CCN Hygroscopicity Variability

Lack of variability of CCN sizes led Dusek et al. (2006) to suggest that CCN could be deduced from particle size measurements alone, which are easier, more readily available, less controversial and more amenable to remote sensing. However, Hudson and Da (1996) and Hudson (2007) showed much more CCN size variability that makes it impossible to deduce CCN concentrations from size distribution measurements using a single relationship between size and CCN. But Dusek et al. (2006) also suggested that CCN sizes may be consistent within various air masses. This was indeed suggested by Hudson and Da (2006) and Hudson (2007). CCN were generally found to be larger (less hygroscopic) in more polluted air masses. This led Andreae and Rosenfeld (2008) to reassert the claim by Dusek et al. that as long as the air mass can be characterized as maritime or continental/polluted then particle size measurements can be used to approximate CCN concentrations. Here we present further measurements of CCN sizes that discount even this. The recent measurements, all of which are from aircraft, come from the PASE project in Aug.-Sep., 2007 at Christmas Island 2 degrees N latitude directly south of Honolulu, the ICE-L project, Nov.-Dec., 2007 over Colorado and Wyoming, and the POST project July-Aug., 2008 off the central California coast. The extremely remote PASE measurements were generally consistent with previous maritime measurements in that the CCN were very hygroscopic, similar to soluble salts like ammonium sulfate. However, the larger particles (>~150nm) were consistently less hygroscopic. This mixture of hygroscopicities (B) where B decreased with particle size was unexpected and would complicate deductions of CCN concentrations from size distribution measurements. Although the POST measurements displayed nearly identical total particle (CN) and CCN concentrations to the PASE measurements, the CCN were significantly less hygroscopic than PASE. The POST B values were intermediate to previous maritime measurements (including PASE) and measurements in polluted air masses. The ICE-L measurements showed consistently low B values that are consistent with measurements in very polluted air masses. This was the case even at high altitudes where total CN and CCN concentrations were well below the PASE, POST or most maritime measurements. CCN size measurements provide important clues about the origins of CCN, namely natural (high B) or anthropogenic (low B). The variations of B and inconsistencies with particle concentrations displayed here demonstrate enormous difficulties for attempts to use size distribution measurements to deduce CCN concentrations. This means that although particle size may matter more than particle chemistry (Dusek et al. 2006; Andreae and Rosenfeld 2008) the latter cannot be ignored; i.e., there is no substitute for CCN measurements. Andreae and Rosenfeld (2008), Earth Science Reviews, 89 (1), 13-41. Dusek et al., (2006), Science, 312, 1375-1378. Hudson, J.G., (2007), Geophys. Res. Let., 34, L08801, doi:10.1029/2006GL028850. Hudson and Da, (1996), J. Geophys. Res., 101, 4435-4442.