= Horizontally averaged plume concentration
{title2=$\overline C=c/w$}
For a plume of full width $w(z)$, its horizontally averaged <concentration> is $\overline C(z,t)=c(z,t)/w(z)$, where $c$ is <horizontally integrated plume concentration>. In a <line plume> with $w=\mu z$, division by $z$ generally changes the location of maximum <concentration>. A gradient closure for $c$ differs from one for $\overline C$: $w\partial_z\overline C=c_z-c/z$, so integrated diffusive flux $-Kw\partial_z\overline C$ equals $-K(c_z-c/z)$.
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