Are outdoor concerts considered "green" as opposed to indoor venues?

Answer 1

Not really.
Outdoor venues do not always have enough bathroom facilities or trash pickup. Constant foot traffic can wear away grass at parks. Concerts at public parks are nice but the garbage left behind is something awful. Food not cleaned up gets eaten more often by rats, mice, European pigeons and starlings that are considered to be invasive pests, and sodas, barf, beer, and human urine may one one hand fertilize the grass and on the other hand do a lot of damage in spreading disease to the plants by being food for bacteria and microbes that move to the shrubs and trees when the grass is gone. Litter may be picked up but a lot does not get cleaned up and blows into the the roads and can clog up storm drains. And the electronic equipment still runs on electricity.
Indoor venues have bathroom facilities that are cleaned before and after every performance. The concessions may be costly but they help pay for the cleanup crew after the show. Parking lots prevent parking problems on the streets. The cost of electricity for the stage lights and electronic equipment are factored into the cost of the ticket. There is less environmental damage (and sometimes the sound is better) in an arena or theater.
Indoor venues are actually much more "green" than the environmentalists would have you believe.

Answer 2

Outdoor is more oxygen during concerts but more allergies too during allergy season.

Answer 3

I don't really think there is a difference