The impact of boundary layer dynamics on mixing of pollutants Janet F.Barlow 1, Tyrone Dunbar 1, Eiko Nemitz 2, Curtis Wood 1, Martin Gallagher 3, Fay Davies 4 and Roy Harrison 5 1 University of Reading 2 Centre for Ecology and Hydrology 3 University of Manchester 4 University 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 Tower DAPPLE 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) 24 th Oct to 14 th 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) 15 th Oct to 15 th Nov 20Hz sampling frequency height z = 190 m (BT), 17 m (roof) wind velocity, sonic temperature, fluxes Sonic anemometers

Lidar observations – 5 th Nov, frontal rain

Lidar observations – 6 th Nov, convective conditions

Lidar observations – 7 th Nov, tracer release day

Mixing height: all days

Mixing height: clear days only

6 th Nov: daytime convective BL

6 th : 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? Estimate time taken to transport pollutant from street level up to BT Tower: Simple approach! z is height of BT Tower (190m)  w is standard deviation of vertical windspeed...compares well with REPARTEE tracer experiments (see Martin et al. ACP, or Stock et al. poster)

Estimated time to diffuse from surface up to BT Tower Time (seconds)

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 ~ m 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 turbulence How far will pollutants be transported vertically (horizontally)? 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 6 th Nov normalised by convective velocity wherez i =BL depth w’T’ = heat flux θ = 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

Windspeed during campaign Tracer

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

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