It's not a congestion charge if its purpose is to reduce emissions

According to the Norwich Evening News, Norfolk (UK) County Council, in its Proposed Transport for Norwich Strategy has suggested congestion charging as part of its strategy to improve air quality. I used to live in Norwich, so I have a particular interest in this, so reviewing the Proposed Transport for Norwich Strategy does reveal that congestion charging is mentioned four times.  Three times in the context of improving air quality and once to 

The second part of its "vision" is "improving the quality of our air" and this includes "road charging/congestion charge" presumably as a tool to achieve this.  This is far from helpful, because a congestion charge by definition is established to ease congestion.  Yes it should also reduce emissions, but because it should only operate at the times and locations of congestion it isn't a scheme to comprehensively address emissions, like the London Ultra Low Emission Zone. 

This is where confusion appears, because if you sell a congestion charge as a public policy measure based on it actually being a low emission zone, then it isn't a congestion charge.  

A low emission zone operates much longer hours (indeed up to 24/7) because its purpose is to exclude higher polluting vehicles from the zone.  It isn't to collect revenue (the only revenue are effectively fines or permits to drive in the zone for such vehicles).  A congestion charge shouldn't operate at times of low or zero congestion, because then it would be overpricing the road.

There is some hope that the Norfolk County Council does actually mean congestion charging for the sake of improving trip reliability, for under the statement of policy "Journey times and reliability", the strategy states:

Journey times and reliability will be improved on the local highway network with particular emphasis to support fast and frequent bus services

We will ensure that journeys by bus are consistent and journey times are reduced where possible and consider the feasibility of demand management approaches such as congestion charging and workplace parking levies to facilitate traffic reduction to free up road space for essential travel.

THIS is a reason to introduce congestion charging, to actually reduce congestion.  However, this can't just be for buses, it needs to improve journey times for the vehicles that are being charged, otherwise it is simply a tax to punish driving.

If local authorities WANT to improve journey times and trip reliability, then sure introduce a congestion charge, and it will happen to reduce emissions as well, because there should be less traffic and the traffic that remains will flow more efficiently, wasting less fuel. Then those paying are getting a benefit from improved travelling conditions, and it happens to reduce pollution too.

HOWEVER, if your objective is to reduce emissions first and foremost, then a congestion charge on peak traffic isn't the tool for the job.  The related tool is a low-emission zone, that penalises vehicles that are not rated as having low emissions, and it should operate at all times there are issues around pollution.  It isn't priced to optimise traffic flow, but rather operates to improve air quality.

I THINK Norfolk County Council actually wants a low emission zone, but is calling it a congestion charge.