Primary NO2 emissions and their role in the development of NO2 concentrations in a traffic environment

An assessment of the formation of NO2 concentrations in heavily traffic-influenced environments in Helsinki, Finland was carried out. The proportion of primary NO2 emissions from road traffic was estimated using a statistical model for the relationship between the mixing ratios of nitrogen oxides (NO + NO2) and total oxidant (O3 + NO2) measured in 1994–2009. Based on this analysis, a quantitative estimate was derived for the relative importance of the primary NO2 emissions, ambient NO–NO2–O3 equilibrium and background concentrations in the observed NO2 concentrations. The proportion of primary NO2 in the vehicular NOx emissions increased from below 10% in the 1990s to about 20% in 2009, with a more distinctive increase during the most recent years. This development was related to the changes in the proportion of diesel-powered passenger cars in Finland. Between 1994 and 2004, the photochemical NO-to-NO2 conversion comprised on average 51% of the mean NO2 concentration, while the primary NO2 emissions contributed 31%. The role of the primary NO2 emissions was limited by the steeply-decreasing total NOx emissions. More recent data (2005–2009) yielded higher primary NO2 emission fractions (15 –21%), with a clearly increasing trend. As a result, the contribution of chemical conversion steadily decreased from 54% in 2005 to 43% in 2009, while that of the primary NO2 emissions increased from 32 to 44%. In order not to exceed in future the annual limit of NO2 concentration, set by the European Union, in the busiest street canyons in downtown Helsinki, the primary NO2 emissions need to be addressed alongside the total NOx emissions.