Hi,

It might be difficult to answer this without knowing the meteorological conditions at the site. I think that in cases where the diurnal cycle of of more than one compound is showing the same pattern and these compounds don't have a common source and sink, then it may result from the meteorological coniditons, in other words,I would look at the boundary layer conditions.

Where are you making the measurements and when?

In addition to where and when of the measurement of your unsaturated NMHCS  peaks, consider what you have upwind at the peak time.

Have you monitored these compunds upwind?

Are you monitoring at an urban street canyon, is there any high traffic area nearby, an industrial site ?

Location is most relevant

Acetylene and isopropene have been mostly reported as emission products from traffic with higher levels around noon, or at times with corresponding higher traffic volumes. Check out some articles of Agnes Borbon, http://www.lisa.univ-paris12.fr/fr/component/comprofiler/userprofile/aborbon or via researchgate.