11 research outputs found
Total ozone variability and trends over the South Pole during the wintertime
The Antarctic polar vortex creates unique chemical and dynamical conditions when the stratospheric air over Antarctica is isolated from the rest of the stratosphere. As a result, stratospheric ozone within the vortex remains largely unchanged for a 5-month period from April until late August when the sunrise and extremely cold temperatures create favorable conditions for rapid ozone loss. Such prolonged stable conditions within the vortex make it possible to estimate the total ozone levels there from sparse wintertime ozone observations at the South Pole. The available records of focused Moon (FM) observations by Dobson and Brewer spectrophotometers at the Amundsen–Scott South Pole Station (for the periods 1964–2022 and 2008–2022, respectively) as well as integrated ozonesonde profiles (1986–2022) and MERRA-2 reanalysis data (1980–2022) were used to estimate the total ozone variability and long-term changes over the South Pole. Comparisons with MERRA-2 reanalysis data for the period 1980–2022 demonstrated that the uncertainties of Dobson and Brewer daily mean FM
values are about 2.5 %–4 %. Wintertime (April–August) MERRA-2 data have a bias with Dobson data of −8.5 % in 1980–2004 and 1.5 % in 2005–2022. The mean difference between wintertime Dobson and Brewer data in 2008–2022 was about 1.6 %; however, this difference can be largely explained by various systematic errors in Brewer data. The wintertime ozone values over the South Pole during the last 20 years were about 12 % below the pre-1980s level; i.e., the decline there was nearly twice as large as that over southern midlatitudes. It is probably the largest long-term ozone
decline aside from the springtime Antarctic ozone depletion. While wintertime ozone decline over the pole has hardly any impact on the environment, it can be used as an indicator to diagnose the state of the
ozone layer, particularly because it requires data from only one station.
Dobson and ozonesonde data after 2001 show a small positive, but not statistically significant, trend in ozone values of about 1.5 % per decade that is in line with the trend expected from the concentration of the ozone-depleting substances in the stratosphere.</p
The site-specific primary calibration conditions for the Brewer spectrophotometer
The Brewer ozone spectrophotometer (the Brewer) is one of the
World Meteorological Organization (WMO) Global Atmosphere Watch (GAW)'s
standard ozone-monitoring instruments since the 1980s. The entire global
Brewer ozone-monitoring network is operated and maintained via a
hierarchical calibration chain, which started from world reference
instruments that are independently calibrated via the primary calibration
method (PCM) at a premium site (National Oceanic and Atmospheric
Administration's (NOAA) Mauna Loa Observatory, Hawaii). These world
reference instruments have been maintained by Environment and Climate Change
Canada (ECCC) in Toronto for the last 4 decades. Their calibration is
transferred to the travelling standard instrument and then to network
(field) Brewer instruments at their monitoring sites (all via the
calibration transfer method; CTM). Thus, the measurement accuracy for the
entire global network is dependent on the calibration of world reference
instruments. In 2003, to coordinate regional calibration needs, the Regional
Brewer Calibration Center for Europe (RBCC-E) was formed in Izaña, Spain.
From that point, RBCC-E began calibrating regional references also via PCM
instead of CTM. The equivalency and consistency of world and regional
references are then assured during international calibration campaigns. In
practice, these two calibration methods have different physical
requirements, e.g., the PCM requires a stable ozone field in the short term
(i.e., half-day), while the CTM would benefit from larger changes in slant ozone
conditions for the calibration periods. This difference dictates that the
PCM can only be implemented on Brewer instruments at certain sites and even in certain
months of the year. This work is the first effort to use long-term
observation records from 11 Brewer instruments at four sites to reveal the challenges
in performing the PCM. By utilizing a new calibration simulation model and
reanalysis ozone data, this work also quantifies uncertainties in the PCM
due to short-term ozone variability. The results are validated by real-world
observations and used to provide scientific advice on where and when the PCM
can be performed and how many days of observations are needed to achieve the
calibration goal (i.e., ensure the calibration uncertainty is within a
determined criterion, i.e., ≤5 R6 units; R6 is a measurement-derived
double ratio in the actual Brewer processing algorithm). This work also
suggests that even if the PCM cannot be used to deliver final calibration
results for mid- or high-latitude sites, the statistics of the long-term PCM
fitting results can still provide key information for field Brewer instruments as
stability indicators (which would provide performance monitoring and data
quality assurance).</p