The main air pollutant of concern in Bath - nitrogen dioxide - reduced since the introduction of the Covid-19 restrictions in late March 2020. The graph below shows the percentage changes in provisional data from three continuous air quality analyser sites around the city to the end of September 2020, compared to the average over the last five years (2015-2019). Continuous analysers are the gold standard in terms of nitrogen dioxide measurements.
Traffic flows remain approximately 15% lower than usually expected for this time of year and this has had an effect on measured nitrogen dioxide concentrations. However, since May, levels of nitrogen dioxide have begun returning to similar levels as in 2019, particularly at the Lower Bristol Road (nr junction with Windsor Bridge Road) analyser location.
The september average nitrogen dioxide level at Lower Bristol Road (nr junction with Windsor Bridge) was 4% higher than for the same period in 2019. This is shown in the graph below, that plots percentage changes from 2019 to 2020.
To supplement these continuous analysers, we also have over 200 nitrogen dioxide tubes around the district. The following graph shows the percentage change from 2019 at some of the sites with the highest recorded nitrogen dioxide concentrations. The month of April had no measurements because the laboratory that analyses the diffusion tubes was closed due to the virus.
The effect of the virus restrictions can be seen from March to July, where diffusion tube measurements of nitrogen dioxide were up to 60% lower than for the corresponding month in 2019. Measurements in August at some locations were higher than the corresponding month in 2019.
These graphs are indicative, in that the data used to create the graphs above is provisional and is subject to possible adjustment. This processed is explained and reported in our Annual Status Report. The Annual Status Report for final 2019 data is now available on the 'Reports' page. Air pollution is affected by a number of factors including meteorological conditions (wind, rain, sunshine, temperature); road layout (eg. how close buildings are to the road); temporary weight limits (such as the Cleveland Bridge 18t limit); traffic levels and the composition of traffic (how many of the vehicles are cars / vans / buses / lorries etc). 2020 has already had exceptional weather with a very windy February (that disperses pollution) and the lockdown also coincided with the start of spring and finer weather. This makes a comparison with 2019 troublesome in terms of assessing the impact of reduced traffic or altered fleet compositions. There is some national modelling in progress that will enable us to adjust the figures to take into account the effects of weather, i.e. to identify what proportion of the changes was due to traffic flow changes.