Hi Brook,
That’s tricky. There are equivalents for GHG emissions as you identified, as well as some other indicators such as litres of water used (equivalent volume of Olympic size swimming pools).
The reason why there aren’t as many other analogue values is probably because they are less consistent. E.g. the GHG emissions of an average car can be easily calculated using average fuel type and fuel efficiency. This becomes a bit harder for Nitrogen emissions (although it should be possible to have a decent stab at it).
As most studies report on GHG emissions as their main indicator, it is logical that this is the most reported analogous example. You may also need to consider the potentially confusing results of expressing both CO2 and N in analogues. Imagine a simple hypothetical process that burns 1 L of petrol and 1 L of diesel. The GHG emissions may be equivalent to driving a car for 15 km. The NOx emissions may be equivalent to driving a car for 20 km. You could get different analogue results for a process depending on how your analogue reference is defined. That would obviously be unhelpful.
You would also have to be wary of the uncertainty that is much higher in the non-GHG indicators than it is for carbon. e.g. NOx emissions of an engine are not only a result of the fuel used, but perhaps more so of the way the engine is run. That makes it incredibly difficult to come up with meaningful analogue equivalents.
Finally, LCA indicators are expressed in equivalents already. Translating these values to analogues adds a layer of interpretation/inaccuracy to the LCA results. Your best approach could be to find a reliable normalisation set (e.g. looking at the total impacts of an average global or North American person across the impact categories you are interested in.) That allows you to express results of an LCA according to how it relates to the annual impacts of an average person. For example, “the nitrogen emissions of the process are equal to 1.5 times the average person’s annual nitrogen emissions”.
Hope this helps.
Rob