The largest mobile source of pollution in the City of Riga is motorised vehicles. The number of vehicles has been growing by 10-12% per year (see Figure 1).Most emissions from mobile sources come in the form of nitrogen oxides (NO, NO₂, N₂O₅), carbon monoxide (CO) and volatile organic compounds (VOC). In 2001, about 63-67% of the total NO_x and 61-65% of the CO emissions were caused by mobile sources (data provided by the Air Protection Division of the Department of Environment).

Analysis of air pollution caused by traffic shows that in average meteorological conditions, the NO₂ concentration in most of the city is 40 µg/m³, while in the centre it is higher - 60 µg/m³ and more. According to the air quality standards (see National air quality limit values), effective in Latvia as of January 1, 2000, the permissible level of NO₂ for the year 2001 is 56 µg/m³. At present, this level is being exceeded in the centre of Riga, but by January 1, 2010 it has to be reduced to 40 µg/m³. Consequently, the city will have to make serious efforts to develop air quality improving plans in the city centre. They could be connected with changes in traffic infrastructure, fuel, new parking places around city centre etc.

High levels of troposphere (ground level) ozone (O₃) have been observed in Riga, caused by effects of the sun radiation in the presence of exhaust gases - nitrogen oxides and hydrocarbons. Troposphere ozone is a harmful oxidant for people, plants and animals. According to the data of the Air Protection Division of the Department of Environment, the O₃ concentration exceeded the air quality standards - the maximum permissible 24-hour average concentration of 65 µg/m³ during some weeks in Spring and Summer (see Figure 2).

Air pollution with benzene is another serious problem in Riga (due to high traffic intensity, large number of petrol stations, and oil transfer enterprises in the Port). For instance, in Tvaika Street where measurements have been taken since 1999, the average benzene concentrations are 12-18 µg/m^3, sometimes reaching 60-100 µg/m³.