This is quite a good explainer piece covering exercise outdoors.
It’s really not relevant to group rides though.
Obviously it’s not written by a group cyclist as it doesn’t even go into riding behind someone for 30 mins.
But if we focus solely on the aerodynamics of particle dispersion, we obscure more crucial unanswered questions: How hardy is the virus when it’s in aerosol form? How long can it linger in the air before its protective coat of moisture dries up and it falls apart, ceasing to be infectious?
I think the answer is very likely longer than 10 seconds.
Or to take the quote
“The risks of virus transmissibility in the air outdoors is likely quite low in those contexts, although this risk hasn’t been definitively measured,” Rasmussen said. “Outside, things like sunlight, wind, rain, ambient temperature, and humidity can affect virus infectivity and transmissibility, so while we can’t say there’s zero risk, it’s likely low unless you are engaging in activities as part of a large crowd (such as a protest). Solitary outdoor exercise is likely low-risk.”
You can see why, if you’re standing outdoors several feet away from an infected person, the virus might have a hard time making its way over to you at a high enough dose to actually infect you.