Abstract
Abstract. In situ airborne sampling of refractory black carbon (rBC) particles and Ice Nuclei (IN) was conducted in and near an extratropical cyclonic storm in the western Pacific Ocean during the Pacific Dust Experiment, PACDEX, in the spring of 2007. Airmass origins were from Eastern Asia. Clouds associated primarily with the warm sector of the storm were sampled at various locations and altitudes. Cloud hydrometeors were evaporated by a counterflow virtual impactor (CVI) and the residuals were sampled by a single particle soot photometer (SP2) instrument, a continuous flow diffusion chamber ice nucleus detector (CFDC) and collected for electron microscope analysis. In clouds containing large ice particles, multiple residual particles were observed downstream of the CVI for each ice particle sampled on average. The fraction of rBC compared to total particles in the residual particles increased with decreasing condensed water content, while the fraction of IN compared to total particles did not, suggesting that the scavenging process for rBC is different than for IN. In the warm sector storm midlevels at temperatures where heterogeneous freezing is expected to be significant (here −24 to −29 °C), IN concentrations from ice particle residuals generally agreed with simultaneous measurements of total ice concentrations or were higher in regions where aggregates of crystals were found, suggesting heterogeneous freezing as the dominant ice formation process in the mid levels of these warm sector clouds. Lower in the storm, at warmer temperatures, ice concentrations were affected by aggregation and were somewhat less than measured IN concentrations at colder temperatures. The results are consistent with ice particles forming at storm mid-levels by heterogeneous freezing on IN, followed by aggregation and sedimentation to lower altitudes. Compositional analysis of the aerosol and back trajectories of the air in the warm sector suggested a possible biomass burning source for much of the aerosol. Comparison of the particles from the CFDC with the other aerosol in the residuals of ice particles suggested that the largest portion of IN had similar inferred origins (from biomass burning with minor amounts of rBC) as the other aerosol, but contained slightly elevated amounts of calcium and less influence from sea salt.
Highlights
Understanding the origins of ice in storms and the relationship between ice in the clouds and aerosols continues to be a fundamental problem in cloud physics
Understanding and predicting the concentration of ice in clouds has been remarkably difficult (e.g. Cantrell and Heymsfield, 2005), so these results suggest that predicting the levels of Ice forming Nuclei (IN) present in regions where heterogeneous freezing is active should be helpful in predicting ice concentrations in storms
The evidence presented here suggests that large ice crystals, such as we observed in the midlevels of an extratropical storm, release multiple aerosol particles upon evaporation in the counterflow virtual impactor (CVI)
Summary
Understanding the origins of ice in storms and the relationship between ice in the clouds and aerosols continues to be a fundamental problem in cloud physics. Dusts and other aerosols from Asia or Africa are known to be a source of both Ice forming Nuclei (IN) and Cloud Condensation Nuclei (CCN), but their impact on storms is not well known. Cozic et al (2008) observed enrichment in the BC aerosol mass fraction in ice particle residuals, compared to the mass fraction outside of cloud They suggested that this result might be due to BCcontaining particles acting preferentially as IN. Extensive airborne sampling of both warm and cold clouds associated with the warm front was conducted In this part of the storm, Hallett-Mossop secondary ice production was not likely, which allowed relatively uncontaminated comparison between IN and measured ice concentrations. Time-integrated filter and impactor samples of aerosols were collected for chemical and morphological analysis to allow the characteristics of the IN to be compared with other aerosols, including rBC
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