Century-long column ozone records show that chemical and dynamical influences counteract each other

Abstract

The recovery of the ozone layer, which is expected as stratospheric chlorine levels have decreased over the past 25 years, remains difficult to detect. Column ozone has been monitored from 1924 to 1975 in Oxford, UK. Here, I present a century-long Oxford column ozone record, extended to the present based on re-discovered material and neighbouring series, and analyse it together with a record from Arosa, Switzerland that starts in 1926. Neither series shows a clear increase over the past 25 years but suggest stratospheric circulation. I separate chemical and dynamical effects using a regression approach and find that chemical recovery amounts to +8 DU between peak stratospheric chlorine concentrations in 1997 to 2021, consistent with model simulations. However, this trend is counteracted by a −5 DU dynamical trend. Century-long ozone records provide a window to the past dynamical, chemical and radiative state of the stratosphere and help better constrain circulation effects on ozone recovery.

Evolution of total column ozone prior to the era of ozone depletion

“…I re-assessed the homogenized series, as is further described in Section 2.5. This collection was complemented by recently digitized segments from Oxford (13 further monthly means, Brönnimann 2022), Wellington, NZ (86 months), and Downham Market (11 months, Bröninmann and Nichol, 2020). Here, I also considered data from the Smithsonian Institution from Montezuma (Angione and Roosen, 1983), although these are clearly of lower quality.…”

“…In order to calculate a monthly mean, it required 10 days with measurements, as in HISTOZ. This accounts for the fact that total column ozone could not be measured under all weather conditions but retains sufficient information for a monthly mean (Brönnimann, 2022). In particular, for the series from the Smithsonian Institution, with often only weekly measurements, this led to a considerable reduction of data points.…”

“…Trends in the pre-CFC era have not been addressed systematically in these records. Brönnimann (2022); analyzing Oxford and Arosa, found an increase of ca. 2 DU over the 1926-1970 period.…”

“…Regular total column ozone measurements began in 1924, when Dobson started the observations in Oxford, which he continued for more than half a century, albeit with gaps (Dobson and Harrison, 1926). There are only few long, homogenized series, namely, Arosa (Staehelin et al, 1998), Oxford (Brönnimann, 2022), and Tromsø (Hansen and Svenøe, 2005). Many total column ozone series began in 1957 in the context of the International Geophysical Year (IGY), which, however, cover only little of the pre-CFC era.…”

Tropospheric ozone data assimilation in the NASA GEOS Composition Forecast modeling system (GEOS-CF v2.0) using satellite data for ozone vertical profiles (MLS), total ozone columns (OMI), and thermal infrared radiances (AIRS, IASI)

“…Tropospheric ozone is the most measured of all atmospheric trace gases with thousands of surface network sites around the world, regular sonde and aircraft profiles, multiple observing satellites from space, and a long history of observational records dating back to the 19th century (Schultz et al 2017, Archibald et al 2020, Brönnimann 2022). Yet there are large uncertainties in the interpretation of ozone trends, and consistency between different observational data sets is often unclear (Christiansen et al 2022).…”