  We present a spatially resolved map of integrated-intensity and abundance of
Neptune's stratospheric hydrogen cyanide (HCN). The analyzed data were obtained
from the archived 2016 observation of the Atacama Large
Millimeter/submillimeter Array. A 0.42 $\times$ 0.39 arcseconds synthesized
beam, which is equivalent to a latitudinal resolution of $\sim$20 degrees at
the disk center, was fine enough to resolve Neptune's 2.24 arcseconds diameter
disk. After correcting the effect of different optical path lengths, a spatial
distribution of HCN emissions is derived over Neptune's disk, and it clearly
shows a band-like HCN enhancement at the equator. Radiative transfer analysis
indicates that the HCN volume mixing ratio measured at the equator was 1.92 ppb
above the 10$^{-3}$ bar pressure level, which is 40$\%$ higher than that
measured at the southern middle and high latitudes. The spatial distribution of
HCN can be interpreted as either the effect of the transportation of N$_{2}$
from the troposphere by meridional atmospheric circulation, or an external
supply such as cometary collisions (or both of these reasons). From the
meridional circulation point of view, the observed HCN enhancement on both the
equator and the pole can be explained by the production and accumulation of HCN
at the downward branches of the previously suggested two-cell meridional
circulation models. However, the HCN-depleted latitude of 60 S does not match
with the location of the upward branch of the two-cell circulation models.
