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2.3.6.3 Photochemical reactions in the atmosphere<br />

ENVIRONMENTAL EXPOSURE ASSESSMENT<br />

Although <strong>for</strong> some substances direct photolysis may be an important breakdown process, the<br />

most effective elimination process in the troposphere <strong>for</strong> most substances results from reactions<br />

with photochemically generated species like OH radicals, ozone <strong>and</strong> nitrate radicals. The<br />

specific first order degradation rate constant of a substance with OH-radicals (kOH in<br />

cm 3 .molecule -1 .s -1 ) can either be determined experimentally (OECD, 1992c) or estimated by<br />

(Q)SAR-methods <strong>and</strong> other methods described in Chapter 4 (US EPA, 2002). By relating kOH to<br />

the average OH-radical concentration in the atmosphere, the pseudo-first order rate constant in<br />

air is determined:<br />

Explanation of symbols<br />

kdegair = kOH • OHCONCair • 24 • 3600 (28)<br />

kOH specific degradation rate constant with OH-radicals [cm 3. molec -1. s -1 ] data set/Ch.4<br />

OHCONCair concentration of OH-radicals in atmosphere [molec . cm -3 ] 5 . 10 5 *<br />

kdegair pseudo first order rate constant <strong>for</strong> degradation in air [d -1 ]<br />

*The global annual average OH-radical concentration can be assumed to be 5.10 5 molecules.cm -3 (BUA, 1992).<br />

Degradation in the atmosphere is an important process <strong>and</strong> it is essential to consider whether it<br />

can affect the outcome, particularly <strong>for</strong> high tonnage substances when the regional concentration<br />

may be significant. Photodegradation data in the atmosphere must be evaluated with some<br />

care. Highly persistent substances may be reported as rapidly degraded in air under<br />

environmental conditions where the chemical could be in large amounts in the gas phase. In<br />

the real environment, most of the substance may be associated to particles or aerosol <strong>and</strong> the<br />

real atmospheric half-life could be orders of magnitude higher.<br />

2.3.6.4 Biodegradation in a sewage treatment plant<br />

The assessment of biodegradability <strong>and</strong>/or removal in sewage treatment plants should preferably<br />

be based on results from tests simulating the conditions in treatment plants. Such a test may be<br />

the OECD 303 test (2001b) or equivalent. For further <strong>guidance</strong> on use of STP simulation test<br />

results, see Section 2.3.7.<br />

The ready biodegradability tests that are used at the moment are aimed at measuring the ultimate<br />

biodegradability of a substance. They do not give a quantitative estimate of the removal<br />

percentage in a wastewater treatment plant. There<strong>for</strong>e, in order to make use of the<br />

biodegradation test results that are available <strong>and</strong> requested in the present chemical legislation, it<br />

is necessary to assign rate constants to the results of the st<strong>and</strong>ard tests <strong>for</strong> use in STP-models.<br />

These constants are based on a relatively limited number of empirical data. However, since<br />

direct measurements of degradation rates at environmentally relevant concentrations are often<br />

not available, a pragmatic solution to this problem has been found. For the purpose of modelling<br />

a sewage treatment plant (STP), the rate constants of Table 6 were derived from the<br />

biodegradation screening tests. All constants in Table 6 have the following prerequisites:<br />

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