Mixing State of Size-Selected Submicrometer Particles in the Arctic in May and September 2012

Abstract

Aerosols have been associated with large uncertainty in estimations of the radiation budget and cloud formation processes in the Arctic. This study reports in-situ measurements of hygroscopicity, fraction of volatile species, and mixing state, and off-line morphological and elemental analysis of Aitken and accumulation mode particles in the Arctic (Ny-Alesund, Svalbard) in May and September. The accumulation mode particles more dominated in May than September. This occurred because of more air mass flow from lower latitude continent, weaker vertical mixing, and less wet scavenging in May compared to September, which could lead to the accumulation of the higher amount of long-range transport aerosols entering the Arctic in spring. The Aitken mode particles observed intermittently in May were produced via nucleation with no significant external mixing, whereas the accumulation mode particles displayed a significant external mixing. The occurrence of an external mixing state was more often observed in May than September, in accumulation mode particles than the Aitken mode particles, and with continental air mass (Siberia) than other air masses. The external mixing of the accumulation mode particles in May could be caused by multiple sources (i.e., long-range transport aerosols with aging and marine aerosols). Those external groups of particles were sub-divided into different mixing structures (internal mixtures of predominantly sulfates and volatile organics without non-volatile species versus internal mixtures of sulfates and non-volatile components, such as sea salts, minerals, and soot) also identified by TEM/EDS data. The variation in the mixing states and chemical species of the Arctic aerosols in terms of sizes, air masses, and seasons suggests that the continuous size-dependent measurements presented in this work are useful to obtain better estimates of their effects on climate change.