  Planets between the sizes of Earth and Neptune are the most common in the
Galaxy, bridging the gap between the terrestrial and giant planets in our Solar
System. Now that we are firmly in the era of JWST, we can begin to measure, in
more detail, the atmospheres of these ubiquitous planets to better understand
their evolutionary trajectories. The two planets in the TOI-836 system are
ideal candidates for such a study, as they fall on either side of the radius
valley, allowing for direct comparisons of the present-day atmospheres of
planets that formed in the same environment but had different ultimate end
states. We present results from the JWST NIRSpec G395H transit observation of
the larger and outer of the planets in this system, TOI-836c (2.587
R$_{\oplus}$, 9.6 M$_{\oplus}$, T$_{\rm eq}$$\sim$665 K). While we measure
average 30-pixel binned precisions of $\sim$24ppm for NRS1 and $\sim$43ppm for
NRS2 per spectral bin, we do find residual correlated noise in the data, which
we attempt to correct using the JWST Engineering Database. We find a
featureless transmission spectrum for this sub-Neptune planet, and are able to
rule out atmospheric metallicities $<$175$\times$ Solar in the absence of
aerosols at $\lesssim$1 millibar. We leverage microphysical models to determine
that aerosols at such low pressures are physically plausible. The results
presented herein represent the first observation from the COMPASS (Compositions
of Mini-Planet Atmospheres for Statistical Study) JWST program, which also
includes TOI-836b and will ultimately compare the presence and compositions of
atmospheres for 12 super-Earths/sub-Neptunes.
