Antarctic ozone depletion due to Erebus volcano gas emissions
New Antarctic study shows hydrogen chloride and sulphur dioxide emitted by Erebus volcano are a significant factor in ozone depletion. Erebus volcanic gases reach the ozone layer via cyclones and the polar vortex. High Erebus volcanic activity in the early 1980s resulted in the ozone hole area increase.
Abstract
Heterogeneous chemical reactions releasing photochemically active molecular chlorine play a key role in Antarctic stratospheric ozone destruction, resulting in the Antarctic ozone hole. Hydrogen chloride (HCl) is one of the principal components in these reactions on the surfaces of polar stratospheric clouds (PSCs). PSCs form during polar nights at extremely low temperatures (lower than −78 °C) mainly on sulfuric acid (H2SO4) aerosols, acting as condensation nuclei and formed from sulfur dioxide (SO2). However, the cause of HCl and H2SO4 high concentrations in the Antarctic stratosphere, leading to considerable springtime ozone depletion, is still not clear. Based on the NCEP/NCAR reanalysis data over the last 35 years and by using the NOAA HYSPLIT trajectory model, we show that Erebus volcano gas emissions (including HCl and SO2) can reach the Antarctic stratosphere via high-latitude cyclones with the annual average probability P¯ann. of at least ∼0.235 (23.5{154653b9ea5f83bbbf00f55de12e21cba2da5b4b158a426ee0e27ae0c1b44117}). Depending on Erebus activity, this corresponds to additional annual stratospheric HCl mass of 1.0–14.3 kilotons (kt) and SO2 mass of 1.4–19.7 kt. Thus, Erebus volcano is the natural and powerful source of additional stratospheric HCl and SO2, and hence, the cause of the Antarctic ozone depletion, together with man-made chlorofluorocarbons.
1. Introduction
The ozone layer is known to absorb the bulk of solar ultraviolet B (UVB) rays, i.e. only a small part of UVB reaches the Earth’s surface, and therefore, it protects Earth’s biological systems from this dangerous radiation (Stolarski et al., 1992; Zerefos et al., 1997). However, this layer is depleted due to various reasons, especially over Antarctica. Based on ozone observations in 1982 at Syowa station (69°00′ S, 39°35′ E) in Antarctica, Chubachi (1984) revealed the smallest value of total ozone since 1966. Soon after, based on the Halley Bay station (75°35′ S, 26°34′ W) data, Farman et al. (1985) revealed a smooth decrease since 1972 and a considerable depletion in the early 1980’s in the total ozone also over Antarctica. The ozone depletion was attributed to man-made chlorofluorocarbons (CFCs) and the region, wherein the total ozone value is less than 220 Dobson Units (DU), was called later the “ozone hole”. For more than twenty years the springtime ozone hole area has exceeded 20 million km2 and spread over biologically-rich Antarctic waters. As a consequence, enhanced solar UVB radiation adversely affects Antarctic marine ecosystems and leads to a reduction in bioresources (diatoms, phytoplankton, etc.) (Smith et al., 1992; Kondratyev and Varotsos, 1996, 2000; Karentz and Bosch, 2001).
Solomon et al. (1986) proposed the key chemical processes and catalytic cycles describing the Antarctic springtime ozone depletion. Heterogeneous reactions, occurring on the surfaces of polar stratospheric clouds (PSCs) of both types I and II (Rex et al., 1998; Finlayson-Pitts and Pitts, 2000) and releasing photochemically active molecular chlorine (Cl2), play a key role in these processes. PSCs form during winter–spring periods in Antarctica at extremely low stratospheric temperatures (lower than −78 °C) on sulfuric acid (H2SO4) aerosols, acting as condensation nuclei and formed from sulfur dioxide (SO2) (Solomon et al., 2005; Finlayson-Pitts and Pitts, 2000). The release of Cl2 occurs mainly in the reactions of chlorine nitrate (ClONO2) with hydrogen chloride (HCl) (Solomon et al., 1986; Solomon, 1999):
or
Subsequently, the Cl2 is photolyzed into two chlorine atoms (Cl) by weak solar radiation
where hv = hc/λ is the photon energy required to break a chemical bond, λ is the wavelength, and h and c are the Planck constant and speed of light, respectively. Ozone molecules are destroyed by Cl in the catalytic reaction:
Generally, ozone holes appear over Antarctica in springtime due to the following factors: 1) winter–spring formation of a stable polar vortex, which isolates the Antarctic stratosphere and cools it to extremely low temperatures (lower than −78 °C); 2) PSCs formation on condensation nuclei (H2SO4 aerosols) at these temperatures; and 3) ozone destruction in reactions (1)–(5) (Finlayson-Pitts and Pitts, 2000; Newman, 2010).
CFCs are assumed to be the main source of inert chlorine reservoir molecules HCl and ClONO2. After entering the equatorial (tropical) stratosphere, the CFCs are photolyzed by UV radiation, releasing Cl (Newman, 2010). In the middle and upper stratosphere, Cl atoms are converted into HCl via
and chlorine monoxide (ClO) radicals produced in reaction (5) are converted into ClONO2 via
where M is a third body (Newman, 2010). These HCl and ClONO2 molecules are then transported to both the Arctic and Antarctic polar stratospheres via the Brewer–Dobson circulation.
However, we revealed that HCl vertical column density (VCD) over the Arrival Heights station in Antarctica is considerably higher than that observed over other Earth’s regions including Arctic one, whereas ClONO2 is distributed homogeneously enough in the Earth’s stratosphere. Moreover, we also revealed that concentration of H2SO4aerosols formed from SO2 is higher in the Antarctic stratosphere in comparison with that in the Arctic one (see Section 3.1). These facts cannot be explained by only the Brewer–Dobson circulation and are indicative of an effective source of HCl and SO2within the Antarctic continent. Thus, the aims of the present work are: 1) to identify the source of HCl and SO2 in Antarctica; 2) to determine the delivery mechanisms of HCl and SO2 from the source to the Antarctic stratosphere; and 3) to estimate the amount of additional annual HCl and SO2 masses reaching the stratosphere.
2. Data and methods
We used the following databases in this study. The information on the global distribution of the vertical column densities of HCl and ClONO2 over different stations, as well as on the aerosol backscattering coefficients over both the Arctic and Antarctic stations, is contained in the NOAA’s National Weather Service Network for the Detection of Atmospheric Composition Change (NDACC, http://www.ndsc.ncep.noaa.gov) online database. The total ozone data over Halley Bay station used in this study were taken from the World Ozone and Ultraviolet Data Center (WOUDC, http://www.woudc.org). The border of the ozone hole in 2014 and the ozone hole area (1979–2014) were determined based on the NASA’s Goddard Space Flight Center (GSFC, http://ozonewatch.gsfc.nasa.gov/SH.html) data. To determine the air-mass forward trajectories, we used the HYSPLIT-compatible NOAA meteorological data from GDAS (Global Data Assimilation System) half-degree archive. The geopotential heights of constant pressure surfaces were retrieved from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR, http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html) global reanalysis data (Kalnay et al., 1996).
All the air-mass forward trajectories started from the summit of Mount Erebus were calculated by using the NOAA’s Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model (Draxler and Hess, 1998; http://ready.arl.noaa.gov/HYSPLIT.php).
3. Results
3.1. HCl and SO2 source identification
The VCDs of HCl and ClONO2 correspond to their values in the stratosphere due to the following reasons. First, ClONO2-producing reaction (7) is not effective due to the very low amount of ClO molecules in the troposphere. Second, HCl is removed rather rapidly from the troposphere by wet scavenging (Marcy et al., 2004). Note that the gas-phase HCl is hydrophilic, and hence, has a low lifetime in the troposphere. The monthly average VCD values of HCl and ClONO2 over four middle- and high-latitude stations in both the Northern and Southern Hemispheres are presented in Fig. 1. ClONO2 is seen in Fig. 1 to be rather homogeneously distributed in the Earth’s stratosphere, whereas the VCD of HCl over the Arrival Heights station in Antarctica is about 1.5–2 times higher than that over other Earth’s regions including the Arctic one.
Fig. 1. The monthly average VCD values of HCl and ClONO2 over different stations. The HCl minimum values in both polar regions are caused by HCl adsorption on PSCs during winter–spring periods. There is no information on the VCD values for nighttime periods.
In addition to the high VCD of HCl, a higher concentration of aerosols is also observed in the Antarctic stratosphere in comparison with that in the Arctic one. Fig. 2 shows the vertical profiles of aerosol backscattering coefficients, retrieved from NDACC lidar data for the wavelength λ = 532 nm over the McMurdo station (77°51′ S, 166°40′ E) in Antarctica and the Ny-Alesund station (78°55′ N, 11°55′ E) in the Arctic. The profiles are averaged over March–April period (for Antarctica) and November (for the Arctic region) of 2001 and 2002. Since PSCs do not form in the polar stratosphere at relatively high temperatures during these months, the retrieved aerosol profiles mainly represent H2SO4 aerosol ones. As seen from Fig. 2, the stratospheric H2SO4 aerosol concentration over Antarctica was higher than that over the Arctic, even after an explosive eruption occurred on May 22, 2001 at Shiveluch volcano (56°39′ N, 161°22′ E, elevation 3307 m), whose eruption column reached an absolute height of 20 km and increased the aerosol concentration (GVP, 2001).
Fig. 2. Vertical profiles of aerosol backscattering coefficients averaged over March–April period (for the McMurdo station) and November (for the Ny-Alesund station) of 2001 and 2002.
These facts (presented in Figs. 1 and 2) lead us to a conclusion that there should be a powerful and permanent additional source of volcanic nature for HCl and SO2 within the Antarctic continent. Note that the high stratospheric H2SO4 aerosol concentration is usually related to oxidation of SO2 erupted into the stratosphere by volcanoes (Finlayson-Pitts and Pitts, 2000).
Erebus volcano (77°32′ S, 167°09′ E, summit elevation 3794 m) located on Ross Island, Ross Sea, is known to be the only burning volcano in Antarctica and one of the most active volcanoes on the Earth. The volcanic activity restarted in 1972 and is ongoing at the present time. At the beginning of the 1980s, the activity was extremely high, and therefore, degassing volumes were considerably higher compared to the present-day ones (Rose et al., 1985; Kyle et al., 1994; Zreda-Gostynska et al., 1993). Erebus volcano is noted for its persistent and permanent gas and aerosol emissions mostly occurring via lava lake degassing (Oppenheimer and Kyle, 2008). The predominant components of Erebus volcano gas emissions are H2O, CO2, CO, SO2, HF and HCl (Oppenheimer and Kyle, 2008). Note that Erebus gas emissions have high HCl/SO2 mass ratio of 0.28–0.92 (Zreda-Gostynska et al., 1993; Oppenheimer and Kyle, 2008; Wardell et al., 2008), one of the highest in the world (Boichu et al., 2011).
Erebus volcano eruptions are of the Strombolian type, which volcanic ejecta and gases are known to reach heights of 1–2 km above the volcano summit and, therefore, cannot directly reach the stratosphere (Boichu et al., 2010, 2011; Dibble et al., 2008). However, according to aircraft observations in 1989 at a height of 8 km over Antarctica, the detected aerosol particles were identified as volcanic ejecta of Erebus (Chuan, 1994). Together with HCl and H2SO4 high concentrations in the Antarctic stratosphere, it is indicative of delivery mechanisms of Erebus volcanic gases into the Antarctic stratosphere.
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