The PCM Model and Modelling

Uncertainty

Dr Christine McHughMarilena Karyampa

12 October 2016

2

• What is the PCM model?

• Background concentrations

• Model trends and uncertainty

• Performance at roadside sites in London

• Quality of previous forecasts

• Conclusions

Contents

3

• PCM: Pollution Climate Mapping

• Collection of models used by Defra to report on UK’s compliance with EU Directive 2008/50/EC

• Base year model and projections model for each pollutant (incl. NOx and NO2)

• Outputs on a 1x1km2 grid of background conditions and approx. 9,000 road links

PCM model

4

• https://uk-air.defra.gov.uk/data/pcm-data

• Produced and updated on an annual basis

• “These maps should not be mistaken for the Local Authority-specific maps which are alternative maps based on the same model results as presented here but which provide source-sector splits and projections to future years by Local Authority to aid the Local Air Quality Management process.”

• Values are the same as the LAQM background maps.

PCM model – background concentrations

5

PCM road links

• road link within 1x1km2 grid• background value and

roadside value• background concentration

varies by road link

https://uk-air.defra.gov.uk/library/no2ten/2015-

no2-projections-from-2013-data

LAQM background maps

• 1x1km2 grid• source-sector splits• available for each local

authority• annual projections to 2030• use within LAQM process• base year 2013 (published

July 2016)

https://uk-air.defra.gov.uk/data/laqm-

background-maps?year=2013

Background concentrations

6

PCM model resultsLAQM background maps

Background concentrations

– – 1 km – –

––

1 k

m –

–

– – 1 km – –

––

1 k

m –

–

7

• Comparison originally undertaken in Autumn 2015

• Years of assessment: 2013 and 2025

• Background concentrations from LAQM maps (2011 based) and PCM road links (2013 based)

• Conclusions:

- Magnitude of difference greater for 2013 than 2025

- 2013: PCM > LAQM by 0-6μg/m3

- 2025: PCM < LAQM by 0-4μg/m3

Background concentrations

8

Background concentrations

-15

-10

-5

0

5

10

15

PC

M 2

01

3 -

LA

QM

20

13

(N

O2)

-15

-10

-5

0

5

10

15

PC

M 2

02

5 -

LA

QM

20

25

(N

O2)

< -6 -6 to -4 -4 to -2 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +4 +4 to +6 > +6

< -6 -6 to -4 -4 to -2 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +4 +4 to +6 > +6

2013

2025

9

Background concentrations

• Comparison of NO2 concentrations between LAQM backgrounds and PCM road link background values for Zone 1 (Greater London)

0

10

20

30

40

50

60

0 10 20 30 40 50 60

LA

QM

20

13

(N

O2)

PCM 2013 (NO2)

0

10

20

30

40

50

60

0 10 20 30 40 50 60

LA

QM

20

25

(N

O2)

PCM 2025 (NO2)

2013 2025

10

Background concentrations

• Difference in NO2 concentrations between LAQM backgrounds and PCM road link background values for Zone 1 (Greater London)

• 20% of values with a difference in concentrations

-15

-10

-5

0

5

10

15

PC

M 2

01

3 -

LA

QM

20

13

(N

O2)

-15

-10

-5

0

5

10

15

PC

M 2

02

5 -

LA

QM

20

25

(N

O2)

2013 2025

11

Background concentrations

• Distribution of differences in NO2 concentrations using “bins” of 1μg/m3 or 2μg/m3

< -6 -6 to -4 -4 to -2 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +4 +4 to +6 > +6

< -6 -6 to -4 -4 to -2 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +4 +4 to +6 > +6

2013

2025

PCM NO2 – LAQM NO2

PCM NO2 – LAQM NO2

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• Comparison of NO2 concentrations between LAQM backgrounds 2013 and 2011 based.

• For the year 2013, the new maps have higher NO2 concentrations typically by 0-6μg/m3

Expected since the new base year is 2013 and the model has been adjusted based on measurements for that year.

• For the year 2025, the new maps have lower NO2 concentrations typically by 0-4μg/m3

The new maps predict a greater improvement in background concentrations for the future years.

Background concentrations

13

Background concentrations

-10

-8

-6

-4

-2

0

2

4

6

8

10

LA

QM

20

13

-L

AQ

M 2

01

1 (

NO

2)

-10

-8

-6

-4

-2

0

2

4

6

8

10

LA

QM

20

13

-L

AQ

M 2

01

1 (

NO

2)

< -6 -6 to -4 -4 to -2 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +4 +4 to +6 > +6

< -6 -6 to -4 -4 to -2 -2 to -1 -1 to 0 0 to +1 +1 to +2 +2 to +4 +4 to +6 > +6

2013

2025

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• AQC report on future background concentrations1

• They predict that concentrations will reduce with time, but not in the same extent as predicted in the Defra maps.

• Method for uplifting future backgrounds

Chiswick example: suggests that

future NO2 backgrounds should

be 15% higher than the LAQM

values

Background concentrations

1 Air Quality Consultants (2016) Deriving background concentrations

of NOx and NO2 for use with ‘CURED V2A’

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Background concentrations

0

5

10

15

20

25

30

35

2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030

An

nu

al m

ean

NO

2 c

once

ntr

atio

ns

(μg/m

3)

Year

Future background concentrations

AQC CURED v2.0 LAQM 2011 based maps LAQM 2013 based maps

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• The figure shows the outcome of model verification for roadside sites

• Most modelled values are within 30% of the monitored value

• ….need to delve deeper than a scatter plot which masks the overall picture, especially around 40μg/m3

Model trends and uncertainty

40

40

from Defra AQ Plan Technical Report (Dec 2015)

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• Analysis of the frequency of the difference between modelled and monitored concentrations

- as % and as μg/m3

- negative values indicate

model under-prediction

• The model mostly under-predicts

Model trends and uncertainty

Frequency of different percentage errors

Frequency of different absolute errors

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• Crucially, the model has a greater tendency to under-predict for

monitored concentrations close to 40µg/m3 (35-45µg/m3) and over

40µg/m3, than it does overall

Model trends and uncertainty

Difference in concentration (modelled – monitored)

as function of monitored concentration

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- If the limit value is used as

the assessment level, i.e.

40µg/m3, then there is a

48% likelihood that the

monitored concentration

would exceed 40µg/m3;

- Even if an assessment level

is selected that is 10% lower

than the limit value (i.e.

36µg/m3) then there is still a

37% likelihood that the

monitored concentration

would exceed 40µg/m3.

Model trends and uncertainty

Use of a lower, surrogate

threshold increases

confidence that the actual

threshold of 40µg/m3 will

be achieved.

7%

16%

32%

37%

48%

cf M. Bull (2010), H13-244, harmo.org

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• Model predictions are on a

road link basis: the same

concentration is predicted for all

roadside locations

• No account is taken of local

differences: junctions, bus stops

or particular traffic conditions,

such as queuing traffic

• Predictions are calculated at a

distance of 4m from kerbside

Performance at roadside sites in London

Monitored vs Modelled annual mean concentrations

at 46 automatic roadside monitoring stations (2013)

40

40

Model over-estimates at the lower

concentrations and under-estimate

at the higher concentrations.

There is substantial spread in the

difference between monitored and

predicted concentrations. 10 out of

the 46 model forecasts (22%), lie

outside the ±30% envelope

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• As the national model, PCM

has been used for over 10 years to

predict future pollutant

concentrations and hence to

predict UK compliance with the

EU limit values

• Consider predictions (for

London) published in 2007 to

support the 2006 review of the Air

Quality Strategy

• Top graph shows model

predictions for 2001 (a past year).

- Max under-estimate 8µg/m3

- Max over-estimate 14µg/m3

• Bottom graph shows model

predictions for 2010.

- Max under-estimate 26µg/m3

- Max over-estimate 4µg/m3

C. Quality of past predictions

2001 modelled and monitored

2010 modelled and monitored

Data from AEAT/ENV/R/2456/Issue 1 report to Defra, January 2007

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• Forecasts are sensitive to input data and to the

‘base year’

- For 2001 (a past year), max difference: 14µg/m3

(average 5µg/m3)

- For 2010, max difference: 11µg/m3 (avg 4µg/m3)

- For 2020, max difference: 12µg/m3 (avg 4µg/m3)

Quality of past predictions

Different base years: 2003, 2004

Sensitivity to base year is striking

No sensitivity of the model

predictions to base year or similar

inputs has been presented within

Defra’s 2015 Air Quality Plan

From CERC FM642/TR04/R1/06 report to Defra & devolved authorities, 2006

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• The background concentrations of the PCM road links are from the count point, even

though the links may cross through multiple grid squares.

• The latest background maps predict an even greater improvement in future background

concentrations than the previous ones.

• To increase confidence in forecasts of compliance with the 40mg/m3 limit value,

forecasts of achieving a lower, surrogate level of concentration should be used. Using

40mg/m3 there is a 48% likelihood of exceeding the limit value

• At roadside sites in London the model over-estimates at the lower concentrations

and under-estimates at the higher concentrations

• Previous forecasts

- Forecasts of concentrations at the highest recording sites are over-optimistic with respect to

predicting compliance with EU limit value;

- Model forecasts are sensitive to the input data and default parameters. Choice of a different

base year caused a difference of up to 4.5mg/m3 in model predictions (over 10% of the EU

limit value)

Conclusions

Thank you

Any questions?