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Information on what aerosol particle types are the major sources of ice
nucleating particles (INPs) in the atmosphere is needed for climate
predictions. To determine which aerosol particles are the major sources
of immersion-mode INPs at a coastal site in Western Canada, we
investigated correlations between INP number concentrations and both
concentrations of different atmospheric particles and meteorological
conditions. We show that INP number concentrations are strongly
correlated with the number concentrations of fluorescent bioparticles
between -15 and -25 degrees C, and that the size distribution of INPs is
most consistent with the size distribution of fluorescent bioparticles.
We conclude that biological particles were likely the major source of
ice nuclei at freezing temperatures between -15 and -25 degrees C at
this site for the time period studied. At -30 degrees C, INP number
concentrations are also well correlated with number concentrations of
the total aerosol particles >= 0.5 mu m, suggesting that non-biological
particles may have an important contribution to the population of INPs
active at this temperature. As we found that black carbon particles were
unlikely to be a major source of ice nuclei during this study, these
non-biological INPs may include mineral dust. Furthermore, correlations
involving chemical tracers of marine aerosols and marine biological
activity, sodium and methanesulfonic acid, indicate that the majority of
INPs measured at the coastal site likely originated from terrestrial
rather than marine sources. Finally, six existing empirical
parameterizations of ice nucleation were tested to determine if they
accurately predict the measured INP number concentrations. We found that
none of the parameterizations selected are capable of predicting INP
number concentrations with high accuracy over the entire temperature
range investigated. This finding illustrates that additional
measurements are needed to improve parameterizations of INPs and their
subsequent climatic impacts.
