Aerosol Transport in Urban Areas

A contribution to subproject SATURN

Ian Colbeck and Meng-chen Chung

Department of Biological Sciences, University of Essex, Colchester UK

Summary

Aerosol concentrations were measured at two sites in the Athens basin. It was concluded that meteorology played a significant role in determining the concentrations at each site.

Aim of research

To investigate physico-chemical transformations of aerosol in an urban plume.

Activities during the year

Field campaign in Athens as well as continuing measurements within Colchester.

Principal results

Air pollution in the Mediterranean urban centres is characterised by intense photochemical activity, high temperatures and low relative humidities during the summer period of the year. The effect of these climatic conditions on atmospheric chemistry coupled by complex topography and specific emissions can be the generation of secondary pollutants different in composition and concentration to those measured in other parts of the world. Previously we have investigated the concentration of common ionic species in the atmospheric aerosol at the centre of Athens, as well as that of secondary gaseous pollutants such as nitric and hydrochloric acids (Eleftheriadis et al., 1998). It was shown how the built-up of atmospheric aerosol and acidic species in the centre of the city was dependant upon the wind flow patterns. In this paper we present concentrations of atmospheric aerosols two sites: one upwind and one downwind of Athens.

Sampling was performed over 24 hour periods with an annular denuder system. The distance between the two sampling sites was 22 km and were located at the coastal and inland boundaries of the Athens Metropolitan area along the NE-SW axis. The latter coincides with the main surface wind flow pattern over the summer period of the year. The coastal site was located on the top of the Castella hill 90 m a.s.l. in Pireas and the inland site on the edge of the Ag. Stefanos residential area. Samples were analysed for Cl-, NO3-, SO42 -, NH4+, Na+, K+, Mg2 +, Ca2 +.
Meteorological parameters such as wind speed and direction and relative humidity were provided by routine measurements of the Greek Meteorological service. These measurements were taken at sites 6 and 7 km from the inland and coastal site respectively and were selected so that they are representative for the ambient air at the sampling sites.

The preliminary results from the campaign are presented here. Figures 1 and 2 show the 24h average concentrations for atmospheric aerosol at Ag. Stefanos and Pireas respectively. It is evident that much higher concentrations of all the aerosol species were found at Ag. Stefanos during the first half of the sampling period, while similar or slightly lower concentrations are observed during the rest of the period. This is clearly shown in figure 3 where the difference in concentration for NO3-, SO42 -, NH4+, Na+ between the two sites is plotted. The concentration patterns at the two sites can be explained by the dominant wind flow patterns present in the area during the measurements. The high concentrations observed at Ag. Stefanos are related to south westerly winds mainly due to sea-breeze circulation while the lower concentrations coincides with cleaner north-easterly winds. At this time of the year a semi-persistent wind circulation, consisting of a northerly flow (Etsesians), occurs. Additionally the region is subjected to a sea-breeze circulation. Over the sampling period each day was classified according to the wind pattern: (i) days with a strong northerly wind, which suppresses the sea breeze (Estesian day); (ii) days with a fully developed sea breeze circulation; and (iii) days with moderate northerly winds and a weak sea breeze cell near the coast. The results of this investigation are being used for a detailed evaluation of the processes governing formation and transport of secondary pollutants, such as NO3- and SO42 - , arising from emissions within the Metropolitan area.


Figure 1 Atmospheric aerosol ionic species ambient concentrations at Ag. Stefanos
 

Figure 2 Atmospheric aerosol ionic species ambient concentrations at Castella, Pireas


Figure 3 Difference in NO3-, SO42 -, NH4+ and Na+ concentrations between Ag. Stefanos and Pireas
 

Main conclusions

The wind climatology illustrates the variation in local meteorological conditions over the Athens basin. Pollution levels depend upon the wind regime. Estesian days result in small differences between the sites whereas on 'normal' days the sites are influenced by local emissions.
Aim for the coming year
To extend the measurements in the Athens basin to different times of the year.

References

Eleftheriadis K., D. Balis, I. Ziomas, I. Colbeck and N. Manalis; Atmospheric aerosol and gaseous species in Athens, Greece, Atmos. Environ. 32, (1998) 2183-2191.