Virkkula, A., Ahlquist, N. C., Covert, D. S., Arnott, W. P., Sheridan, P. J., Quinn, P. K., Coffman, D. J. (2005). Modification, calibration and a field test of an instrument for measuring light absorption by particles, Aerosol Sci & Technol, 39 (1), 68-84