**Definition of terms and symbols**

C - The atmospheric concentration of a gas; typical units are parts per million (ppm).

C_{0} - 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.

C_{eq} - Equilibrium concentration. When the emission rate
equals the removal rate.

t_{1/2} - atmospheric half-life of a gas. t_{1/2}
= 0.7 t

RR - Removal rate; typical units of ppm/year. RR= ^{C}/_{t}

Basic relationship

(1)

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.

C_{eq}=S**t**
(2)