Long-term analysis of NO, NO2 and O3 concentrations in North Rhine-Westphalia, Germany

Legislative norms developed at the end of the 1970s, structural changes in the economy, and the implementation of filters in both vehicles and industries improved the air quality of North Rhine-Westphalia, the largest western state in Germany in terms of population and economic output. Because these changes occurred unevenly across the state, the main aim of the given study is to analyse the historical development of air pollutants in the various environments (e.g., industrial, traffic or rural background) within the state. NO, NO2 and O-3 concentrations observed at six stations (one industrial, two traffic, one urban and two rural background) from 1981 to 2007 have been obtained from the Environmental State Agency for Nature, Environment and Consumer Protection (LANUV NRW). These data have been evaluated to investigate time trends of these pollutants and the impact of weekly cycles on ozone formation in NOx-limited areas (rural background) and VOC-limited areas (urban areas with traffic network). The relationships between ozone production mechanisms and ozone precursors (NO, NO2 and their ratio) have also been taken into account. In contrast to the sharp decrease in NO concentrations (65%), only a moderate, insignificant decrease in NO2 (10%) was observed because catalytic filters in vehicles support emissions of NO2 as a primary pollutant. Changes in NO/NO2 ratio and higher temperature favoured an increase in ozone concentrations (20%), which appears to be the indicator of summer smog. The results showed that, although ozone forming potential is higher at background stations due to higher biogenic VOC and lower NO emissions, time trends were steeper at the industrial stations because of radical changes in NOx concentrations. Examination of the weekly and diurnal cycles of NO and NO2 showed that not only the absolute average values of the pollution levels decreased (with a turning point in 1998) but also the shape of the diurnal cycle (differences between primary and secondary peaks) changed in response to developed legislative norms (Bayerisches Landesamt fur Umweltschutz, 2011). (c) 2012 Elsevier Ltd. All rights reserved.