This module introduces stratospheric sulfate aerosol (SSA) formation from gaseous sulfuric acid (H₂SO₄) in the upper atmosphere, following the framework proposed by Hamill et al. (1977).
The process is activated when the compilation flag
#IFPATMO_useAerosolformation
in patmo_ode.f90
#IFPATMO_useAerosolformation
!aerosol formation
do i=13,34
if (va(i) <= n(i, patmo_idx_H2SO4) .AND. pa(i) >= n(i, patmo_idx_H2SO4)) then
dn(i, patmo_idx_H2SO4) = dn(i, patmo_idx_H2SO4) - (n(i, patmo_idx_H2SO4) - va(i))
dn(i, patmo_idx_SO4) = dn(i, patmo_idx_SO4) + (n(i, patmo_idx_H2SO4) - va(i))
end if
end do
#ENDIFPATMO
i = 13–34: Altitude layers between 13 km and 34 km (assuming 1 km layer resolution).
n(i, patmo_idx_H2SO4): Current number density (molecule cm⁻³) of gaseous H₂SO₄.
va(i): Critical vapor-phase H₂SO₄ density threshold (nucleation limit).
pa(i): Ambient H₂SO₄ partial pressure.
dn(i, patmo_idx_H2SO4): Rate of change (Δn/Δt) for gaseous H₂SO₄.
dn(i, patmo_idx_SO4): Rate of change (Δn/Δt) for condensed SO₄ aerosol.
The code checks whether the ambient partial pressure of H₂SO₄ exceeds the vapor-phase equilibrium value, which indicates supersaturation and thus triggers aerosol formation.
When the condition is satisfied, the excess H₂SO₄ is removed from the gas phase and added to the sulfate aerosol pool (SO4).
The implemented mechanism is based on the stratospheric aerosol scheme described by Hamill et al. (1977), where the formation of sulfate aerosol nuclei occurs once the ratio between the molecular H₂SO₄ partial pressure and the H₂O vapor pressure exceeds unity:
At this point, sulfuric acid–water clusters aggregate to form stable aerosol particles (sulfate nuclei).
Subsequent gravitational settling is parameterized following Kasten (1968) using a representative particle radius r = 0.33μm.
The layer index range (13–34) corresponds to 13–34 km only if your vertical resolution is 1 km per model layer.
For different resolutions, you must adjust this range manually to match the desired stratospheric region.
This process is typically applied above the tropical tropopause, where H₂SO₄ nucleation and aerosol growth dominate stratospheric sulfur chemistry.
To include this scheme in your simulation, ensure that:
The #IFPATMO_useAerosolformation preprocessor flag is enabled in your compile configuration.
patmo_idx_H2SO4 and patmo_idx_SO4 are properly initialized in the species index list.
The condensation sink and settling terms are balanced within the overall dn array.
Hamill, P., Toon, O. B. and Kiang, C. S. (1977) Microphysical processes affecting stratospheric aerosol particles. J. Atmos. Sci. 34, 1104–1119.
Kasten, F. (1968) Falling speed of aerosol particles. J. Appl. Meteorol. (1962–1982) 7, 944–947.
Li, Y., Kamezaki, K., & Danielache, S. O. (2024). Photo-oxidation pathway as a potential CS₂ sink in the atmosphere. Geochemical Journal, 58(5), 169–183.