A global three‐dimensional model of the circulation and chemistry of CFCl₃, CF₂Cl₂, CH₃CCl₃, CCl₄, and N₂O

Abstract

An efficient three‐dimensional spectral model is used to investigate the tropospheric and stratospheric circulation, chemistry, and photochemistry of the long‐lived atmospheric species CFCl 3 , CF 2 Cl 2 , CH 3 CCl 3 , CCl 4 , and N 2 O. The model uses precalculated three‐dimensional spectral fields of vorticity, vertical velocity, temperature, and ozone concentration and current estimates of the magnitudes and distributions of industrial and natural sources of the various species. Explicit calculations of instantaneous photodissociation rates for each species are carried out, and reactions with atmospheric OH and O( 1 D ) and dissolution in the ocean are also included in the model where relevant. Available measurements of the horizontal, vertical, and temporal distribution of the modeled atmospheric species generally compare favorably with our calculations. However, there are some disagreements, particularly between the observed and calculated surface distributions of CH 3 CCl 3 and the vertical distributions of CCl 4 , which are critically discussed. Assuming that their destruction is dominated by photodissociation and reaction with O( 1 D ), the calculated present‐day global atmospheric lifetimes of CFCl 3 , CF 2 Cl 2 , CCl 4 , and N 2 O are 73, 210, 48, and 182 years, respectively. Decreasing the O 2 absorption cross sections in the Schumann‐Runge continuum (2000–2200 Å) by 40% results in a decrease of the CFCl 3 lifetime to 44 years. The calculated atmospheric lifetime of CH 3 CCl 3 due to photodissociation and reaction with OH and O( 1 D ) is 12 years, but this result depends on the prescribed seasonally dependent zonally invariant OH distribution used in the model.