Characterization of size segregated particles collected over Alaska and the CAnadian high Arctic, AGASP-II flights 204–206

Abstract

Ten aircraft-collected cascade impactor samples from the North American Arctic were analyzed using analytical electron microscopy. Morphological, mineralogical and elemental information were obtained from individual particles, as well as compositional data and size distribution estimates of the bulk aerosol. Categorization of carbonaceous material into organic-type and combustion-type carbon particles was performed in this study. This was accomplished through the use of a new ultra-thin window X-ray spectrometer, which can directly detect carbon X-rays emitted from particles, and through interpretation of morphological and electron diffraction data. Verification of graphite as a specific carbon mineral phase present in Arctic soot particles was performed in this manner.

Several classes of particles were present in most of the aerosol samples and size fractions. These included liquid H₂SO₄ droplets, which were always present in the highest numbers, and crustal-type and composite SO₄⁻² particles. A small fraction (0–30%) of a random sampling of SO²⁻₄particles from all impactor stages were found to contain detectable nitrogen, suggesting that partial neutralization by NH₃ may have occurred in this minority of the SO²⁻₄ droplets. Particles rich in non-combustion carbon and thought to be composed of organic material were also observed in most samples. Haze samples collected off the coast of Alert, NWT, show moderate loadings of H₂SO₄ droplets. Judging from these loadings and those from higher-altitude samples, ambient aerosol particle concentrations must have been considerably higher in the haze. The extent to which local activity at Alert has influenced these haze samples is not known, although a major contribution is not expected. Stratospheric samples did not contain several classes of particles thought to have major anthropogenic source inputs to the Arctic, such as black carbon and coal-fired combustion spheres. The lightest particle loadings in any samples were collected in the upper troposphere near the tropopause, where condensation nuclei counts during sampling fell to as low as 10 cm⁻³.