Grantee Research Project Results
1999 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, 1998 through September 30, 1999
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 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. Results will help determine how successfully a downward trend in stratospheric ozone can be detected by monitoring the surface UV-B irradiance from a network of ground-based stations in the United States. The database will also help refine the forecast of the UV-B Index.
Progress Summary:
The optical depth at seven UV wavelengths was derived from the direct irradiance component measured by the UVMFR (Ultraviolet Multi-Filter Radiometer). Preliminary analysis of data collected at a mountain-top site reveals a mean optical depth of 2.95 - 0.21 at 300 nm. The optical depth decreases as wavelength increases; it reaches a minimum at the longest wavelength channel, with the mean optical depth being 0.05 - 0.04 at 368 nm. The large change in optical depth over UV wavelengths is not unexpected. Rayleigh scattering has been removed from the data. Determination of the ozone optical depth still needs to be accomplished in order to obtain an aerosol optical depth from this data.
Research at the University of Chicago has examined the behavior of ground-level solar UV irradiance measured from several locations during the 1990s. The data sets come from four Brewer spectrophotometers located in Japan (Naha, Kagoshima, Tateno, and Sapporo) and three Brewer spectrophotometers in Canada (Saturna, Winnipeg, and Montreal). The goal is to define the variability that occurs in UV irradiance over time scales ranging from days to several years. Since the duration of the data sets is five years or less, conclusions concerning systematic trends should not be made; however, any trends associated with a decline in or recovery of stratospheric ozone amounts is meshed with (and obscured by) the behavior identified in this work. Spectral information provided by the Brewer instruments allows the observed variability to be interpreted in terms of cause and effect. The 24-hour integrated irradiance at a wavelength of 322.5 nm and the erythemally-weighted irradiance both show day-to-day variations at ranges typically - 30-35 percent of their monthly average values. This similarity in interday variability, specifically the absence of a wavelength dependence, eliminates fluctuations in ozone as the primary cause. A similar study of monthly integrated UV irradiances at selected wavelengths from 300-322.5 nm demonstrates that cloudiness is by far the major driver of interannual variability. This is true, in particular, of the biologically weighted irradiance for erythema. Changes in ozone from one year to the next are a secondary factor that account for approximately one-fifth of the interannual variance in erythemal irradiance for the sites and time period studied.
A conventional delta-Eddington radiative transfer code was modified to simplify the modeling of the highly anisotropic scattering properties of cloud droplets and aerosol particles in the atmosphere. The modification involves independent computation of the singly scattered and multiply scattered radiation, allowing the singly scattered radiation to be computed analytically. The standard Eddington approximation is applied to only the multiply scattered radiation component. This modified delta-Eddington model adequately handled strong absorption in the stratosphere and the anisotropic scattering within optically thin cloud layers, with results better than those from a simple two-stream or conventional delta-Eddington model.
The wavelength dependence of aerosols in cloudy atmospheres was investigated using the modified delta-Eddington model (Erlick et al., 1998). When clouds are superimposed on an aerosol profile with the cloud drops and aerosol particles externally mixed, the shape of the normalized transmission spectrum is dominated by the effect of the cloud drops, unless the optical depth of the aerosols begins to approach the optical depth of the cloud, such as when an optically thin stratus cloud is superimposed on an urban aerosol profile. If cloud drops and aerosol particles are internally mixed through coagulation, the shape of the normalized transmission spectrum is again dominated by the cloud drops, unless there is an unrealistically high volume fraction of strongly absorbing inclusions inside the droplets. The number density of aerosol particles outside the cloud has a greater influence on the magnitude and shape of the transmission spectrum than variations in interstitial aerosol optical depth or in the volume fraction of absorbing inclusions in the cloud drops.
The Brewer spectral UV measurements from the valley research site were analyzed to determine the wavelength-dependent effects of clouds and aerosols on solar transmission. Normalizing the irradiance measurements to a clear-sky measurement, it was found that the normalized irradiance increased with wavelength on certain days and decreased with wavelength on other days. This is consistent with the modeled results discussed above, indicating that aerosols in cloudy atmospheres can cause normalized transmission to increase with wavelength in the ultraviolet and visible, while cleaner clouds cause normalized transmission to decrease with wavelength from 320 nm through 700 nm. The implications are that the attenuation of clouds in the UV cannot always be accurately estimated from visible sunlight data when aerosols are present. In continental or urban regions prone to high aerosol loadings, the attenuation of clouds with optical depths on the order of 10 or less may either increase or decrease with wavelength depending on atmospheric conditions. Measurements of ground-level irradiance at both UV and visible wavelengths may give an indication of the level of aerosol loading in the atmosphere under clear skies or in the presence of optically thin clouds.
An intercomparison of total ozone measurements was conducted using two independent pairs of Brewer and Microtops instruments (Wenny et al., 1999). The measurements demonstrated good agreement, with a mean percentage difference of - 3 percent for both simultaneous individual measurements and daily averaged ozone column values. A comparison of total ozone (daily average) measurements as recorded by Brewer, Microtops II, and TOMS (Total Ozone Mapping Spectrometer) demonstrated a consistent trend in the magnitude of ozone column amounts. The consistency and quality of the agreement indicated that the Microtops may be a suitable instrument to deploy in the field for total column ozone measurements, as long as it is subject to periodic calibrations against an accepted standard.
The occurrence of a long-lasting forest fire in Mexico during May 1998 provided evidence that the research sites are indeed influenced by the long-range transport of aerosols. A smoke plume originating in Mexico was observed by the TOMS (Total Ozone mapping Spectrometer) instrument, and subsequent images showed the plume passing over the southeastern United States. A substantial increase in the black carbon (BC) mass concentration measurement at the mountain site was observed, coinciding with the passage of the plume (an increase on the order of seven times the pre-plume BC concentrations). Light scattering measurements at the valley site also showed a sharp increase coinciding with passage of the plume.
Future Activities:
The field campaign up to this point has resulted in a large body of data, the analysis of which is under way. 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 the Surface Radiation Research Branch of the National Oceanic and Atmospheric Administration. Some of the anticipated directions of future work are as follows: (1) continuing to derive aerosol optical depth in the UV from the UVMFR instrument; (2) retrieving aerosol optical properties in the UV using spectrally resolved Brewer measurements; (3) defining the temporal variability in the Chicago Brewer UV data set and examine its dependence on cloudiness, visibility, and air quality; (4) investigating the use of polarized UV measurements from the Brewer as a method to determine aerosol properties; (5) testing the sensitivity of the UV Index determination using additional information on aerosols not currently included in the UV Index calculations; (6) analyzing the long-term database for seasonal trends; and (7) incorporating aerosol physico-chemical measurements into radiative transfer model calculations.
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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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) |
not available |
Supplemental Keywords:
atmosphere, stratospheric ozone, 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.