  Nitrogen dioxide (NO$_2$) on Earth today has biogenic and anthropogenic
sources. During the COVID-19 pandemic, observations of global NO$_2$ emissions
have shown significant decrease in urban areas. Drawing upon this example of
NO$_2$ as an industrial byproduct, we use a one-dimensional photochemical model
and synthetic spectral generator to assess the detectability of NO$_2$ 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 NO$_2$
concentrations increase on planets around cooler stars due to less
short-wavelength photons that can photolyze NO$_2$. In cloud-free results,
present Earth-level NO$_2$ on an Earth-like planet around a Sun-like star at
10pc can be detected with SNR ~5 within ~400 hours with a 15 meter LUVOIR-like
telescope when observed in the 0.2 - 0.7micron range where NO$_2$ has a strong
absorption. However, clouds and aerosols can reduce the detectability and could
mimic the NO$_2$ feature. Historically, global NO$_2$ levels were 3x higher,
indicating the capability of detecting a 40-year old Earth-level civilization.
Transit and direct imaging observations to detect infrared spectral signatures
of NO$_2$ on habitable planets around M-dwarfs would need several 100s of hours
of observation time, both due to weaker NO$_2$ absorption in this region, and
also because of masking features by dominant H$_2$O and CO$_2$ bands in the
infrared part of the spectrum. Non-detection at these levels could be used to
place upper limits on the prevalence of NO$_2$ as a technosignature.
