The impact of boundary layer dynamics on mixing of pollutants

Janet F.Barlow1, Tyrone Dunbar1, Eiko Nemitz2, Curtis Wood1, Martin Gallagher3, Fay Davies4 and Roy Harrison5

1University of Reading2Centre for Ecology and Hydrology

3University of Manchester4University of Salford

5 University of Birmingham

Funded by The BOC Foundation

Urban Boundary Layer dynamics: effect on aerosol distribution

City scale:

(from Oke, 1987)

Urban Boundary Layer dynamics: effect on aerosol distribution

Neighbourhood scale:

• heat and moisture sources / drag sinks are patchy• aerosol / precursor gas sources are patchy too!• vertical structure of turbulence and aerosols varies spatially and diurnally

Simultaneous measurements required to attribute variability in pollutant concentrations to both dynamical and chemical processes

REPARTEE campaign 2007

Regents Park

Lidar Site BT TowerDAPPLE roof site

Aim:Intensive observations of aerosols, trace gases and boundary layer dynamics to quantify variability in urban pollutants due to both chemical and meteorological processes.

Special Issue in Atmospheric Chemistry and Physics!

• Collaborators: Birmingham, York, Manchester, Salford (FGAM), Reading, CEH, Lancaster, Leicester, Cambridge, Environment Agency, Bristol

c. 1.6 km

• Facility for Ground-based Atmospheric Measurement (FGAM) 1.5 micron scanning Doppler lidar (Halo-photonics)

• 24th Oct to 14th Nov 2007 • vertical stare• 30 m resolution gates• integration every 4 sec

• backscatter• along beam Doppler velocity (vertical component)

Doppler lidar measurements

• R3 sonic anemometers (Gill)• 15th Oct to 15th Nov • 20Hz sampling frequency• height z = 190 m (BT), 17 m (roof)• wind velocity, sonic temperature, fluxes

Sonic anemometers

Lidar observations – 5th Nov, frontal rain

Lidar observations – 6th Nov, convective conditions

Lidar observations – 7th Nov, tracer release day

Mixing height: all days

Mixing height: clear days only

6th Nov: daytime convective BL

6th: clear night – jet??

Influence of boundary layer on mixing

• Big changes in: mixing height, strength of turbulence, vertical structure of turbulence

What is the impact on upward mixing of pollutants?

Estimated time to diffuse from surface up to BT Tower

Tim

e (

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Conclusions• Doppler lidar gives reliable measurements of turbulent structure of urban boundary layer

• Lidar measurements indicate well-mixed profiles by day, separate layers of aerosol and turbulence at night of depth ~100-500m

Occasionally during night-time ground-level and BT Tower measurements can diverge, i.e. the flow is decoupled from the surface• Mixing timescales depend on boundary layer turbulenceHow far will pollutants be transported vertically (horizontally)?

j.f.barlow@reading.ac.uk

Latest work: Advanced Climate Technology Urban Laboratory (ACTUAL) Further Doppler lidar measurements in London

Intercompare BT Tower and lidar

• standard deviation of vertical velocity• big difference during calm, clear night

• across all days, near 1:1 comparison

• across all nights, lidar sometimes less than BT sonic, esp low values

Intercompare BT Tower and lidar

• BT sonic resampled to 0.25 Hz to match lidar response Lidar sampling rate “too slow” at night to capture small scale turbulence

Lidar observations – vertical velocity variance

• hourly averages 6th Nov• normalised by convective velocity

where zi=BL depth

w’T’ = heat fluxθ = potential

temp• Compare with Sorbjan (1991), CBL over homogeneous terrain• error bars = standard deviation

Peak below mid-boundary layer, indicating strong shear contribution to mixing

BT Tower observations

• convective day, positive sensible heat flux, negative at night• clear skies throughout• northerly flow (i.e. from Park)• moderately strong winds

Intercompare sonic data at rooftop and BT Tower

• Despite statically stable conditions, Rb indicates turbulence still maintained at BT Tower so not completely decoupled

Rb ~ 0.1

Wind bearing during campaign

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Figure 10: Example of stable conditions (2nd May). a) Mean and standard deviation of sonic temperature,

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Fig 10 b) Ratio of virtual potential temperatures, and normalised turbulent kinetic energy at BT Tower and Lib sites.

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