Scavenging of biomass burning refractory black carbon and ice nuclei in a Western Pacific extratropical storm
1National Center for Atmospheric Research, Boulder, Colorado, USA
2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA
3Department of Atmospheric Sciences, Colorado State University, Fort Collins, Colorado, USA
4Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico City, Mexico
5Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA
6Mechanical and Aerospace Engineering, Arizona State University, Tempe, Arizona, USA
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. Cloud hydrometeors were evaporated by a counterflow virtual impactor and the residue was sampled by a single particle soot photometer (SP2) instrument and a continuous flow diffusion chamber ice nucleus detector. Clouds associated primarily with the warm sector of the storm were sampled at various locations and altitudes. In storm midlevels at temperatures where heterogeneous freezing is expected to be significant (here −24 to −29 °C), IN measurements from ice particle residues generally agreed well with simultaneous measurements of total ice concentrations provided that the measurements were made at ambient temperatures similar to those in the CFDC chamber, 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 (−22 to −6.4 °C), ice particle concentrations were similar to IN concentrations at CFDC chamber temperatures representative of colder temperatures. This is consistent with ice particles forming at storm mid-levels by heterogeneous freezing on IN, followed by sedimentation to lower altitudes. Homogeneous freezing did not appear to contribute significantly to midlevel ice concentrations and rime-splintering was also unlikely due to the absence of significant supercooled liquid water in the warm sector clouds. IN number concentrations were typically about a~factor of five to ten lower than simultaneous measurements of rBC concentrations in cloud.