Abiotic Effects

Stratospheric Ozone Depletion

The ozone-depleting potential (ODP) of a substance is defined as the ratio of calculated ozone column change for each unit mass of a gas emitted into the atmosphere relative to the depletion calculated for an equal mass of reference gas chlorofluorocarbon-11 (CFC-11), an ozone-depleting substance with an ODP of 1. The formula used to calculate ODP is applicable only for substances with chlorine or bromine atoms. As DNOC does not contain any chlorine or bromine atoms, it has an ODP of zero, and it is concluded that DNOC will not contribute to ozone depletion.

Ground-Level Ozone

Formation To estimate ground-level ozone formation, a Photochemical Ozone Creation Potential (POCP) index is used (Environment Canada, 1996). The POCP is a measure of the relative effectiveness for ozone formation of a unit mass of organic substance compared with that of an equivalent mass of ethene. By definition, ethene has a POCP value of 100.

The episodic ozone formation can be estimated from a reactivity scale based on the rate constant for the hydroxyl–hydrocarbon reaction and the molecular weight of the subject substance relative to those properties of ethene:

POCP = (k_s/M_s) (M_ethene/k_ethene) × 100

where:

k_s = reaction rate constant at 298 K for the reaction with the hydroxyl radical for the substance (3.0 × 10⁻¹¹ cm³/mol per second)
k_ethene = reaction rate constant at 298 K for the reaction with the hydroxyl radical (8.5 × 10−12 cm³/mol per second)
M_s = molecular mass of the substance (198.13 g/mol for DNOC)
M_ethene = molecular mass of ethene (28 g/mol).

The POCP value for DNOC is 50.

Although it is recognized that DNOC may have the potential to contribute to ground-level ozone formation, the relative contribution is dependent on the concentration as well as on the reactivity of DNOC relative to other volatile organic compounds (VOCs) in an area. Nevertheless, DNOC is a reactive VOC. VOCs “that participate in atmospheric photochemical reactions” were added as a class to Schedule 1 of CEPA 1999 (List of Toxic Substances) in July 2003 and are subject to a range of risk management activities focusing on total VOCs, rather than on individual compounds. Since VOCs that participate in atmospheric photochemical reactions already appear on Schedule 1, a given existing substance’s ability to participate in photochemical reactions is not used as a basis for concluding that it is toxic under Section 64 of CEPA 1999.

It is further worthy of note that releases of DNOC from non-pesticidal uses are believed to be largely to aquatic systems, with little partitioning to air. The extent and mechanism of possible DNOC formation in the atmosphere from precursor species are not yet well understood.

Global Warming Potential

Global Warming Potential (GWP) is defined as the ratio of calculated warming for each unit mass of a gas emitted into the atmosphere relative to the calculated warming for a reference gas, CFC-11. The GWP for DNOC was estimated to be 7.2 × 10⁻¹², and therefore DNOC is not expected to contribute significantly to climate change.