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. Ice nucleation is a fundamental atmospheric process that impactsprecipitation, cloud lifetimes, and climate. Challenges remain to identifyand quantify the compositions and sources of ice-nucleating particles (INPs).Assessment of the role of black carbon (BC) as an INP is particularlyimportant due to its anthropogenic sources and abundance at upper-tropospheric cloud levels. The role of BC as an INP, however, is unclear.This is, in part, driven by a lack of techniques that directly determine thecontribution of refractory BC (rBC) to INP concentrations. One previouslydeveloped technique to measure this contribution uses the Single ParticleSoot Photometer (SP2) as a pre-filter to an online ice-nucleating particlecounter. In this technique, rBC particles are selectively heated to theirvaporization temperature in the SP2 cavity by a 1064 nm laser. From previouswork, however, it is unclear under what SP2 conditions, if any, theoriginal rBC particles were fully vaporized. Furthermore, previous work alsoleft questions about the effect of the SP2 laser on the ice-nucleatingproperties of several INP proxies and their mixtures with rBC. To answer these questions, we sampled the exhaust of an SP2 with a ScanningMobility Particle Sizer and a Continuous Flow Diffusion Chamber. UsingAquadag® as an rBC proxy, the effect of several SP2 instrumentparameters on the size distribution and physical properties of particles inrBC SP2 exhaust were explored. We found that a high SP2 laser power (930 nW ∕ (220 nm PSL)) is required to fully vaporize a ∼  0.76 fg rBCparticle. We also found that the exhaust particle size distribution isminimally affected by the SP2 sheath-to-sample ratio; the size of theoriginal rBC particle, however, greatly influences the size distribution ofthe SP2 exhaust. The effect of the SP2 laser on the ice nucleationefficiency of Snomax®, NX-illite, and Suwannee River FulvicAcid was studied; these particles acted as proxies for biological,illite-rich mineral dust, and brown carbon INPs, respectively. The originalsize distribution and ice nucleation efficiency of all non-rBC proxies wereunaffected by the SP2 laser. Furthermore, the ice nucleation efficiencies ofall proxies were not affected when externally mixed with rBC. These proxies,however, always show a reduction in ice-nucleating ability when internallymixed with rBC. We end this work with recommendations for users who wish touse the SP2 as a pre-filter to remove large rBC particles from an aerosolstream.

atmospheric measurement techniques

Use of the Single Particle Soot Photometer (SP2) as a pre-filter for ice nucleation measurements: effect of particle mixing state and determination of SP2 conditions to fully vaporize refractory black carbon