Where can I find data about critical loads, levels and background concentration and deposition for UK national (SSSIs) and international protected sites?

You can use the Site Relevant Critical Loads tool to look up individual sites to get critical load information and deposition and concentration data at a particular site. . Information about critical loads themselves can be found on the quick link pages. The full downloadable data is also available on UK Centre for Ecology and Hydrology's Environmental Information Data Centre (EIDC).