I joined the UMD Meteorology Department in April of 1997. My primary research collaborations are still with the excellent scientists in that branch. A trace constituent is one which is only present in the atmosphere in small quantities.
Ozone is a good example of such a constituent.
Although its concentration never gets above a few parts per million by volume, it's presence is tremendously important both to the biosphere and to the structure of our atmosphere. If you want more detail on the topics I spend my life thinking about, you can find it here. A Proposal on the Use of a Two-Dimensional Model for Understanding Past Ozone Downward Trends and Future Ozone Recovery. C. H. Jackman, NASA Goddard Space Flight Center. Which Control Stratospheric Ozone Using UARS Constituent Measurements.