Routes to Clean Air 2019 Air Quality Consultants

1. Prof. Duncan Laxen
Associate
Trends in NO2 and NOx
Concentrations across the UK

2. Why look at trends
➢ It is important to understand trends in measured
concentrations:
➢ They tell us if air quality is changing in the way it is
expected based on policies and measures in place
➢ They highlight successes – SO2, lead, CO
➢ They identify failures – NO2
➢ Raise questions – PM2.5

3. Trend in NO2 1998-2010
Diesel NOx Euro Standards (g/km) 1992 - 2018
Expected a reduction
Measurements 1998-2009 showed
reduction not delivered

4. Recent past
➢ At the beginning of 2018 AQC issued a report
looking at the trends in NO2 and NOx over the period
2005 to 2016
Report available at: www.aqconsultants.co.uk/Resources/Download-Reports.aspx
NO2➢ This showed an overall downward trend
for NOx and NO2 across all sites of
~1.5% per year
➢ The trend was greater at rural sites than
road or urban sites
➢ But, around a third of sites did not have
a downward trend and some sites had
upward trend

5. Trends in NO2
Concentrations in
UK 2005-2016
156 Automatic sites
Area 1 -1.51% per year
Area 2 - n/s
Area 3 -1.61% per year
Note different trends at different sites
➢ A geographic variation in
trends was noted

6. The Presentation will:
➢ Provide an update considering two periods:
➢ 2005 to 2018 (112 sites)
➢ 2010 to 2018 (183 sites)
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/
file/796887/Air_Quality_Statistics_in_the_UK_1987_to_2018.pdf
Analysis is based on sites that were present
throughout the periods (unlike the recent
Government statistics release that added new sites as
time went on, which will bias the underlying trend)
Recent Government publication

7. Overall Trends – All Sites
Period Trend (%/yr)
NO2 NOx
2005 - 2016 -1.50 -1.48
2005 - 2018 -1.82 -1.86
2010 – 2018 -3.13 -3.07
2010 – 2018 21% not signf. 29% not signf.
Important
There are still sites
with no significant
downward trend
(p>0.05)

8. Example of Scale of Change at National Level
2005
45 mg/m3
2010
45 mg/m3
2018
32.3 mg/m3
2016
36.9 mg/m3
2018
33.5 mg/m3

9. Overall Trends – All
Sites 2005 - 2018
NO2
NOx
112 Sites

10. Overall Trends – All
Sites 2010 - 2018
NO2
NOx
183 Sites

11. NO2 Trends by Site
Type 2005 - 2018
Road
Rural
Urban
Site Type
NO2 Trend (%/yr)
2005-18 2010-18
Road -1.80 -3.10
Urban -1.65 -3.09
Rural -2.46 -3.41

12. Magnitude and Significance of NO2
Trends – All Sites 2010 - 2018
2010 – 2018
Central/Northern
England catching up
2005 – 2016
Trends

13. Magnitude and Significance of
NO2 and NOx Trends
SE England
2010 – 2018
NO2
NOx

14. Subtleties of the Results
The relationship between
NOx and NO2 is non-linear
As example, if the downward trend in
NOx is -3.1 %/yr then
NO2 would be ~ -2.3 to -2.6 %/yr
(in NO2 range 30-50 µg/m3)
The overall NOx and NO2 trends
essentially the same although the
reduction in NO2 has speeded up –
likely reason for both these is a
declining NO2:NOx ratio in recent years
Period Trend (%/yr)
NO2 NOx
2005 - 2016 -1.50 -1.48
2005 - 2018 -1.82 -1.86
2010 – 2018 -3.11 -3.04

15. Changing NO2:NOx Ratio
Graph shows measured ratios in on-road emissions
of diesel light duty vehicles by year of manufacture
Carslaw et al (2019) Atmos. Env. Available at: http://dx.doi.org/10.1016/j.aeaoa.2018.100002
The current rate of reduction for
NO2 of ~3.1 %/yr
would become ~ -2.3 to -2.6 %/yr
or even less if NO2:NOx ratio
increases again

16. NO2 Road Trend + Defra Projection
Defra Projection for
Roadside Sites
(Rest of UK HDV<10%)
Defra projection now
seems to be correct –
overall in short term
But it does not apply to all
sites – some still showing
no significant trend - and
what happens after 2020
Source for blue dots: https://laqm.defra.gov.uk/tools-monitoring-data/roadside-no2-projection-factor.html

17. Final Words
➢ The overall picture is of downward trends in both NO2 and NOx
➢ Part of the trend for NO2 seems to be driven by reduction in
primary NO2 emissions
➢ Defra projections of roadside NO2 now seem to be working at a
national level, at least in the short term
➢ Importantly not all sites show a significant downward trend so
you cannot generalise from the national to the local
➢ The central/northern areas previously showing no downward
trend now appear to be catching up – this could well relate to
fleet mixes/ages being different in different parts of the UK
➢ A report will be added to the AQC website shortly setting out the full results

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