Results from the SMEAR III urban measurement station

Leena Järvi et al.

URPO – Urban and Rural Air Pollution seminar

22.11.2007

Introduction

• The SMEAR III measurement station started in Helsinki in autumn 2004

• To get information about the sources and dispersion of air pollutants, turbulence and air chemistry

• Continuous measurements in aerosol particle dynamics atmospheric chemistry micrometeorology weather monitoring ecophysiology of trees

• Turbulent exchange (momentum, heat and CO2 fluxes) and meteorological parameters are measured in a 31 m high tower

• Aerosol particle and trace gas instrumentation are located in a container next to the tower

• Surrounding area can be divided into three land use sectors: urban, road and vegetation

Methods

• Aerosol particle measurements are carried out with twin DMPS (Differential mobility particle sizer) and APS (Aerodynamic

particle sizer)

• Aerosol particle data is divided into three classes which have different dynamics and sources

• In urban areas, ultrafine particles (UFP, d<0.1 μm) are mainly produced

in combustion processes • Accumulation particles (AP, 0.1< d < 1 μm) are also combustion related

and the size of the is favorable for LRT• Coarse particles (>1 μm) are mainly resuspended from the soil of roads

• Data between May 2005 and June 2007 has been analyzed

Measurements and calculation of turbulent fluxes

• Flux is the transfer of some substance/area/time

• Turbulent fluxes can be calculated with eddy covariance (EC) technique

Flux is the covariance of vertical wind speed and scalar in

question

• Setup includes Metek ultrasonic anemometer and LiCor infrared gas analyzer (10 Hz)

• Data from Dec 2005 to Jun 2007 was analyzed

Wind direction dependence of aerosol particles

• UFP’s and AP’s have dependence on land use cover

• Highest coarse particle concentrations are measured in 180-250°

• Concentrations typical for cities

UFP: 3000-18000 cm-3

AP: 500-3300 cm-3

Coarse: 0.3-1.7 cm-3

Diurnal behavior of aerosol particles for different seasons and separately for weekdays and weekends

• The influence of traffic clear in UFP and AP concentrations

• Winter UFP values highest

• Low mixing heights• Combustion

sources

Weekdays Weekends

Diurnal pattern of CO2 fluxes in different seasons and land use sectors

• Most of the time, surrounding area acted as a source

• In spring and summer days, vegetation uptake exceeded the effect of anthropogenic sources in vegetation sector

Correlation between traffic rates and CO2-fluxes

• Traffic data available for Jan-Aug 2006

• CO2 flux and traffic rates were positively correlated

• Similar results from Edinburgh (Nemitz et al.

2002) Vesala et al. (2007)

Diurnal cycle of sensible heat (black) and latent heat (red) fluxes

• Sensible heat fluxes are elevated in urban sector (heat island effect)

• Latent heat fluxes are raised by the vegetation cover

Conclusions

• Heterogeneous measurement site enables the studying the effect of different land use covers to air pollutants and turbulent fluxes

• Ultrafine and accumulation particle concentrations were typical for urban areas and were affected by the road next to the measurement site

• The heat island effect was distinguishable in sensible heat fluxes

• Surroundings of the site acted as a source for CO2 most of the time