The vertical distribution of NOx and its variability above metropolitan areas in America, Europe, and Asia, as observed by IAGOS

Nitrogen oxide (NOX) is an important air quality indicator and one of the main precursors of ozone (O3). These trace gases have natural and anthropogenic sources at ground level and in the troposphere. At ground level, the main sources are transport emissions, industry, agriculture, and biomass burning. In the troposphere, additional sources include lighting and aircraft emissions and in the upper troposphere, downmixing from the stratosphere also makes a significant contribution to the ozone budget.The European Research Infrastructure IAGOS (www.iagos.org) uses in-service passenger aircraft as observation platforms, equipped with instruments to measure gaseous species, aerosols, and cloud particles. The IAGOS-CORE NOx instrument (Package 2b) is designed to measure NO, NO2, and total NOX. Since its operation started on one Lufthansa aircraft in 2015 and further expansion of the fleet in 2023, a fourth aircraft was equipped with this instrument type in 2025. These four IAGOS-CORE aircraft from Air France, China Airlines, Iberia, and Lufthansa cover routes to North, Central and South America, Europe, Africa and Asia. From this unique in situ measurements, we discuss the vertical NOX profiles from ground up to about 12 km altitude. Thereby we are not only focusing on the abundance of NOX within the planetary boundary layer but also how NOX is distributed in the free troposphere and how this vertical distribution varies globally above different metropolitan aeras. Cities such as New York, Montevideo, Frankfurt, Madrid, Hong Kong, Taipei, and Tokyo were selected to represent different global regions and have a statistical base of at least ten and up to about 300 individual profiles for each city available. Acknowledgments: We thank all members of IAGOS-CORE, in particular the airlines for enabling these IAGOS-CORE observations. The data were created with support from the European Commission, national agencies in Germany (BMBF), France (MESR), and the UK (NERC), and the IAGOS member institutions (http://www.iagos.org/partners).