‘Weather a factor’ in hourly NO2 drop

A reduction in exceedances of the hourly NO2 limit at sites in London could be in part due to weather conditions early in the year, as well as a reduction in emissions of pollutants, an air quality expert has said.

This year, the hourly NO2 target was breached at sites at Brixton Road and Putney High Street much later than in previous years.

Brixton Road was one of the first monitoring sites to exceed the NO2 hourly limit in 2018

UK objectives and EU limits stipulate a maximum nitrogen dioxide concentration — 200 µgm/m3 — that must not be exceeded at a single monitoring site for more than 18 hours over the whole year. These limits came into force in 2010.

In 2018, the Brixton Road monitoring site recorded its 18th hour above the threshold on 31 January, compared to having hit the limit after just six days in 2017. Similarly, the monitoring site situated on Putney High Street reached the same threshold in March 2018, compared to the first week of 2017.

To some extent this improvement was seen as having been a result of policies aimed at reducing vehicle emissions in the capital (see story).

However, speaking at last week’s London Air Quality Network (LAQN) Conference, Timothy Baker, principal air quality analyst at King’s College London explained that there are likely to be other factors having contributed to the drop in exceedances of the hourly limit.


“All the last ten years [exceedances] have been in the first week. It was the last day of January this year. Why? There are a number of things going into this,” Mr Baker told the Conference.

“Generally there has been a reduction in levels. But there is also some significant differences which help explain why things [happened] early in the month in 2017 and late in the month this year.”

Mr Baker suggested that a milder January in 2018 is likely to have created conditions that would have been conducive to lower overall levels of pollutants.

“This year it was warmer than normal. That helps explain it,” he said. “Generally during winter, if it is colder, unless we have got an absolute blasting easterly wind coming through, that tends to lead to a build-up in pollution as the air is more stable and doesn’t move around as much.

“We did have a difference in wind directions [in 2018] as well, so there was a number of factors. There was a decrease in emissions levels but there was also a change in weather that helped.”


Addressing the yearly average 40 µgm/m3 limit value, Mr Baker said: “The worst sites are getting better. But, that improvement is not uniform. We are seeing a big improvement at some locations and not such a big improvement at other ones.”

He added: “Some of the background sites as well as some of the roadside sites are still exceeding the objective 40 µgm/m3 annual mean. But, we are down to about twice the annual mean where we used to be at about three or getting close to four.”

He also added that monitoring of ozone levels in the capital has become ‘increasingly important’, pointing to two pollution episodes in June 2017 and May 2018, where high levels of the pollutant have been recorded in the capital.

“You need two things for ozone,” he explained. “You need the fuel, that is the ozone precursor chemicals. They have to be emitted, and they need time and strong sunshine to power the conversion.

“That is what happened on 21 June [2017]. We had an import of pollution, we had the strong sunshine and it drove Ozone to high. What was significant was that this was the first time that we recorded index level ‘8’ since the daily air quality index was created in 2012.”

For May 2018, he suggested that the episode may have been linked to later spreading of fertilisers for agriculture due to a cooler Spring.

He added: “Spring was very cool and wet and agriculture they didn’t put the fertiliser out until later in the season. As a result there was more fertiliser out there in May, which was lifted and transported, and the unusually high temperature drove that pollutant from the particle phase to the gas phase, and when it cooled off at night it went back again.”


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