First ground-based Fourier transform infrared (FTIR) spectrometer observations of HFC-23 at Rikubetsu, Japan, and Syowa Station, Antarctica

We have developed a procedure for retrieving atmospheric abundances of HFC-23 (CHF3) with a ground-based Fourier transform infrared (FTIR) spectrometer and analyzed the spectra observed at Rikubetsu, Japan (43.5∘ N, 143.8∘ E), and at Syowa Station, Antarctica (69.0∘ S, 39.6∘ E). The FTIR retrievals were carried out with the SFIT4 retrieval program, and the two spectral windows of 1138.5–1148.0 cm−1 and 1154.0–1160.0 cm−1 in the overlapping ν2 and ν5 vibrational–rotational transition bands of HFC-23 were used to avoid strong H2O absorption features. We considered O3, N2O, CH4, H2O, HDO, CFC-12 (CCl2F2), HCFC-22 (CHClF2), peroxyacetyl nitrate (PAN) (CH3C(O)OONO2), HCFC-141b (CH3CCl2F), and HCFC-142b (CH3CClF2) to be interfering species. Vertical profiles of H2O, HDO, and CH4 are preliminarily retrieved with other independent spectral windows because these profiles may induce large uncertainties in the HFC-23 retrieval. Each HFC-23 retrieval has only one piece of vertical information with sensitivity to HFC-23 in the troposphere and the lower stratosphere. Retrieval errors mainly arise from the systematic uncertainties of the spectroscopic parameters used to obtain HFC-23, H2O, HDO, and CH4 abundances. For comparison between FTIR-retrieved HFC-23 total columns and surface dry-air mole fractions provided by AGAGE (Advanced Global Atmospheric Gases Experiment), FTIR-retrieved HFC-23 dry-air column-averaged mole fractions (XHFC-23) were calculated. The FTIR-retrieved XHFC-23 values at Rikubetsu and Syowa Station have negative biases of −15 % to −20 % and −25 % compared to the AGAGE datasets, respectively. These negative biases might mainly come from systematic uncertainties of HFC-23 spectroscopic parameters. The trend of the FTIR-retrieved XHFC-23 data at Rikubetsu was derived for December to February (DJF) observations, which are considered to represent the background values when an air mass reaching Rikubetsu has the least influence by transport of HFC-23 emissions from nearby countries. The DJF trend of Rikubetsu over the 1997–2009 period is 0.810 ± 0.093 ppt yr−1 (ppt: parts per trillion), which is in good agreement with the trend derived from the annual global mean datasets of the AGAGE 12-box model for the same period (0.820 ± 0.013 ppt yr−1). The DJF trend of Rikubetsu over the 2008–2019 period is 0.928 ± 0.108 ppt yr−1, which is consistent with the trend in the AGAGE in situ measurements at Trinidad Head (41.1∘ N, 124.2∘ W) for the same period (0.994 ± 0.001 ppt yr−1). The trend of the FTIR-retrieved XHFC-23 data at Syowa Station over the 2007–2016 period is 0.819 ± 0.071 ppt yr−1, which is consistent with that derived from the AGAGE in situ measurements at Cape Grim (40.7∘ S, 144.7∘ E) for the same period (0.874 ± 0.002 ppt yr−1). Although there are systematic biases in the FTIR-retrieved XHFC-23 at both sites, these results indicate that ground-based FTIR observations have the capability to monitor the long-term trend of atmospheric HFC-23. If this FTIR measurement technique were extended to other Network for the Detection of Atmospheric Composition Change (NDACC) ground-based FTIR sites around world, the measurements reported from these sites would complement the global AGAGE observations by filling spatial and temporal gaps and may lead to improved insights about changes in regional and global emissions of HFC-23 and its role in global warming.

Characterization of aerosol number size distributions and their effect on cloud properties at Syowa Station, Antarctica

We took aerosol measurements at Syowa Station, Antarctica, to characterize the aerosol number–size distribution and other aerosol physicochemical properties in 2004–2006. Four modal structures (i.e., mono-, bi-, tri-, and quad-modal) were identified in aerosol size distributions during measurements. Particularly, tri-modal and quad-modal structures were associated closely with new particle formation (NPF). To elucidate where NPF proceeds in the Antarctic, we compared the aerosol size distributions and modal structures to air mass origins computed using backward trajectory analysis. Results of this comparison imply that aerosol size distributions involved with fresh NPF (quad-modal distributions) were observed in coastal and continental free troposphere (FT; 12 % of days) areas and marine and coastal boundary layers (1 %) during September–October and March and in coastal and continental FT (3 %) areas and marine and coastal boundary layers (8 %) during December–February. Photochemical gaseous products, coupled with ultraviolet (UV) radiation, play an important role in NPF, even in the Antarctic troposphere. With the existence of the ozone hole in the Antarctic stratosphere, more UV radiation can enhance atmospheric chemistry, even near the surface in the Antarctic. However, linkage among tropospheric aerosols in the Antarctic, ozone hole, and UV enhancement is unknown. Results demonstrated that NPF started in the Antarctic FT already at the end of August–early September by UV enhancement resulting from the ozone hole. Then, aerosol particles supplied from NPF during periods when the ozone hole appeared to grow gradually by vapor condensation, suggesting modification of aerosol properties such as number concentrations and size distributions in the Antarctic troposphere during summer. Here, we assess the hypothesis that UV enhancement in the upper troposphere by the Antarctic ozone hole modifies the aerosol population, aerosol size distribution, cloud condensation nuclei capabilities, and cloud properties in Antarctic regions during summer.

A new cosmic ray observation at Syowa Station in the antarctic

A set of Cosmic Ray detectors was newly installed in Syowa Station, in the Antarctic, to observe CR neutrons and muons simultaneously at the same location. The observing system has started working in February 2018 and is in stable operation with a high operation rate, >90%. We describe the new systems and show its stability.

Dynamical analysis of extreme tropopause folding events in the coastal region of Antarctica

<p>Rapid and deep descent in the tropopause (the so-called tropopause folding; TF) is often observed in the extratropics. Previous studies pointed out that the frequency of deep TF is maximized along the coast of Antarctica. However, the dynamics of TF in the Antarctic region have not yet been studied adequately. In the present study, the extreme TF in the Antarctic are examined using the state-of-art reanalysis data to clarify the uniqueness of TF in the Antarctic.</p><p>First, the distribution of TF frequency in the extra-tropics of the Southern hemisphere is examined. In austral winter, extreme TF often occurs along the coast of Antarctica. Around Syowa Station (69.0S, 39.6E), the frequency of extreme TF is maximized in August while the frequency is small in austral summer. It is interesting that the coast of Antarctica is located to the south of the maximum of the synoptic-scale eddy kinetic energy. This implies that the maximum of TF frequency along the coast of Antarctica cannot be explained only by the geographical distribution of the storm track.</p><p>Next, to examine the dynamics of the extreme TF events along the coast of Antarctica, we perform composite analyses of the extreme TF events at Syowa Station. When the negative anomaly of tropopause height is maximized, the significant downwelling is observed at the location of the extreme TF. From the analyses using the quasi-geostrophic Q-vector, it is found that the divergence of the Q-vector is observed around Syowa Station. The distribution of Q-vector is explained by the local westerly jet and strengthening of the frontal structure associated with a synoptic low-pressure system extending west-east centered at 70°S over Antarctica. The mechanism of the low-pressure system extending along the coast of Antarctica based on ray-tracing theory under the WKB approximation is also discussed.</p>

D-region impact area of energetic electron precipitation during pulsating aurora

A total of 10 radars from the Super Dual Auroral Radar Network (SuperDARN) in Antarctica were used to estimate the spatial area over which energetic electron precipitation (EEP) impacts the D-region ionosphere during pulsating aurora (PsA) events. We use an all-sky camera (ASC) located at Syowa Station to confirm the presence of optical PsAs, and then we use the SuperDARN radars to detect high frequency (HF) radio attenuation caused by enhanced ionisation in the D-region ionosphere. The HF radio attenuation was identified visually by examining quick-look plots of the background HF radio noise and backscatter power from each radar. The EEP impact area was determined for 74 PsA events. Approximately one-third of these events have an EEP impact area that covers at least 12∘ of magnetic latitude, and three-quarters cover at least 4∘ of magnetic latitude. At the equatorward edge of the auroral oval, 44 % of events have a magnetic local time extent of at least 7 h, but this reduces to 17 % at the poleward edge. We use these results to estimate the average size of the EEP impact area during PsAs, which could be used as a model input for determining the impact of PsA-related EEP on the atmospheric chemistry.

Characterization of aerosol number size distributions and their effect on cloud properties at Syowa Station, Antarctica

We took aerosol measurements at Syowa Station, Antarctica to characterize the aerosol number–size distribution and other aerosol physicochemical properties. Four modal structures (i.e., mono-, bi-, tri-, and quad-modal) were identified in aerosol size distributions during measurements. Particularly, quad-modal structures were associated closely with new particle formation (NPF). To elucidate where NPF proceeds in the Antarctic, we compared the aerosol size distributions and modal structure to air mass origins computed using backward trajectory analysis. Results of this comparison imply that NPF occurred in free troposphere during spring and autumn, and in the free troposphere and boundary layer during summer. Photochemical gaseous products, coupled with UV radiation, play an important role in NPF, even in the Antarctic troposphere. With the appearance of the ozone hole in the Antarctic stratosphere, more UV radiation can enhance atmospheric chemistry, even near the surface in the Antarctic. However, linkage among tropospheric aerosols in the Antarctic, ozone hole, and UV enhancement is unknown. Results demonstrated that NPF started in the Antarctic free troposphere already in the end-August – early September by UV enhancement resulting from the ozone hole. Then, aerosol particles supplied from NPF during spring grow gradually by vapor condensation, suggesting modification of aerosol properties such as number concentrations and size distributions in the Antarctic troposphere during summer. Here, we assess the hypothesis that UV enhancement in the upper troposphere by the Antarctic ozone hole modifies the aerosol population, aerosol size distribution, cloud condensation nuclei capabilities, and cloud properties in Antarctic regions during summer.

First ground-based FTIR observations of HFC-23 at Rikubetsu, Japan, and Syowa Station, Antarctica

We have developed a procedure for retrieving atmospheric abundances of HFC-23 (CHF3) with a ground-based Fourier transform infrared spectrometer (FTIR) and analysed the spectra observed at Rikubetsu, Japan (43.5° N, 143.8° E), and at Syowa Station, Antarctica (69.0° S, 39.6° E). The FTIR retrievals were carried out with the SFIT4 retrieval program, and the two spectral windows of 1138.5–1148.0 cm−1 and 1154.0–1160.0 cm−1 in the overlapping ν2 and ν5 vibrational-rotational transition bands of HFC-23 were used to avoid strong H2O absorption features. We considered O3, N2O, CH4, H2O, HDO, CFC-12 (CCl2F2), HCFC-22 (CHClF2), PAN (CH3C(O)OONO2), HCFC-141b (CH3CCl2F), and HCFC-142b (CH3CClF2) as interfering species. Vertical profiles of H2O, HDO, and CH4 are preliminarily retrieved with other independent spectral windows because these profiles may induce large uncertainties in the HFC-23 retrieval. Each HFC-23 retrieval has only one piece of vertical information with sensitivity to HFC-23 in the troposphere and the lower stratosphere. The retrieval errors mainly arise from the systematic uncertainties of the spectroscopic parameters used to obtain the HFC-23, H2O, HDO, and CH4 abundances. For comparison between FTIR-retrieved HFC-23 total columns and surface dry-air mole fractions provided by AGAGE (Advanced Global Atmospheric Gases Experiment), the FTIR-retrieved HFC-23 dry-air column-averaged mole fractions (XHFC-23) were calculated. The FTIR-retrieved XHFC-23 at Rikubetsu and Syowa Station have negative biases compared to AGAGE datasets. The trend derived from the FTIR-retrieved XHFC-23 data at Rikubetsu for December to February (DJF) data over the 1997–2010 period is 0.817 ± 0.087 ppt (parts per trillion) year−1, which is in good agreement with the trend derived from the annual global mean datasets of the AGAGE 12-box model for the same period (0.820 ± 0.011 ppt year−1). The trend of the FTIR-retrieved XHFC-23 data at Rikubetsu for DJF data over the 2007–2020 period is 0.894 ± 0.099 ppt year−1, which is smaller than the trend in the AGAGE in-situ measurements at Trinidad Head (41.1° N, 124.2° W) for the 2007–2019 period (0.984 ± 0.002 ppt year−1). The trend computed from the XHFC-23 datasets at Syowa Station over the 2007–2016 period is 0.823 ± 0.075 ppt year−1, which is consistent with that derived from the AGAGE in-situ measurements at Cape Grim (40.7° S, 144.7° E) for the same period (0.874 ± 0.002 ppt year−1). Although there are systematic biases on the FTIR-retrieved XHFC-23 at both sites, these results indicate that ground-based FTIR observations have the capability to monitor the trend of atmospheric HFC-23.