The aim of our study was to assess the effects of the CCS on
pollution concentrations in London. We approached this task in
three ways. Our first, and principal, objective was to study the
potential impact of the CCS on pollutant concentrations within the
congestion charging zone (CCZ) itself. Secondly, we investigated
the possible impact of the scheme upon pollution concentrations
on the area surrounding the charging zone and finally we assessed
the possibility that the CCS may have had an impact upon pollutant
concentrations at weekends (when the CCS does not operate).
Pollution concentrations in London are influenced by local
factors and by London-wide and regional (including trans-
boundary) sources and determinants of pollution (e.g. trends in
vehicle numbers in London, vehicle technology, meteorological
conditions and long-range transport of pollutants). To identify the
impact of the CCS on pollution levels within the CCZ therefore we
compared temporal changes in pollutant concentrations at sites
within the zone with those located in a control area where
temporal changes in pollutant concentrations were assumed to
indicate London-wide and regional temporal patterns in pollution
only. The control area was defined as the area 8 km or more from
the CCZ centre but within Greater London.
To assess the possible impact of the scheme upon pollution
concentrations on the area surrounding the charging zone, termed
the boundary zone, we plotted the temporal changes in pollutants
at all available monitors within Greater London (subject to data
availability) ordered by distance from the centre of the CCZ. This
analysis was then repeated using data for congestion charging
hours (07:00–18:00) during weekends, when the scheme was not
in operation to provide an assessment of the impact of the CCS on
non-working days.