The theme for the special issue is the EMEP as "an integrated system" of models, observations and overall assessment, with scientific work and findings in the hands of the parties and the centres in conjunction. The focus in ACP is on the research issues, with a discussion of the relevant methodological issues. We will look at the past, the present and the possible future.

This special issue serves as a central location for the analysis of a range of scientific issues based on the EMEP programme (EMEP models and measurements programme including overall assessment), cf. www.emep.int and www.unece.org/env/lrtap/welcome.html.

This includes European as well as global acidification, eutrophication, tropospheric ozone and other atmospheric contaminants:

• their state and trends;
• emissions and their trends;
• transboundary source–receptor relationships and their change;
• ecosystem recovery;
• air pollution changes as climate variability and trends change;
• particulate matter;
• the reactive nitrogen cycle and its change;
• toxic substances;
• the role of atmospheric and atmosphere–surface processes in the cycling of biogeochemical substances, e.g. deposition.