Scavenging of East-Asian Black Carbon by a Western Pacific Extratropical Storm

In situ airborne sampling of black carbon (BC) 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 Ice Nucleus detector. Clouds associated primarily with the warm sector of the storm were sampled at various locations and altitudes. Insoluble trace gas concentration measurements of Carbon Monoxide and Ozone were made in the warm sector airmass outside of clouds and were found to be significantly correlated with BC concentrations. The relationship between trace gas concentrations and BC was used to predict the expected total (cloud plus interstitial) BC concentrations in the cloud based on the measured trace gases concentration in cloud. These were compared with simultaneous BC measurements of the cloud residue. The ratio of BC in residue to expected total BC in the clouds in the warm sector at temperatures warmer than freezing was substantially lower than in nearby ice-containing clouds at colder temperatures (higher altitudes), suggesting that ice hydrometeors are preferred over liquid ones as carriers of BC particles in storms such as this one. IN measurements in ice particle residue generally agreed well with measurements of total ice concentrations and were typically about a factor of five lower than simultaneous measurements of BC concentrations.