Esko Valkonen

A measurement campaign in a street canyon in Helsinki and comparison of results with predictions of the OSPM model

In 1997, a measuring campaign was conducted in a street canyon (Runeberg St.) in Helsinki. Hourly mean concentrations of CO, NOx, NO2 and O3 were measured at street and roof levels, the latter in order to determine the urban background concentrations. The relevant hourly meteorological parameters were measured at roof level; these included wind speed and direction, temperature and solar radiation. Hourly street level measurements and on-site electronic traffic counts were conducted throughout the whole of 1997; roof level measurements were conducted for approximately two months, from 3 March to 30 April in 1997. CO and NOx emissions from traffic were computed using measured hourly traffic volumes and evaluated emission factors. The Operational Street Pollution Model (OSPM) was used to calculate the street concentrations and the results were compared with the measurements. The overall agreement between measured and predicted concentrations was good for CO and NOx (fractional bias were −4.2 and +4.5%, respectively), but the model overpredicted the measured NO2 concentrations (fractional bias was +22%). The agreement between the measured and predicted values was also analysed in terms of its dependence on wind speed and direction; the latter analysis was performed separately for two categories of wind velocity. The model qualitatively reproduces the observed behaviour very well. The database, which contains all measured and predicted data, is available for further testing of other street canyon dispersion models. The dataset contains a larger proportion of low wind speed cases, compared with other available street canyon measurement datasets.

THE INFLUENCE OF VEHICLE EMISSION CHARACTERISTICS AND METEOROLOGICAL CONDITIONS ON URBAN NO2 CONCENTRATIONS

A major fraction of nitrogen oxides released from traffic is commonly nitrogen monoxide (NO); however, the proportion of NO2 in exhaust emissions can range from less than 1% to more than 50%. The released NO is chemically transformed into the more harmful nitrogen dioxide (NO2); the reaction time-scale is typically a few minutes in urban daylight conditions. This paper presents numerical results concerning the influence of the tailpipe NO2/NOx fraction and meteorological conditions on the ambient air NO2 concentrations. We have applied the road network dispersion model CAR-FMI in the computations.

THE EMISSIONS, DISPERSION AND CHEMICAL TRANSFORMATION OF TRAFFIC-ORIGINATED NITROGEN OXIDES IN THE HELSINKI METROPOLITAN AREA

We have developed a modelling system for evaluating the emissions, dispersion and chemical transformation of nitrogen oxides from traffic. This paper presents an application of the system for estimating the NO2 and NOx concentrations in the Helsinki metropolitan area in 1993. The highest concentrations of NO2 are located in the vicinity of the main roads and streets, and in the downtown area of Helsinki. The model predictions were compared with the results of the urban air quality measurement network of the Helsinki Metropolitan Area Council. The predicted annual average concentrations and statistical concentration parameters of NO2 agree well with the measured data.

Development and Verification of a Modelling System for Predicting Urban NO2 Concentrations

This paper describes the development of a modelling system for predicting the emissions, dispersion and chemical transformation of nitrogen oxides in an urban area. The system takes into account of all source categories, including stationary point and area sources, and vehicular sources. We also present a statistical comparison of predicted results and measured concentrations.

Modelling urban air pollution in Espoo, Finland

Abstract The main objective of this study is to analyse quantitatively the influence of urban air pollution on the health of children. For this aim, one has to analyse the concentrations of pollutants in the atmosphere, analyse the indoor air concentrations, and conduct epidemiological surveys. This paper addresses the first part of the study, i.e. the application of atmospheric dispersion models for analysing outdoor air quality. The area considered is Espoo, the neighbouring city of Helsinki. Numerical results have been computed with an urban dispersion modelling system, using emissions and meteorological data for the year 1990. The computations included carbon monoxide, nitrogen oxides and sulphur dioxide; this paper focuses on the results for nitrogen dioxide.

Model Predictions on Urban Air Quality and Their Comparison to Measurements

We discuss studies on urban air quality in the Helsinki metropolitan area. This area includes four cities, which have a total population of 850 000. We have conducted comprehensive emission surveys comprising mobile and stationary sources, in 1990 and 1993. The emission time series were formed using the LIISA system, developed at the Technical Research Centre of Finland, and the transportation planning system EMME/2. The atmospheric dispersion was evaluated by the models UDM-FMI (The Urban Dispersion Modelling system of the FMI) and CAR-FMI (Contaminants in the Air from a Road) (Harkonen et al., 1994). Both dispersion models are connected to a meteorological preprocessing model, which is based on Monin-Obukhov type boundary layer scaling.