The figures published today by DEFRA reveal that annual mean UK roadside concentrations, as measured by the national air quality monitoring network, were 31 micrograms per cubic metre in 2019, down from 45.8μg/m3 ten years before. But the declining trend accelerated hugely last year, falling to only 23μg/m3. Rural and urban background concentrations fell in similar fashion, down to levels not seen in modern times.
The fall in fine and coarse particulate pollution last year was not quite so dramatic, in part reflecting how vehicular emissions have been cleaned up in recent times. The greatest difference compared to previous years was seen in February 2020, when concentrations were about 31% lower than 2017-19 average. Being before the first lockdown period, this can be attributed to the unusually mild, wet and windy weather at the time, rather than Covid-19. This in turn, “perhaps resulted in less domestic burning for heating, more deposition of particulates via precipitation, and more clean air being blown in from the Atlantic than usual, respectively,” according to DEFRA.
The enduring influence of domestic burning on air quality is also reflected in how it changed according to the time of day. PM2.5 concentrations tend to be greater in the evening, when homes are most likely to be burning wood, coal or other solid fuels in stoves or open fires.
While benefiting the planet by absorbing ultraviolet light in the stratosphere, ozone is far more problematic at ground level, triggering respiratory inflammation and asthma attacks and damaging vegetation and crops. Rather than being emitted directly, it is formed by a variety of complex processes in the atmosphere, through interactions of non-methane volatile organic compounds with sunlight and nitrogen oxides (NOx).
But high concentrations of nitric oxide (NO) destroy it, so concentrations tend to be lower in urban areas than the countryside. It is therefore unsurprising that urban background concentrations of the gas hit 64.4μg/m3 last year (expressed as the annual mean daily eight-hour mean concentration), accelerating a long-term increasing trend.
Climate change appears to be exacerbating the situation, with more spells of hot and sunny weather prone to generating photochemical smog. The highest average concentrations of ozone levels in the UK on record have all been observed in the past three years.
However, there was also a major dip in ozone concentrations in the middle of the year, when levels would be expected to be high. This is because strong westerly winds brought cleaner air and blew away pollution, according to DEFRA.