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Sodium lighting doesn't provide a "continuous spectrum" (just "spikes" in the orange/yellow part of the spectrum) so it has no true "Color Temperature".
You just have to correct such images manually to the best of your ability particularly when different parts of a single image may be receiving light from several different kinds of light source.
>Is there any reason why 2000K is the minimum coolest temperature available on the ACR white balance selector? Wikipedia list match flame at 1700, and candle-light at 1850K.
If you took a picture under candle light or a match flame, you probably would want to keep a reddish cast rather than balancing all the way to white. 2000K is usually sufficient for most conditions. If you want complete control over WB, try DCRaw, which allows you to seet the white balance multipliers manually.
>I've taken some photos under a railway bridge with very dim sodium lights, and the grey pavement came up around 2000K +1 with the ACR white balance selector. I'm wondering if in fact a cooler temperature might be more accurate, and why it's not possible to go below 2000.
Sodium lights come in two flavors: high pressure and low pressure. Low pressure sodium lamps were introduced in the 1930's and have a tubular shape, resembling florescent lights. There is a picture of a low pressure lamp in the link. Such lamps are often found in viaducts and under bridges. They emit essentially monochromatic light and are not suitable for color photography.
The high pressure lamps are more recent and resemble an incandescent bulb. Like Ann said they have a discontinuous spectrum (shown in the Wikipedia link) and a low color rendering index, but you might be able to coax come color out of the picture.