On board measurement of black carbon aerosols over the Arctic Ocean in summer

This paper presents aerosol black carbon (BC) concentrations measured at deck level on board the R/V XUE LONG icebreaker.The vessel cruised the Arctic Ocean carrying an in situ aethalometer during the summers of 2008 and 2010.The courses of the third Chinese National Arctic Research Expedition (3rd CHINARE-Arctic,August 2008) and fourth Chinese National Arctic Research Expedition (4th CHINARE-Arctic,from late July to August 2010) were bounded by 173°W-143°W and 178°E-150°W,with northernmost points 85°25′N and 88°26′N,respectively.Results show low surface BC concentrations over the ocean throughout the courses,with means (standard error) of 6.0 (±4.7) ng·m-3 for 3rd CHINARE-Arctic,and 8.4(±7.1) ng·m-3 for 4th CHINARE- Arctic.It is clear that these onboard BC concentrations are similar to reported data from coastal stations in the Arctic region.The latitude-average BC concentration varied from 3.0-26.2 ng·m-3 for 3rd CHINARE-Arctic,to 4.2-20.5 ng·m-3 for 4th CHINARE-Arctic. At latitudes higher than 72°N for 3rd CHINARE-Arctic and 75°N for 4th CHINARE-Arctic,BC concentrations were lower and had negligible latitudinal gradients.Analysis indicates that the presence of the Arctic front isolates the lower atmosphere of the high-latitude Arctic Ocean from low-latitude terrestrial transport.This maintains the very low BC concentrations and negligible concentration gradients at high latitudes of the Arctic Ocean during summer.Calculated airmass backward trajectories for the two expeditions show that the Arctic front in 2010 was further north than in 2008,which caused different latitudinal variation of BC concentration in the two years.