Characterization of NOx Emission Sources in Innsbruck

There is widespread concern about the influence of human activities on the chemical composition of the earth atmosphere including their effects on human health (WHO, 2006). The processes that control the composition of the atmosphere are still not fully understood or quantified, but it is clear that trace gas emissions are primary drivers of changes in regional air quality. Technological advances have been successful to mitigate air pollutants in some areas and for some compounds. For example, in Western Austria, benzene, CO and aerosols have seen a trend reversal, where immission levels have considerably decreased, despite increasing traffic volume and population growth. Yet, the concentrations of certain compounds, for example nitrogen dioxide (NO2), have not exhibited such a trend reversal. NOx has many sources in the troposphere, but is believed to be mainly emitted from various combustion sources in urban environments. Emission inventories of stationary and mobile sources are still highly uncertain and often hard to assess. Here, we propose to provide an important novel and complementary aspect for conventional urban air quality management systems by further developing the concept of urban trace gas flux measurements, allowing to provide unique top- down constraints for important urban trace gas emissions in Innsbruck. The measurements will be compared to the most commonly used pollutant emission inventories implemented in state of the art air quality models. The goal of the project is to provide essential constraints for air quality models and local air quality decision systems.

Long-term measurements of air pollutants in the urban area of Innsbruck, Austria, allowed to constrain emissions of nitrogen oxides (NO+NO2 = NOx) and study their cycling with ozone (O3). The 40-meter-high monitoring tower of the Innsbruck Atmospheric Observatory near the city center of Innsbruck in Austria, continuously provides data on the composition of the atmosphere near the surface. Every hour, 36.000 data points for each compound are recorded. Using a special measuring method - the so-called eddy covariance method - the emissions and cycling of air components can be continuously monitored. Analyses show that traffic restrictions during the first COVID lockdown in 2020 led to a sharp drop in air pollutant emissions, significantly more than for carbon dioxide (CO2). The findings highlight that traffic is responsible for only about 50% of urban CO2, but for more than 90% of NOx. The project results confirm the assumption that the mobility sector is still the dominating source of nitrogen oxide pollution in cities, responsible for over 90 percent of its emissions. Within the project it was possible to compare surface flux observations with bottom-up emission inventories. The analysis revealed, that NO emissions from the mobility sector are still underestimated, but that primary nitrogen dioxide (NO2) emissions are significantly overestimated. Direct emissions of NO2 from urban traffic are largely negligible in comparison to secondary formation (e.g. 90%). As a consequence more than 90% of nitrogen oxides are emitted as nitrogen monoxide (NO), and the high proportion of diesel vehicles in European cities still leads to a significant enhancement of NO. NO reacts with ozone to produce nitrogen dioxide (NO2). In the atmosphere, nitrogen dioxide decomposes again to nitrogen monoxide and atomic oxygen, which immediately combines with atmospheric oxygen to form ozone. This chemical cycle was described mathematically over 60 years ago in the first air pollution textbook by Philip Leighton and has been used to constrain the fast chemistry between NO, NO2 and O3. The relationship between two major chemical reactions has since been referred to as the Leighton ratio. In this project it was possible to show that, in the urban surface layer, this common assumption needs to be re-interpreted. It was possible to show that the effect of chemistry - turbulence interactions between NO, NO2 and O3 in the presence of high NO emissions plays a significant role in the lowest layer of the urban atmosphere (e.g. up to 200 meters above the ground). This leads to a perturbation of the Leighton ratio, and as a consequence can cause an overestimation of modelled urban O3. Findings from this project call for future investigations on urban ozone formation.