Ultraviolet-B radiation, ozone and plant biology

Environmental Pollution 110 (2000) 193±194

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Commentary

Ultraviolet-B radiation, ozone and plant biology S.V. Krupa* Department of Plant Pathology, University of Minnesota, 1991 Upper Buford Circle, 495 Borlaug Hall, St. Paul, MN 55108, USA Received 5 October 1999; accepted 25 October 1999

``Capsule'': Some plant scientists believe that increased UV-B at the surface will also generate more ozone. The underlying rationale does not appear to be wholly correct.

Clearly, at the present time, everyone recognizes the bene®cial role of stratospheric versus the deleterious e€ects of tropospheric or surface level ozone (O3) on human health and welfare (vegetation, materials, etc.). One of the bene®cial e€ects of stratospheric O3 is to substantially reduce the level of harmful ultraviolet (UV)-B radiation (280±315 nm wavelength) from arriving at the surface (UV-B is approximately 0.5% of the total radiation at the surface). There is much concern about the observed and/or predicted losses in the stratospheric O3 column and the consequent increases in UV-B radiation at the surface. In this regard, periodically I have heard some vegetation e€ects scientists state that more UV-B will result in more O3 formation at the surface. Most recently I have seen this conclusion in print, authored by a plant scientist. Some degree of clari®cation is required regarding this matter. In the stratosphere, formation of O3 occurs from oxygen (O2) molecules through the Chapman cycle (Finlayson-Pitts and Pitts, 1986). While O2 absorbs radiation at < 220 nm (shorter wavelengths of UV-C), wavelengths  O3 absorbs at 220±290 nm (longer wavelengths of UV-C and shorter wavelengths of UV-B). < 220 nm† ! 2O O2 ‡ hv …l 

…R1†

O ‡ O2 ‡ M ! O3 ‡ M

…R2†

O ‡ O3 ! 2O2

…R3†

O3 ‡ hv ! O ‡ O2

…R4†

* Tel.: +1-612-625-8200; fax: +1-612-625-9728. E-mail address: [email protected] (S.V. Krupa).

where M is a third body of matter such as N2 serving to remove the excess energy produced in the reaction. Thus, in the stratosphere the photolysis of O2 leads to the formation of O3, while the photolysis of O3 leads back to the formation of O2, essentially resulting in a steady state of the reaction sequence, as long as there are no other intervening factors. In contrast, in the troposphere, atmospheric oxidation and the formation of photochemical oxidants are radical driven. The hydroxyl radical (OH) is the most important oxidizing radical in the troposphere. The primary source of OH is the UV-B photolysis of O3 in the presence of water vapor. < 320 nm† ! O…1 D† ‡ O2 O3 ‡ hv …l 

…R5†

O…1 D† ‡ H2 O ! 2OH

…R6†

O…1 D† ‡ M ! O…3 P† ‡ M

…R7†

where O(1D) is the electronically excited atom and O(3P) is the atom at ground state. The primary mechanism for the formation of O3 at the surface is through the photolysis of nitrogen dioxide (NO2) [(R9)] (Kley et al., 1999). This takes place at < 420 nm, with a peak yield of wavelengths of radiation  the oxygen (O) atoms at 397.8 nm (UV-A). There is no evidence to indicate that surface level UV-A radiation is increasing (Stanhill and Moreshet, 1992), although there are data to show that surface concentrations of O3 precursor pollutants are on the increase at many urban centers across the world. An example of the reaction sequence leading to O3 production in the troposphere is: HO2  ‡NO ! NO2 ‡ OH

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…R8†

194

S.V. Krupa / Environmental Pollution 110 (2000) 193±194

Fig. 1. Relationship between ultraviolet (UV)-B radiation and column ozone (O3) at Uccle, Belgium. One Dobson unit is de®ned as 0.01-mm thick O3 layer at STP or approximately 0.67 ppm O3. Figure courtesy of Belgian Institute for Space Aeronomy: www.oma.be/BIRA-IASB.

< 420 nm† ! NO ‡ O NO2 ‡ hv …l  O ‡ O2 ‡ M ! O3 ‡ M

…R9† …R10†

where HO2 is the peroxy radical and NO is nitric oxide. Also note that (R2) and (R10) are the same. Thus, the driving radiative mechanisms for O3 formation are UVC in the stratosphere and UV-A in the troposphere. As noted previously, absorption of UV-B by O3 in the troposphere would lead to its photolysis [(R5)]. In the presence of H2O, the product of O3 photolysis is OH [(R6)] that can be further converted to HO2 leading to reaction (R8). However, any increase in the subsequent O3 concentration will depend on the mix and the ratios of volatile organic compounds and the oxides of nitrogen (Kley et al., 1999). On a geographic scale, this will be highly patchy both in time and space. In addition, there are reactions other than the formation of O3 that will also consume OH radicals. Nevertheless, disproportionate losses of O3 in the stratosphere (80% of the total column O3) cannot be o€set beyond a point, by any increases in the tropospheric O3 (10% of the total column).

Therefore, levels of UV-B and O3 at the surface should be anti-correlated (Fig. 1). In addition to O3, both ®ne and coarse particles at the surface also act as ®lters of solar radiation. Frequently, at the surface high levels of O3 and ®ne particulate sulfate co-occur in polluted areas. Therefore, plant scientists in their future studies should consider the consequences of the random, sequential daily occurrences of high levels of O3 and UV-B radiation to achieve realistic results (Krupa et al., 1998).

References Finlayson-Pitts, B.J., Pitts Jr., J.N., 1986. Atmospheric Chemistry: Fundamentals and Experimental Techniques. Wiley, New York. Kley, D., Kleinman, M., Sandermann, H., Krupa, S., 1999. Photochemical oxidants: state of the science. Environmental Pollution 100, 19±42. Krupa, S.V., Kickert, R.N., JaÈger, H.-J., 1998. Elevated Ultraviolet (UV)-B Radiation and Agriculture. Springer, Heidelberg and Landes Bioscience, Georgetown, TX. Stanhill, G., Moreshet, S., 1992. Global radiation climate changes: the world network. Climatic Change 21, 57±75.