OCCURRENCE OF AN ULTRAFINE PARTICLE MODE LESS-THAN 20 NM IN DIAMETER IN THE MARINE BOUNDARY-LAYER DURING ARCTIC SUMMER AND AUTUMN

The International Arctic Ocean Expedition 1991 (IAOE-91) provided a pl atform to study the occurrence and size distributions of ultrafine par ticles in the marine boundary layer (MEL) during Arctic summer and aut umn. Measurements of both aerosol physics, and gas/particulate chemist ry were taken aboard the Swedish icebreaker Oden. Three separate submi cron aerosol modes were found: an ultrafine mode (D-p < 20 nm), the Ai tken mode (20 < D-p < 100 nm), and the accumulation mode (D-p > 100 nm ). We evaluated correlations between ultrafine particle number concent rations and mean diameter with the entire measured physical, chemical, and meteorological data set. Multivariate statistical methods were th en used to make these comparisons. A principal component (PC) analysis indicated that the observed variation in the data could be explained by the influence from several types of air masses. These were characte rised by contributions from the open sea or sources from the surroundi ng continents and islands. A partial least square (PLS) regression of the ultrafine particle concentration was also used. These results impl ied that the ultrafine particles were produced above or in upper layer s of the MBL and mixed downwards. There were also indications that the open sea acted as a source of the precursors for ultrafine particle p roduction. No anti-correlation was found between the ultrafine and acc umulation particle number concentrations, thus indicating that the sou rces were in separate air masses.