Abstract
Abstract. In situ measurements with a four-channel stratospheric condensation particle counter (CPC) were conducted at up to 20 km altitude on board the aircraft M-55 Geophysica from Kiruna, Sweden, in January through March (EUPLEX 2003, RECONCILE 2010) and in December (ESSenCe 2011). During all campaigns air masses from the upper stratosphere and mesosphere were subsiding inside the Arctic winter vortex, thus initializing a transport of refractory aerosol into the lower stratosphere (Θ < 500 K). The strength and extent of this downward transport varied between the years depending on the dynamical evolution of the vortex. Inside the vortex and at potential temperatures Θ ≥ 450 K around 11 submicron particles per cm3 were generally detected. Up to 8 of these 11 particles per cm3 were found to contain thermo-stable (at 250 °C) residuals with diameters of 10 nm to about 1 μm. Particle mixing ratios (150 mg−1) and fractions of non-volatile particles (75% of totally detected particles) exhibited highest values in air masses having the lowest content of nitrous oxide (70 nmol mol−1 of N2O). This indicates that refractory aerosol originates from the upper stratosphere or the mesosphere. Derived from the mixing ratio of the simultaneously measured long-lived tracer N2O, an empirical index serves to differentiate probed air masses according to their origin: inside the vortex, the vortex edge region, or outside the vortex. Previously observed high fractions of refractory submicron aerosol in the 2003 Arctic vortex were ascribed to unusually strong subsidence during that winter. However, measurements under perturbed vortex conditions in 2010 and during early winter in December 2011 revealed similarly high values. Thus, the abundance of refractory aerosol in the lower stratosphere within the Arctic vortices appears to be a regular feature rather than the exception. During December, the import from aloft into the lower stratosphere appears to be developing; thereafter the abundance of refractory aerosol inside the vortex reaches its highest levels in March. The correlations of refractory aerosol with N2O suggest that, apart from mean subsidence, diabatic dispersion inside the vortex significantly contributes to the transport of particles to the Arctic lower stratosphere. A measurement-based estimate of the total mass of refractory aerosol inside the vortex is provided for each campaign. Based on the derived increase of particle mass in the lower stratospheric vortex (100–67 hPa pressure altitude) by a factor of 4.5 between early and late winter, we estimate the total mass of mesospheric particles deposited over the winter 2009/2010 in the entire Arctic vortex to range between 77 × 103 and 375 × 106 kg. This estimate is compared with the expected atmospheric influx of meteoritic material (110 ± 55 × 103 kg per day). Such estimates at present still hold considerable uncertainties, which are discussed in this article. Nevertheless, the results enable placing constraints on the shape of the so far unknown size distribution of refractory aerosol within the vortex.
Highlights
In situ measurements of the aerosol vertical distribution inside the Arctic winter vortex of 1989 revealed an increase of the mixing ratios of sub-micrometer sized particles with altitude (Wilson et al, 1990)
Considering the observations by Curtius et al (2005) from EUPLEX together with our findings presented in this study, we hypothesize that this particle import is a regular feature of the late winter polar vortex in the Northern Hemisphere, provided that the vortex develops throughout the season without being strongly diluted by midlatitude in-mixing
We surmise that earlier observations of increased aerosol number concentration inside the Arctic vortex at about 20 km altitude (Wilson et al, 1990) and above 20 km (Hofmann et al, 1990) comprise refractory aerosol and that new particle formation by homogeneous nucleation at the considered high altitudes plays a less crucial role than previously thought
Summary
In situ measurements of the aerosol vertical distribution inside the Arctic winter vortex of 1989 revealed an increase of the mixing ratios of sub-micrometer sized particles with altitude (Wilson et al, 1990). In the Arctic (from Kiruna, Northern Sweden), in the year 2003, similar vertical profiles were observed (Curtius et al, 2005) and here it was reported that an increase of particle concentrations coincided with an increasing fraction of non-volatile aerosol compounds Both studies demonstrated that the abundance of particles is closely linked to air mass subsidence inside the vortex from upper stratospheric or mesospheric altitudes down to the Arctic lower stratosphere. Such particles provide surfaces for heterogeneous reactions and deposition of condensable gases. A downward import of non-volatile particles potentially occurring on a regular basis inside the vortex could imply an important natural process linked to atmospheric ozone chemistry
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