Don Coen has a painting in the new Hamilton Building of the Denver Art Museum. Most of his paintings are large-scale (8' long or more), hyper-realistic images of the rural West. Some are air-brushed acrylic on canvas, some are oil-stick on paper and some are monotypes. With their size and level of detail, they could never be mistaken for plein-air.
His ink-and-wash drawings, however, do look like they were done on the spot. That's the only way you could get that combination of accuracy & spontaneity.