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Space-based measurements of NO2 tropospheric column density began over two decades ago with the launch of the Global Ozone Monitoring Experiment (GOME) instrument on board the ERS-2 satellite in 1996 [5]. Only NO2, rather than total NOx is measured due to its spectroscopic properties. Since then, several additional instruments have been launched, including the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) [6], Ozone Monitoring Instrument (OMI) [7], and GOME-2 [8]. All these instruments are carried on board polar orbiting satellites which allows them to observe the entire globe in 1–6 days, depending on the instrument and operational mode. Space-based observations of NO2 offer a level of combined spatial and temporal coverage not possible with groundor aircraftbased instruments. This offers several notable advantages, such as the ability to observe an entire urban and suburban area, to compare multiple urban areas across the globe using the same instrument, and the ability to monitor episodic events (biomass burning, lightning) difficult to track with other types of instruments. Multiple papers have made use of these properties to investigate both anthropogenic [9–18] and natural NOx emissions [19–24]. The result of these measurements is a “tropospheric vertical column density” (tVCD), usually in units of molecules/cm. This is the total number of molecules of NO2 over one square centimeter of the Earth’s surface between the surface and the top of the troposphere (typically ∼ 12 km). Rural areas considered “clean” typically have tVCDs of ≤ 1 × 10 molec. cm−2. Highly polluted areas such as Los Angeles, CA, USA or Beijing, China have tVCDs in excess of 1× 10 molec. cm−2. Measurements of O3 from space can be done similarly to measurements of NO2 using ultraviolet-visible spectroscopy and are almost always measured by the same satellites. However, measurements of tropospheric O3 are complicated by the high concentration of O3 in the stratosphere. Whereas the tropospheric and stratospheric components of the total NO2 vertical column density are similar orders of magnitude, the tropospheric compoABSTRACT

Illustrating Trends in Nitrogen Oxides Across the United States Using Sonification