Defra reports rise in hourly particulate concentrations

Defra has reported that ‘small’ decreases in concentration of nitrogen dioxide (NO2) were recorded in the UK in 2017 in its annual air quality statistical release, published today (26 April).

The report also suggests a downward trend in overall concentrations of particulate matter (PM2.5 & PM10) recorded, but suggests that the number of hours of moderate or high levels of both pollutants increased in 2017 compared to the previous 12 months.

Burning of wood and coal by households in stoves and open fires is a large contributor to emissions of particulate matter, Defra notes

The statistical release provides an overview of the concentrations of air pollutants monitored across the Automatic Urban and Rural Network — the national web of air quality monitoring stations — covering particulate matter, ozone and nitrogen dioxide.

The report suggests that in 2017, there were a greater number of days on average when air pollution was moderate or higher at rural sites compared to urban sites, compared to 2016.

According to Defra, the main drivers of the average number of days when air pollution is Moderate or higher are particulate matter and ozone, for urban and rural pollution monitoring sites in the UK respectively.

The report adds that urban background ozone (O3) pollution has remained fairly stable between 2003 and 2017, although concentrations have shown a long-term increase since monitoring began.


Rural background ozone pollution has shown no clear long-term trend, according to Defra, with both of these indicators having increased slightly from 2016 to 2017; continuing fluctuation in recent years.

Looking in detail at NO2, the report indicates that the annual mean concentration of the pollutant has reduced by an average of 0.9 μg/m3 between 2006 and 2017. “Emissions of NO2 in the UK and Europe have continued to decrease as newer road vehicles subject to stricter emission standards enter the fleet and power generation moves away from the use of coal,” the report suggests.

Average concentrations of roadside NO2 have decreased to 33.7μg/m3 in 2017, a low since monitoring began in 1997, Defra adds, noting that the NO2 concentrations are significantly higher at roadside sites compared to those in urban background zones.

It notes: “At roadside sites between 2016 and 2017 there was a decrease in the number of hours for which the mean NO2 concentration over the previous hour exceeded 200 μg/m3; from 5.8 hours to 3.3 hours per site.”

Looking at trends in NO2 pollution, the report adds that concentrations tend to be much lower at the weekend compared to during weekdays; particularly for roadside sites. “This is likely to be primarily driven by far less road traffic at the weekends,” the report notes. “In 2017, the Monday-to-Friday mean concentration at roadside sites was 35.9 μg/m3; significantly greater than the mean concentration at the weekend (28.4 μg/m3).”

Nitrogen dioxide

On daily concentrations, it adds: “NO2 concentrations tend to be much greater during the morning and evening rush hour compared to other times of the day, for roadside and urban background sites.

“This is most likely due to commuter traffic during the working week; as this observation does not apply to Saturdays and Sundays. This pattern of concentrations follows the distribution of road traffic by time of day.”

On particulates, the report suggests that spikes tend to occur in winter months and in the evening.

The month with the greatest average concentration in 2017 was January, Defra noted, and the time of day with the greatest average concentration was 9pm.

Burning of wood and coal by households in stoves and open fires is a large contributor to emissions of particulate matter both in the UK and across Europe, and is most common in winter months and during the evenings, the report adds.

Related Links
Defra Air Quality Statistical Release


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