A comparison of long-term trends in observations and emission inventories of NO<sub><i>x</i></sub>

Abstract

Abstract. Air pollution is a pressing issue that is associated with adverse effects on human health, ecosystems, and climate. Despite many years of effort to improve air quality, nitrogen dioxide (NO2) limit values are still regularly exceeded in Europe, particularly in cities and along streets. This study explores how concentrations of nitrogen oxides (NOx = NO + NO2) in European urban areas have changed over the last decades and how this relates to changes in emissions. To do so, the incremental approach was used, comparing urban increments (i.e. urban background minus rural concentrations) to total emissions, and roadside increments (i.e. urban roadside concentrations minus urban background concentrations) to traffic emissions. In total, nine European cities were assessed. The study revealed that potentially confounding factors like the impact of urban pollution at rural monitoring sites through atmospheric transport are generally negligible for NOx. The approach proves therefore particularly useful for this pollutant. The estimated urban increments all showed downward trends, and for the majority of the cities the trends aligned well with the total emissions. However, it was found that factors like a very densely populated surrounding or local emission sources in the rural area such as shipping traffic on inland waterways restrict the application of the approach for some cities. The roadside increments showed an overall very diverse picture in their absolute values and trends and also in their relation to traffic emissions. This variability and the discrepancies between roadside increments and emissions could be attributed to a combination of local influencing factors at the street level and different aspects introducing inaccuracies to the trends of the emission inventories used, including deficient emission factors. Applying the incremental approach was evaluated as useful for long-term pan-European studies, but at the same time it was found to be restricted to certain regions and cities due to data availability issues. The results also highlight that using emission inventories for the prediction of future health impacts and compliance with limit values needs to consider the distinct variability in the concentrations not only across but also within cities.

Mobile mini-DOAS measurements of the outflow of nitrogen dioxide from the Toluca Valley Metropolitan Area, Mexico

“…applied to generate emissions inventories (Van Amstel et al, 1999). Differences can also be attributed to a combination of local influencing factors at the street level and different aspects introducing inaccuracies to the used emission inventories, including deficient emission factors (Macdonald et al, 2021).…”

Rapid O3 assimilations – Part 2: Tropospheric O3 changes accompanied by declining NOx emissions in the USA and Europe in 2005–2020

“…The successful emission regulations employed in the United States (USA) and Europe (Crippa et al, 2016;US EPA, 2023a) have led to dramatic decreases in anthropogenic NO x emissions (Di et al, 2020;Macdonald et al, 2021;. As an important air pollutant, tropospheric ozone (O 3 ) is produced when volatile organic compounds (VOCs) are photochemically oxidized in the presence of nitrogen oxides (NO x ).…”

Rapid assimilations of O3 observations – Part 2: tropospheric O3 changes in the United States and Europe in 2005–2020

“…The successful emission regulations employed in the United States (US) and Europe (Crippa et al, 2016;EPA, 2017) have led to dramatic decreases in anthropogenic NOx emissions (Di et al, 2020;Macdonald et al, 2021;. As an important air pollutant, tropospheric ozone (O3) is produced when volatile organic compounds (VOC) are photochemically oxidized in the presence of nitrogen oxides (NOx).…”