You are free to download and use the source code freely under an MIT License. For any details contact me under email@example.com.
You can access it via subversion: svn://orgis.org/pep-tracer/trunk
The main publication featuring PEP-Tracer is "Influence of interactive stratospheric chemistry on large-scale air mass exchange in a global circulation model" by Orgis et al., published in the European Physical Journal Special Topics, vol. 174, with the DOI 10.1140/epjst/e2009-01105-8. The article is also available here in my personal layout: The article, A4 full-page PDF. I am preparing a PDF formatted for small eReader screens, it will be published here once I tested it on my own device.