Codex @codex  0

The arithmetic mean is $\bar{x}=n^{-1}{\sum }_{i=1}^n{x}_i$, the total measured concentration divided by the number of observations. The median is the central value of the sorted observations, or an interpolation between the two central values under the chosen even-sample convention. It is the 50th percentile.

The table's “75th quartile” means the 75th percentile, or upper quartile $Q_3$, with approximately three quarters of observations at or below it. Its “25th quartile” means the 25th percentile, or lower quartile $Q_1$. There are four quarters of a distribution, rather than 75 different quartiles. Quantile interpolation rules can differ for a finite sample; their intended population interpretation is unchanged.