Overall, there was a decrease in total traffic between 2005 and 2014, mainly explained by the reduction in cars and taxis. However, despite the reduced traffic, levels of roadside pollutants did not decrease accordingly.

The findings showed significant variability across the city with some roads showing significant decreases but others did not improve. Examples include a notable improvement in nitrogen dioxide alongside Putney High Street and the deterioration along Upper Thames Street; the improvements in airborne particles on Marylebone Road in central London contrasting with the increase in coarse particles alongside some busy roads in outer London including Westhorne Avenue, part of the south circular in Eltham.

Among the specific findings:

• The majority of sites in London had a significant downward trend in their roadside increment for NO₂ (8–13%·year⁻¹) and PM_2.5 (15–45%·year⁻¹). This contrasted with the trends for the preceding five year period (2005–2009) when a wider upward tendency was observed in ΔNO₂ concentrations; this can be attributed in part to a 3% rise in the number of diesel buses and coaches between 2005 and 2009 and to the failure of tighter Euro class emissions standards. No data was available to compare trends in ΔPM_2.5 between the two periods.

  Fitting new exhaust clean-up technology to older buses helped to curb nitrogen dioxide along some of London’s roads. Putney High Street, for example, saw a particular improvement with a significant reduction in NO₂ levels after 2010, due largely to the retrofitting of older buses technology to cut emissions. Nevertheless, around three-quarters of air quality monitoring sites still recorded levels exceeding the NO₂ EU Limit Value in 2015.

• The overall downward trend in NO_x and NO₂ in 2010–2014 was likely due to a variety of factors including a general decrease in the traffic flow; a reduction of primary NO₂ emissions from aging diesel passenger cars; and the introduction of the Low Emissions Zone with cleaner HGVs.

• Fine particulate matter measured as PM_2.5 dropped by 28% per year after 2010 and black carbon decreased by 11% per year, which the authors attribute to the success of traffic exhaust abatement technologies such as particle filters on newer diesel vehicles.

• Roadside PM_2.5 and carbon black decreased at similar rates on roads with collocated measurements; decreases were attributed to the effectiveness of diesel particle filters. However, the number of black carbon measurement sites was small and an extended number of sites will be needed to assess this fully, the team cautioned—especially given the heterogeneity in the behavior of other pollutants.

• Despite a reduction in the total number of vehicles on London roads by 0.5% and improvements in exhaust emissions during 2010-2014, levels of PM₁₀ from traffic showed unexpectedly no significant overall change over this period. The authors suggested that the decrease in exhaust emissions may have been offset by greater releases of coarse particles from dust resuspension and wear-and-tear on tires and brakes associated with a larger number of heavy goods vehicles (HGVs). This was found on some roads in outer London, such as Westhorne Avenue and Eltham that experienced decreases in PM_2.5levels but increased of PM₁₀.

• Heavy vehicles were an important factor in urban air pollution. Increasing the number of buses may be desirable for many social and environmental reasons, the researchers observed, but this has to be in conjunction with investment in cleaner emissions technologies such as the successful installation of low NO₂ SCRT on some of London’s buses. Greater management of HGVs is also needed to ensure that increased numbers do not offset benefits from emissions abatement and to control the increase in coarse PM. A greater investigation of sources of PM coarse, the factors which control emission rates and options for managing them is needed, the authors said.

• Changes in CO₂ from 2010 onwards did not match the downward predictions from reduced traffic flows and improved fleet efficiency. CO₂ increased along with increasing HGVs and buses.

This study showed that there was considerable heterogeneity in the outcome of policy interventions to control air pollution in London's roads and this is likely to be the case in other cities. This highlights the need for detailed measurement and to feedback into the policy making process. In some locations emissions abatement policies were offset by changes in traffic flow. Abatement technologies for tail-pipe emissions might be beneficial to control fine particulate and black carbon emissions from diesel; Euro 6 standards for diesel cars are also expected to lead to decreased NO_x and primary NO₂ emissions under real driving conditions. However, increasing traffic flows especially of those from heavy vehicles will enhance non-exhaust emissions. This suggests that stronger policy packages are therefore needed to control both exhaust and non-exhaust traffic emissions to ensure that all areas, and therefore all communities, benefit from improved air pollution. Given that the pollution trends in London are partially the result of European-wide policies, our study suggests that there is an urgent need for detailed analysis across other European cities and regions to ensure that these policies are working well and everywhere.

—Font and Fuller

Resources

• Anna Font, Gary W. Fuller (2016) “Did policies to abate atmospheric emissions from traffic have a positive effect in London?” , Environmental Pollution doi: 10.1016/j.envpol.2016.07.026