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

Basic relationship


From this relationship it is easy to see that,

if S=RR (S= C/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.

                                 Ceq=St                (2)