STREET SCALE MODELLING OF NANOPARTICLES USING A SIMPLIFIED

APPROACH AND AN OPERATIOAL MODEL

7TH INT. CONF. ON AIR QUALITY – SCIENCE & APPLICATION, ISTANBUL, 24-27 MARCH 09

PRASHANT KUMAR

MATTHIAS KETZEL

ALAN ROBINS

REX BRITTER

POINTS FOR DISCUSSION

BACKGROUND

MEASUREMENTS

Application of a DMS500 for street canyon measurements

MODELLING

Formulation of a simple dispersion model (a modified Box model)

CFD (FLUENT) simulations, and OSPM

Comparison of measurements with CFD, OSPM and Box models

SUMMARY AND CONCLUSIONS

ACKNOWLEDGEMENTS

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 2

BACKGROUND

Stringent emissions: particle mass emissions (↓), number (↑)

Current regulations address atmospheric particulate matter as PM10, PM2.5 mass concentration; not particle number concentration (PNC)

Ultrafine particles (< 100 nm); main component of ambient particles by number, produced mainly by vehicles, contribute most to PNC but little to PMC; these

are more toxic than coarse particles per unit mass (Brugge et al., 2007)

Progress hampered by lack of proven methods and instrumentation to measure PNCs

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This work addresses:

Application of a fast response DMS500, its suitability and best operating conditions for the measurements of PNDs in street canyons

To apply an operational (OSPM), a CFD (using FLUENT) and the modified Box model to one of our previously studied street canyon and to compare the model predictions with measured PNCs

To investigate the effect of different sizes of emission sources on the distribution of the mean PNCs in CFD simulations

To compare measured and modelled vertical PNC profilesPRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 3

MEASUREMENTS

Measurements: Street canyon (Pembroke Street, Cambridge)

Instrument: Differential Mobility Spectrometer (DMS500) Response: 10 Hz, real time continuous (used 0.5 Hz) Sampling flow rate: 8.0 lpm at 250 mb for 5-1000 nm

2.5 lpm at 160 mb for 5-2738 nm

Movie: Diesel drive by (Courtesy: Cambustion Ltd.)

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PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 4

APPLICATION OF DMS500

Check the sensitivity level of the instrument

Identify the suitable operating conditions (mainly sampling frequency) of the instrument which maximised its utility

2 of 3MEASUREMENTS

0.E+00

2.E+04

4.E+04

6.E+04

8.E+04

1.E+05

1 10 100 1000D p (nm)

0.1 s Av Noise (10 Hz)1 s Av Noise (1 Hz)10 s Av Noise (0.1 Hz)0.1 s Av Roadside background (10 Hz)0.1 s Roadside (10 Hz)

0.E+00

2.E+04

4.E+04

6.E+04

8.E+04

1.E+05

1 10 100 1000

dN/d

logD

p (#

cm

–3)

0.8

0.6

0.4

0.2

0.0

1.0 105

Sensitivity of the DMS500. Both typical roadside and background PNDs were measured at the fastest (10 Hz) sampling frequency.

Smaller (1 Hz or lower) rather than maximal (10 Hz) sampling frequencies found appropriate, unless experiments relied critically upon fast response data

Suggested sampling frequencies used in later experiments (Kumar et al., 2008a-c, 2009a-c):

measured PNDs well above instrument’s noise level

reduced size of data files to manageable proportions

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 5

See Kumar et al. (2009d) for details

STREET CANYON 3 of 3MEASUREMENTS

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 6

4-way solenoid switching system

Pembroke Street, Cambridge

Pseudo-simultaneous measurements Measurements at four heights z/H = 0.09, 0.19, 0.4

and 0.64 Lengths of sampling tubes: 5.17, 5.55, 8.9 and 13.4 m Switching time: 60 s; Sampling frequency: 0.5 Hz Size range: 5–2738 nm (range considered here 10-

300 nm) Sampling tunes i.d.: 7.85 mm Cross-canyon winds (NW)

See Kumar et al. (2008b) for details

br

x

n

xjix

CzkWUb

TEC

11

1,

exp

THE MODIFIED BOX MODEL

Vertical Concentration profile

Empirical constant for exchange velocity 1% of Ur

(Bentham and Britter, 2005)

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when z = max (z , h0), Ur = max (Ur, Ur,crit) and k1 = 0.11 m–1

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 7

MODELLING

C and Cb are the predicted and background PNCs (# cm-3)

Ur and Ur,crit are in cm s-1, k1 is exponential decay coefficient in cm-1

b1 = σ0 = 11 dimensionless parameter (Rajaratnam, 1976)

Ex,i-j (PNEF # veh-1cm-1 in any particle size range of any vehicle class x )

Tx = veh s-1 of a certain class

h0 (= 2 m) is assumed initial dispersion height close to road level

W (width in cm); z (vertical height in cm above road level)

04/1

CFD SIMULATIONS – COMPUTATIONAL DOMAIN

CFD code: FLUENT

Standard k- model

2D domain; Ht. = 6H

Inlet Ur profile: constant

53824 grid cells, expansion factor 1.10 near walls

TKE profile k = IUin2 (I = 0.1)

Turbulent dissipation profile

115.175.0 zkCz

Constant discharge emission sources of 4 various sizes used

24 set of simulations were made for 24 h selected data

ρ and Ta changed every hour

with Cμ = 0.09 and κ = 0.40

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PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 8

MODELLING

Vortex

0.20 m

Ws

Input Length (5H) Output Length (5H) W

Symmetry

H

Inlet velocity profile

Shear Layer

Hs

Roof (downstream) Roof (upstream)

Side Wall Side Wall

Outflow

Winds from NW

Sampling points

CFD SIMULATIONS – EFFECT OF SOURCE SIZE 3 of 5

(a) (b)

(d)(c)

Sa (0.53 x 0.11 m)

Sb (5.08 x 1.98 m)

Sc (1 x 0.75 m)

Sd (2 x 1.5 m)

Shows the advection of PNCs from the sources to the leeward side of the canyon; selection of the source size is critical to determine PNC distributions

In case of smallest source Sa largest concentrations in the bottom corner of the canyon and the region near to the street wall up to 0.50 m in the leeward side

In other cases with larger source area, particles first accumulate on the leeward side corner of the source, where concentrations are largest, and then advected upwards in the leeward side by the canyon vortex.

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 9

MODELLING

Wind

COMPARISON OF VERTICAL PNC PROFILES 4 of 5

Important aspects shape and magnitude; General trend – conc. (↓) with (↑) height

Box and OSPM assume constant PNCs up to 2 m and then follows general trend, but CFD profiles does not show this decrease, suggesting that it does not predict enough mixing in region of leeward wall

Measurements showed positive concentration gradient; reasons identified were: dry deposition, recirculating vortex, trailing vortices (Kumar et al., 2008b)

This gradient was not shown by Box and OSPM, but reproduced by CFD suggesting that size of source which is closest to vehicle dimensions may be a better representation for setting up a source in CFD simulations

See Kumar et al. (2009c) for details

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 10

MODELLING

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80Normalised concentration (C * )

z/H

OSPMCFD_ScBox

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80

z/H

MeasuredOSPMCFD_ScBox

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80Normalised concentration (C * )

(a) Leeward (b) Windward

COMPARISON OF MEASURED AND MODELLED PNCs 5 of 5

0.0E+00

4.0E+04

8.0E+04

1.2E+05

1.6E+05

0.0E+00 4.0E+04 8.0E+04 1.2E+05 1.6E+05

0.0E+00

4.0E+04

8.0E+04

1.2E+05

1.6E+05

0.0E+00 4.0E+04 8.0E+04 1.2E+05 1.6E+05

0.0E+00

4.0E+04

8.0E+04

1.2E+05

1.6E+05

0.0E+00 4.0E+04 8.0E+04 1.2E+05 1.6E+050.0E+00

4.0E+04

8.0E+04

1.2E+05

1.6E+05

0.0E+00 4.0E+04 8.0E+04 1.2E+05 1.6E+05

OSPM

CFD_Sc

Box

0

0.4

0.8

1.2

1.6

0 0.4 0.8 1.2 1.6

0

0.4

0.8

1.2

1.6

0 0.4 0.8 1.2 1.6

0

0.4

0.8

1.2

1.6

0 0.4 0.8 1.2 1.6

0

0.4

0.8

1.2

1.6

0 0.4 0.8 1.2 1.6

105

105

(a) (b)

(d)(c)

z/H = 0.40 z/H = 0.64

z/H = 0.09 z/H = 0.19

0

0.4

0.8

1.2

1.6

0 0.4 0.8 1.2 1.6

Measured N 10-300 (# cm-3)

0

0.4

0.8

1.2

1.6

0 0.4 0.8 1.2 1.6

Mo

del

led

N1

0-3

00

(# c

m-3)

The measured PNCs at different heights compared well within a factor of 2-3 to those modelled using OSPM, Box model and CFD simulations, suggesting that if model inputs are given carefully, even the simplified approach can predict the concentrations as well as more complex models.

See Kumar et al. (2009c) for details

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 11

MODELLING

SUMMARY AND CONCLUSIONS

An advanced particle spectrometer was successfully applied to measure PNDs and PNCs in street canyons and was found to be quite useful when fast response nature of an instrument is essential.

Model comparison suggested that If model inputs are given carefully, a simplified approach can predict the PNCs to accuracy comparable with that obtained using more complex models.

Study for the selection of the source size in CFD simulations showed that a source size scaling the vehicle dimension, not the size of the exhaust pipe, better represented the measured PNC profiles.

The PNC differences were largest between idealised (CFD and Box) and operational (OSPM) models at upper sampling heights; these were attributed to weaker exchange of clean air between street and roof-above in the upper part of canyon in case of idealised models.

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PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 12

RELATED ARTICLES FOR DETIALED INFORMATION 1 of 1

JOURNAL• Kumar, P., Garmory, A., Ketzel, M., Berkowicz, R., 2009c. Comparative study of measured and modelled number concentration

of nanoparticles in an urban street canyon. Atmospheric Environment 43, 949-958.• Kumar, P., Fennell, P., Symonds, J., Britter, R., 2009b. Treatment for the losses of ultrafine aerosol particles in long sampling

tubes during ambient measurements. Atmospheric Environment 42, 8831-8838.• Kumar, P., Fennell, P., Hayhurst, A., Britter, R., 2009a. Street versus rooftop level concentrations of fine particles in a Cambridge Street Canyon. Boundary–Layer Meteorology 131, 3-18. • Kumar, P., Fennell, P., Britter, R., 2008c. Effect of wind direction and speed of the dispersion of nucleation and accumulation

mode particles in an urban street canyon. Science of the Total Environment 402, 82-94.• Kumar, P., Fennell, P., Britter, R., 2008b. Pseudo-simultaneous measurements for the vertical variation of coarse, fine and

ultrafine particles in an urban street canyon. Atmospheric Environment 42, 4304-4319.• Kumar, P., Fennell, P., Britter, R., 2008a. Measurements of the Particles in the 5-1000 nm range close to the road level in an

urban street canyon. Science of the Total Environment 390, 437-447.

CONFERENCE• Kumar, P., Robins, A., Britter, R., 2009d. Fast response measurements for the dispersion of nanoparticles in vehicle wake and

street canyon. 89th AMS meeting on the Urban Environment, Phoenix, Arizona (USA), 11-15 January 2009.• Kumar, P., Fennell, P., Britter, R., 2008e. The influence of Ambient Meteorology on Nanoparticle Concentration in an Urban

Setting. Cambridge Particle meeting, Cambridge (UK), 16 May 2008.• Kumar, P., Britter, R., 2008d. Measurements and dispersion modelling on traffic-emitted particles in the urban environment .

National Environment Research Institute (Denmark), 7 May 2008. • Kumar, P., Fennell, P., Britter, R., 2007d. Measurement and dispersion behaviour of particles in various size (5 nm>Dp<1000

nm) ranges in a Cambridge Street Canyon. Proceedings of the 11th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Cambridge (UK), 2-5 July 2007, pp. 368-372.

• Kumar, P., Fennell, P., Britter, R., 2007c. The measurement of fine particles for the study of their dispersion and of street-scale air quality. UK Atmospheric Aerosol Network (UKAAN) Workshop, University of Reading, Berkshire (UK), 6-7 June 2007.

• Kumar, P., Britter, R., 2007b. Particulate Matter: Importance, Regulations and Historical Perspective. ‘Nirmaan’, IIT Delhi Civil Engineering Society, Issue 2, May 2007 pp. 38-42.

• Kumar, P., Britter, R., Langley, D., 2007a. Street versus rooftop level concentrations of fine particles in a Cambridge Street Canyon. 6th International Conference on Urban Air Quality, Limassol (Cyprus), 27-29 March 2007, pp. 135-138.

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 13

ACKNOWLEDGEMENTS

World Meteorological Organisation – bursary award

Cambridge Nehru Scholarship and ORS Award – PhD funding

Dr. Paul Fennell (Imperial College, London) – helping in experiments

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PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 14

THANK YOU

CONTACT

PRASHANT KUMAR

Email: pp286@cam.ac.uk

Webpage: http://people.pwf.cam.ac.uk/pp286

CFD SIMULATIONS – EFFECT OF SOURCE SIZE Extra slide

See Kumar et al. (2009c) for details

PRASHANT KUMAR UAQ 2009, ISTANBUL, TURKEY, 24-27 MARCH 09 16

MODELLING

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80

Normalised concentration (C * )

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80

z/H

CFD_SaCFD_SbCFD_ScCFD_Sd

At W = 0.40 m from leeward side wall

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80

CFD_SaCFD_SbCFD_ScCFD_Sd

At W = 0.40 m from windward side wall

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80

z/H

At W = 1.50 m from leeward side wall

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80Normalised concentration (C*)

z/H

At W = 2.50 m from leeward side wall

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80

At W = 1.50 m from windward side wall

0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80Normalised concentration (C * )

At W = 2.50 m from windward side wall0.0

0.3

0.6

0.9

1.2

1.5

0 20 40 60 80

Nor

mal

ised

hei

ght

(z/H

)

At W = 2.50 m from windward side wall

(a) (b)

(e) (f)

(c) (d)

At w/H = 0.034 (0.40 m) from leeward side wall

At w/H = 0.13 (1.50 m) from leeward side wall

At w/H = 0.034 from windward side wall

At w/H = 0.22 from windward side wall

At w/H = 0.13 from windward side wall

At w/H = 0.22 (2.50 m) from leeward side wall

Considerably larger PNC variations in leeward side, but modest on windward side ( 0.50 m), while changing size of the source

PNCs increases from road level to a certain height; the height at which this maximum occurs could be related to the height of various sources used

The largest sources shows similar profile suggesting that effect of source size is minimal after a certain cross-sectional area

Unlike leeward side, concentration profiles in windward side shows similar trend with consistent increase in concentrations with increasing distance from windward wall