Summary

Many pollutants are of importance for assessing the adverse impact of the air pollution, e.g. NO₂, CO, lead, VOCs and particulate matter. In addition to monitoring of traditional pollutants, measurements of benzene, toluene and xylenes have been carried out in central Copenhagen since 1994. Significant correlation was observed between VOCs and CO concentrations, indicating that the petrol engine vehicles are the major sources of VOC air pollution in central Copenhagen. A method to determine emissions from the actual car fleet under realistic driving conditions has been developed. The method is based on air quality measurements, traffic counts and inverse application of street air quality models. Using this method average emission factors of CO, NO_x and benzene for the fleet of petrol fuelled vehicles was estimated for the period 1994-1997. The trends were related to the measures taken to reduce the emissions from vehicles, i.e. percentage of catalyst cars and fuel composition.
 

Dispersion and transformation of particulate matter in streets are studied by using a combination of a street pollution dispersion model, the Operational Street Pollution Model (OSPM) and a particle transformation model. The particle model implements a very fast implicit method for solving the changes in particle size distribution due to coagulation, condensation and dilution processes. Using available measurements of emitted particle size distributions, impact analysis of the various transformation processes affecting the size distribution of particles was performed. For this purpose, a plume model simulating dilution of diesel exhaust has been developed and coupled with the particle coagulation model. The results show that due to the rapid dilution of the exhaust plume, the impact of the coagulation process is insignificant. Calculations with the street pollution dispersion model OSPM, using the diesel exhaust emissions as the only source, show that the mass concentrations of emitted particles in the street are predicted to be significantly lower than the measured concentrations.
 

Aim of the research

The main goal is to provide model tools for determination of traffic generated air pollution. The study is based on the activities in Copenhagen. They are, experimental studies of air pollution from traffic in a local (street) scale, development of local scale models and combination of these activities with experimental studies and model development in urban scale.
 
 

Activities during the year

The monitoring of NO_x/NO, CO, TSP, O₃ etc. has been continued during 1998. The measurements of benzene, toluene and xylenes were continued. In addition, size fractionated particle measurements in 30 fractions, 0.2-20 m m were performed by an optical particle analyser.
 

The experimental data has been analysed and applied for determination of emissions and emission factors for CO, NO_x and benzene of the car fleet in Copenhagen.
 

A plume model simulating dilution of diesel exhaust has been developed and coupled with the particle coagulation model. The model has been applied for estimates of the particle growth of directly emitted particles in diesel exhaust in a street.

Principal results
 
 

Figure 4. Evolution in annual (background subtracted) mean NOx, CO and benzene concentrations and calculated traffic emissions in Jagtvej.

Atmospheric pollution dispersion models are usually used for calculation of air quality based on known theoretical relationships between emissions, meteorology and air concentrations. On the other hand, combining model calculations with ambient pollution measurements allows in-situ estimations of emissions.
 
 

Considering dispersion in streets of non-reactive or only slowly reactive car exhaust gases, the chemical transformations can be disregarded, and we formulate the problem in the following way,
 
 

        (1)
 
 

where C is the concentration of a particular pollutant in the street, Q is the emission of pollutants from the traffic in the street and F(meteorology) is a function describing dispersion processes. C_background is the contribution to pollution concentrations in the street from all other sources than the traffic in the street.
 
 

The dispersion function F(meteorology) is given by the Operational Street Pollution Model (OSPM). OSPM describes the dispersion in a street canyon based on meteorological parameters, mainly wind speed and direction above roof tops. In comprehensive tests on measurements from a number of monitoring sites, OSPM has shown to give a satisfactory description of the air pollutant dispersion in urban streets (Berkowicz et al., 1997).
 
 

Equation (1) can be used for calculations of hourly emissions from traffic, provided that both street and background concentrations are available on an hourly basis.
 
 

                 (2)
 
 

In (2) the index h refers to a particular hour of the day. Different regression methods were tested and applied.
 
 

The method has been applied on continuos one hour average measurements at street level and urban background. Very strong correlation was observed between NO_x, CO, benzene and toluene in the street indicating traffic as the main source.
 
 

Calculating the diurnal emission profiles for several years provides estimation of the trends in the traffic contribution to air pollution. Concentrations depend on both emissions and meteorology, whereas the trend analysis of emissions is independent of the inter-annual variations in the meteorological conditions. Trend analysis of the average diurnal NO_x, CO and benzene emissions were performed for the years 1993(4)-1997. Results are shown in Fig. 1 together with the measured annual average concentrations, but subtracted the background contribution. The calculated emissions and measured concentrations show a similar long-term trend but the inter-annual variation is different, illustrating the influence of meteorology on air pollution levels. The benzene content in the major part of petrol sold in Copenhagen was reduced from approx. 3.5% in 1994 to approx. 2 % by the end of 1995. This explains the lower graph of Fig. 1 (Palmgren et al., 1998).

The emission factors were also estimated for different vehicle categories. This was possible because the diurnal traffic patterns of the different vehicle categories were different.
 
 

In order to evaluate the effect of coagulation on modification of the size distribution of traffic emitted particles, the coagulation-dilution model was applied to a hypothetical plume emitted from a diesel engine into the street air. A size distribution for the emitted particles was estimated based on laboratory results.
 
 
 

Fig. 2. Characteristic time scale for coagulation of particles of various sizes as function of the time after emission. The dashed area represents typical residence times of particles in a street.

The plume dilution is calculated assuming very low wind speed (u=0.1 m/s) and traffic created turbulence corresponding to 900 passenger cars/h and 100 heavy vehicles/h traveling with

speed of 40 km/h. The particle size distribution in the plume at 0.1s after emission is clearly affected by the emitted particles. After somewhat longer time, the dilution of the emitted particles becomes so large that the plume concentration is totally dominated by the background particles. No visible effect of the coagulation can be observed. This leads to the conclusion that, due to the rapid dilution, coagulation is not expected to significantly change the particle size distribution in vehicle exhaust plumes. This conclusion is supported by analysis of the characteristic times for coagulation and dilution shown in Fig. 2 (Vignati et al., 1998).

Main conclusions
 

A model tool has been established for estimates of emissions and emission factors of the actual vehicle fleet based on experimental data on air quality, traffic density and meteorology. These data are very important as a basis for determination of the air quality in urban areas by model calculations. This type of calculation are e.g. necessary for estimation of the human exposure of air pollutants in urban areas.
 

A newly developed particle model shows that coagulation of diesel exhaust particles will not change the particle size distribution significantly in the street due to the very short residence time. It means that most of the particles from the local traffic are in the size range 20-300 nm inside the streets. Further test and investigations have to be made, especially under other traffic and meteorological conditions.
 

Aim for the coming year

The experimental studies of emission from traffic will be continued in Copenhagen and supplemented with studies in other cites in Denmark. In addition, similar studies will be carried out in St. Petersburg in co-operation with Main Geophysical Observatory.
 

Measurement campaigns of ultrafine and fine particles will be carried out in Copenhagen by DMA, optical analysers and PM_2.5/PM₁₀ in order to investigate the possibilities to include particles in the street pollution model.
 

Acknowledgements

Genikhovich, E., Ziv, A. and Iakovleva, E., Main Geophysical Observatory, St. Petersburg are acknowledged for very fruitful co-operation during their work as visiting scientist at NERI and the co-operation about traffic related air pollution measurements and modelling in St. Petersburg.
 
 

References