Grantee Research Project Results
1997 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, 1996 through September 30, 1997
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:
The intensity level of surface UV-B radiation is the driving factor in important photochemical reactions that determine tropospheric air quality. 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 to 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 U.S.?
Progress Summary:
Products: The initial focus of the study is the analysis of variations in spectral UV-B transmission and aerosol physico-chemical properties due to air masses of contrasting source regions overlying the research sites. To this end, analysis involved data collected both during the 1997 and 1995 field campaigns. Methods of analysis were developed based on the 1995 data set and will be applied to the data currently being collected. A list of products obtained from the collected data sets are as follows. 1) The UV-B transmission over the layer defined by the research sites characterized by air mass type. 2) The characteristics of optical depths at three wavelengths (415, 500 and 673 nm) and diffuse-direct solar irradiance ratio as a function of air mass type. 3) The columnar lognormal aerosol size distributions retrieved by inverting the optical depth measurements. 4) Background levels of black carbon (BC) mass concentrations for different air mass types. 5) Aerosol scattering coefficients characterized by air mass type.
Approach: Two sites 1 kilometer vertically and 10 kilometer horizontally apart situated in the Blue Ridge Mountains of western North Carolina were outfitted with instrumentation beginning in June 1997. The instrumentation was installed, as it became available to us from NOAA-SRRB and U.S. EPA-AREAL. The experimental site and instrumentation are described in detail by Schafer et al. (1996) and Wenny et al. (1997). The instruments deployed relevant to this project are the Yankee Multi-Filter Rotating Shadowband Radiometer (MFRSR), Yankee Ultraviolet Multi-Filter Radiometer (UVMFR), Yankee UVB-1 Radiometer (UVB1), Brewer Spectrophotometer (Brewer), TSI Differential Mobility Particle Sizer (DMPS), TSI Integrating Nephelometer (TSIN), Radiance Research Integrating Nephelometer (RRN), and a Magee Scientific Aethalometer. The MFRSR provides simultaneous measurements of global, diffuse and direct components of solar irradiance at seven wavelengths (415, 500, 615, 673, 870 and 940 nm). The UVMFR provides simultaneous measurements of global, diffuse and direct components of solar irradiance at six wavelengths (300, 305.5, 311.5, 317.5, 325, 332.5, and 368 nm). The UVB1 provides measurements of broadband UV-B global irradiance (280-320 nm). The Brewer provides measurements of both total ozone column and spectral UV global irradiance (286.5 - 363 nm in 0.5 nm increments). The TSIN measures total scattering and backscattering coefficients at 3 wavelengths (450, 550, and 700 nm). The RRN measures total scatting coefficient at 530 nm. Backward air parcel trajectories were computed using the Hybrid Single Particle Lagrangian Integrated Trajectory (HY-SPLIT) model. The criterion for designating air masses as P, C and M are based on the emission inventories of the U.S. Environmental Protection Agency. An Aethalometer manufactured by Magee Scientific was used to make Black Carbon measurements. The TSI Differential Mobility Particle Sizer (DMPS) measures particle size distributions (0.016-0.6 mðm).
Aerosol optical depth was derived from the MFRSR direct irradiance measurements using the Bouguer-Langley method (Lenoble, 1993). UV-B transmission over the layer defined by the two sites was determined by the ratio of the UVB1 measurements at the two sites. Search Graph Method The ground albedo and imaginary part of refractive index are calculated using a unique procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved columnar aerosol size distribution, aerosol optical depth and diffuse-direct ratio.
During November 1997, an intensive field campaign was conducted in conjunction with researchers from the University of Miami. The focus of this campaign was the collection of aerosol samples using both optical sizing instruments (TSI Scanning Mobility Particle Sizer, TSI Ultra-Fine Particle Counter, TSI Condensation Particle Counter, TSI Aerodynamic Particle Sizer)
Results-to-date:
The clear-sky broadband UV-B tranmsission over the layer defined by the two sites was determined for a number of days with contrasting air mass histories. The average UV-B tranmsission was 77.8%±ð4.6%, 81.9%±ð2.9%, and 83.5%±ð2.8% for polluted, marine, and continental air masses, respectively. Our observed range of broadband UV-B attenuation of 14% to 31% for the 1-kilometer layer is consistent with other reported values of transmission through 1-kilometer in the lower troposphere (Blumthaler et al., 1992, 1994; Reiter et al., 1982). The observed differences for layer transmission for the three air mass types are attributed to the contrasting aerosol properties of the three air mass types.
The Brewer spectral UV measurements from the valley site were analyzed to determine the wavelength-dependent effects of clouds and aerosols on solar transmission. Normalizing the irradiance measurements to a clear-sky measurment, 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 of Erlick and Frederick (1997a, 1997b), 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 the visible.
The measurement of the physico-chemical properties of aerosols in the atmospheric layer between the two sites was made in order to estimate the optical properties of the intervening layer. During the months from July to December 1995, the total representative optical depths at 500 nm at the valley site were 0.29±0.19, 0.10±0.04 and 0.68±0.33 for M, C, and P air masses, respectively. Total optical depths at 500 nm on Mt. Mitchell were 0.10±0.09, 0.032±0.01 and 0.19±0.12 for M, C, and P air mass types, respectively. The ratios of mean 1-kilometer layer optical depth between the mountain and the valley to total mean optical depth at 500 nm from the valley site were 71%, 68% and 73% for M, C, and P air masses, respectively. This indicates that the major part of atmospheric aerosols is located within the lowest 1-kilometer boundary layer of the troposphere. The average diffuse-direct ratios at the valley site for P, M and C air masses at 500 nm were 0.98±0.33, 0.37±0.29 and 0.14±0.07 respectively. There was a significant linear correlation between the diffuse-direct ratio and the total aerosol optical depth at both the mountain and the valley site in conformity with theoretical predictions. A search-graph method was developed (Yu et al., 1997) and used to retrieve the columnar lognormal size distribution (N, rg, and sðg) on the basis of the optical depth at three wavelengths (415, 500, and 673 nm). The ground albedo and imaginary part of the refractive index were calculated using an unique procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved aerosol size distribution, aerosol optical depth, and diffuse to direct ratio. It is found that rg and sðg were in the range of 0.022 to 0.18 mðm and 1.37 to 2.95, respectively. The asymmetry parameter and single scattering albedo were in the range of 0.607 to 0.729 and 0.849 to 0.989, respectively. The average ground albedo and imaginary part of the refractive index were 0.17 and 0.019, respectively.
The role of BC aerosols on cloud microphysical/optical properties was investigated. Fifteen minute BC mass concentrations have been obtained for clear air days in June-October 1996 and March-October 1997. Table 1 shows the monthly average BC mass concentrations for each sector obtained at the site and a comparison with those found at other sites. In general, average BC mass concentrations from the P sector were higher than the C and M sectors. These results compare well with those obtained by Chylek et al. (1996) in southern Nova Scotia and those found at Mace Head, Ireland by Jennings et al. (1993). Average values of transatlantic BC transport to Mace Head ranged from 7 to 21 ng m-3. The average BC data collected at Mt. Mitchell for M air was 62 ng m-3. The higher value can be expected for Mt. Mitchell because the marine air may have been modified by traversing over land before reaching the site.
References:
Bahrmann C.P., and V.K. Saxena, The influence of air mass history on black carbon concentrations in the southeastern U.S., J. Geophys. Res., (in review), 1997.
Chylek, P., C.M. Banic, B. Johnson, P.A. Damiano, G.A. Isaac, W.R. Leaitch, P.S.K. Liu, F.S. Boudala, B. Winter, and D. Ngo, Black carbon: atmospheric concentrations and cloud water content measurements over southern Nova Scotia, J. Geophys. Res., 101, 29105-29110, 1995.
Jennings, S.G., F.M. McGovern, and W.F. Cooke, Carbon mass concentration measurements at Mace Head, on the west coast of Ireland, Atmos. Environ., 27A, 1229-1239, 1993.
Lenoble, J., Atmospheric Radiative Transfer, 532 pp., A. Deepak, Virginia, 1993.
Schafer, J.S., V.K. Saxena, B.N. Wenny, W.F. Barnard, and J.J. DeLuisi, Observed influence of clouds on ultraviolet-B radiation, Geophys. Res. Let., 23, 2625-2628, 1996.
Wenny, B.N., J.S. Schafer, J.J. DeLuisi, V.K. Saxena, W.F. Barnard, I.V. Petropavlovskikh, and A.J. Vergamini, A study of regional aerosol radiative properties and effects on ultraviolet-B radiation, J. Geophys. Res.,(conditionally accepted), 1997.
Yu, S, V.K. Saxena, B.N. Wenny, J.J. DeLuisi, G.K. Yue, and I.V. Petropavlovskikh, A study of the diffuse to direct solar irradiance ratio and aerosol optical depth in the southeastern US, J. Geophys. Res., (in review), 1997.
Journal Articles:
No journal articles submitted with this report: View all 32 publications for this projectSupplemental Keywords:
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.