  The New Horizons spacecraft, which flew by Pluto on July 14, 2015, revealed
the presence of haze in Pluto's atmosphere that were formed by CH4/N2
photochemistry at high altitudes in Pluto's atmosphere, as on Titan and Triton.
In order to help the analysis of the observations and further investigate the
formation of organic haze and its evolution at global scales, we have
implemented a simple parametrization of the formation of organic haze in our
Pluto General Circulation Model. The production of haze in our model is based
on the different steps of aerosol formation as understood on Titan and Triton:
photolysis of CH4 in the upper atmosphere by Lyman-alpha UV radiation,
production of various gaseous species, and conversion into solid particles
through accumulation and aggregation processes. We compared two reference
simulations ran with a particle radius of 50 nm: with, and without South Pole
N2 condensation. We discuss the impact of the particle radius and the lifetime
of the precursors on the haze distribution. We simulate CH4 photolysis and the
haze formation up to 600 km above the surface. Results show that CH4 photolysis
in Pluto's atmosphere in 2015 occured mostly in the sunlit summer hemisphere
with a peak at an altitude of 250 km, though the interplanetary source of
Lyman-alpha flux can induce some photolysis even in the Winter hemisphere. We
obtained an extensive haze up to altitudes comparable with the observations,
and with non-negligible densities up to 500 km altitude. In both reference
simulations, the haze density is not strongly impacted by the meridional
circulation. With no South Pole N2 condensation, the maximum nadir opacity and
haze extent is obtained at the North Pole. With South Pole N2 condensation, the
descending parcel of air above the South Pole leads to a latitudinally more
homogeneous haze density with a slight density peak at the South Pole.
