Sorry, I’m new to game programming and especially 3D, so probably my original thinking was incorrect. I didn’t understand much of what you said but I used some of your keywords to google and found out what is actually probably needed: horizontal billboarding instead of vertical billboarding.
I just need a way to orient the XY plane of the particle system to the ‘ground’, like this:
In unity, they call this horizontal billboarding.
I changed the title to reflect this.