Models predict that CO_2 doubling in the atmosphere cools the stratosphere by as much as 10°C. We argue that this effect alone, through the temperature dependence of the ozone spectrum, can result in an increase of a few percent in UV-B (280–320 nm) penetration at the Earth's surface. The increase in Erythema-weighted UV radiation is about 1%. Future spectral observations of UV at the surface could distinguish this effect from any result of changes in ozone abundance

Gerstell, M. F.

Jiang, Yibo

Liao, Hong

Yung, Yuk L.

Caltech Authors - Main

GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 24, PAGES 3229-3231, DECEMBER 15, 1997 
Enhanced UV penetration due to ozone cross-section changes 
induced by doubling 
Yuk L. Yung, Yibo Jiang • 
Division of Geological nd Planetary Sciences, California Institute of Technology, Pasadena, California 
Hong Liao 
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 
M. F. Gerstell 
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 
Abstract. Models predict that CO• doubling in the desefi• by Miohelangeli et al. (1992) and it includes the 
atmosphere cools the stratosphere by as much as 10øC. We effects ofabsorption, Rayleigh and aerosol scattering without 
argue that this effect alone, through the temperature polarization. 
dependence of the ozone spectrum, can result in an increase In this model, the temperature-dependent ozone absorption 
of a few percent inUV-B (280-320 nm) penetration at the cross ection data in the UV region are taken from Malicet et 
Earth's surface. The increase in Erythema-weighted UV el. (1995) with resolution of 0.01 nm at 5 temperatures (218 
radiation is about 1%. Future spectral observations ofUV at K, 228 K, 243 K, 273 K, 295 K). Our calculations are 
the surface could distinguish this effect from any result of performed at the resolution of 0.2 nm. The vertical profile of 
changes inozone abundance. ozone concentration (Fig. lb) is taken from a standard model 
Introduction 
The increase of carbon dioxide in the atmosphere has two 
distinct consequences [Hansen et el., 1984]. On the one hand, 
temperature increases, by a few degrees C, at the surface and 
in the troposphere. On the other hand, the stratosphere cools 
by as much as 10øC. As we shall show, the latter may have a 
significant impact on the penetration of solar UV radiation to 
the surface of the Earth. 
Since 90% of atmospheric ozone resides in the 
stratosphere, and its absorption cross sections in the UVB 
(280 - 320 nm) region are strongly temperature dependent, he 
cooling effect induerod by a CO2 increase will cause a 
decrease in the absorption cross sections of ozone in the 
stratosphere, and hence an increase of solar UV radiation at 
the surface. In this note, we quantify how cooling due to CO2 
doubling should affect the UV radiation at the surface of the 
earth, and the biologically effective UV-dose. 
In our work the feedbacks on the ozone layer due to 
temperature change are not considered. 
Model 
To investigate quantitatively the radiative properties of 
atmospheric ozone in the IJV region, we employ a radiative 
transfer model consisting of 80 vertical layers from 0 km to 
80 km with vertical resolution of I kin. This model 
accurately computes multiple scattering using a method 
1Now at Jet Propulsion Laboratory, California Institute of 
Technology, Pasadena, CA 91109 
Copyright 1997 by the American Geophysical Union. 
Paper number 97GL03311. 
0094-8534/97/97GL-03311505.00 
for the tropics [Yang et el., 1991]. The total column ozone is 
267.23 DU. 
The aerosol vertical profile and the size distribution are 
taken from Demerjian et el. (1980). The maximum radius of 
this distribution and the complex refractive index of the 
particles are 0.07pm and 1.5-0.1i, respectively. The surface 
albedo is set to 0.1 [Eck et el., 1987; Jiang and Yung, 1997]. 
The temperature profile for a recent CO: condition is also 
taken from a standard model for the tropics [Yang et al., 
1991], and that for the doubled CO: case is from Pitari et al. 
(1992), who predicted temperature changes due to CO: 
doubling by using a three dimensional spectral model of the 
atmosphere. We adopt a representative tropical profile of 
temperature change from this model (see Fig. 1). The model 
predicts a temperature increase of 2.0øC at the surface and 
2.5øC in the upper troposphere. In the stratosphere the trend 
is negative. There is maximum cooling of 10øC around 50 km 
and 5øC around 30 kin, where most of the ozone resides. The 
results are comparable to those obtained by other groups 
[Schlesinger and Mitchell, 1985]. 
Results and Discussion 
Figure 2 gives the spectral change in UV transmissions due 
to CO2 doubling for solar zenith angles (SZA) 0 ø, 30 ø and 60 ø. 
As shown in this figure, most of the change is in the UVB 
region with up to 8% of increase around 280 nm when SZA = 
0 ø, and 10% increase when SZA = 60 ø. In all the pertur• 
calculations, the ozone column density remains the same. 
The changes shown in Figure 2 are solely due to the effect of 
temperature change on the cross-sections. 
It has been shown that larger than normal evels of UV 
radiation inhibit photosynthesis in marine microflora [Smith 
and Baker, 1980] and damage DNA in all organisms 
[Friedberg, 1985]. The biological effect of UV radiation may 
be conveniently expressed in terms of the effective UV4ose 
rate defined as the convolution of an action speetnm• of the 
3229 
3230 YUNG ET AL.: ENHANCED UV PENETRATION 
10 9 
8O 
Ozone Concentration (cm -3) 
1010 10 TM 1012 
' ' ..... I ' ' ' ' .... I 
lO 13 
, i , , , 11 
60 
o 4o 
20 
\ 
\ 
\ 
\ 
\ 
i I i i I i 
o 
0 , I , , , , I 
-10 -5 5 
Temperoture Chonge (K) due to Doubled CO 2 
Figure 1. Altitude profiles of temperature change (solid line) and ozone concentrations (dashed line) adopted in the model. 
particular biological response with the solar irradiance 
spectrum in the biosphere [Tsay and Stamnes, 1992; Stamnes, 
1993]. Action spectra are empirical wavelength dependent 
Table 1. The Percentage Change of Integrated UV-Dose at 
the Earth Surface Due to CO2 Doubling and 1% Column 
Ozone Reduction 
Solar Zenith 1% Column 
Angle (degree) CO2 Doubling Ozone Reduction 
DNA 
0 1.04 0.68 
30 1.08 2.31 
60 1.20 2.39 
Plant 
0 0.72 0.47 
30 0.78 1.71 
60 1.10 2.33 
Erythcma 
0 0.66 0.41 
30 0.67 1.31 
60 0.77 1.35 
functions that measure the adverse impact of UV radiation on 
life processes. 
The UV-dose rate change (280-400 nm) for DNA, Plant 
and Erythema ction spectrum is surmna•ed in Table 1. The 
changes increase as SZA increases, mainly because of the 
increased optical path of ozone absorption. The change is 
larger than 1.0% at SZA 60 ø, which may have a measurable 
effect on the biosphere. For comparison, Table I also 
includes the UV-dose rate changes we calculate for 1% 
reduction in the ozone column density. The UV dosage 
change induced by CO2 cooling is equivalent o that caused 
by 1% reduction in the ozone column. Since efforts to 
decrease the use of chlorofluorocarbons seem to be ahead of 
any agreement to reduce the burning of carbon [Houghton et 
al., 1996], the relative importance of the effect we are 
describing might be expected to increase in the future. 
The comparisons we have shown against 1% ozone 
decreases erve mainly to illustrate that the effect we are 
describing need not be negligible. High precision 
measurements of the spectral characteristics of UV fluxes in 
the Huggins bands at the Earth's surface in the future could 
reveal the contribution due to each mechanism. 
YUNG ET AL.: ENHANCED • PElqETRATION 3231 
c 
c• 
c 
c• 
_c 
lO 
,, 
o 
29o 
.. o 
. ,, , 
ß -, 
295 300 305 310 315 320 
Wavelength (nm) 
Figure 2. Change oftransmission at SZA 0 ø (solid line), 30 ø (dotted line) and 60 ø (dashed line) due to ozone cross-section 
changes induced by CO• doubling. 
Acknowledgments. We thank A- P. Ingersoll, M. Allen, J. Kaye, 
D. V. Michelangeli, S. Sander for valuable comments. We are 
grateful to M. Ko and two anonymous referees for suggestions on 
focusing and shortening the paper. This research is supported by 
NASA grant NAGI-1806 to the California Institute of Technology. 
Contribution number 5674 from the Division of Geological and 
Planetary Sciences, California Institute of Technology. 
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Yuk L. Yung, Yibo Jiang, and M. F. Getstell, Division of 
Geological and Planetary Sciences, California Institute of 
technology, Pasadena, CA 91125 (e-mail: 
yly(•mercu I .gps.caltech. edu; ybj @mercu I .gps.caltech. edu; 
mfg•m ercu I .gps.caltech. edu )
Hong Line, Division of Engineering and Applied Science, 
California Institute of Technology, Pasadena, CA 91125. (c-mail: 
hong@cco.caltech.edu) 
(Received July 17, 1997; revised November 4, 1997; accepted 
November 7, 1997) 


Enhanced UV penetration due to ozone cross-section changes induced by doubling

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Enhanced UV penetration due to ozone cross-section changes induced by doubling

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