Laboratory study of the heterogeneous ice nucleation on black-carbon-containing aerosol
Authors: Nichman Leonid , Wolf Martin , Davidovits Paul , Onasch Timothy B. , Zhang Yue , Worsnop Doug R. , Bhandari Janarjan , Mazzoleni Claudio , Cziczo Daniel J. 
Published: October 2019

Abstract. Soot and black carbon (BC) particles are generated in the
incomplete combustion of fossil fuels, biomass, and biofuels. These airborne
particles affect air quality, human health, aerosol–cloud interactions,
precipitation formation, and climate. At present, the climate effects of BC
particles are not well understood. Their role in cloud formation is obscured
by their chemical and physical variability and by the internal mixing
states of these particles with other compounds. Ice nucleation in field
studies is often difficult to interpret. Nonetheless, most field studies
seem to suggest that BC particles are not efficient ice-nucleating particles
(INPs). On the other hand, laboratory measurements show that in some cases,
BC particles can be highly active INPs under certain conditions. By working
with well-characterized BC particles, our aim is to systematically establish
the factors that govern the ice nucleation activity of BC. The current study
focuses on laboratory measurements of the effectiveness of BC-containing
aerosol in the formation of ice crystals in temperature and ice
supersaturation conditions relevant to cirrus clouds. We examine ice nucleation on BC particles under water-subsaturated cirrus
cloud conditions, commonly understood as deposition-mode ice nucleation.
We study a series of well-characterized commercial carbon black particles
with varying morphologies and surface chemistries as well as ethylene
flame-generated combustion soot. The carbon black particles used in this
study are proxies for atmospherically relevant BC aerosols. These samples
were characterized by electron microscopy, mass spectrometry, and optical
scattering measurements. Ice nucleation activity was systematically examined
in temperature and saturation conditions in the ranges of 217≤T≤235 K and 1.0≤Sice≤1.5 and 0.59≤Swater≤0.98, respectively, using a SPectrometer for Ice Nuclei (SPIN) instrument,
which is a continuous-flow diffusion chamber coupled with instrumentation to
measure light scattering and polarization. To study the effect of coatings
on INPs, the BC-containing particles were coated with organic acids found in
the atmosphere, namely stearic acid, cis-pinonic acid, and oxalic acid. The results show significant variations in ice nucleation activity as a
function of size, morphology, and surface chemistry of the BC particles. The
measured ice nucleation activity dependencies on temperature,
supersaturation conditions, and the physicochemical properties of the BC
particles are consistent with an ice nucleation mechanism of pore
condensation followed by freezing. Coatings and surface oxidation modify the
initial formation efficiency of pristine ice crystals on BC-containing
aerosol. Depending on the BC material and the coating, both inhibition and
enhancement in INP activity were observed. Our measurements at low
temperatures complement published data and highlight the capability of some
BC particles to nucleate ice under low ice supersaturation conditions. These
results are expected to help refine theories relating to soot INP activation
in the atmosphere.