CATCH introduced advanced and innovative pollution monitoring techniques developed by the University of Liverpool 's Centre for Intelligent Monitoring Systems (CIMS). A CCTV-based system using light scattering techniques was used and cameras were positioned at fixed sites within the city and on the buses themselves. The monitoring lead to the mapping of high concentrations of particulates within Liverpool city centres and identified the times at which pollution was at its greatest.

Monitoring with this system on Car Free Day in September 2003 and 2004 recorded a significant fall in the level of particulates in the air by between 50 and 75% when traffic was removed from a particular area. This drop in particulate concentration was outside the day to day variations caused by changing weather and other ambient conditions and suggests that the ‘car free day' did have a measurable and positive impact on air quality in the immediate locality of the road closure.

The Liverpool Air Quality Action Plan can be found here

The full evaluation of the pollution monitoring component of the CATCH demonstration is available in chapter 8 of the CATCH Evaluation Report