A study of the A470 at Pontypridd has found vegetation could help reduce the spread of nitrogen dioxide from the road to neighbouring properties.
The Welsh Government collected data at two sites by the A470 using simultaneous monitoring at the roadside and points downwind. Instruments measured wind speed and direction and concentrations of NO2, ozone and PM10 particulate.
At the site with a vegetation barrier, the monitored concentration in the nearby gardens was lower than the estimated concentration, as modelled using the measured roadside data and Defra’s calculator of NO2 fall off with distance.
At the site without the green barrier, the monitored concentrations were higher than the estimated values.
'The green barrier was considered to have an overall beneficial impact on pollutant concentrations outside of the barrier,' said a government spokesman.
'Although benefits of green vegetation have been established through the survey, it would not be advisable to draw direct conclusions on the suitability of vegetative barriers elsewhere as a measure to reduce NO2 concentrations arising from road vehicle emissions.'
Further analysis is being undertaken.
In 2018, the Welsh Government reduced the speed limit from 70mph to 50mph at Pontypridd and four other places on its trunk road network where NO2 concentrations had to be brought below the legal limit in the shortest possible time.
The annual monitoring report shows that concentrations were below the limit at all five locations in 2021, but COVID-related travel restrictions were in place that year.
The government is developing potential additional measures, including Air Quality Barriers – walls six or nine metres tall – alongside the A470 in Pontypridd, A494 in Deeside and A483 in Wrexham.
However, the study suggests the presence of trees and other vegetation beside the A470, in combination with the 50mph limit, could prevent neighbours being re-exposed to NO2 above the limit.
