Hand, J., Schichtel, B., Malm, W., Copeland, S., Molenar, J., Frank, N., Pitchford, M. L. (2014). Widespread reductions in haze across the United States from the early 1990s through 2011, Atmospheric Environment, 94, 671-679, 10.1016/j.atmosenv.2014.05.062