Grantee Research Project Results
2001 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, 2000 through September 30, 2001
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:
Ultraviolet radiation is a driving factor in atmospheric photochemical processes, and an increase in UV radiation could lead to damaged vegetation and an increase in incidences of skin cancer and cataracts. The overall objective of this project is to provide a pilot database for interpreting the measurements of surface ultraviolet-B (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?
Progress Summary:
During the past year, project research focused on developing a procedure to retrieve single scattering albedo (), the ratio of scattering coefficient to total extinction coefficient, in the UV wavelengths. It is an important aerosol parameter in determining the aerosol effect on UV radiation. A procedure was successfully devised, involving coupling measurements of surface UV irradiance from the Ultraviolet Multifilter Rotating Shadowband Radiometer (UVMFR-SR) with the tropospheric ultraviolet radiative transfer model TUV4.1 from the National Center for Atmospheric Research. The UVMFR-SR was deployed near the town of Black Mountain, NC (35.63N, 82.33W, 951 m amsl) atop a 10 m tower. Work conducted under this project in the previous year focused on deriving total ozone column (TOC) and aerosol optical depth (AOD) from UVMFR-SR measurements taken in the latter halves of 1997 and 1999. TOC and AOD values from 1999 were used as inputs to TUV4.1. Assumed values of asymmetry parameter (g) and ground albedo (ga) of 0.70 and 0.04, respectively, also were inputs. TUV4.1 was modified to output diffuse-to-direct ratio (DDR) for the seven wavelengths of the UVMFR-SR at solar noon. The initial output of DDR from TUV4.1 was compared with UVMFR-SR measurements. The DDR was derived from the UVMFR-SR measurements as the ratio between the recorded diffuse and direct irradiances. The TUV4.1 was iterated by varying until the output matched that of the UVMFR-SR, ultimately yielding for each wavelength of the UVMFR-SR for each day. Only days with cloudless conditions at solar noon were considered.
Additionally, back trajectory analysis was used to determine if values of could be correlated with air mass classifications as determined at the research site. The three sectors are classified according to Environmental Protection Agency (EPA) source inventories. If directly north of the site is defined at 0° with values increasing clockwise back around to north (360°), marine air masses originate from 65° to 240°, continental air masses originate from 240° to 290°, and polluted air masses originate from 290° to 65°. The marine sector was found to be dominated by salt aerosols, the continental sector by a combination of silicate and soot aerosols, and the polluted sector is dominated by soot and sulfate aerosols. The 48-hour back trajectories were computed from the site using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HY-SPLIT) Model for each value of to determine from which sector the air over the site came. If the computed trajectory went over more than one sector, that trajectory was classified by which sector it remained over the longest.
Values of were obtained for the seven wavelengths of the UVMFR-SR for 9 days from July 26 to October 5, 1999. The value of ranged from 0.53-0.94 at 300 nm, 0.58-0.99 at 305.5 nm, 0.59-1.00 at 311.4 nm, 0.59-1.00 at 317.6 nm, 0.59-1.01 at 325.4 nm, 0.59-1.01 at 332.4 nm, and 0.55-1.03 at 368 nm. For a few of the days, the upper range of for the longer wavelengths (325.4, 368 nm) is not physically possible. However, if the uncertainty in the procedure used to obtain these values is factored in, they can be said to be unity or very close to unity. There was no evidence of correlation between air mass type and at these wavelengths. This can be attributed to the widely varying aerosol content of air masses coming from each sector. This is especially true for air masses originating in the polluted sector, where efficient scatterers (sulfate) and absorbers (soot) abound.
It was necessary to perform sensitivity studies so that error in this retrieval technique could be determined. Tests were conducted to see which input played the greatest role in determining DDR. One parameter was varied holding the others constant. TUV4.1's output of DDR was expected to be more sensitive to a more turbid atmosphere so two AOD scenarios (0.3 and 0.8) were used. From these tests, the order of importance for parameters for determining DDR, based on the rate of increase of DDR with respect to the variable in question, is: AOD, , g, ga, and TOC. It was found that DDR has little dependence to realistic values of TOC. This can be attributed to the good absorption and poor scattering properties of ozone in the UV wavelengths. Variations of tropospheric ozone do little to influence UV radiation when compared to stratospheric ozone. It was found that the sensitivity of DDR to increased from AOD = 0.3 to AOD = 0.8, and this also was found to occur for g and ga. A further test was conducted to see how much a change in DDR of 0.02 would affect the retrieval of as AOD is varied. Here it was found that, as AOD decreases, the change in increases. Hence, error analysis in retrieval was conducted at AOD, from 0.05 to 1 in 0.05 increments to see how AOD affects the error.
At each AOD, the uncertainty in DDR values due to uncertainty in g, ga, and AOD was determined. This was done by summing together the greatest possible error in DDR due to these three parameters. This error in DDR was combined with the uncertainty in the instrument's DDR measurements through the root mean square error (RMSE) formula. After the total error in DDR was found, how much this error in TUV4.1 output of DDR affected the values retrieved could be determined. The value of was allowed to vary from a value of 0.86. The following displays the assumed values of the model parameters and the uncertainty assigned to these parameters and the instrument measurements:
Assumed Value | Error (+/-) | |
uncertainty in aerosol optical depth due to Vo | 0.01 | |
asymmetry parameter | 0.7 | 0.05 |
ground albedo | 0.04 | 0.02 |
diffuse-to-direct ratio | 0.02 | |
single scattering albedo | 0.86 |
The uncertainty in AOD should be greater than + 0.01 because in the UV wavelengths AOD is not the dominating attenuator in the UV wavelengths, and is subject to changes in the other attenuators. Ozone absorption is most important in the two shortest wavelengths of the UVMFR-SR, and Rayleigh scattering is most important in the five longest wavelengths. AOD accounts for only 7 percent of the TOD at 300 nm, increasing to 34 percent at 368 nm. The error related to this phenomenon has not yet been quantified, but is expected to give greater uncertainty to AOD measurements as wavelength decreases.
The results of the error/sensitivity analysis are as follows. There is no correlation between wavelength and uncertainty in . However, the increasing uncertainty in AOD retrieval with decreasing wavelength in the UV spectrum also is expected to increase uncertainty in retrieval as wavelength decreases. Uncertainty in this technique seems to decay exponentially with increasing AOD, from an average error of + 0.19 for AOD = 0.5 to an average error of + 0.2 for AOD = 1.0. For AOD less than 0.3, the estimated error is + 0.04. This was deemed to be the threshold for a reasonable retrieval, so results from days with AOD less than 0.3 were excluded.
The values of found in the current work encompass a wider range than the few previous studies of in the UV wavelengths. One possible cause of the wide range of values of in this study, as compared to previous work, is the high temporal and spatial variability of tropospheric aerosols. The vast differences in sources, transformation and removal processes, and lifetimes all contribute to this variability. Through comparison of the values of found here to the values found at the site in 1995 at 312 nm (0.75-0.93), it is possible that there has been a recent change in the aerosol composition in the air masses influencing the site. An increase in the scattering ability or decrease in the absorbing ability of the aerosols at the site would explain higher values of, and vice versa for lower values.
It was found that, on average, increases with increasing wavelength, from 0.75 at 300 nm to 0.83 at 368 nm. SAS (Statistical Analysis System) analysis indicated that the increase is statistically significant at the 0.05 level in these wavelengths. It has been suggested previously that an assumption that black carbon absorption is independent of wavelength can force to decrease with wavelength. This may explain the increase in wavelength of the current study, as TUV4.1 has no unique input for black carbon aerosol properties. Absorption and scattering properties of soot aerosols are allowed to vary within the input of.
The values of found here can be used for better estimation of the parameter in these wavelengths for the southeastern United States. This will lead to further development of UV radiative transfer models and lessen error in estimation of surface UV irradiances for the region.
Future Activities:
The project has been completed.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 32 publications | 13 publications in selected types | All 13 journal articles |
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Type | Citation | ||
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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) |
not available |
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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) |
not available |
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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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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, UV-B radiation, solar radiation, modeling, southeast, United States, 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.