The impact of aerosols on solar ultraviolet radiation and photochemical smog

Science

Volumn 278, Issue 5339, 1997, Pages 827-830

  Dickerson, R R ^a   Kondragunta, S ^a   Stenchikov, G ^a   Civerolo, K L ^a   Doddridge, B G ^a   Holben, B N ^b  

^a UNIVERSITY OF MARYLAND   (United States)

^b NASA GODDARD SPACE FLIGHT CENTER   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

HYDROCARBON; OZONE; SULFUR;

AEROSOL; ATMOSPHERIC POLLUTION; PHOTOCHEMICAL SMOG; SCATTERING; SULPHATE; ULTRAVIOLET RADIATION;

AEROSOL; AIR POLLUTION; AIR POLLUTION CONTROL; AIR QUALITY; ARTICLE; MINERAL DUST; PHOTOCHEMISTRY; PRIORITY JOURNAL; RADIATION SCATTERING; SMOG; SOLAR RADIATION; SOOT; ULTRAVIOLET RADIATION;

AEROSOLS; COMPUTER SIMULATION; MODELS, THEORETICAL; OZONE; PHOTOCHEMISTRY; SCATTERING, RADIATION; SMOG; ULTRAVIOLET RAYS;

ARIA;

EID: 0030685149     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.278.5339.827     Document Type: Article

References (55)

1. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
2. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
3. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
4. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
5. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
6. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
7. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
8. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
9. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
10. Photochemical production of tropospheric ozone has been studied extensively [J. H. Seinfeld, Science 243, 745 (1989); National Academy of Sciences, Rethinking the Ozone Problem in Urban and Regional Air Pollution (National Academy of Sciences, Washington, DC, 1991); U.S. Environmental Protection Agency, National Air Quality and Emissions Trends Report, 1995 (EPA454/R-96-005, 1996); B. J. Finlayson-Pitts and J. N. Pitts Jr., Science 276, 1045 (1997). The effect of aerosols on UV flux and photochemical ozone production near Earth's surface has generally been assumed to be small and negative, that is, the presence of aerosols reduces slightly the rate of smog formation [L. Zafonte, P. Rieger, J. Holmes, Environ. Sci. Technol. 11, 483 (1977); S. Liu, S. McKeen, S. Madronich, Geophys. Res. Lett. 18, 2265 (1991); Y. Lu and M. A. K. Khalil, Chemosphere 32, 739 (1996)]. Operation of many smog models, such as the Urban Airshed Models, is generally conducted without regard to the radiative effects of aerosols [U.S. Environmental Protection Agency, Guidelines for Regulatory Application of Urban Airshed Model (EPA-450/4-91-013, 1991); SAI, Users Guide to the Variable Grid Urban Airshed Model (UAM-V) (Systems Applications International, San Rafael, CA, 1995); R. Pielke et al., Meteorol. Atmos. Phys. 49, 69 (1992)].
11. Photochemical reactions progress at a rate that depends on the intensity of radiation. For any particular species, the photolysis rate coefficient can be calculated by integrating the product of the actinic flux, the quantum yield, and the absorption cross section with respect to wavelength. This coefficient can also be measured directly with a chemical actinometer. See, for example, J. Peterson and K. Demerjian, Atmos. Environ. 10, 459 (1976); J. Peterson, ibid. 11, 689 (1977); R. Dickerson, D. Stedman, A. Delany, J. Geophys. Res. 87, 4933 (1982); T. Blackburn, S. Bairai, D. Stedman, ibid. 97, 10109 (1992); A. Ruggaber, R. Dlugi, T. Nakajima, J. Atmos. Chem. 18, 171 (1994); C. Blindauer, V. Rozanov, J. Burrows, ibid. 24, 1 (1996).
12. Photochemical reactions progress at a rate that depends on the intensity of radiation. For any particular species, the photolysis rate coefficient can be calculated by integrating the product of the actinic flux, the quantum yield, and the absorption cross section with respect to wavelength. This coefficient can also be measured directly with a chemical actinometer. See, for example, J. Peterson and K. Demerjian, Atmos. Environ. 10, 459 (1976); J. Peterson, ibid. 11, 689 (1977); R. Dickerson, D. Stedman, A. Delany, J. Geophys. Res. 87, 4933 (1982); T. Blackburn, S. Bairai, D. Stedman, ibid. 97, 10109 (1992); A. Ruggaber, R. Dlugi, T. Nakajima, J. Atmos. Chem. 18, 171 (1994); C. Blindauer, V. Rozanov, J. Burrows, ibid. 24, 1 (1996).
13. Photochemical reactions progress at a rate that depends on the intensity of radiation. For any particular species, the photolysis rate coefficient can be calculated by integrating the product of the actinic flux, the quantum yield, and the absorption cross section with respect to wavelength. This coefficient can also be measured directly with a chemical actinometer. See, for example, J. Peterson and K. Demerjian, Atmos. Environ. 10, 459 (1976); J. Peterson, ibid. 11, 689 (1977); R. Dickerson, D. Stedman, A. Delany, J. Geophys. Res. 87, 4933 (1982); T. Blackburn, S. Bairai, D. Stedman, ibid. 97, 10109 (1992); A. Ruggaber, R. Dlugi, T. Nakajima, J. Atmos. Chem. 18, 171 (1994); C. Blindauer, V. Rozanov, J. Burrows, ibid. 24, 1 (1996).
14. Photochemical reactions progress at a rate that depends on the intensity of radiation. For any particular species, the photolysis rate coefficient can be calculated by integrating the product of the actinic flux, the quantum yield, and the absorption cross section with respect to wavelength. This coefficient can also be measured directly with a chemical actinometer. See, for example, J. Peterson and K. Demerjian, Atmos. Environ. 10, 459 (1976); J. Peterson, ibid. 11, 689 (1977); R. Dickerson, D. Stedman, A. Delany, J. Geophys. Res. 87, 4933 (1982); T. Blackburn, S. Bairai, D. Stedman, ibid. 97, 10109 (1992); A. Ruggaber, R. Dlugi, T. Nakajima, J. Atmos. Chem. 18, 171 (1994); C. Blindauer, V. Rozanov, J. Burrows, ibid. 24, 1 (1996).
15. Photochemical reactions progress at a rate that depends on the intensity of radiation. For any particular species, the photolysis rate coefficient can be calculated by integrating the product of the actinic flux, the quantum yield, and the absorption cross section with respect to wavelength. This coefficient can also be measured directly with a chemical actinometer. See, for example, J. Peterson and K. Demerjian, Atmos. Environ. 10, 459 (1976); J. Peterson, ibid. 11, 689 (1977); R. Dickerson, D. Stedman, A. Delany, J. Geophys. Res. 87, 4933 (1982); T. Blackburn, S. Bairai, D. Stedman, ibid. 97, 10109 (1992); A. Ruggaber, R. Dlugi, T. Nakajima, J. Atmos. Chem. 18, 171 (1994); C. Blindauer, V. Rozanov, J. Burrows, ibid. 24, 1 (1996).
16. Photochemical reactions progress at a rate that depends on the intensity of radiation. For any particular species, the photolysis rate coefficient can be calculated by integrating the product of the actinic flux, the quantum yield, and the absorption cross section with respect to wavelength. This coefficient can also be measured directly with a chemical actinometer. See, for example, J. Peterson and K. Demerjian, Atmos. Environ. 10, 459 (1976); J. Peterson, ibid. 11, 689 (1977); R. Dickerson, D. Stedman, A. Delany, J. Geophys. Res. 87, 4933 (1982); T. Blackburn, S. Bairai, D. Stedman, ibid. 97, 10109 (1992); A. Ruggaber, R. Dlugi, T. Nakajima, J. Atmos. Chem. 18, 171 (1994); C. Blindauer, V. Rozanov, J. Burrows, ibid. 24, 1 (1996).
17. R. E. Shelter et al., J. Geophys. Res. 101, 14631 (1996)
18. o is the same radiance under an aerosol-free atmosphere. When τ equals or exceeds unity, the atmosphere is said to be optically thick [R. J. Charlson et al., Science 255, 423 (1992); J. T. Kiehl and B. P. Briegleb, ibid. 260, 311 (1993); J. T. Houghton et al., Climate Change 1995: The Science of Climate Change (Cambridge Univ. Press, Cambridge, 1996)].
19. o is the same radiance under an aerosol-free atmosphere. When τ equals or exceeds unity, the atmosphere is said to be optically thick [R. J. Charlson et al., Science 255, 423 (1992); J. T. Kiehl and B. P. Briegleb, ibid. 260, 311 (1993); J. T. Houghton et al., Climate Change 1995: The Science of Climate Change (Cambridge Univ. Press, Cambridge, 1996)].
20. o is the same radiance under an aerosol-free atmosphere. When τ equals or exceeds unity, the atmosphere is said to be optically thick [R. J. Charlson et al., Science 255, 423 (1992); J. T. Kiehl and B. P. Briegleb, ibid. 260, 311 (1993); J. T. Houghton et al., Climate Change 1995: The Science of Climate Change (Cambridge Univ. Press, Cambridge, 1996)].
21. C. Bruehl and P. J. Crutzen, Geophys. Res. Lett. 16, 703 (1989).
22. S. Madronich, J. Geophys. Res. 92, 9740 (1987).
23. The photolysis rate coefficients presented here were determined with an instrument described by P. Kelley, R. Dickerson, W. Luke, and G. Kok [Geophys. Res. Lett. 22 (no. 19), 2621 (1995)]. Most natural surfaces (except snow) reflect 8% or less of incident UV irradiance. For these measurements, a black surface was placed below the actinometer to ensure a constant and known local albedo, ensuring that radiation was measured only from the downwelling 2 π steradians. This device has an absolute uncertainty of about 7% at the 95% confidence level for zenith angles less than 60°; the precision is a few percent.
24. K. L. Civerolo, thesis, University of Maryland, College Park, MD (1996); W. Ryan, B. Doddridge, R. Dickerson, K. Hallock, J. Air Waste Manage. Assoc., in press.
25. K. L. Civerolo, thesis, University of Maryland, College Park, MD (1996); W. Ryan, B. Doddridge, R. Dickerson, K. Hallock, J. Air Waste Manage. Assoc., in press.
26. The instruments monitor direct solar radiation and sky radiance at the almucanter (on the celestial circle parallel to the horizon and at the same zenith angle as the sun) and principal plane (the plane perpendicular to the horizon through the apparent position of the sun) [Y. Kaufman et al., J. Geophys. Res. 99, 10341 (1994); T. Nakajima et al., Appl. Opt. 35, 2672 (1996); B. N. Holben et al., Sixth International Symposium of Physical Measurements and Signatures in Remote Sensing, Val D'Isere, France, 17 to 21 January 1994 (Center National d'Etudes Spatiales, Toulouse, France, 1997), pp. 75-83; L. Remer, S. Gasso, D. Hegg, Y. Kaufman, B. Holben, J. Geophys. Res., 102, 16849 (1997).
27. The instruments monitor direct solar radiation and sky radiance at the almucanter (on the celestial circle parallel to the horizon and at the same zenith angle as the sun) and principal plane (the plane perpendicular to the horizon through the apparent position of the sun) [Y. Kaufman et al., J. Geophys. Res. 99, 10341 (1994); T. Nakajima et al., Appl. Opt. 35, 2672 (1996); B. N. Holben et al., Sixth International Symposium of Physical Measurements and Signatures in Remote Sensing, Val D'Isere, France, 17 to 21 January 1994 (Center National d'Etudes Spatiales, Toulouse, France, 1997), pp. 75-83; L. Remer, S. Gasso, D. Hegg, Y. Kaufman, B. Holben, J. Geophys. Res., 102, 16849 (1997).
28. The instruments monitor direct solar radiation and sky radiance at the almucanter (on the celestial circle parallel to the horizon and at the same zenith angle as the sun) and principal plane (the plane perpendicular to the horizon through the apparent position of the sun) [Y. Kaufman et al., J. Geophys. Res. 99, 10341 (1994); T. Nakajima et al., Appl. Opt. 35, 2672 (1996); B. N. Holben et al., Sixth International Symposium of Physical Measurements and Signatures in Remote Sensing, Val D'Isere, France, 17 to 21 January 1994 (Center National d'Etudes Spatiales, Toulouse, France, 1997), pp. 75-83; L. Remer, S. Gasso, D. Hegg, Y. Kaufman, B. Holben, J. Geophys. Res., 102, 16849 (1997).
29. The instruments monitor direct solar radiation and sky radiance at the almucanter (on the celestial circle parallel to the horizon and at the same zenith angle as the sun) and principal plane (the plane perpendicular to the horizon through the apparent position of the sun) [Y. Kaufman et al., J. Geophys. Res. 99, 10341 (1994); T. Nakajima et al., Appl. Opt. 35, 2672 (1996); B. N. Holben et al., Sixth International Symposium of Physical Measurements and Signatures in Remote Sensing, Val D'Isere, France, 17 to 21 January 1994 (Center National d'Etudes Spatiales, Toulouse, France, 1997), pp. 75-83; L. Remer, S. Gasso, D. Hegg, Y. Kaufman, B. Holben, J. Geophys. Res., 102, 16849 (1997).
30. E. Flowers, R. McCormick, K. Kurfis, J. Appl. Meteorol. 8, 955 (1969); J. Peterson, E. Flowers, G. J. Berri, C. L. Reynolds, J. H. Rudisill, ibid. 20, 229 (1981); R. B. Husar, J. M. Holloway, D. E. Patterson, W. E. Wilson, Atmos. Environ. 15, 1919 (1981); Y. Kaufman and R. S. Fraser, J. Appl. Meteorol. 22, 1694 (1983); R. B. Husar and W. E. Wilson, Environ. Sci. Technol. 27, 12 (1993).
31. E. Flowers, R. McCormick, K. Kurfis, J. Appl. Meteorol. 8, 955 (1969); J. Peterson, E. Flowers, G. J. Berri, C. L. Reynolds, J. H. Rudisill, ibid. 20, 229 (1981); R. B. Husar, J. M. Holloway, D. E. Patterson, W. E. Wilson, Atmos. Environ. 15, 1919 (1981); Y. Kaufman and R. S. Fraser, J. Appl. Meteorol. 22, 1694 (1983); R. B. Husar and W. E. Wilson, Environ. Sci. Technol. 27, 12 (1993).
32. E. Flowers, R. McCormick, K. Kurfis, J. Appl. Meteorol. 8, 955 (1969); J. Peterson, E. Flowers, G. J. Berri, C. L. Reynolds, J. H. Rudisill, ibid. 20, 229 (1981); R. B. Husar, J. M. Holloway, D. E. Patterson, W. E. Wilson, Atmos. Environ. 15, 1919 (1981); Y. Kaufman and R. S. Fraser, J. Appl. Meteorol. 22, 1694 (1983); R. B. Husar and W. E. Wilson, Environ. Sci. Technol. 27, 12 (1993).
33. E. Flowers, R. McCormick, K. Kurfis, J. Appl. Meteorol. 8, 955 (1969); J. Peterson, E. Flowers, G. J. Berri, C. L. Reynolds, J. H. Rudisill, ibid. 20, 229 (1981); R. B. Husar, J. M. Holloway, D. E. Patterson, W. E. Wilson, Atmos. Environ. 15, 1919 (1981); Y. Kaufman and R. S. Fraser, J. Appl. Meteorol. 22, 1694 (1983); R. B. Husar and W. E. Wilson, Environ. Sci. Technol. 27, 12 (1993).
34. E. Flowers, R. McCormick, K. Kurfis, J. Appl. Meteorol. 8, 955 (1969); J. Peterson, E. Flowers, G. J. Berri, C. L. Reynolds, J. H. Rudisill, ibid. 20, 229 (1981); R. B. Husar, J. M. Holloway, D. E. Patterson, W. E. Wilson, Atmos. Environ. 15, 1919 (1981); Y. Kaufman and R. S. Fraser, J. Appl. Meteorol. 22, 1694 (1983); R. B. Husar and W. E. Wilson, Environ. Sci. Technol. 27, 12 (1993).
35. x and NMHC emissions may not lead to proportionate reductions in particulate loading [J. R. Odum, T. P. W. Jungkamp, R. J. Griffin, R. C. Flagan, J. H. Seinfeld, Science 276, 96 (1997); Z. Meng, D. Dabdub, J. H. Seinfeld, ibid. 277, 116 (1997)].
36. x and NMHC emissions may not lead to proportionate reductions in particulate loading [J. R. Odum, T. P. W. Jungkamp, R. J. Griffin, R. C. Flagan, J. H. Seinfeld, Science 276, 96 (1997); Z. Meng, D. Dabdub, J. H. Seinfeld, ibid. 277, 116 (1997)].
37. R. Fraser and Y. Kaufman, J. Geosci. Rem. Sens. 23, 525 (1985);
38. Y. Kaufman, T. Brakke, E. Eloranta, J. Atmos. Sci. 43 (no. 11), 1135 (1986).
39. W. Wiscombe, Appl. Opt. 19, 1505 (1980).
40. K. Stamnes, S.-C. Tsay, W. Wiscombe, K. Jayaweera, Appl. Opt. 27 (no. 12), 2502 (1988).
41. NASA Jet Propulsion Laboratory, Chemical Kinetics and Photochemical Data for use in Stratospheric Modeling, Evaluation No. 11, 1994.
42. S. Kondragunta, thesis, University of Maryland, College Park, MD (1997).
43. J. Lelieveld and P. J. Crutzen, Nature 343, 277 (1990); F. Dentener and P. Crutzen, J. Geophys. Res. 98 (no. 4), 7149 (1993); C. Walcek, H.-H. Yuan, W. Stockwell, Atmos. Environ. 31 (no. 8), 1221 (1997); J. Liang and D. Jacob, J. Geophys. Res. 102 (no. 5), 5993 (1997); M. Andreae and P. Crutzen, Science 276, 1052 (1997); A. R. Ravishankara, ibid., p. 1058.
44. J. Lelieveld and P. J. Crutzen, Nature 343, 277 (1990); F. Dentener and P. Crutzen, J. Geophys. Res. 98 (no. 4), 7149 (1993); C. Walcek, H.-H. Yuan, W. Stockwell, Atmos. Environ. 31 (no. 8), 1221 (1997); J. Liang and D. Jacob, J. Geophys. Res. 102 (no. 5), 5993 (1997); M. Andreae and P. Crutzen, Science 276, 1052 (1997); A. R. Ravishankara, ibid., p. 1058.
45. J. Lelieveld and P. J. Crutzen, Nature 343, 277 (1990); F. Dentener and P. Crutzen, J. Geophys. Res. 98 (no. 4), 7149 (1993); C. Walcek, H.-H. Yuan, W. Stockwell, Atmos. Environ. 31 (no. 8), 1221 (1997); J. Liang and D. Jacob, J. Geophys. Res. 102 (no. 5), 5993 (1997); M. Andreae and P. Crutzen, Science 276, 1052 (1997); A. R. Ravishankara, ibid., p. 1058.
46. J. Lelieveld and P. J. Crutzen, Nature 343, 277 (1990); F. Dentener and P. Crutzen, J. Geophys. Res. 98 (no. 4), 7149 (1993); C. Walcek, H.-H. Yuan, W. Stockwell, Atmos. Environ. 31 (no. 8), 1221 (1997); J. Liang and D. Jacob, J. Geophys. Res. 102 (no. 5), 5993 (1997); M. Andreae and P. Crutzen, Science 276, 1052 (1997); A. R. Ravishankara, ibid., p. 1058.
47. J. Lelieveld and P. J. Crutzen, Nature 343, 277 (1990); F. Dentener and P. Crutzen, J. Geophys. Res. 98 (no. 4), 7149 (1993); C. Walcek, H.-H. Yuan, W. Stockwell, Atmos. Environ. 31 (no. 8), 1221 (1997); J. Liang and D. Jacob, J. Geophys. Res. 102 (no. 5), 5993 (1997); M. Andreae and P. Crutzen, Science 276, 1052 (1997); A. R. Ravishankara, ibid., p. 1058.
48. J. Lelieveld and P. J. Crutzen, Nature 343, 277 (1990); F. Dentener and P. Crutzen, J. Geophys. Res. 98 (no. 4), 7149 (1993); C. Walcek, H.-H. Yuan, W. Stockwell, Atmos. Environ. 31 (no. 8), 1221 (1997); J. Liang and D. Jacob, J. Geophys. Res. 102 (no. 5), 5993 (1997); M. Andreae and P. Crutzen, Science 276, 1052 (1997); A. R. Ravishankara, ibid., p. 1058.
49. D. Hauglustaine, B. Ridley, S. Solomon, P. Hess, S. Madronich, Geophys. Res. Lett. 23, 2609 (1996); D. Lary et al., J. Geophys. Res. 102, 3671 (1997).
50. D. Hauglustaine, B. Ridley, S. Solomon, P. Hess, S. Madronich, Geophys. Res. Lett. 23, 2609 (1996); D. Lary et al., J. Geophys. Res. 102, 3671 (1997).
51. M. Naja and S. Lal, Geophys. Res. Lett. 23, 81 (1996); C. Varshney and M. Aggarwal, Atmos. Environ. 26, 291 (1992); J. Herman et al., J. Geophys. Res. 102, 16911 (1997); K. Rhoads et al., ibid., p. 18981.
52. M. Naja and S. Lal, Geophys. Res. Lett. 23, 81 (1996); C. Varshney and M. Aggarwal, Atmos. Environ. 26, 291 (1992); J. Herman et al., J. Geophys. Res. 102, 16911 (1997); K. Rhoads et al., ibid., p. 18981.
53. M. Naja and S. Lal, Geophys. Res. Lett. 23, 81 (1996); C. Varshney and M. Aggarwal, Atmos. Environ. 26, 291 (1992); J. Herman et al., J. Geophys. Res. 102, 16911 (1997); K. Rhoads et al., ibid., p. 18981.
54. M. Naja and S. Lal, Geophys. Res. Lett. 23, 81 (1996); C. Varshney and M. Aggarwal, Atmos. Environ. 26, 291 (1992); J. Herman et al., J. Geophys. Res. 102, 16911 (1997); K. Rhoads et al., ibid., p. 18981.
55. We thank P. Crutzen, Y. Kaufman, I. Laszlo, and W. Ryan for helpful comments. Supported by Electric Power Research Institute through the NARSTO-Northeast Program, by the National Science Foundation-funded Center for Clouds, Chemistry, and Climate, and by NASA EOS Interdisciplinary Science Investigation.