DOAS MEASUREMENTS ABOVE AN URBAN STREET CANYON IN A MEDIUM SIZED CITY

摘要

The concentrations of ozone, NO_2and SO_2measured by a Differential Optical Absorption Spectroscopy (DOAS) system in the city of Kozani in Northwestern Greece are presented and compared against data from conventional ground stations. This was made in order to assess DOAS performance in a complicated area in terms of sources variety (i.e. coal fired power stations operation and urban sources), terrain complexity and meteorological conditions. Measurements of aromatic hydrocarbon substances (benzene, toluene and xylene) are also presented for air quality purposes.Results of the regression analysis between the conventional station and the DOAS system indicated the existence of significant correlations among some of the monitored trace gases, confirming this way the strong compatibility between the two systems. For example the two data sets were correlated for sulfur dioxide (R~2=0.89) and ozone (0.63) but the very low correlation for NO_2 measurements (0.21) was mainly attributed to differences in traffic flow between the locations covered by the two systems. The mean period concentrations measured by DOAS and the ground station for NO_2 were 25.33 ug/m3 and 12.34 ug/m~3, respectively. SO_2 took values 7.99 μg/m~3 and 5.07 μg/m~3, respectively. The low values of sulfur dioxide especially during the winter were attributed to the absence of central heating in the most of the buildings due to the use of district heating provided by the Greek Public Power Corporation from the power stations. Seasonal differences n ozone measurements have been found without clear correlation. In the summer the correlation between the two systems is stronger due to the very well mixed boundary layer. The averaged monthly benzene values ranged between 5-11 ug/m3. Toluene, xylene and styrene levels are still under evaluation of their reliability. In conclusion, the level of agreement between the conventional ground monitoring station and the DOAS system can be affected by the variations in the spatial mixing conditions. Apart from resolving the remaining uncertainties the meteorological parameters remain vital in the development of a clear understanding in DOAS performance due to its ability to cover a wide spatial scale over the open path length. Means of micro and meso scale modeling techniques could also be employed to further investigate the influence in the relation between DOAS and surface station data sets due to distant sources impacts under specific mesoscale conditions. This mostly concerns ozone, NO_2, and SO_2 concentrations. For benzene the case is clearer where contributions were attributed to traffic emissions.Keywords: differential optical absorption spectroscopy, DOAS, urban street canyon;;ozone;;NO_2;;SO_2;;benzene;;toluene;;p;;m-xylene;;aromatic hydrocarbons;;VOCs