not previously been used to infer economic activity. The reason is that there may be significant differences between concentrations measured by satellites (stock) and emissions more indicative of ongoing economic activity (flow). For many air pollutants including CO2, there may be substantial movement between the source of emissions and place of measurement due to wind, necessitating a sophisticated atmospheric chemical transport model (e.g. Middleton (1995)).

**Why NO** 2 **?** It is specifically the fact that NO2 stays in place, and is thus directly indicative of combustion below it, which makes it a preferable signal of economic activity among all air pollutants. Indeed, scientific papers have supported the veracity of satellite retrievals of NO2 densities in particular (Geddes et al., 2016). The short atmospheric life of NO2 (less than a day) means that densities are closely correlated to emissions (Seinfeld and Pandis, 2016).<sup>3</sup> While these densities are also possibly subject to other errors, such as non-human sources (e.g., soil emissions, wildfires, and lightning), these have comparatively small effects. In this paper, we use near-Earth densities of NO2, which are closely linked with anthropogenic emissions due to NO2’s low transport, and apply a statistical procedure to account for measurement errors.

**Sample.** We make use of vertical column densities (VCDs) of tropospheric NO2 from Ozone Monitoring Instrument (OMI), a nadir spectrometer onboard NASA’s Earth Observing System (EOS) Aura satellite (Lamsal et al., 2008, 2010). The units of densities are molecules per square centimeter, typically on the order of 1e15. The column has a high spatial resolution of 0.1° _×_ 0.1° (latitude by longitude), which is equivalent to 13 km _×_ 24 km at the nadir, i.e. directly downwards towards the center of the Earth in the field of view.<sup>4</sup> The height of the columns is the troposphere, about the maximum cruising altitude of commercial aircraft, and more indicative of ground-level human activity than readings higher in the atmosphere.

OMI is a spectrometer, a scientific instrument which uses solar backscatter visible and ultraviolet radiation to deduce aerosol densities. Consequently, measurements always occur during daylight, since solar wavelengths are required for a spectrometer to function. This is inherently opposite the conditions required to measure night lights, a key feature we make use of later towards identification. The satellite on which OMI is mounted, Aura, is part of the “A-Train” afternoon constellation, a sun-synchronous line of satellites moving in a poleto-pole route. These satellites are oriented as such purposefully to provide data from multiple

> 3NO2 is in fact so easy to measure relative to other air pollutants that it has been used to validate measures of these other atmospheric species, like CO2, over China (Berenzin et al., 2013). It is also well-known that NO2 serves as an effective proxy for human-made nitrogen oxides (NO _x_ = NO + NO2) (Leue et al., 2001; Velders et al., 2001). During the daytime, a chemical process converts nitrogen oxide (NO) into NO2 (and vice versa) within minutes. It is common practice to use measurements of NO2 densities as a proxy for human-made NO _x_ .

> 4Although the grid is of uniform size in degrees, due to the curvature of the Earth, the geographic area of this grid varies in the field of view i.e. swath. For instance, longitude lines approach each other when moving from the equator to either pole. In the analysis, we convert the degree grid from latitude and longitudinal coordinates to geographic area using elementary trigonometric principles and weight the measured density by geographic area, consistent with accepted practices.

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