2.2. Data
Hourly average pollutant concentrations recorded at monitoring
sites across Greater London were extracted from the London Air
Quality Network (LAQN) database (LAQN, 2005). NOX, NO and NO2
were monitored at 102 sites, PM10 at 87 sites, CO at 32 sites and O3
at 29 sites. Daily average concentrations for the hours the CCS was
in operation (Monday–Friday, 07:00–18:00) were calculated for
a 4-year period, 2 years before and 2 years after the implementation
of the scheme in February 2003. Completeness criteria applied
to the calculation of the daily average values (75% of hourly
observations available) and to the selection of sites for analysis
(daily average values available for at least 75% of days in the fouryear
period). Data from both roadside (sample inlets between 1 m
and 5 m from the kerbside of a road) and urban background
(sample inlets sites at least 10 m from any major local sources and
broadly representative of city-wide background concentrations)
monitoring stations were selected for analysis. Rural concentrations
of ozone in the south east of England (7 monitoring sites)
were also extracted from the LAQN database. Fig. 1 illustrates these
zones together with the location of the individual monitoring
stations selected for this study