Fundamental differences between Arctic and Antarctic ozone depletion

Abstract

Antarctic ozone depletion is associated with enhanced chlorine from anthropogenic chlorofluorocarbons and heterogeneous chemistry under cold conditions. The deep Antarctic "hole" contrasts with the generally weaker depletions observed in the warmer Arctic. An unusually cold Arctic stratospheric season occurred in 2011, raising the question of how the Arctic ozone chemistry in that year compares with others. We show that the averaged depletions near 20 km across the cold part of each pole are deeper in Antarctica than in the Arctic for all years, although 2011 Arctic values do rival those seen in less-depleted years in Antarctica. We focus not only on averages but also on extremes, to address whether or not Arctic ozone depletion can be as extreme as that observed in the Antarctic. This information provides unique insights into the contrasts between Arctic and Antarctic ozone chemistry. We show that extreme Antarctic ozone minima fall to or below 0.1 parts per million by volume (ppmv) at 18 and 20 km (about 70 and 50 mbar) whereas the lowest Arctic ozone values are about 0.5 ppmv at these altitudes. At a higher altitude of 24 km (30-mbar level), no Arctic data below about 2 ppmv have been observed, including in 2011, in contrast to values more than an order of magnitude lower in Antarctica. The data show that the lowest ozone values are associated with temperatures below −80°C to −85°C depending upon altitude, and are closely associated with reduced gaseous nitric acid concentrations due to uptake and/or sedimentation in polar stratospheric cloud particles.stratosphere | atmospheric chemistry T he extensive springtime depletion of Antarctic ozone has attracted both public and scientific interest since its discovery (1) and explanation in the 1980s. The ozone hole has been linked to the coupling of human-made chlorofluorocarbons with surface chemistry on and in polar stratospheric clouds (PSCs) that form during extreme cold conditions (2). Polar stratospheric clouds are composed of nitric acid hydrates, liquid solutions of sulfuric acid, water, and nitric acid, and (under very cold conditions) water ice (e.g., ref. 3 and citations therein). Some of the key reactions are photochemical, so that the ozone hole does not form during midwinter when the polar cap is dark, but rather in late winter/spring as sunlight returns, provided that temperatures remain low. Although the same basic chemical mechanisms operate in both hemispheres, the Arctic winter stratosphere is generally warmer than the Antarctic, and it warms up earlier in the spring. These two factors taken together explain why ozone depletion in the Arctic is generally much smaller than in the Antarctic. A particularly cold Arctic stratospheric winter and spring in 2010/2011 displayed much larger ozone depletion than typical years, as highlighted by Manney et al. (4). This noteworthy geophysical event has intrigued scientists and raised several important questions: Could this be the first Arctic ozone hole? Are Arctic ozone losses ever observed to be as extreme...

Contribution of liquid, NAT and ice particles to chlorine activation and ozone depletion in Antarctic winter and spring

“…For the ice nucleation 50 % H 2 O supersaturation is assumed to be necessary (Tabazadeh et al, 1997;Peter and Grooß, 2012).…”

“…They further argue that even the heterogeneous reactivity on cold binary aerosol is sufficient to explain the major fraction of polar stratospheric chlorine activation. In contrast, Solomon et al (2014) have argued that in sunlit air in the polar stratosphere, deactivation, i.e. the reformation of HCl and ClONO 2 , increases the rate at which heterogeneous reactions must proceed to keep pace with deactivation.…”

Future Arctic ozone recovery: the importance of chemistry and dynamics

“…While similar chemical processes operate in the Arctic as in the Antarctic, where substantial springtime loss occurs every year, meteorological conditions in the Arctic are generally less favourable to cause extreme ozone depletion (Solomon et al, 2007. In particular, the Arctic stratospheric polar vortex in wintertime is, on average, more dynamically disturbed, resulting in higher temperatures and greater transport of ozone into the polar regions, and consequently the Arctic is generally not subject to the very large springtime ozone losses observed in the Antarctic (Tilmes et al, 2006;Solomon et al, 2007Solomon et al, , 2014. However, when the winter/spring Arctic lower stratosphere is cold for a long period, substantial ozone depletion is to be expected (WMO, 2014).…”

Radiative and Dynamical Influences on Polar Stratospheric Temperature Trends

“…While in austral spring the Antarctic lower-stratospheric temperatures are consistently cold enough for heterogeneous ozone depletion, Arctic lowerstratospheric temperatures are often near the threshold for polar stratospheric cloud formation (e.g., Solomon et al 2014). Thus, in dynamically quiescent winters, the Arctic can also experience significantly lower ozone abundances, such as those observed in the winters of the mid-to late 1990s and 2011 (Newman et al 1997;Manney et al 2011), owing both to chemical depletion and weakened transport.…”