Abstract of PhD Thesis

Air quality monitoring and modelling at motoway and roundabout sites in Ireland

Una Budd (2004), Trinity College Dublin

EU legislation requires Ireland to set ambient air quality objectives, assess the ambient air quality, gather information on ambient air quality and make it available to the public, and to maintain or improve the ambient air quality.  These ambient air quality objectives are specified in the Daughter Directives of the Air Quality Framework Directive (CEC, 1996; 1999; 2000; 2002).  The Irish EPA considered that NO2 and PM10 would present the greatest challenges in meeting the EU standards and warned of the implications for traffic management and transport policy (EPA, 2000).  “Road traffic has replaced stationary combustion sources as the greatest threat” to air quality in Ireland, according to the EPA (2000).  The transport sector is estimated to contribute 50% of total NOx emissions and 80% of total CO emissions in Ireland.  Sources of PM10 in Ireland are under investigation.  Although classified as priority pollutants, there has been no historical national monitoring campaign for CO, NO2, PM10 and benzene, as there has been for smoke and sulphur dioxide.

Air quality modelling can be used to complement monitoring networks, and at a lower cost than that of monitoring.  Modelling can be used in conjunction with monitoring for the estimation of present air quality.  Unlike monitoring, modelling can also be used for the prediction of future air quality.  Air quality modelling is used to predict the future impact of road improvements, often as part of an Environmental Impact Assessment. The accuracy of such modelling is dependent not only on the quality of the data used, but also on the methodology of the model and its user. The air pollution dispersion models in common usage in Ireland, DMRB and CALINE4, were both developed outside the country and have not been validated for use in local conditions.  The novel aspect of this work is to validate the use of such models for road and motorway situations in Ireland, although the results will also be of wider international interest.

Two sites were identified for monitoring and modelling which satisfied the criteria for proximity to a national primary route with high and quantifiable traffic flows, power, security, accessibility and with the receptor location downwind of the source for the prevailing wind direction.  Databases of traffic flows, meteorological conditions and pollutant concentrations were collated for each site.  The database of hourly traffic, of meteorological conditions from the monitoring site and from Met Eireann at Casement Aerodrome, Dublin and Shannon Airports, and of concentrations for CO, NO, NO2, total NOx and PM10 by TEOM and Partisol (daily) was analysed diurnally, directionally, seasonally and statistically for 8786 hours between 15th September 2001 and 15th September 2002.  Concentrations for comparison with the limit values were predicted by DMRB modelling for both sites.  Hourly concentrations were predicted by CALINE4 modelling for both sites, and analysed diurnally, directionally, seasonally and statistically for the entire monitoring period.  A collaborative modelling exercise was designed and executed, culminating in a modelling workshop, from which the techniques, data sources and assumptions of four modellers were assessed.  Predictions using four different models were compared with observed concentrations and with the limit values set by the EU Daughter Directives (CEC, 1999; 2000).

The absolute values of concentrations of CO, NO2 and PM10 were low at both sites compared to the limit values, except for PM10 at the roundabout which will require monitoring to comply with the EU Daughter Directive (CEC, 1999).  The source effect was clearly discernible but often low, thus the background concentrations were crucial for accurate predictions.  The databases (of traffic flows, meteorological conditions and pollutant concentrations) collated for the motorway and roundabout are a valuable resource.  Irish databases of calibrated background concentrations (for rural, urban and suburban situations) and of regulatory emission factors (comparable to the US MOBILE and the UK EFD and DMRB) would reduce the disparity between modellers’ assumptions.

DMRB modelling predictions were within 1% to 436% of observed concentrations (relative to the limit values).  CALINE4 modelling predictions were within -45% to 20% of observed concentrations (relative to the limit values).  Predictions by other modellers (using ADMS and CAL3QHCR in the collaborative modelling exercise) were within -48% to 30% of observed concentrations (relative to the limit values).

Modelling performed better for prediction of NO2 and PM10 than of CO.  Concentrations of NO2 and PM10 are of more concern than concentrations of CO since NO2 and PM10 are closer to their respective limit values than CO is.  NO2 and PM10 are thus considered by the EPA to present the greatest challenges in meeting the EU standards (EPA, 2000).  Therefore it is of great interest that modelling of NO2 (specifically the 19th highest hourly NO2 concentration or 99.8th percentile) and of PM10 (specifically the annual mean PM10 concentration) were shown to be the most accurate.