Observation of large and all-season ozone losses over the tropics featured. Qing-Bin Lua. AIP Advances 12, 075006 (Jul 5 2022); https://doi.org/10.1063/5.0094629
Abstract: This paper reveals a large and all-season ozone hole in the lower stratosphere over the tropics (30°N–30°S) existing since the 1980s, where an O3 hole is defined as an area of O3 loss larger than 25% compared with the undisturbed atmosphere. The depth of this tropical O3 hole is comparable to that of the well-known springtime Antarctic O3 hole, whereas its area is about seven times that of the latter. Similar to the Antarctic O3 hole, approximately 80% of the normal O3 value is depleted at the center of the tropical O3 hole. The results strongly indicate that both Antarctic and tropical O3 holes must arise from an identical physical mechanism, for which the cosmic-ray-driven electron reaction model shows good agreement with observations. The whole-year large tropical O3 hole could cause a great global concern as it can lead to increases in ground-level ultraviolet radiation and affect 50% of the Earth’s surface area, which is home to approximately 50% of the world’s population. Moreover, the presence of the tropical and polar O3 holes is equivalent to the formation of three “temperature holes” observed in the stratosphere. These findings will have significances in understanding planetary physics, ozone depletion, climate change, and human health.
The present finding of a tropical ozone hole is closest to the observation by Polvani
et al.26 of large O
3 losses at the altitude of 18.5 km or 67/68 hPa over the tropics (30°S–30°N) from 1979 to 1997 with data from three independent datasets (TOST, BDBP, and GOZCARDS)
28–30 although they did not reveal the tropical O
3 hole. Polvani
et al.26 also showed that global and tropical LST cooling had disappeared since 1997, while the tropical O
3 concentration reached the minimum around 2005. When explaining their observed results, however, Polvani
et al. argued for the low abundance of active chlorine and hence no local chemical O
3 destruction in the tropical lower stratosphere. Instead, they argued that the observed tropical O
3 and cooling trends were primarily driven by
tropical upwelling caused by ODSs rather than greenhouse gases (GHGs) (mainly non-halogenated GHGs, as widely believed). As we have noted in the Introduction and will further discuss later, however, even a low level of active halogen can cause significant ozone depletion in the tropical lower stratosphere.
Interestingly, Polvani
et al.26 also performed simulations from a chemistry–climate model (CCM) with incrementally added single forcings (sea surface temperatures—SSTs, GHGs, ODSs, volcanic eruptions, and solar variations) to detail the contribution of each forcing to tropical ozone and LSTs. Although their simulated results showed that ODSs were the dominant forcing of tropical ozone loss over GHGs, it must be pointed out that their simulated values of sum ozone loss (−28 ± 13 ppbv per decade; see their Table 1) were about
five times smaller than their observed results (∼−150 ppbv per decade for the 1980s and 1990s), even ignoring that not all individual ensemble members showed statistically significant trends (see their Table 2). Moreover, in contrast to their claim that tropical lower stratospheric ozone would be closely tied to tropical upwelling w*, their simulated value of the w* increase by ODSs is
not dominant but very close to that by GHGs, each force contributing to an increment by ∼0.1 km yr
−1 decade
−1 at 85 hPa, namely, 0.04 ± 0.09 for SSTs, 0.10 ± 0.11 for SSTs + GHGs, and 0.21 ± 0.11 for SSTs + GHGs + ODSs (see their Table 1). The results of the latter were also consistent with their simulated results of tropical LST trends (see their
Fig. 3).
The simulated results of CCMs by Randel and Thompson
37 and others
38 had some differences from but were overall similar to the above-mentioned CCM results by Polvani
et al.26 In most simulated results of CCMs, the strength of tropical upwelling was projected to increase from 1960 to 2100 by ∼
2% per decade with the largest trends occurring in JJA, corresponding to tropical O
3 reductions at 50 hPa of 0.15–0.35 ppmv (11–25 ppbv
per decade).
38 This ozone loss trend resulting from CCM simulations is
about 10
times less than the observations by Polvani
et al.26 and the present observations shown in
Figs. 1–
5. More crucially, the observed data have robustly shown that significant tropical ozone loss and LST cooling occurred
in the 1980s and 1990s only, which is in drastic discrepancy from the simulated results of CCMs. Polvani
et al.26 were then led to the open question: How could ODSs affect the stratospheric circulation? They conceded that the underlying mechanism for ODSs being a key forcing for tropical lower stratospheric O
3 and temperature trends remained largely unexplored. Knowing the wide belief in CCMs that the key drivers of tropical upwelling and thus tropical O
3 or LST trends since 1960 are non-halogenated GHGs (mainly CO
2), Polvani
et al.26 were forced to suggest that polar O
3 depletion caused by ODSs would cause tropical upwelling and hence large tropical O
3 losses. This explanation cannot be correct either, as the polar O
3 hole is seasonal and appears only in the springtime, whereas the tropical O
3 hole is all-season and has no changes in its central location over the seasons and over the decades since its appearance in the 1980s [
Figs. 1–
3 and
4(e)].
The present observed results in
Figs. 1–
6 and Figs. S1–S4 strongly indicate that, like the Antarctic O
3 hole that was once incorrectly explained by the misconceived air transport mechanism (“dynamical theory”), the tropical O
3 hole must not result from changes in normal atmospheric circulation patterns over the tropics since the 1960s or 1970s but result from an identical physical/chemical mechanism to that for the polar O
3 hole. Obviously, the tropical O
3 hole varies closely with the atmospheric level of CFCs [as seen in
Fig. 4(b)], so it must originate from a CFC-related mechanism. The postulated stratospheric cooling and tropical upwelling effects of increasing non-halogenated GHGs have disappeared in observed O
3 and temperature data for the Antarctic lower stratosphere
6–8 and for the tropical lower stratosphere.
26 It is obvious that the simulated results from CCMs
26,37,38 do not agree with the observed results shown in
Figs. 4(b),
5(a),
5(b), and
6(f), which show that the negative O
3 trends were about 10 times larger (−25
to −30%
per decade) in the 1980s and 1990s and there have been
no significant O
3 or LST trends in the tropical/Antarctic since the mid-1990s. The latter are actually consistent with the observations summarized in the newest IPCC report (Chap. 2).
36 Moreover, the proposed
enhanced tropical upwelling directly contradicts with the observed CFC depletion in the lower tropical stratosphere [
Fig. 4(d)], as increased upward motion would transport CFC-rich air from the troposphere. Indeed, the observed data robustly show no shifts in the positions of both Antarctic and tropical O
3 holes that have constantly been centered in the altitude region corresponding to the CR ionization peak since the 1960s/1980s and circularly symmetric O
3 depletion cyclones are formed with the largest depletion at the centers [
Figs. 1–
3 and
4(e)]. These major features cannot be explained by tropical upwelling due to non-halogenated GHGs (mainly CO
2) that have kept rising since the industrial revolution starting in 1760. All the observed data strongly indicate that tropical upwelling cannot be the major mechanism for the observed large, deep, and all-season tropical O
3 hole. The simultaneous depletions of both CFCs and O
3 in the lower tropical stratosphere are most likely due to a physical reaction mechanism that occurs locally. For the latter, the CRE mechanism, supported by the observed data in
Figs. 1–
6 and the substantial datasets obtained from both laboratory and atmospheric measurements,
3–9,39–41 has provided the best and predictive model.
It is well known that the presence of PSCs is crucial for the formation of the Antarctic O
3 hole.
42–45 It was proposed that on the surfaces of PSCs, chlorine reservoir molecules (HCl and ClONO
2) are converted into photoactive forms (Cl
2) that can then undergo photolysis to destroy O
3. There are two types of PSCs, namely, Type I and Type II PSC. The composition of Type II PSC is water ice, while Type I PSC is composed of mixtures of nitric acid (HNO
3), water vapor (H
2O), and sulfuric acid (H
2SO
4). The temperatures required for the formation of Type I and II PSCs are 195 and 188 K, respectively. Thus, it is very likely that TSCs, at least Type I PSC-like TSCs, can also form in the tropical lower stratosphere over the seasons due to the observed low temperatures of 190–200 K [Figs. S4 and
Fig. 4(f)]. CRs may also play a certain role in forming PSCs and PSC-like TSCs.
16,19 Note that the tropical lower stratosphere is very different from the polar lower stratosphere in both composition and climate. The former is rich in CFCs and other halogen-containing gases, whereas the latter are composed of inorganic chlorine species and lower-level CFCs. However, the CRE mechanism put forward two decades ago has proposed that O
3-depleting reactions of both CFCs and inorganic halogen species can effectively occur on the surfaces of PSCs.
3–9,39–41 Therefore, there are required and sufficient conditions for O
3-depleting reactions occurring on the surfaces of proposed PSC-like TSCs in the tropical lower stratosphere. As noted in the Introduction, the constant co-presence of low-temperature TSCs and intense sunlight should lead to a unique active halogen evolution in the tropical lower stratosphere, in which halogen-catalyzed reactions are much more efficient for O
3 destruction than those in the polar lower stratosphere.
The tropics (30°N–30°S) constitutes 50% of the Earth’s surface area, which is home to about 50% of the world’s population. O
3 depletion in the tropics could cause a great global concern. In areas where O
3 depletion is observed to be smaller in absolute O
3 value, UV-B increases are more difficult to detect as the detection can be complicated by changes in cloudiness, local pollution, and other difficulties. However, it is generally agreed that the depletion of the O
3 layer leads to an increase in ground-level UV radiation because ozone is an effective absorber of solar UV radiation. Exposure to enhanced UV-B levels could increase the incidence of skin cancer and cataracts in humans, weaken human immune systems, decrease agricultural productivity, and negatively affect sensitive aquatic organisms and ecosystems.
46 Indeed, there was a report called HIPERION published by the Ecuadorian Space Agency in 2008.
47 The study using ground measurements in Ecuador and satellite data for several countries over 28 years found that the UV radiation reaching equatorial latitudes was far greater than expected, with the UV index as high as 24 in Quito. This Ecuadorian report concluded that O
3 depletion levels over equatorial regions are already endangering large populations in the regions. Further delicate studies of O
3 depletion, UV radiation change, increased cancer risks, and other negative effects on health and ecosystems in the tropical regions will be of great interest and significance.
Another important result is that the global lower stratospheric temperature is essentially governed by the O
3 layer, which is expected as ozone is the main and dominant molecule that absorbs solar radiation in the stratosphere. As a result, the presence of the tropical and polar O
3 holes will play a major role in stratospheric cooling and regulating the global lower stratospheric temperature, as seen previously
6–9,26 and in the results shown in Fig. S4 and
Figs. 4(f) and
6(e)–
6(f). As seen in Fig. S4 and
Fig. 4(f), this is equivalent to the formation of three “temperature holes” in the stratosphere, corresponding to the Antarctic, tropical, and Arctic O
3 holes, respectively. This interesting result will be further explored in a subsequent paper.