Overview of retrofitting in various countries
More information is available from the pages on the individual LEZs, and each LEZs' own websites, but in general in terms of fitting particulate filters:
In Germany and the Czech Republic you can retrofit to Euro 2, 3 or 4 with a particulate filter.
In the Netherlands since July 2013 the full Euro 4 standard is required to access to the environmental zones, and retrofitted vehicles are no longer allowed. More information (in Dutch).
In London, vehicles can be retrofitted as long as the retrofit device is certified as meeting the required emissions standard of Euro 4 in 2012. Only full filters are certified in London.
In Denmark, particulate filters should be fitted on Euro 3 or earlier vehicles after 2010. Only full filters are certified in Denmark.
In North Italy, generally, particulate filters can be fitted to allow access into LEZs. Details depend on the individual city.
In Sweden retrofitting is allowed if you meet fit both a particulate filter and SCR to meet the required standard.
More information on the certification for particulate filters in the different countries can be found in finding a certified filter.