Jari Härkönen

The Spatial and Temporal Variation of Measured Urban PM10 and PM2.5 in the Helsinki Metropolitan Area

We have studied particulate matter (PM) concentrations,PM10 and PM2.5, measured in an urban air qualitymonitoring network in the Helsinki Metropolitan Area during1997–1999. The data includes PM10 concentrationsmeasured at five locations (two urban traffic, one suburbantraffic, one urban background and one regional backgroundsite) and PM2.5 concentrations measured at twolocations (urban traffic and urban background sites). Theconcentrations of PM10 show a clear diurnal variation,as well as a spatial variation within the area. Bycontrast, both the spatial and temporal variation of thePM2.5 concentrations was moderate. We have analysedthe evolution of urban PM concentrations in terms of therelevant meteorological parameters in the course of oneselected peak pollution episode during 21–31 March, 1998.The meteorological variables considered included wind speedand direction, ambient temperature, precipitation, relativehumidity, atmospheric pressure at the ground level,atmospheric stability and mixing height. The elevated PMconcentrations during the 1998 March episode were clearlyrelated to conditions of high atmospheric pressure,relatively low ambient temperatures and low wind speeds inpredominantly stable atmospheric conditions. The resultsprovide indirect evidence indicating that the PM10concentrations originate mainly from local vehiculartraffic (direct emissions and resuspension), while thePM2.5 concentrations are mostly of regionally andlong-range transported origin.

A Three-Step Method for Estimating the Mixing Height Using Ceilometer Data from the Helsinki Testbed

AbstractA new three-step idealized-profile method to estimate the mixing height from vertical profiles of ceilometer backscattering coefficient is developed to address the weaknesses found with such estimates that are based on the one-step idealized-profile method. This three-step idealized-profile method fits the backscattering coefficient profile of ceilometer measurements into an idealized scaled vertical profile of three error functions, thus having the potential to determine three aerosol layers (one for the surface layer, one for the mixing height, and one for the artificial layer caused by the weakened signal). This three-step idealized-profile method is tested with ceilometer and radiosounding data collected during the Helsinki Testbed campaign (2 January 2006–13 March 2007). Excluding cases with low aerosol concentration in the boundary layer, cases with clouds present, and cases with precipitation present, the resulting dataset consists of 97 simultaneous backscattering coefficient profiles and ...

Validation of the dispersion model CAR-FMI against measurements near a major road

A field measurement campaign was conducted near a major road in southern Finland from 15 September to 30 October 1995. The concentrations of NO, NO2 and O3 were measured simultaneously at three locations, at three heights (3.5, 6 and 10 m) on both sides of the road. The relevant meteorological parameters and traffic densities were also measured on site. We have compared measured concentration data with the predictions from the road network dispersion model CAR-FMI, used in combination with a meteorological preprocessing model. According to statistical analyses used, the overall agreement of the model predictions and measured data was reasonably good. For all data (N = 587), the modelling system overpredicts NOx concentrations with a fractional bias FB = +0.13 and O3 concentrations with FB = +0.08, while for NO2 concentrations FB = -0.02. The experimental dataset is available for the evaluation of other roadside dispersion models.

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.

Utilization of Meandering in a Receptor-Oriented Solution of the Line Source Dispersion Equation

Lateral dilution in line source dispersion models is usually treated as the sum of short and long period variations of the wind direction and dilution by wind speed perpendicular to the road. We present an alternative solution, where the influence of meandering is estimated assuming normally distributed long term fluctuations of wind direction. The average of receptor-oriented wind direction, which is based on probability distribution, differs substantially from the meteorological mean value in case of wind directions nearly parallel to the road.

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.

A Semi-empirical Model For Evaluating UrbanParticipate Matter Concentrations

In previous work, an integrated modelling system was developed, in order to analyze the urban NO* and NO; concentrations, and this model was successfully tested against the data from an urban measurement network. This paper discusses the extension of the modelling system in order to evaluate urban particulate matter concentrations. The model includes a treatment of traffic-originated pollution, pollution from stationary sources, regional and long-range transported background concentrations and resuspension of particles from street surfaces. The modelling system is based on urban dispersion models and a semiempirical model, which utilizes data from the air quality monitoring network of Helsinki Metropolitan Area Council. We have determined a linear relationship between the observed urban hourly PMio and NO% -concentrations, based on this data. Using the previously computed spatial distributions of NO% -concentrations and the measured regional background particulate matter concentrations, we can evaluate PMio and PM2.5 concentrations in the Helsinki Metropolitan Area. The key parameters of the derived semi-empirical correlations have been tested against an independent dataset. The model presented is suitable for predicting long-term average (for instance, annual average) particle concentrations. The model assumes that the influence of local stationary emissions on the ground-level NO% or PM concentrations is not substantial. We have applied the modelling system for analyzing the effects of various traffic emission reduction strategies. Transactions on Ecology and the Environment vol 29