Abstract
Abstract. The sedimentation of HNO3 containing Polar Stratospheric Cloud (PSC) particles leads to a permanent removal of HNO3 and thus to a denitrification of the stratosphere, an effect which plays an important role in stratospheric ozone depletion. The polar vortex in the Arctic winter 2009/2010 was very cold and stable between end of December and end of January. Strong denitrification between 475 to 525 K was observed in the Arctic in mid of January by the Odin Sub Millimetre Radiometer (Odin/SMR). This was the strongest denitrification that had been observed in the entire Odin/SMR measuring period (2001–2010). Lidar measurements of PSCs were performed in the area of Kiruna, Northern Sweden with the IRF (Institutet för Rymdfysik) lidar and with the Esrange lidar in January 2010. The measurements show that PSCs were present over the area of Kiruna during the entire period of observations. The formation of PSCs during the Arctic winter 2009/2010 is investigated using a microphysical box model. Box model simulations are performed along air parcel trajectories calculated six days backward according to the PSC measurements with the ground-based lidar in the Kiruna area. From the temperature history of the backward trajectories and the box model simulations we find two PSC regions, one over Kiruna according to the measurements made in Kiruna and one north of Scandinavia which is much colder, reaching also temperatures below Tice. Using the box model simulations along backward trajectories together with the observations of Odin/SMR, Aura/MLS (Microwave Limb Sounder), CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) and the ground-based lidar we investigate how and by which type of PSC particles the denitrification that was observed during the Arctic winter 2009/2010 was caused. From our analysis we find that due to an unusually strong synoptic cooling event in mid January, ice particle formation on NAT may be a possible formation mechanism during that particular winter that may have caused the denitrification observed in mid January. In contrast, the denitrification that was observed in the beginning of January could have been caused by the sedimentation of NAT particles that formed on mountain wave ice clouds.
Highlights
Polar stratospheric clouds (PSC) play a key role in stratospheric ozone destruction in the polar regions (Solomon et al, 1986; Crutzen and Arnold, 1986)
Khosrawi trajectories together with the observations of Odin/Sub-Millimetre Radiometer (SMR), Aura/MLS (Microwave Limb Sounder), CALIPSO (CloudAerosol Lidar and Infrared Pathfinder Satellite Observations) and the ground-based lidar we investigate how and by which type of PSC particles the denitrification that was observed during the Arctic winter 2009/2010 was caused
PSC Ia particles are composed of nitric acid trihydrate (NAT), the stable HNO3 hydrate under stratospheric conditions (Toon et al, 1986; Hanson and Mauersberger, 1988) and PSC type II of pure water ice (Steele et al, 1983; Browell et al, 1990)
Summary
Polar stratospheric clouds (PSC) play a key role in stratospheric ozone destruction in the polar regions (Solomon et al, 1986; Crutzen and Arnold, 1986). The formation of solid PSC type Ia and II particles is generally assumed to be initiated by the homogeneous freezing of supercooled ternary solution particles at temperatures 3–4 K below the ice frost point Tice ≈ 185 K (Koop et al, 1995). Fahey et al (2001) suggested that Arctic denitrification could be caused by a selective, but yet unknown, nucleation mechanism responsible for the formation of a small number of large solid particles as was first observed in the 1999/2000 Arctic winter stratosphere. In a recent study by Pitts et al (2011) analysing CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) observations for the Arctic winter 2009/2010 an increase of ice observations with a coincident decrease in NAT mixtures under a synopticscale cooling event was observed, suggesting that under these conditions heterogeneous nucleation on NAT particles may be an important process for ice PSC formation. The RECONCILE campaign was performed during January– March 2010 (von Hobe et al, 2011)
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