In northern Europe, meteorological conditions vary widely in terms of the seasons. The seasonal variation of urban air pollution is therefore to a large extent ...
International Journal of Environment and Pollution The seasonal variation of urban air quality in northern European conditions Jaakko Kukkonen, Mervi Konttinen, Pia Bremer, Timo Salmi and Helena Saari Finnish Meteorological Institute Sahaajankatu 20 E, FIN - 00810 Helsinki, Finland
Abstract In northern Europe, meteorological conditions vary widely in terms of the seasons. The seasonal variation of urban air pollution is therefore to a large extent determined by the meteorological variables, which are important for air quality. We present first results on the yearly variation of atmospheric stability and the occurrence of inversion layers, wind velocity, ambient temperature, solar radiation, precipitation and snow cover. The seasonal variation of the guideline exceedences for CO, NO2, SO2, TSP and PM10 in urban areas in Finland is then interpreted in the light of the meteorological variation. Keywords: urban, air quality, seasonal variation, measurement, meteorological 1. Introduction The long-term trends of pollutant concentrations (in the course of tens of years) are mainly caused by the trends in emissions. The temporal variation of pollutant concentrations is caused by the variations in emissions and meteorological conditions. In northern European conditions, the seasonal variation of traffic emissions is moderate, compared with the influence of the widely varying meteorological factors. The seasonal variation of urban pollutant concentrations is therefore mainly caused by the changing meteorological conditions. However, the variation of traffic emissions has a dominating influence on the diurnal urban pollutant concentration variations. Meteorological conditions influence pollutant concentrations by changing the atmospheric diffusion characteristics, and also by changing the conditions, which affect their chemical transformation. The main objective of this study is to analyze the interdependence of urban air quality and meteorological conditions during the seasons of the year in the Northern European conditions. The meteorological variables considered include atmospheric stability and the occurrence of inversion layers, wind velocity, ambient temperature, solar radiation, precipitation and snow cover. The results of the air quality in Finnish cities and towns are based on the data from urban measurement networks. The data was compiled from 42 Finnish cities and towns, from measurements conducted in 1990-1993. Measured data shows that the proposed national air quality guidelines have been fairly often exceeded in urban areas (Kukkonen et al., 1998). The occurrence of the exceedences is interpreted in terms of meteorological quantities. 2. Materials and methods
2.1 The meteorological data The results presented here have been mainly compiled from the meteorological yearbooks published by the Finnish Meteorological Institute (FMI, 1991a,b, 1993a,b,c and 1994). The global radiation in 1991-1993 was obtained from the meteorological database of the FMI. We have considered the time interval 1990-1993, as this is the time period of the urban concentration measurements considered. We have compiled the meteorological data from three observatories, located in southern, central and northern Finland. Figure 1 shows the location of these stations. 2.2 The concentration data The results of the seasonal variation of air pollutants have been obtained from Kukkonen et al. (1998). This study evaluated exceedences of the new national air quality guidelines issued in 1996, in urban areas in 1990-1993. The data was compiled from 42 Finnish cities and towns. The measured concentration data has been annually collected to the National Air Quality Database. The measured concentrations were used for computing statistical concentration parameters, which were compared with national air quality guidelines and European Union limit values.
3. The seasonal variation of meteorological quantities Figures 2a-h present the seasonal variation of four atmospheric variables at the observatories of Jokioinen and Sodankylä during four years, 1990-1993. The data presented are monthly average values of the temperature, the precipitation and the snow cover, and the cumulative sum of the global radiation. Figures 2a-b show that the seasonal variation of temperature is substantial in Finland. In 1990-1993 the yearly difference of the monthly average temperatures in summer and winter was approximately 19-24 ºC in southern Finland and 23-32 ºC in northern Finland, respectively. Figures 2c-d show the monthly sums of global radiation in 19901993. The global radiation is the sum of the downward direct and diffuse solar radiation (the latter is the downward solar radiation scattered by the sky). Similarly to the corresponding curves for the temperature, the seasonal variation of the global radiation is substantial. Comparing the global radiation and the temperature curves, one observes a phase change, which is caused by the heat capacity of the soil and the waters. Figures 2e-f show the precipitation amounts in 1990-1993. The data includes the total contribution from all kinds of precipitation (e.g. water, snow, hail and frost), measured in liquid form. The irregularity of precipitation is typical for the Finnish climate (for instance, Heino, 1994), and the variation of precipitation from one year to another is substantial. However, the precipitation amount is usually larger in summer, compared with winter. Figures 2g-h presents the snow depth on the ground in 1990-1993. The snow depth values are based on measurements in two days in each month (the 15th and the last day of the month). Similarly to precipitation, the snow depth varies significantly from year to year. Most unfavourable meteorological conditions for the efficient mixing of pollution in an urban area include stable atmospheric stratification, low wind speed (or calm), and the presence of a strong ground-based inversion. The meteorological conditions in Finland are
most unfavourable for the efficient mixing of pollution during the winter half-year, and particularly in winter (December-February).
4. The seasonal variation of urban air quality The seasonal variation of urban air quality is caused by the variation in emissions and meteorological conditions. The seasonal variation of traffic volumes in major Finnish cities is typically smaller than 20 %. The seasonal variation of the emission factors (g/km) depends on the pollutant considered, the vehicle class and the distance travelled. For instance, cold ambient temperature and cold start emissions in winter in Finland can cause a substantial increase of emissions of CO for short travel distances (Laurikko, 1997). However, the seasonal variation of the emissions of NOx from traffic is moderate (Laurikko, 1997). The emissions caused by energy production have a more pronounced seasonal variation. Clearly, this has influence on urban SO2 concentrations, and it can also affect urban NO2 and particulate concentrations. Figures 3a-e show the seasonal variation of the guideline exceedences for CO, NO2, SO2, TSP and PM10. The data was compiled from 42 Finnish cities and towns. The exceedences of half of the guideline have also been shown, in order to achieve a better statistical reliability. Carbon monoxide The seasonal variation of the guideline exceedences of CO is controlled by the variation of meteorological conditions and the variation of emissions from traffic. Both controlling factors tend to cause the largest concentrations to occur in winter. Figure 3a shows that the CO concentrations exceed the guidelines or half of the guidelines most frequently in winter, late autumn and early spring (during OctoberMarch). The maximum number of the exceedences takes place in December. However, the total number of measurements for CO is moderate. The statistical reliability of the results for CO is therefore worse, compared with the other compounds presented. Nitrogen dioxide The guideline exceedences for NO2 are controlled by the variation of meteorological conditions, the emissions from traffic and stationary sources and the chemical transformation processes. In urban areas, the contribution of the emissions from traffic is more important on the average, although the contribution of emissions from stationary sources can be substantial at some specific monitoring stations. The seasonal variation of the emissions of NOx from traffic is smaller, compared with the corresponding variation of the emissions of CO. Figure 3 shows that the seasonal variation of guideline exceedences for NO2 is similar, compared with the corresponding curve for CO. However, the temporal variation of the NO2 exceedences is more evenly distributed, compared with the corresponding results for CO. The NO2 concentrations exceed the guidelines or half of the guidelines most frequently in late autumn, winter and spring. In summer, the net transformation of NO into NO2 is more efficient, due to a higher temperature and the better availability of tropospheric ozone, despite the increased dissociation of NO2 by solar radiation (cf. Härkönen et al., 1998 and Laurila, 1996). This tends to increase the concentrations of NO2 during the summer half-year, compared with
the winter half-year. The chemical transformation processes therefore tend to smooth out the seasonal variation of the guideline exceedences of NO2. Sulphur dioxide The seasonal variation of SO2 is controlled by the temporal variation of the meteorological conditions and of the emissions from energy production and industry. The emissions from energy production in northern European conditions are largest during the cold winter months. Because of increased emissions and unefficient atmospheric dispersion, SO2 concentrations exceed the guidelines or half of the guidelines most frequently in winter. Particulates The particles originate not only directly from combustion processes, but also from the resuspension of the material deposited on the street and road surfaces. In Finland, this material is due to, e.g., the wintertime sanding of streets, and the residue of studded tires. The resuspension affects not only TSP concentrations, but it is also important for the PM10 concentrations. In northern European conditions, the particles are resuspended from street and road surfaces particularly in spring, after the snow cover has melted. In spring, resuspension often involves material, which has been deposited on the street surface during an extensive period of time. A similar resuspension can also take place in late autumn, as snow can be melted (sometimes several times), before a permanent snow cover is sustained. Resuspension of street dust is also influenced by other meteorological factors, e.g., the type of precipitation (snow or rain), the relative humidity and the wind speed. Resuspension is commonly initiated by traffic-induced turbulence, and it is affected by the properties of the vehicle fleet and street maintenance procedures. Figures 3d-e show that the TSP concentrations exceed the guidelines most frequently in spring, but exceedences are fairly common also in late autumn, late winter and early summer. The seasonal variation of PM10 exceedences is similar, compared with the corresponding curves for TSP. The PM10 concentration exceeds the guideline or half of the guideline most frequently in spring, but exceedences are fairly common also in late autumn and late winter. We assume that the maxima of exceedences for particulates in spring and late autumn are mainly caused by the resuspension of dust from the street surfaces. Contrary to the corresponding curves for the gaseous pollutants considered, the exceedences for TSP and PM10 have a local minimum value in winter (December or January). This may be partly caused by the binding together of street dust by the snow cover in winter. It is interesting to compare the seasonal variation of the particulate guideline exceedences with the seasonal variation of the snow cover. However, the measurements of the snow cover have been conducted in rural environments. Systematically collected data of the snow cover in cities is not available. The snow cover in the streets of major cities is present for a much shorter time, compared with the rural environment, owing to, e.g., street maintenance procedures. The binding together of street dust by the snow cover is also influenced by other meteorological factors, e.g., type of precipitation, relative humidity and wind speed. Figure 2g shows that in southern Finland the permanent snow cover in rural areas is sustained on the average from November to March. This is somewhat longer than the time period, during which the local minimum of the particle exceedences occur. It is therefore
possible that the binding together of the street dust by the snow cover would have an influence on the guideline exceedences of urban particles.
5. Conclusions Most unfavourable meteorological conditions for the efficient mixing of pollution in an urban area include stable atmospheric stratification, low wind speed (or calm), and the presence of a strong ground-based inversion. The meteorological conditions in Finland are most unfavourable for atmospheric dispersion during the winter half-year, and particularly in winter (December-February). For instance, strong ground-based inversions have a clear seasonal variation, occurring mainly in winter and in late autumn. CO and NO2 at the ground level in cities are mainly originated from traffic, and SO2 is mainly due to energy production and industry. The air quality guidelines and half of the guidelines for CO, NO2 and SO2 were most frequently exceeded in Finnish towns and cities in winter. Some exceedences occurred also in spring and in autumn. The seasonal variation of exceedences for these pollutants is in agreement with the time variation of the unfavourable meteorological conditions. However, the seasonal variation of traffic emissions and enhanced chemical transformation processes in summer cause the seasonal variation of NO2 exceedences to be more evenly distributed, compared with CO. The corresponding exceedences for TSP and PM10 occurred most frequently in spring, but exceedences were fairly common also in late autumn and late winter. The particles originate not only directly from combustion processes, but also from the resuspension of the material deposited on the street and road surfaces. Seasonal variation of exceedences for particles is also in agreement with the time variation of the unfavourable meteorological conditions, but it is also significantly influenced by, e.g., the existence of a snow cover on the street surfaces.
7. References Center of Statistics, 1992. Traffic and environment (in Finnish). Center of Stataistics, Environmental reports 1992:2. Helsinki, 272 s. Finnish Meteorological Institute, 1991a. Climatological Data 1990, Meteorological Yearbook of Finland, Vol. 90, 1-1990, Helsinki, Finland. Finnish Meteorological Institute, 1991b. Climatological statistics in Finland 1961-1990. Supplement to the meteorological Yearbook of Finland, Vol. 90, 1-1990, Helsinki, Finland. Finnish Meteorological Institute, 1993a. Measurements of Solar Radiation 1981-1990. Meteorological Yearbook of Finland, Vol. 81-90, 4:1, Helsinki, Finland. Finnish Meteorological Institute, 1993b. Meteorological Yearbook of Finland 1991-1993, 1235-0419, Helsinki, Finland. Heino, R., 1994. Climate in Finland during the period of meteorological observations. Finnish Meteorological Institute Contributions 12. Helsinki, Finland.
Huovila, S., Luukkanen M.L., Tuominen A. (1991) Some features of ground inversions in Finland. Meteorological Publications 17, Finnish Meteorological Institute, Helsinki, Finland. Härkönen, J., Kukkonen, J., Valkonen, E. and Karppinen, A., 1998. The influence of vehicle emission characteristics and meteorological conditions on urban NO2 concentrations. International Journal of Vehicle Design, Vol. 20, Nos. 1-4 (in print). Kolkki, O., 1969. Review of Finnish climate. (in Finnish), Finnish Meteorological Institute, Helsinki, 64 p. Jaakko Kukkonen, Timo Salmi, Helena Saari, Mervi Konttinen and Raimo Kartastenpää, 1998. Review of urban air quality in Finland. Boreal Environment Research (in print). Laurikko, J., 1997. Regulated and unregulated exhaus emissions from in-use catalyst cars at normal and low ambient temperatures. 4th Intern. Symposium "Transport and Air Pollution", Proceedings, INRETS, Avignon, 9-13 June, France, 161-168. Laurila, T., Effects of environmental conditions and transport on surface ozone concentrations in Finland. Geophysica (1996), 32, 167-193.
Figure captions Figure 1. Location of the meteorological observatories of Jokioinen, Luonetjärvi and Sodankylä in Finland. Figures 2a-h. The monthly average of ambient temperature, sum of the total global radiation, the total monthly precipitation and the monthly average snow depth. The data has been compiled from four years, 1990–1993, from the observatories of Jokioinen (the left-hand side figures) and Sodankylä (the right-hand side figures). Figures 3a-e. Seasonal variation of the exceedences of national guidelines in Finnish cities and towns in 1990-1993. The exceedences of half of the guideline have been shown as dashed lines (Kukkonen et al., 1997).
70 o
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