Long-term trends of primary and secondary NO2 production in the Athens area. Variation of the NO2/NOx ratio

This paper examines the contribution of primary and secondary NO2 production in NOx concentrations and offers a comprehensive analysis of the long-term trends of NOx, NO2 and O3 concentrations, as well as of the NO2/NOx ratio, in the Athens urban conurbation. Long-term pollutant concentration time series show that NO2 concentrations in Athens have decreased since 1987 but at a slower rate than those of NOx, resulting to an increasing NO2/NOx concentration ratio. However, this increasing trend is much smaller than those observed in urban areas of other European countries. The possible causes of this trend are examined and especially the interaction with ozone and the amount of direct NO2 traffic emissions. The results indicate that the increasing NO2/NOx ratio in the Athens area is mainly attributable to an increased secondary formation of NO2 through photochemical reactions in the atmosphere. More specifically, two different empirical methodologies were applied to examine the primary NO2 concentration fraction in Athens, using ambient monitoring data from a kerbside station. Both methods indicate that the primary NO2 concentration share has not altered significantly between 1998 and 2006. This is mainly attributed to the fact that in the Athens area diesel passenger cars are not allowed and after-treatment technologies such as particle filters and oxidation catalysts are not yet applied in Greece. A probable future penetration of diesel passenger cars in Athens should be combined with inventories of primary NO2 emissions and with the development of appropriate policies to reduce ambient NO2 concentrations below the EU limit values.

► The NO2/NOx concentration ratio in Athens is increasing slowly since 1990. ► The primary NO2 concentration share in Athens has not altered notably with time. ► The behaviour of primary NO2 is due to diesel passenger cars not allowed in Athens. ► The NO2/NOx trend in Athens is attributable to increased secondary NO2 formation.