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# Atmospheric degradation rates

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hannaholmquist
Atmospheric degradation rates

Hello!

The organic substances database manual state that "the kOH is multiplied with the [OH]" ([OH]=1,5E6 molecules/cm3 per 12 hrs daylight). Does that mean that the kDegA (s-1) is calculated by taking the estimated (when no experimental data are available) overall OH rate constant (cm3/molecule-sec) times the [OH] as it is (i.e. radical formation only during daylight and assuming a 12 hrs daylight period) or should the calculation extrapolate the 12-hrs OH-concentration to a 24 hrs day? If so, how should that be done?

Many thanks in advance,

Hanna

USEtox Team
Degradation rate constants in air

Whenever using EPISuite estimated degradation in air, there are two ways to obtain the degradation rate constant in air based on the hydroxyl radicals reaction:

1. Start from the overall OH rate constant, multiply it by the OH concentration in air and divide by a factor of 2 to reflect degradation during a 12 hours/day period. Example for CAS 100-00-5: 1.714E-13 [cm³/mol-OH/s] x 1.5E+06 [mol-OH/cm³] / 2 = 1.3E-07 [1/s]
2. Start from the half-life and convert it to a rate constant including unit conversion from day to second. Example for CAS 100-00-5: LN(2)/(62.39 [d] x 86400 [s/d]) = 1.3E-07 [1/s]