2021ApJ...908..164K

Nitrogen dioxide (NO₂) on Earth today has biogenic and anthropogenic sources. During the Covid-19 pandemic, observations of global NO₂ emissions have shown a significant decrease in urban areas. Drawing upon this example of NO₂ as an industrial byproduct, we use a one-dimensional photochemical model and synthetic spectral generator to assess the detectability of NO₂ as an atmospheric technosignature on exoplanets. We consider cases of an Earth-like planet around Sun-like, K-dwarf, and M-dwarf stars. We find that NO₂ concentrations increase on planets around cooler stars because there are fewer short-wavelength photons that can photolyze NO₂. In cloud-free results, present Earth-level NO₂ on an Earth-like planet around a Sun-like star at 10 pc can be detected with signal-to-noise ratio ∼5 within ∼400 hr with a 15 m LUVOIR-like telescope when observed in the 0.2-0.7 µm range where NO₂ has a strong absorption. However, clouds and aerosols can reduce the detectability and could mimic the NO₂ feature. Historically, global NO₂ levels were 3x higher, indicating the capability of detecting a civilization at the stage where Earth's civilization was 40 yr ago. Transit and direct imaging observations to detect infrared spectral signatures of NO₂ on habitable planets around M-dwarfs would need several hundred hours of observation time, both due to weaker NO₂ absorption in this region and because of masking features by dominant H₂O and CO₂ bands in the infrared part of the spectrum. Non-detection at these levels could be used to place upper limits on the prevalence of NO₂ as a technosignature.