“…In a comprehensive review, Graber and Rudich (2006) concluded that, although it is difficult to distinguish atmospheric HULIS from terrestrial and aquatic humic substances on a chemical level, there are significant differences in physical properties such as hygroscopicity and CCN activity. This has been substantiated in a series of studies on the microphysical properties of HULIS extracted from real aerosol samples (Dinar et al, 2006a(Dinar et al, , b, 2007Asa-Awuku and Nenes, 2007;Taraniuk et al, 2007;Wex et al, 2007). There is indication that HULIS can be of primary origin, e.g., wood combustion, but may also be associated with secondary particle-phase production (Gelencsér et al, 2002;Samburova et al, 2005a, b;Feczko et al, 2007;Surratt et al, 2007aSurratt et al, , 2008.…”