Direct NO2 in the UK: evidence from ambient

measurementsDr David Carslaw

Institute for Transport Studies, University of LeedsAir Quality Expert Group (AQEG)

Recent report • UK Air Quality

Expert Group (AQEG)– Emissions– Measurements

analysis– Modelling– Review of potential

causes

Report available at: www.defra.gov.uk/environment/airquality/aqeg/index.htm

Contents

• Focus on UK situation– Mostly London

• Recent trends in NOX and NO2

• Estimated trends in primary NO2

• Types of change• How have these changes affected

NO2 concentrations?• Concluding remarks

Recent trends at selected sites around the UK

1998 2000 2002 20040

50

100

150

200

250

GLASGOW KERBSIDE

NO

X (µg

m-3)

Year

0

50

100

150

200

250

OXFORD CENTRE ROADSIDE

NO

X (µg

m-3)

0

50

100

150

200

250

300 N

OX (µ

g m

-3)

BATH ROADSIDE

1998 2000 2002 20040

20

40

60

80

GLASGOW KERBSIDE

NO

2 (µg

m-3)

Year

0

20

40

60

80

OXFORD CENTRE ROADSIDE

NO

2 (µg

m-3)

0

20

40

60

80

100

NO

2 (µg

m-3)

BATH ROADSIDE

NOX NO2

UK monitoring• NOX has broadly declined in line with

expectations at all site types• NO2 has not declined with expectations at

many sites (or has increased)– Background sites have generally declined as

expected– Roadside/kerbside sites show a mixed picture:

increases most apparent in England and in particular London

– Availability of sites is important (relatively few roadside sites outside London)

Oxidant partitioning modelMarylebone Road (London)

Bury roadside(motorway, north England)

Jenkin ME (2004). Analysis of sources and partitioning of oxidant in the UK – Part 2:contributions of nitrogen dioxide emissions and background ozone at a kerbside location inLondon. Atmospheric Environment 38, 5131-5138.

Trends in NO2/NOX emission ratio in London

1997 1998 1999 2000 2001 2002 2003 2004 20050

5

10

15

20

25

30es

timat

ed N

O2/N

OX e

mis

sion

ratio

(% b

y vo

l.)

year

Marylebone Road

1997 1998 1999 2000 2001 2002 2003 2004 2005 20060

10

20

30

NO

2/NO

X e

mis

sion

ratio

(% b

y vo

l.)

year

Estimated weekly NO2/NOX ratio

Change-point detection• Certain sites around the UK have shown

reasonably abrupt changes in NO2– Can this be detected and what might the

reasons be?– Cumulative sum (CUSUM) technique applied– Uncertainty level and intervals in timing of

change-point quantified using bootstrap re-sampling methods

– Two sites considered: Marylebone Road and Oxford Centre

Change-point detection -Method overview

0 12 24 36 48 60 72 84 9680

85

90

95

100

105

110

115

120

-160

-140

-120

-100

-80

-60

-40

-20

0

20

0

25

50

75

100

µ2

valu

e

time (arbitrary units)

µ1

CU

SUM

nor

mal

ised

pro

babi

lity

(%)

Carslaw D.C., K. Ropkins and M.C. Bell (2006). Change-point detection of gaseous and particulate traffic-related pollutants at a roadside location. Environmental Science and Technology. (In press).

Change-point detection applied to Oxford Centre

1998 1999 2000 2001 2002 2003 2004 20050

10

2030

40

50

0255075

100125150

NO

2 (ppb

)N

OX (p

pb)

-300-250-200-150-100-50

050

1998 1999 2000 2001 2002 2003 2004 2005

-150

-100

-50

0

0

25

50

75

100

0

25

50

75

100

NO

X CU

SU

MN

O2 C

US

UM

nor

mal

ised

pro

babi

lity

(%)

norm

alis

ed p

roba

bilit

y (%

)

•Two NO2 change-points detected (July 1999 and August 2002)

•July 1999 – Oxford “bus gate”•August 2002 – particle filters on buses?

Estimated NO2/NOX emission ratio at Oxford Centre

1998 1999 2000 2001 2002 2003 2004 20050

5

10

15

20N

O2/N

OX e

mis

sion

ratio

(% b

y vo

l.)

Year

Change-point detection applied to Marylebone Road

0200400600800

1000

1998 1999 2000 2001 2002 2003 2004 2005-250-200-150-100-50

0

0

25

50

75

100

0

25

50

75

100

NO

X CU

SU

MN

O2 C

US

UM

prob

abilit

y (%

)pr

obab

ility

(%)

0

50

100150

200

250

1998 1999 2000 2001 2002 2003 2004 20050

1020304050

NO

X (ppb

)N

O2 (p

pb)

• Clear NO2 change-point detected in February 2003• Coincides with introduction of congestion charging• More buses + more buses with continuously regenerating diesel particulate filters

•No evidence of a change in PM10•Some evidence of a decrease in PM2.5 beforecongestion charging, BUT…•Increase in coarse particles (PM2.5-10) – why?

1998 1999 2000 2001 2002 2003 2004 20050

10

20

30

0

10

20

30

0

10

20

30

40

PM

coar

se (µ

g m

-3)

year

PM

2.5 (µ

g m

-3)

PM

10 (µ

g m

-3)

1998 1999 2000 2001 2002 2003 2004 2005-100-80-60-40-20

0

-250-200-150-100-50

0

-200

20406080

100

-60

-40

-20

0

20

0

25

50

75

100

0

25

50

75

100

0

25

50

75

100

0

25

50

75

100

PM

coar

se

year

NO

2 CU

SUM

PM

2.5 C

USU

MP

M10

CU

SU

M

prob

abilit

y (%

) pr

obab

ility

(%)

prob

abilit

y (%

) pr

obab

ility

(%)

Effect on particles?

Origins of NO2 as a function of NOX concentration

primary

background

Secondary (NO + O3)

Based on 10 % primary NO2 at Marylebone Road

Contributions to NO2concentrations > 200 µg m-3 at

Marylebone Road

1998 1999 2000 2001 2002 2003 20040

100

200

300

400

500

600ho

urs

> 20

0 µg

m-3 N

O2

year

primary NO2

local NO-O3

background

Sensitivity of hourly concentrations to direct NO2

0 5 10 15 20 25 30

0

200

400

600

800

1000

1200

1400ho

urs

whe

re N

O2 >

200

µg

m-3

primary NO2 (% by vol.)

Based on year 2000 meteorology and emissions

Worst-case site in Europe?

• Site in inner London• 5th highest flow of buses

in UK• 2005: annual mean NO2

219 µg m-3 and 2532 hours > 200 µg m-3

• Driver/passenger exposure?

Source: www.londonair.org.uk

What if increases in NO2/NOXratio had not occurred?

1998 2000 2002 2004 20060

25

50

75

100

125N

O2 (µg

m-3)

measured NO2

constant prirmary NO2

2005 difference:28 µg m-3 (33 % higher)

1998 2000 2002 2004 20060

20

40

60

2005 difference:13 µg m-3 (25 % higher)

NO

2 (µg

m-3)

measured NO2

constant prirmary NO2

1996 1998 2000 2002 2004 20060

20

40

60

80

2005 difference:10 µg m-3 (19 % higher)

NO

2 (µg

m-3)

measured NO2

constant prirmary NO2

Central London, high traffic flow, typical urban speed(Marylebone Road)

Outer London, high traffic flow, high speed, similar to a motorway(A3)

Inner London, average traffic flow(Tower Hamlets)

Note: 12-month LOESS smooth applied.

Derivation of vehicle NO2/NOXratio from measurements

• Estimate primary NO2 at 37 London sites

• Multiple regression with traffic NOXemissions:

Buses = 39 (± 2) %HGVs = 12 (± 5) %Cars/vans = 18 (± 5) %

Note: Results depend on the relative accuracy of NOX emission factors.

Concluding remarks• Increasing evidence that the ratio of

NO2/NOX is increasing across the UK– Mostly at roadside sites, most apparent in

London– By no means uniform

• Some sites show an abrupt change in NO2concentration– Most likely due to rapid introduction of

catalytically regenerating DPF on bus fleets• More analysis is required to understand

effects at specific sites– Delineate between diesel car/van oxy. cat and

bus/HGV catalytically regenerating DPF

Acknowledgements

Thanks to the AQEG for providing the material for the presentation

…and for listening