Design principles#

Here we provide an overview of the key design principles and choices in OpenSCM-Units.

Unit handling makes use of the Pint library. This allows us to easily define units as well as contexts. Contexts allow us to perform conversions which would not normally be allowed e.g. in the ‘AR4GWP100’ context we can convert from CO2 to CH4 using the AR4GWP100 equivalence metric.

An illustration of how the unit_registry can be used is shown below

import traceback

from pint.errors import DimensionalityError

from openscm_units import unit_registry

unit_registry("CO2")

1 CO2

emissions_aus = 0.34 * unit_registry("Gt C / yr")
emissions_aus

0.34 C gigametric_ton/a

emissions_aus.to("Mt CO2 / yr")

1246.666666666667 CO2 megametric_ton/a

with unit_registry.context("AR4GWP100"):
    print((100 * unit_registry("Mt CH4 / yr")).to("Mt CO2 / yr"))

2500.0 CO2 * megametric_ton / a

More details on emissions unit#

Emissions are a flux composed of three parts: mass, the species being emitted and the time period e.g. “t CO2 / yr”. As mass and time are part of SI units, all we need to define in OpenSCM-Units are emissions units i.e. the stuff. Here we include as many of the canonical emissions units, and their conversions, as possible.

For emissions units, there are a few cases to be considered:

fairly obvious ones e.g. carbon dioxide emissions can be provided in ‘C’ or ‘CO2’ and converting between the two is possible
less obvious ones e.g. NOx emissions can be provided in ‘N’ or ‘NOx’, we provide conversions between these two which can be enabled if needed (see below).
case-sensitivity. In order to provide a simplified interface, using all uppercase versions of any unit is also valid e.g. unit_registry("HFC4310mee") is the same as unit_registry("HFC4310MEE")
hyphens and underscores in units. In order to be Pint compatible and to simplify things, we strip all hyphens and underscores from units.

As a convenience, we allow users to combine the mass and the type of emissions to make a ‘joint unit’ e.g. “tCO2”. It should be recognised that this joint unit is a derived unit and not a base unit.

By defining these three separate components, it is much easier to track what conversions are valid and which are not. For example, as the emissions units are all defined as emissions units, and not as atomic masses, we are able to prevent invalid conversions. If emissions units were simply atomic masses, it would be possible to convert between e.g. C and N2O which would be a problem. Conventions such as allowing carbon dioxide emissions to be reported in C or CO2, despite the fact that they are fundamentally different chemical species, is a convention which is particular to emissions (as far as we can tell).

Pint’s contexts are particularly useful for emissions as they facilitate metric conversions. With a context, a conversion which wouldn’t normally be allowed (e.g. tCO2 –> tN2O) is allowed and will use whatever metric conversion is appropriate for that context (e.g. AR4GWP100).

Namespace collisions#

Finally, we discuss namespace collisions.

CH\(_4\)#

Methane emissions are defined as ‘CH4’. In order to prevent inadvertent conversions of ‘CH4’ to e.g. ‘CO2’ via ‘C’, the conversion ‘CH4’ <–> ‘C’ is by default forbidden. However, it can be performed within the context ‘CH4_conversions’ as shown below:

try:
    unit_registry("CH4").to("C")
except DimensionalityError:
    traceback.print_exc(limit=0, chain=False)

pint.errors.DimensionalityError: Cannot convert from 'CH4' ([methane]) to 'C' ([carbon])

With a context, the conversion becomes legal again

with unit_registry.context("CH4_conversions"):
    print(unit_registry("CH4").to("C"))

0.75 C

As an unavoidable side effect, this also becomes possible

with unit_registry.context("CH4_conversions"):
    print(unit_registry("CH4").to("CO2"))

2.75 CO2

N\(_2\)O#

Nitrous oxide emissions are typically reported with units of ‘N2O’. However, they are also reported with units of ‘N2ON’ (a short-hand which indicates that only the mass of the nitrogen is being counted). Reporting nitrous oxide emissions with units of simply ‘N’ is ambiguous (do you mean the mass of nitrogen, so 1 N = 28 / 44 N2O or just the mass of a single N atom, so 1 N = 14 / 44 N2O). By default, converting ‘N2O’ <–> ‘N’ is forbidden to prevent this ambiguity. However, the conversion can be performed within the context ‘N2O_conversions’, in which case it is assumed that ‘N’ just means a single N atom i.e. 1 N = 14 / 44 N2O, as shown below:

try:
    unit_registry("N2O").to("N")
except DimensionalityError:
    traceback.print_exc(limit=0, chain=False)

pint.errors.DimensionalityError: Cannot convert from 'N2O' ([nitrous_oxide]) to 'N' ([nitrogen])

With a context, the conversion becomes legal again:

with unit_registry.context("N2O_conversions"):
    print(unit_registry("N2O").to("N"))

0.3181818181818182 N

NO\(_x\)#

Like for methane, NOx emissions also suffer from a namespace collision. In order to prevent inadvertent conversions from ‘NOx’ to e.g. ‘N2O’, the conversion ‘NOx’ <–> ‘N’ is by default forbidden. It can be performed within the ‘NOx_conversions’ context:

try:
    unit_registry("NOx").to("N")
except DimensionalityError:
    traceback.print_exc(limit=0, chain=False)

pint.errors.DimensionalityError: Cannot convert from 'NOx' ([NOx]) to 'N' ([nitrogen])

with unit_registry.context("NOx_conversions"):
    print(unit_registry("NOx").to("N"))

0.30434782608695654 N

NH\(_3\)#

In order to prevent inadvertent conversions from ‘NH3’ to ‘CO2’, the conversion ‘NH3’ <–> ‘N’ is by default forbidden. It can be performed within the ‘NH3_conversions’ context, analogous to the ‘NOx_conversions’ context:

try:
    unit_registry("NH3").to("N")
except DimensionalityError:
    traceback.print_exc(limit=0, chain=False)

pint.errors.DimensionalityError: Cannot convert from 'NH3' ([NH3]) to 'N' ([nitrogen])

with unit_registry.context("NH3_conversions"):
    unit_registry("NH3").to("N")

Design principles