Recent trends in PM2.5 are discussed using both data from the National Atmospheric Emissions Inventory and evidence from atmospheric measurements. The good news is that airborne measurements provide strong evidence for a recent decline in emissions from vehicle exhaust as a result of the requirement for diesel particle filters on Euro 5 and Euro 6 vehicles. There is also some evidence for a slow but steady decline in airborne concentrations of secondary sulphates and nitrates which make up a substantial proportion of PM2.5 mass. Non-exhaust particle emissions from road vehicles are unregulated and contribute both to the fine (PM2.5) and coarse particle fractions. Evidence for trends derives mainly from the inventory which shows only modest change in recent years. There is also a contribution from cooking aerosol which is not included in the emissions inventories but can be seen clearly in the results of receptor modelling studies based upon chemical composition. The bad news relates primarily to emissions from the combustion of biomass, predominantly wood smoke, for which ambient data are rather scarce, but the emissions inventories show emissions increasing with time. Some future projections are also shown.

1. SOURCES AND TRENDS IN PM2.5
The Good News and the Bad
Roy M. Harrison
University of Birmingham
and
National Centre for Atmospheric Science

2. Loss of Life Expectancy due to PM2.5 Exposure
From: UNECE, 2016

3. From: UNECE, 2016

4. PM2.5 particles are:
- primary (described by NAEI)
- secondary
- sulphates
- nitrates
- secondary organic compounds

5. UK emissions of PM2.5 (NAEI, 2017)

6. Sources of Air Pollutants from a Vehicle
Emissions dependent upon
• vehicle speed (resuspension, tyre and road surface wear)
• engine revs and load (exhaust)
• driving mode (exhaust, brake, tyre, road surface)
• materials (brakes, tyres, road surface)
• fuel and lubricant (exhaust)
• vehicle weight and aerodynamics (resuspension)
• road surface silt loading (resuspension)
exhaust
Resuspension (particles)
tyre wear (particles)
road surface wear (particles)
brake wear
particles
carbon monoxide
oxides of nitrogen (NOx)
particles
VOC (hydrocarbons)

7. 0
20
40
60
80
100
120
140
160
180
1990 1995 2000 2005 2010 2015 2020
Emissions(ktonnes)
Total UK PM2.5 Emissions (kt), 1990-2009, 2015 and 2020
Other (Non-Combustion)
Other (Combustion)
Small Scale Waste Burning
Agriculture
Iron and steel production
Other Mineral products
Quarrying and Mining of Minerals Other Than
Coal
Residential
Off Road Transport
Other Transport (including Rail, National
Navigation and Aviation LTO)
Road Transport (Non-Exhaust)
Road Transport (Exhaust)
Combustion in Industry
Combustion in the Energy Industries

8. 0
5
10
15
20
25
30
35
40
45
1990 1995 2000 2005 2010 2015 2020
Emissions(ktonnes)
Total UK PM2.5 EmissionsfromRoad Sources
(1990-2009, 2015 and 2020)
Road abrasion (all vehicles, non-exhaust)
Brake wear (all vehicles,non-exhaust)
Tyre wear (all vehicles,non-exhaust)
Motorcycles(exhaust)
Busesand coaches(exhaust)
AllHGVs (exhaust)
AllLGVs (exhaust)
Dieselcars (exhaust)
Petrolcars (exhaust)

9. SPECIFIC SOURCES: NON-EXHAUST EMISSIONS
FROM ROAD TRAFFIC
• Emission inventories include tyre wear, brake wear and road
surface wear. They do not include particle resuspension.
• Currently, non-exhaust emissions of PM10 are of a similar
magnitude to exhaust emissions. By 2020, non-exhaust
emissions will be strongly dominant.
• This source contributes similar masses of particles to the fine
(PM2.5) and coarse (PM2.5-10) fractions.
• There are no current measures in place, or planned, to
control emissions from this source.

10. Use a twin site approach:
Polluted Kerbside minus Urban Background
- London, Marylebone Road minus North
Kensington
- Paris, Boulvard Pérphérique Auteuil minus Vitry-
sur-Seine
- Glasgow, Roadside minus Glasgow Central
(2000-2012) or Townhead (2013-2015)
TRENDS IN ROAD TRAFFIC EMISSIONS

11. Trends in PM2.5:
Marylebone Road and North Kensington

12. Trends in EC:
Marylebone Road and North Kensington

13. Trends in PM2.5:
Paris Roadside and Urban Background

14. Trends in NOx:
Marylebone Road and North Kensington

15. Trends in NO2:
Marylebone Road and North Kensington

16. Will electric vehicles reduce PM2.5 emissions?
▪ No exhaust PM emissions
▪ Brake wear reduced due to regenerative braking
▪ Large weight of batteries may increase vehicle
weight
▪ An increased vehicle weight would imply
increased emissions from tyre and road surface
wear and particle resuspension

17. What does receptor modelling
tell us?

18. Daily PM2.5 Source Contribution Estimates with Secondary Biogenic
Components at NK

19. SELECTED MEAN CONTRIBUTION TO PM2.5 MASS (µg m-3)
From: J. Yin et al., ACP 15, 2139-2158 (2015)

22. UK consumption of wood for domestic combustion:
2002-2012. Source: Digest of UK Energy Statistics (DECC, 2014)

23. Trends in Annual UK PM2.5 emissions from biomass
sources according to the NAEI (Passant et al., 2014)

24. Comparative emission factors for domestic consumption of
fuels implied by energy content of fuel

25. Emissions of NOx, PM10 and PM2.5 from open fire places
utilising different fuel types

26. Emissions of NOx, PM10 and PM2.5 from residential boilers
and stoves utilising different fuel types

27. ▪ Sulphur dioxide oxidation leads to sulphuric acid formation, which
reacts irreversibly with ammonia.
SO2 + oxidant  H2SO4 + 2NH3  (NH4)2SO4
▪ Nitrogen dioxide oxidation leads to nitric acid formation, which
reacts reversibly with ammonia
NO2 + oxidant  HNO3 + NH3 ⇌ NH3NO3
▪ A reduction in ammonia emissions would be a highly cost-effective
means of reducing particulate nitrate.
▪ Sulphate and nitrate concentrations are decreasing slowly, but not
linearly (i.e. in proportion) with emissions of SO2 and NOx.
SECONDARY PARTICLES

28. ▪ Road traffic exhaust emissions are declining.
▪ Non-exhaust emissions from traffic are
substantial, and the implications of electric
vehicle use are unclear in this respect.
▪ Cooking emissions are a poorly quantified source.
▪ Biomass burning is a source of increasing
importance.
▪ The major contributors to ambient PM2.5 are
secondary nitrates and sulphates, which are
declining slowly.
▪ Ammonia control is a missed opportunity
CONCLUSIONS

29. 33
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