Fine-Scale Variations in Aerosol Transport within a Street Canyon – a Pilot Field Study

I.D. Longley, M.W. Gallagher, M. Flynn, J.R. Dorsey, P.I. Williams 

Physics Department,University Of Manchester Institute of Science and Technology, Manchester, UK

SASUA Edinburgh Average Diurnal Particle Flux

Time (Local)

00:00 04:00 08:00 12:00 16:00 20:00 00:00

Par

ticle

Flu

x (#

cm

-2 s

-1)

0

10000

20000

30000

40000

50000

Diurnal average vertical flux: fine mode (cm-2s-1)

00:00:00 04:00:00 08:00:00 12:00:00 16:00:00 20:00:00 00:00:00

Dp /

m

0.15

0.2

0.3

0.4

0.1

0 100 200 300 400 500 600

diurnal average of fine mode particle number concentrations

(0.1 m < Dp < 0.2 mm)

0

500

1000

1500

2000

2500

0:00 6:00 12:00 18:00 0:00

N /

cm-3

Diurnal average of particle number flux (0.1 m < Dp < 0.2 m)

and sensible heat flux

-6000

-4000

-2000

0

2000

4000

6000

8000

10000

12000

0:00 6:00 12:00 18:00 0:00

w'N

' / c

m-2

s-1

-40

-20

0

20

40

60

80

hea

t flu

x / W

m-2

particle flux

heat flux

Campaign mean mass size distribution

Ultra-fine mode number concentrations

Fine mode number concentrations and fluxes

1000

10000

100000

1 10 100 1000

Dp / nm

dN

/dlo

g(D

p)

/ cm

-3

SW perp

up-canyon

0

50000

100000

150000

200000

250000

0 2 4 6 8 10

Uroof / ms-1

N0.

1 /

cm-3

SW perp

Up-Canyon

NE perp

Normalised coarse particle concentration decay on 17th Oct

0

0.2

0.4

0.6

0.8

1

1.2

1.4

00:00 06:00 12:00 18:00 00:00

N/N

0 /

cm-3

2 to 4 micron fit

2 to 4 micron data

8 to 10 micron fit

8 to 10 micron data

decay rate constant, evening of 17th Oct, moderate wind

0

0.5

1

1.5

2

2.5

3

3.5

3 5 7 9

Dp / mm

dec

ay r

ate

/ d

ay-1

R2 of exponential decay fit for evening of 17th Oct

0

0.1

0.2

0.3

0.4

0.5

0.6

3 5 7 9

Dp / m

R2

fractional change in coarse mass concentration between 18:00 and 01:00 is dependent upon size and wind speed

0

1

2

3 5 7 9 2 to 10

Dp / mm

M/M

0

moderate wind

low wind

Coarse mode concentrations and fluxes

•One- and ten-minute number concentrations of ultra-fine aerosol (<100nm) concentrations at street level are very sensitive to air flow direction.

•When flow is from the road towards the pavement, concentrations at this side of the canyon are much higher, due mostly to increased numbers of less-diluted particles in the size range 10-80nm. This can lead to brief increases in concentration by an order of magnitude above the mean.

Coarse mode mass concentrations by size

16/10/2001 17/10/2001 18/10/2001 19/10/2001 20/10/2001

0

2

4

6

8

10

12

14

2 to 4 m4 to 6 m6 to 8 m8 to 10 m

Percentage fraction of total PM10-2

16/10/2001 17/10/2001 18/10/2001 19/10/2001 20/10/2001

0

10

20

30

40

50

60

70

2 to 4 m4 to 6 m6 to 8 m8 to 10 m

0

20

40

60

80

100

Frequency histogram of 1-minute N0.1 at 4m, week 1 with cumulative frequency curve

N0.1 / cm-3

0 50x103 100x103 150x103 200x103

n

0.0

0.1

0.2

0.3

0.4

•After wind direction, traffic flow rate and wind speed are the most important determinants of concentration.

•However, in perpendicular flow, wind speed in-canyon is poorly related to roof-level wind speed.

Fine mode (0.1-0.5m) number concentrations follow a diurnal cycle related to anthropogenic activity, particularly traffic flow.

•Emission fluxes also have a similar diurnal cycle, but more closely linked to sensible heat flux.

•The same conclusions were drawn above the urban canopy in the SASUA project in Edinburgh, see above (Dorsey et al, 2002).

The coarse mode behaviour could be split into three periods:

1. Decay following wind-driven re-suspension event,

2. Moderate wind diurnal cycle

3. Low wind diurnal cycle

•Decay rate was dependent upon size and wind speed.

•Particles in the size range 4-8m dominated during moderate winds.

•In high winds re-suspension of extra particles, especially in the range 8-10m, enhanced PM10.

Smoothed hourly average coarse mode mass emission flux, 17/10/01

-40

-20

0

20

40

60

80

100

120

0:00 6:00 12:00 18:00 0:00

FM

/ g

cm-2

s-1

2 to 4 microns

4 to 6 microns

6 to 8 microns

8 to 10 microns

Normalised vertical turbulent variance at 3.5mseperated by wind direction

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.5 1 1.5 2 2.5 3 3.5

U / ms-1

w/U

NE perpendicular

SW perpendicular

Up-Canyon

Turbulence

Vertical profiles of u,v,w/U with U>1 ms-1

02

46

81012

1416

1820

0 0.2 0.4 0.6 0.8 1

u,v,w/U

z /

m

sigma u

sigma v

sigma w

•Longitudinal (u) turbulence roughly double lateral (v) and vertical (w).

•At low wind speeds local sources (including traffic and) dominate.

•Turbulence also enhanced in perpendicular flow.

vertical profiles of parameterised traffic-induced vertical turbulence, wt,

for low and busy traffic flow rates (T / h-1)

0

2

4

6

8

10

12

14

16

18

20

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

wt / ms-1

z/ m

T=300

T=1000

The component modes of PM10 varied independently due to differing transport characteristics.

•A two-week campaign in an asymmetric street canyon in Manchester with busy traffic (~20 000 vehicles.day-1) in October 2001.

•Measurements of ultra-fine (SMPS), fine (ASASP-X) and coarse (FSSP) size-segregated particle concentrations.

•Fine and coarse mode emission fluxes determined by eddy correlation.

SW perp

Up-canyon

NE perp

Down-canyon

Above: canyon site with definition of wind directions

day night

References:

Dorsey, J.R.; Nemitz, E.; Gallagher, M.W.; Fowler, D; Williams, P.I; Bower, K.N.; Beswick, K.M., 2002. Direct Measurements and Parameterisation of Aerosol Flux, Concentration and Emission Velocity Above a City. Atmos. Environ. 36, 791-800.

Longley, I.D.; Gallagher, M.W.; Dorsey, J.R.; Flynn, M.; Allan, J.D.; Alfarra, M.R.; D. Inglis, D.. A case-study of aerosol (4.6nm<Dp<10m)

number and mass size distribution measurements in a busy street canyon in Manchester, U.K. Atmos. Environ. [Accepted, 2002].

Vertical turbulence could be described by the parameterisation

w2 = (U)2 + wt

2 where wt2 = AT + B and T = traffic flow rate

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Dp / m

dV/d

log(

Dp )

/

m3 cm

-3

SMPS

ASASP-X

FSSP

•In low winds, reduced deposition/dispersion in the size range 4-6m had the biggest effect on above-normal night-time coarse mass concentrations.

•Mass emission fluxes (left) followed a diurnal cycle, with an early-afternoon peak. Traffic peaks had more influence on larger sizes.

Above: N0.1 I.e. number concentration (4.6nm < Dp < 100nm)

Mass mode concentrations over 2 weeks

0

10

20

30

40

50

60

10/14 10/16 10/18 10/20 10/22 10/24 10/26 10/28

PM

/ g

m-3

coarse mode 2-10 microns

fine mode 0.1-0.5 microns

ultra-fine mode <0.1 microns