, UV-B radiation, climate variabilityRelevant Websites:
http://www4.ncsu.edu/unity/users/s/saxena/public/cloud.html Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.