Definition of terms and symbols
C - The atmospheric concentration of a gas; typical units are parts per million (ppm).
C0 - The initial atmospheric concentration of a gas.
DC - change in trace gas concentration.
Dt - change in time (or time increment).
S - The emission source strength; typical units are ppm/yr.
r - Fractional change in S per year. If r = 0.01 then S increases by 1% per year.
t - atmospheric life-time; typical units are years.
Ceq - Equilibrium concentration. When the emission rate equals the removal rate.
t1/2 - atmospheric half-life of a gas. t1/2 = 0.7 t
RR - Removal rate; typical units of ppm/year. RR= C/t
From this relationship it is easy to see that,
if S=RR (S= C/t or C=St) then DC =0 and C=Ceq i.e. C does not change.
Thus, the equilibrium concentration can be calculated from S and t as.