Traffic flow in the streets of Riga is mostly responsible for air pollution with nitrogen dioxide and hydrocarbons (toluene and benzene) which are typical primary pollutants of exhaust gases (see Figure 5 ).

In Riga the highest pollution values are observed on Kr. Valdemara and Brivibas streets, which are typical trunk-roads with 2 lines in both directions and deep canyons especially in central part of city (see Figure 6 ).