Question

What enabled the impressionists to practice painting en plein air?

Answer 1

I'm not 100% sure but, Impressionist artists were the first to paint outside (plein-air painting). The technical innovation of collapsible tubes of paint allowed this practice, which enabled the Impressionists to record, first-hand, fleeting effects of color and light in the landscape.
For example the life and work of Claude Monet serves as an allegory to Impressionism itself. One of the revolutionary aspects of Impressionism was its unfinished, sketchy quality seen in works such as Monet’s Impression-Sunrise or Bridge over a Pool of Water Lilies.