Fig.2b. Geometric mean concentrations of pollutants at roadside and background monitoring sites in the CCZ and the control zone measured two years before and two years after the introduction of the CCS. Note: Solid circles indicate monitoring stations inside the congestion charging zone: hollow circles monitoring station in the control zone.
Unusual meteorological conditions but by using four years of data we minimized any potential bias arising from these unusual conditions that may not have already been accounted for by our temporal-spatial study design.
The question of whether the observed reduction in background concentrations of NO.PM10 and CO and increases in background NO2 and O3 observed within the zone are attributable to the implementation of the CCS is problematic. The congestion charging scheme eas one specific action within a general programme of measures to reduce traffic congestion and air pollution across London as a whole. These measures included the implementation of bus lanes, the use of larger “bendy” buses, the fitting of particle
Fig.3. Percentage change in geometric mean pollutant concentrations at background monitoring stations stratified by station location (within zone, boundary and control zones and distance from CCZ centre. Note: Solid circles indicate stations within CCZ, hollow circles with cross indicate station and the X-axis shows the distance in km from the centre of the charging zone.
Table 3
Geometric mean concentrations of pollutants at background monitoring sites during CCS hours at weekends two years before and two years after the introduction of the CCS.
Traps to diesel buses, increased bus frequency and changes to traffic light phases. Whilst these were not confined to the charging zone it is possible that, because of the concentration of traffic in the centre, these other measures may have hed a greater relative impact in central London than outer London, thus explaining the temporal patterns in pollution observed in this analysis.
The boundary of the CCZ is formed by an Inner Ring Road and is the most obvious alternative rute for through traffic wishing to avoid the zone. TfL reported an initial increase in total vehicle-kil-ometres of 4% on the boundary road together with a decrease in congestion resulting from the implementation of effective traffic management on this key route (TfL, 2007). They also reported that measurements taken during 2004 and 2005 suggested that traffic on the Inner Ring Road during weekday charging hours declined very slightly overall compared to 2003 and that flows in 2005 were comparable with pre-charging conditions in 2002. The observed temporal changes in background pollutant concentrations within the CCZ reported in this study were also reflected, to some degree, at monitoring stations just outside the charging zone. There are two potential explanations for this observation: 1) these changes were
Fif.4. Percentage change in geometric mean pollutant concentrations measured during weekends at background monitoring stations stratified by station location (within zone, boundary and control zones) and distance from CCZ centre. Note: Solid circles indicate stations within CCZ, hollow circles with cross indicate stations surrounding the zone boundary and gollow circls indicate stations in control zone (8 km+ from zone centre). The Yaxis shows percent change pre-post-CCS implementation and the X-axis shows the distance in km from the centre of the charging zone.