The state of California is about to become a giant playground for more than 200 atmospheric scientists. Beginning this week and extending into July, aircraft will criss-cross the skies, measuring an array of greenhouse gases, aerosols and other atmospheric properties as they fly over cities, industrial facilities and agricultural areas. Dozens of scientists will man two ground stations, while a ship monitors the air off the coast and two electric vehicles zip about collecting samples upwind and downwind of selected sites.