The French Impressionists weren't the ones who invented painting outdoors. Certainly artists had been carrying sketchbooks outside for centuries. And the British would tell you they were watercoloring outdoors long before Monet painted his "Impression: Sunrise". It just happened that oil paints could now be stored in tubes as opposed to being handground in the studio where it was stored in pots. Adding to that, artists were now painting on linen canvas which could be stretched over supports making it lots lighter than painting on primed wood. Monet even used a boat as a floating studio to paint the populars next to the river bank.
Painting Aspens at Lake TahoePlein air painting can be as rugged a sport as you want to make it. Some artists hike miles in the woods and mountains with their supplies on their backs just to capture their scenes. You can paint in public or not. You can carry large canvases or very small. There just isn't a right or wrong way. So if you see an artist outside set up with their easel and paints, you can say, "Good luck with your plein air painting." And they will know that you know the Secret.