A new computer-based tool has been developed in Sweden to help local authorities evaluate air pollution at street level. It allows the results to be compared easily with EU air quality standards.

SIMAIR is a user-friendly, internet-based tool, designed for the road network throughout Sweden. It can assess concentration levels for four pollutants: fine particles (PM10), nitrogen dioxide (NO2), carbon monoxide (CO) and benzene. Local authorities can calculate total pollution levels for local street sites and, within 10 seconds, receive a report which compares the simulated concentrations at that location with EU limits.

In addition, SIMAIR separates long range, urban and local (street) contributions to total pollution levels. Under the EU Air Quality Directive1, 2008/50/EC, ambient air quality must be assessed and air pollution maintained within limit values for clean air. Swedish legislation requires local authorities to monitor air quality in areas where pollution concentrations are expected to be above certain thresholds. In Sweden this may be the case for urban populations as small as 10,000 people. Setting up and maintaining the necessary air quality monitoring equipment can be expensive.

A major benefit of SIMAIR is that it estimates, or models, the impact on air quality of local street emissions. This cuts costs as it may reduce the number of monitoring facilities needed for an assessment, although it is unlikely to replace them altogether. Air quality data from the remaining stations could be used to evaluate the system.

In urban areas, levels of particulate matter are of particular concern, as fine particles can cause serious health effects. In Sweden, there are special measures to make it safer to travel on icy roads.

For example, vehicles are fitted with studded tyres and icy roads are treated with sand and salt. However, these cause mechanical wear of the road and tyres, creating more particulate pollution. As a result, emission of wear particles in Stockholm is eight times higher than if these safety measures were not in place.

However, SIMAIR can distinguish between street level (kerbside) contributions of PM10 from mechanical wear of roads and tyres and from normal car exhaust emissions. The researchers compared estimates of PM10 produced by SIMAIR with actual monitored values for four street locations. They found that SIMAIR's results for annual concentrations and for 90 percentiles of daily averages, exceedences were within plus or minus 25 per cent of the monitored results for three of the locations and within plus or minus 50 percent at the fourth location. This means that use of the SIMAIR model for assessing air quality standards met EU requirements.

However, further work is needed to validate the SIMAIR system for more locations, representing all geographical parts of Sweden. The SIMAIR system combines information from a number of models containing key details, including street and building layouts, climatic conditions, the type and flow of traffic, as well as emission data on the local, urban and regional scale.

Models drawn upon include SAMPERS and EMME/2 for traffic data, ARTEMIS for emissions data, MATCH for regional background concentrations and OSPM for roadside concentrations. National road and vehicle information comes from the extensive Swedish National Road Database.

The system is currently only applicable to Sweden, but provides an important case study for how air quality could be modelled in other regions.