Nearly every modern city faces problems of traffic and transportation system, that are of most concern. Time we spend on our way to work, traffic congestions, contamination of environment - all of these cause substantiated dissatisfaction among inhabitants.

The problem of high levels of chemical and noise pollution from transport is one of the priority environmental problems in the City of Riga (see Causes).

Presently traffic flow is the main source of air pollution in Riga and is mostly responsible for air pollution with NO 2 and hydrocarbons (see also Figure 5). Because of a low capacity of the streets and inconvenient routes, there are frequent traffic jams causing increased air pollution on Kr. Valdemara iela, Raina bulvaris and Merkela iela, especially at the end of working days.

As it is seen from results collected from all DOAS stations measuring actual air pollution levels in Riga , highest values of air pollution are observed at the street level and close to the busy streets (see Table 1). If traffic volumes in the streets are growing, also air pollution will increase. At present the pollution levels in Riga and particularly in the Old Town are close to the limits set by the national guidelines for NO 2 (see Table 13). Under special weather conditions (calm wind, positive differential temperature) pollution with nitrogen dioxide from transport exhaust doubles in broad areas of Riga . As it can be seen from Table 1 , 24 hours values for ozone are exceeded very frequently, especially in the Spring and Summer. Ozone, as a secondary pollutant in air, is formed from NOx and hydrocarbons in the presence of UV radiation (sun).

Of concern are high benzene concentrations, which today do not exceed Latvian legislation, but in future will rise. Regarding the new EU Directive 2000/69/EC relating to limit values for benzene and carbon monoxide in ambient air, the guideline value for the protection of human health is 10 g/m3 (mean annual). Benzene is a genotoxic carcinogen for humans and there is no identifiable threshold below which there is no risk to human health.

Increased pollution of air with hydrocarbons (benzene, toluene) is observed in the harbour region of Riga, where a large number of enterprises connected with oil transport are situated. Inhabitants, who live close to the harbour region frequently complain about oil product odours in apartment and on the street. Nevertheless, measured concentrations do not exceed the existing standard which is very high.

Traffic intensity and the age and type of the car fleet are main drivers of urban air pollution. Therefore, traffic intensity along the main transport routes in Riga and car age structure could be selected as the indicators.