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Abstract
Abstract. In January 2010 and December 2011, synoptic-scale polar stratospheric cloud (PSC) fields were probed during seven flights of the high-altitude research aircraft M-55 Geophysica within the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interaction) and the ESSenCe (ESSenCe: ESA Sounder Campaign) projects. Particle size distributions in a diameter range between 0.46 and 40μm were recorded by four different optical in situ instruments. Three of these particle instruments are based on the detection of forward-scattered light by single particles. The fourth instrument is a grayscale optical array imaging probe. Optical particle diameters of up to 35μm were detected with particle number densities and total particle volumes exceeding previous Arctic measurements. Also, gas-phase and particle-bound NOy was measured, as well as water vapor concentrations. The optical characteristics of the clouds were measured by the remote sensing lidar MAL (Miniature Aerosol Lidar) and by the in situ backscatter sonde MAS (Multiwavelength Aerosol Scatterometer), showing the synoptic scale of the encountered PSCs. The particle mode below 2μm in size diameter has been identified as supercooled ternary solution (STS) droplets. The PSC particles in the size range above 2μm in diameter are considered to consist of nitric acid hydrates, and the particles' high HNO3 content was confirmed by the NOy instrument. Assuming a particle composition of nitric acid trihydrate (NAT), the optically measured size distributions result in particle-phase HNO3 mixing ratios exceeding available stratospheric values. Therefore the measurement uncertainties concerning probable overestimations of measured particle sizes and volumes are discussed in detail. We hypothesize that either a strong asphericity or an alternate particle composition (e.g., water ice coated with NAT) could explain our observations. In particular, with respect to the denitrification by sedimentation of large HNO3-containing particles, generally considered to be NAT, our new measurements raise questions concerning composition, shape and nucleation pathways. Answering these would improve the numerical simulation of PSC microphysical processes like cloud particle formation, growth and denitrification, which is necessary for better predictions of future polar ozone losses, especially under changing global climate conditions. Generally, it seems that the occurrence of large NAT particles – sometimes termed "NAT rocks" – are a regular feature of synoptic-scale PSCs in the Arctic.
Highlights
The main role of polar stratospheric clouds (PSC) in stratospheric ozone-related chemistry can be divided into two major processes
A backward model calculation starting from the PSC field of the flight track on 11 December 2011 can accommodate a nitric acid trihydrate (NAT) particle of only up to 16 μm, where a 20 % increased growth rate and 10 % reduced sedimentation speed were already assumed in the model (LAGRANTO trajectories based on ERA-Interim data were used here)
A comprehensive data set of optical in situ measurements of PSCs was obtained in two Arctic winter vortices
Summary
The main role of polar stratospheric clouds (PSC) in stratospheric ozone-related chemistry can be divided into two major processes These are the heterogeneous chlorine activation (Solomon et al, 1986; Solomon, 1999) and the redistribution of water and nitric acid due to sedimentation of cloud particles leading to dehydration and denitrification. Regarding the structure of the polar vortex, the temperature maps at 50 hPa, which are provided in the Supplement, look very similar, albeit shifted to somewhat higher temperature values As those cold synoptic conditions are relatively rare in the Arctic, previous in situ measurements have mostly documented mesoscale lee-wave-induced PSCs (Voigt et al, 2003; Lowe et al, 2006); only a few reported PSC observations are representative of synoptic conditions (Larsen et al, 2004; Weisser et al, 2006). COPAS has a maximum detectable particle diameter of a few microns, a limit given by the aspiration efficiency of the aerosol inlet
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