ABSTRACT
Human activities, particularly man-made chemicals, over recent decades have resulted in the depletion of stratospheric ozone and consequently, increased ultraviolet B (UVB) irradiation of the earth's surface. Each year, the ozone layer over the Antarctic pole reaches the thinnest thickness recorded for the year around early October, and the hole is usually largest in early September. The unit used in the measure of the column abundance of ozone in the atmosphere is Dobson Units (DU), corresponding to a layer of ozone 10 μm thick if the ozone was held at standard temperature and pressure (273 K, 1 atm pressure) (American Meteorological Society 2018). The average amount of ozone in the atmosphere is roughly 300 DU. Any recorded level thinner than 220 DU is called an ozone hole. The annual data for 21 September–16 October, 1979 to 2017, over Antarctica are below 220 DU, which represents anthropogenic ozone losses from chlorine and bromine compounds (NASA Ozone Watch 2018a). As the observations show, Antarctica is experiencing an alarmingly thin ozone layer. However, the abundance of ozone in the Arctic has changed between 200 and 250 DU. The average minimum and mean ozone layer over the Arctic from 1 July 2017 to 30 June 2018 were 312 and 350 DU, respectively (NASA Ozone Watch 2018b). During the year 2010, measurements over much of the Northern Hemisphere unexpectedly indicated a high annual mean of the ozone columns. At the mid-latitudes, 45°N to 55°N, the annual mean exceeded 360 DU, which was one of the highest records over the last decades. This unusually high annual mean of the ozone layer over the Northern Hemisphere is surmised to be mostly due to the negative phase of the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) in 2010, coinciding with the easterly wind shear phase of the quasi-biennial oscillation 452of stratospheric winds at the equator (QBO), and partially, the recent decline of stratospheric chlorine and bromine (Steinbrecht et al. 2011). Therefore, it seems that the worst conditions are being experienced in Southern Hemisphere latitudes rather than their Northern counterparts. Apart from the irreversibly depleted ozone layer over the Antarctic continent due to anthropogenic activities, particular weather patterns also cause mini-holes in ozone. The differences between these two are that the mini-hole is a natural phenomenon, covering a smaller area, and reversible. For example, the highest and lowest minima over recent decades have fluctuated between 170 and 340 DU in the northern polar region (NASA Ozone Watch 2018c).
