Stratospheric ozone, UV radiation, and climate interactions

Ozone in the stratosphere protects life on Earth by absorbing much of the ultraviolet solar radiation, as well as being a major source of ozone to the troposphere. A number of factors can influence stratospheric ozone levels and explain past, present and future changes. While there is a large literature exploring changes in stratospheric ozone due to, for example, ozone depleting substances (ODSs) and climate change, the study of its interactions with the rest of the Earth system is relatively recent. The work presented in this thesis investigates changes in stratospheric ozone and its links with other elements of the Earth system (tropospheric chemical composition and climate), using observations, a global chemistry-climate model (CESM1-WACCM), and existing multi-model output. This work evaluates past changes and explores future evolution of stratospheric ozone and associated climate impacts using multi-model output (ACCMIP simulations) from the pre-industrial period to the end of the 21st century. ACCMIP multi-model mean total column ozone trends compare favourably against observations. These models show a strong link between the Antarctic ozone hole and surface climate, which demonstrates that stratospheric ozone changes are coupled with the troposphere. A series of sensitivity simulations is conducted to investigate multi-decadal variability. A striking finding is that low frequency variability of ozone in the tropical middle stratosphere resembles multi-decadal climate variability in Pacific Ocean sea surface temperatures. This analysis also shows that internally generated climate variability is the leading factor explaining recent negative trends of mid-stratospheric tropical ozone. Finally, an additional set of sensitivity simulations is performed to quantify future radiative forcing between 2000s and 2100s that results from changes in ozone due to climate change, ODSs and methane concentrations. These results highlight the importance of stratosphere-troposphere exchange, as well as the key role of the stratosphere controlling the tropospheric ozone forcing.

Meteorological and dosimetric ultraviolet (UV) erythemal radiation (UVER) measurements were performed in Didcot, England, on 6 and 7 April 2017. Both days were characterized by clear-sky conditions in the morning and the afternoon with development of shallow stratocumulus clouds (SSC) around noon. In addition, a low-ozone event occurred on 7 April characterized by a 34 DU (Dobson Unit) drop in total stratospheric ozone content. Compared to 6 April, the ozone mini-hole caused UVER increases of 2.67 standard erythema dose (SED) for diffuse and 4.32 SED for global radiation characterized by radiation amplification factors (RAF) of 1.62 and 1.52, respectively. The total global UVER dose reductions due to SSC coverage amount to 2.33 SED (6 April) and 2.81 SED (7 April). As innovation the RAF is decomposed into two parts, named cloud ozone factor (COF) and radiation amplification factor based on measured data (RAFm), to quantify the low-ozone event's effect and the SSC influence in independently modifying the UVER doses. Hereby, the weight of each of these two effects acting during the same low-ozone event is expressed by the new COF. In this case, the COF values range between -0.13 and -0.11 for diffuse UVER and -0.03 to -0.07 for the global UV and UV-B parts. A positive COF value (0.18) results for the global UV-A range.

I Ozone Depletion and UV Radiation in the Arctic.- 1 Recent Changes in Surface UV Radiation and Stratospheric Ozone at a High Arctic Site.- 2 Spectral Properties and UV Attenuation in Arctic Marine Waters.- 3 Spectral Properties and UV Attenuation in Arctic Freshwater Systems.- 4 UV and the Optical Properties of Sea Ice and Snow.- II UV Radiation and Arctic Terrestrial Systems.- 5 Effects of UV-B Radiation on Terrestrial Organisms and Ecosystems with Special Reference to the Arctic.- 6 UV Tolerance, Pigmentation and Life Forms in High Arctic Collembola.- III UV Radiation and Arctic Freshwater Systems.- 7 UV Effects on Aquatic Microbial Food Webs in Northern Lakes and Rivers.- 8 UV Radiation and Arctic Freshwater Zooplankton.- 9 UV Radiation and the Pelagic Food Web of Arctic Freshwaters.- IV UV Radiation and Arctic Marine Systems.- 10 UV Radiation Effects on Phytoplankton Primary Production: A Comparison Between Arctic and Antarctic Marine Ecosystems.- 11 UV Radiation and Arctic Marine Macroalgae.- 12 UV Radiation and Pelagic Bacteria.- 13 Impacts of UV Radiation on Crustacean Zooplankton and Ichthyoplankton: Case Studies from Subarctic Marine Ecosystems.- V UV Radiation and the Arctic Human Population.- 14 Human Health and Arctic Ozone Depletion.

Exposure of plants and animals to ultraviolet-B radiation (UV-B; 280-315 nm) is modified by stratospheric ozone dynamics and climate change. Even though stabilisation and projected recovery of stratospheric ozone is expected to curtail future increases in UV-B radiation at the Earth's surface, on-going changes in climate are increasingly exposing plants and animals to novel combinations of UV-B radiation and other climate change factors (e.g., ultraviolet-A and visible radiation, water availability, temperature and elevated carbon dioxide). Climate change is also shifting vegetation cover, geographic ranges of species, and seasonal timing of development, which further modifies exposure to UV-B radiation. Since our last assessment, there has been increased understanding of the underlying mechanisms by which plants perceive UV-B radiation, eliciting changes in growth, development and tolerances of abiotic and biotic factors. However, major questions remain on how UV-B radiation is interacting with other climate change factors to modify the production and quality of crops, as well as important ecosystem processes such as plant and animal competition, pest-pathogen interactions, and the decomposition of dead plant matter (litter). In addition, stratospheric ozone depletion is directly contributing to climate change in the southern hemisphere, such that terrestrial ecosystems in this region are being exposed to altered patterns of precipitation, temperature and fire regimes as well as UV-B radiation. These ozone-driven changes in climate have been implicated in both increases and reductions in the growth, survival and reproduction of plants and animals in Antarctica, South America and New Zealand. In this assessment, we summarise advances in our knowledge of these and other linkages and effects, and identify uncertainties and knowledge gaps that limit our ability to fully evaluate the ecological consequences of these environmental changes on terrestrial ecosystems.

The Montreal Protocol is working. Concentrations of major ozone-depleting substances in the atmosphere are now decreasing, and the decline in total column amounts seen in the 1980s and 1990s at mid-latitudes has not continued. In polar regions, there is much greater natural variability. Each spring, large ozone holes continue to occur in Antarctica and less severe regions of depleted ozone continue to occur in the Arctic. There is evidence that some of these changes are driven by changes in atmospheric circulation rather than being solely attributable to reductions in ozone-depleting substances, which may indicate a linkage to climate change. Global ozone is still lower than in the 1970s and a return to that state is not expected for several decades. As changes in ozone impinge directly on UV radiation, elevated UV radiation due to reduced ozone is expected to continue over that period. Long-term changes in UV-B due to ozone depletion are difficult to verify through direct measurement, but there is strong evidence that UV-B irradiance increased over the period of ozone depletion. At unpolluted sites in the southern hemisphere, there is some evidence that UV-B irradiance has diminished since the late 1990s. The availability and temporal extent of UV data have improved, and we are now able to evaluate the changes in recent times compared with those estimated since the late 1920s, when ozone measurements first became available. The increases in UV-B irradiance over the latter part of the 20th century have been larger than the natural variability. There is increased evidence that aerosols have a larger effect on surface UV-B radiation than previously thought. At some sites in the Northern Hemisphere, UV-B irradiance may continue to increase because of continuing reductions in aerosol extinctions since the 1990s. Interactions between ozone depletion and climate change are complex and can be mediated through changes in chemistry, radiation, and atmospheric circulation patterns. The changes can be in both directions: ozone changes can affect climate, and climate change can affect ozone. The observational evidence suggests that stratospheric ozone (and therefore UV-B) has responded relatively quickly to changes in ozone-depleting substances, implying that climate interactions have not delayed this process. Model calculations predict that at mid-latitudes a return of ozone to pre-1980 levels is expected by the mid 21st century. However, it may take a decade or two longer in polar regions. Climate change can also affect UV radiation through changes in cloudiness and albedo, without involving ozone and since temperature changes over the 21st century are likely to be about 5 times greater than in the past century. This is likely to have significant effects on future cloud, aerosol and surface reflectivity. Consequently, unless strong mitigation measures are undertaken with respect to climate change, profound effects on the biosphere and on the solar UV radiation received at the Earth's surface can be anticipated. The future remains uncertain. Ozone is expected to increase slowly over the decades ahead, but it is not known whether ozone will return to higher levels, or lower levels, than those present prior to the onset of ozone depletion in the 1970s. There is even greater uncertainty about future UV radiation, since it will be additionally influenced by changes in aerosols and clouds.

Dynamical contributions to past long-term changes in the lower stratospheric ozone over the northern mid-latitudes are evaluated using a chemical transport model (CTM) forced by the horizontal wind of the Japanese 25-year Reanalysis (JRA-25). Two simulations (i.e., one is a simulation that prescribes the time-dependent vertical profile of halogens and the other is a simulation which uses the fixed vertical profile of halogens at 1979.) were conducted to estimate chemical and dynamical contributions to the long-term changes in stratospheric ozone during the last three decades. Different from previous similar studies using meteorological data of ECMWF (European Centre for Medium Weather Forecast) 40-year re-analysis (ERA-40), our current simulation does not show a large positive anomaly of simulated total ozone over northern mid-latitudes in the late 1980s, which is consistent with the observation. Because the trend of the fixed halogen simulation amounts to about two-third of that of the time-dependent halogen simulation during 1980–1993 in the northern mid-latitudes, it is evaluated that about two-thirds of the negative trend in total ozone comes from dynamics in the northern mid-latitudes. Since the increasing ozone from 1994 to 1998 is also represented in the fixed halogen simulation, it is considered that the increase of ozone was mainly due to dynamics as pointed out in previous studies. However the dynamical contribution to the trend after 1994 could not be evaluated in our simulation because of simulated ozone gap in 1998. In the same manner, it is evaluated that about two-thirds of the negative ozone trend in the lower stratosphere comes from dynamics in the northern mid-latitudes from 1980 to the mid-1990s. The simulation results indicate that the effect of transport (dynamical influence) is predominant for the negative ozone trend in the lower stratosphere from 1980 to mid-1990s, while the upper stratospheric ozone trend is strongly influenced by long-term changes in halogens (chemical influence).

Stratospheric ozone has begun to recover in Antarctica since the implementation of the Montreal Protocol. However, the effects of ultraviolet (UV) radiation on tundra greenhouse gas fluxes are rarely reported for Polar Regions. In the present study, tundra N2O and CH4 fluxes were measured under the simulated reduction of UV radiation in maritime Antarctica over the last three-year summers. Significantly enhanced N2O and CH4 emissions occurred at tundra sites under the simulated reduction of UV radiation. Compared with the ambient normal UV level, a 20% reduction in UV radiation increased tundra emissions by an average of 8 μg N2O m−2 h−1 and 93 μg CH4 m−2 h−1, whereas a 50% reduction in UV radiation increased their emissions by an average of 17 μg N2O m−2 h−1 and 128 μg CH4 m−2 h−1. No statistically significant correlation (P > 0.05) was found between N2O and CH4 fluxes and soil temperature, soil moisture, total carbon, total nitrogen, NO3−-N and NH4+-N contents. Our results confirmed that UV radiation intensity is an important factor affecting tundra N2O and CH4 fluxes in maritime Antarctica. Exclusion of the effects of reduced UV radiation might underestimate their budgets in Polar Regions with the recovery of stratospheric ozone.

Ultraviolet (UV) radiation is one of the most abundant carcinogens in our environment, and the development of non-melanoma skin cancers, the most common type of human malignancy worldwide, represents one of the major consequences of excessive exposure. Because of growing concerns that the level of UV radiation is increasing as a result of depletion of the stratospheric ozone and climate change, the development of strategies for protection of the skin is an urgent need. Many phytochemicals that belong to various families of secondary metabolites, such as alkaloids (caffeine, sanguinarine), flavonoids [(-)-epigallocatechin 3-gallate, genistein, silibinin], carotenoids ( beta-carotene, lycopene), and isothiocyanates (sulforaphane), offer exciting platforms for the development of such protective strategies. These phytochemicals have been consumed by humans for many centuries as part of plant-rich diets and are presumed to be of low toxicity, an essential requirement for a chemoprotective agent. Mechanistically, they affect multiple signalling pathways and protect against UV radiation-inflicted damage by their ability to act as direct and indirect antioxidants, as well as anti-inflammatory and immunomodulatory agents. Such "pluripotent character" is a critical prerequisite for an agent that is designed to counteract the multiple damaging effects of UV radiation. Especially attractive are inducers of the Keap1/Nrf2/ARE pathway, which controls the gene expression of proteins whose activation leads to enhanced protection against oxidants and electrophiles. Such protection is comprehensive, long-lasting, and unlikely to cause pro-oxidant effects or interfere with the synthesis of vitamin D.

The 5G, the fifth generation of wireless cellular network technology, promises faster speeds, higher data transfer, and lower latency. It is being deployed globally, making it crucial to understand how exposure to 5G networks affects human tissues. In Europe, the 26 GHz frequency band has been approved for 5G telecommunications. While radiofrequency used in 4G networks has been extensively studied, millimeter-wavelength microwaves like those in the 26 GHz range remain less researched. As frequency increases, penetration into human tissue decreases, therefore 26 GHz microwaves mainly absorbed by the skin. Ultraviolet (UV) radiation, part of the electromagnetic spectrum, includes UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm) ranges. While UVA and part of the UVB range reach the Earth's surface, UVC and 95% of UVB are absorbed by the ozone layer. Humans are also exposed to artificial UVC radiation from sterilizing lamps. The genetic material suffers significant damage during the absorption of UV radiation, especially in the UVC and UVB ranges, causing cross-links between constituent nucleic acids. The absorption of UV can result in the formation of intrastrand cyclobutane-pyrimidine dimers in DNA, which can lead to mutations or cell death. Therefore, solar UV radiation has high carcinogenic activity and gives rise to a long-term risk of skin cancer. As a result, solar UV radiation is carcinogenic, classified by the World Health Organization’s IARC as a Group 1 carcinogen to humans. Both 26 GHz microwaves and solar UV radiation are absorbed in the upper layers of the skin, making it essential to study their combined effects. HaCaT cells were cultured in Dulbecco’s Modified Eagle Medium (DMEM) with fetal bovine serum (FBS) and antibiotics, maintained at 37°C with 5% CO2. Cells were seeded into 96- well plates or 35 mm Petri dishes before exposure to UV and microwave radiation. UV exposure was conducted using a SOL-500 solar simulator equipped with a filter to transmit UVB and UVA wavelengths. The UV dose delivered was controlled by adjusting exposure time and lamp distance, with an effective dose of 40 J/m². For 26 GHz microwave exposure, the cells were irradiated using a horn antenna, with an average power density of 5–10 W/m². A software-defined radio (SDR) system generated a 5G signal, up-converted to the 26 GHz band, amplified, and transmitted to the cells. The exposure system was placed in a CO2 incubator, and the temperature was carefully monitored to stay below 37°C. DNA damage was assessed using the T4 enzyme-modified single-cell gel electrophoresis (comet) assay, which detects DNA strand breaks and UV-induced pyrimidine dimers. After irradiation, cells were harvested, mixed with 1% low-melting point agarose, dropped onto agarose pre-coated slides and covered with coverslips. Cells were lysed in lysis solution overnight at 4°C, then treated with a 1:1000 dilution of FPG enzyme (New England Biolabs, USA) and incubated at 37°C for 30 minutes. After alkaline treatment (pH>13) and electrophoresis, the slides were stained and analyzed using Comet Assay IV software (Instem, UK). The aim of this study was to assess the combined effects of 26 GHz microwave and UV radiation exposure on CPD formation in keratinocytes. The findings will provide insight into the potential risks of combined exposure to these non-ionizing radiations, which could have significant implications for human health as 5G technology becomes more widespread.

Abstract. Coupling between the stratosphere and the troposphere allows changes in stratospheric ozone abundances to affect tropospheric chemistry. Large-scale effects from such changes on chemically produced tropospheric aerosols have not been systematically examined in past studies. We use a composition-climate model to investigate potential past and future impacts of changes in stratospheric ozone depleting substances (ODS) on tropospheric oxidants and sulfate aerosols. In most experiments, we find significant responses in tropospheric photolysis and oxidants, with small but significant effects on methane radiative forcing. The response of sulfate aerosols is sizeable when examining the effect of increasing future nitrous oxide (N2O) emissions. We also find that without the regulation of chlorofluorocarbons (CFCs) through the Montreal Protocol, sulfate aerosols could have increased by 2050 by a comparable amount to the decreases predicted due to relatively stringent sulfur emissions controls. The individual historical radiative forcings of CFCs and N2O through their indirect effects on methane (−22.6 mW m−2 for CFCs and −6.7 mW m−2 for N2O) and sulfate aerosols (−3.0 mW m−2 for CFCs and +6.5 mW m−2 for N2O when considering the direct aerosol effect) discussed here are non-negligible when compared to known historical ODS forcing. Our results stress the importance of accounting for stratosphere-troposphere, gas-aerosol and composition-climate interactions when investigating the effects of changing emissions on atmospheric composition and climate.

The Parties to the Montreal Protocol are informed by three Panels of experts. One of these is the Environmental Effects Assessment Panel (EEAP), which deals with two focal issues. The first focus is the effects of UV radiation on human health, animals, plants, biogeochemistry, air quality, and materials. The second focus is on interactions between UV radiation and global climate change and how these may affect humans and the environment. When considering the effects of climate change, it has become clear that processes resulting in changes in stratospheric ozone are more complex than previously believed. As a result of this, human health and environmental issues will be longer-lasting and more regionally variable. Like the other Panels, the EEAP produces a detailed report every four years; the most recent was published as a series of seven papers in 2015 (Photochem. Photobiol. Sci., 2015, 14, 1-184). In the years in between, the EEAP produces less detailed and shorter Progress Reports of the relevant scientific findings. The most recent of these was for 2015 (Photochem. Photobiol. Sci., 2016, 15, 141-147). The present Progress Report for 2016 assesses some of the highlights and new insights with regard to the interactive nature of the direct and indirect effects of UV radiation, atmospheric processes, and climate change. The more detailed Quadrennial Assessment will be made available in 2018.

Ozone in the atmosphere has considerable impacts on the climate system. Changes in ozone in the stratosphere and troposphere cause significant fluctuation in the global radiation budget for both the short-wave and long-wave. Although chemical reaction mechanisms relevant to ozone differ between the stratosphere and troposphere, ozone chemistry is originally driven by photolytic reactions (photolysis) with solar radiation flux for both the stratosphere and troposphere. For the historical period, changes in stratospheric ozone are basically attributed to the increased emissions of ozone depleting species (ODSs) and nitrous oxide (N2O). It is projected that stratospheric circulation (Brewer-Dobson circulation) will be continuously enhanced by the global warming trend expected in the future resulting in a significant impact on the prospective ozone recovery in this century. Ozone increases in the troposphere are attributed to the combination of emission increases for ozone precursors (NOx, CO, and VOCs) and methane. The latest CMIP6 experiments suggest that despite the expected NOx emission decrease, tropospheric ozone will successively increase in this century due to increasing methane concentrations in the future. Future tropospheric ozone will be also affected by climate change as well as anthropogenic emission trends. In particular, increases in ozone transport from the stratosphere under stronger stratospheric circulation induced by warming can cause considerable O3 increases in the troposphere. It can be also expected that increases in some natural emissions like biogenic VOCs due to warming will also play important role in future tropospheric ozone. The net radiative forcing for ozone (as the sum of stratospheric and tropospheric ozone changes) is estimated at 0.35 or 0.39 W m−2 by CMIP6 including a negative radiative forcing of about −0.1 W m−2 due to the stratospheric ozone depletion since the 1970s.

The paper constitutes part 2 of a study performing a first systematic inter-model comparison of the atmospheric responses to stratospheric sulfate aerosol injections (SAI) at various latitudes as simulated by three state-of-the-art Earth System Models – CESM2(WACCM6), UKESM1.0, and GISS-E2.1-G. We use a set of five sensitivity experiments with constant annual injections of SO2 in the lower stratosphere at either 30° S, 15° S, 0°, 15° N or 30° N. We identify the similarities and differences in the simulated responses amongst the models as well as demonstrate the role of biases in the climatological circulation and specific aspects of the model microphysics and chemistry in driving the inter-model differences. Building on part 1 (Visioni et al., 2022), we explain the simulated differences in the aerosol spatial distribution between the models: CESM2 shows significantly faster shallow branches of the Brewer Dobson circulation facilitating transport of the relatively larger-sized aerosol to higher latitudes; UKESM shows a relatively isolated tropical pipe and older tropical age-of-air confining the relatively smaller-sized aerosols to the tropics; and the two GISS versions with either bulk or modal aerosol microphysics show elevated sulfate levels at higher latitudes as the result of smaller aerosol sizes and relatively stronger horizontal mixing (thus very young stratospheric age-of-air). We then elucidate the role of these factors in driving the stratospheric responses to SAI. We find a large spread in the magnitudes of the tropical lower stratospheric warming amongst the models, which can be partially attributed to the differences in aerosol distribution and sizes. Regarding the stratospheric ozone responses, we find a good agreement in the tropics between the models with modal microphysics, with lower stratospheric ozone changes consistent with the SAI-induced modulation of the large-scale circulation and the resulting changes in transport. In contrast to the relative agreement at low latitudes, we find a large inter-model spread in the Antarctic ozone responses that can largely be explained by the differences in the simulated latitudinal distributions of aerosols as well as the degree of implementation of heterogeneous halogen chemistry on sulfate. Finally, we also find large differences in stratospheric water vapour responses amongst the models, with CESM2 and GISS with modal microphysics both showing significant increases in stratospheric water vapour under SAI consistent with the increase in cold point temperatures that were largely not reproduced in UKESM. For the GISS runs with bulk microphysics, the SAI simulations show contrastingly different stratospheric responses to the models using the modal aerosol treatment, including the absence of lower stratospheric warming as well as significant reductions in stratospheric water vapour and ozone. The results point towards the importance of detailed treatment of aerosol processes, although some problems in halogen chemistry in GISS are identified that require further attention. Overall, our results contribute to an increased understanding of the underlying physical mechanisms as well as the sources of uncertainty in model projections of climate impacts from SAI.

This paper uses a state‐of‐the‐art general circulation model to study the impacts of the stratospheric ozone depletion from 1980 to 2000 and the expected partial ozone recovery from 2000 to 2020 on the propagation of planetary waves in December, January, and February. In the Southern Hemisphere (SH), the stratospheric ozone depletion leads to a cooler and stronger Antarctic stratosphere, while the stratospheric ozone recovery has the opposite effects. In the Northern Hemisphere (NH), the impacts of the stratospheric ozone depletion on polar stratospheric temperature are not opposite to that of the stratospheric ozone recovery; i.e., the stratospheric ozone depletion causes a weak cooling and the stratospheric ozone recovery causes a statistically significant cooling. The stratospheric ozone depletion leads to a weakening of the Arctic polar vortex, while the stratospheric ozone recovery leads to a strengthening of the Arctic polar vortex. The cooling of the Arctic polar vortex is found to be dynamically induced via modulating the planetary wave activity by stratospheric ozone increases. Particularly interesting is that stratospheric ozone changes have opposite effects on the stationary and transient wave fluxes in the NH stratosphere. The analysis of the wave refractive index and Eliassen‐Palm flux in the NH indicates (1) that the wave refraction in the stratosphere cannot fully explain wave flux changes in the Arctic stratosphere and (2) that stratospheric ozone changes can cause changes in wave propagation in the northern midlatitude troposphere which in turn affect wave fluxes in the NH stratosphere. In the SH, the radiative cooling (warming) caused by stratospheric ozone depletion (recovery) produces a larger (smaller) meridional temperature gradient in the midlatitude upper troposphere, accompanied by larger (smaller) zonal wind vertical shear and larger (smaller) vertical gradients of buoyancy frequency. Hence, there are more (fewer) transient waves propagating into the stratosphere. The dynamical warming (cooling) caused by stratospheric ozone decreases (increases) partly offsets their radiative cooling (warming).

This assessment by the Environmental Effects Assessment Panel (EEAP) of the United Nations Environment Programme (UNEP) provides the latest scientific update since our most recent comprehensive assessment (Photochemical and Photobiological Sciences, 2019, 18, 595–828). The interactive effects between the stratospheric ozone layer, solar ultraviolet (UV) radiation, and climate change are presented within the framework of the Montreal Protocol and the United Nations Sustainable Development Goals. We address how these global environmental changes affect the atmosphere and air quality; human health; terrestrial and aquatic ecosystems; biogeochemical cycles; and materials used in outdoor construction, solar energy technologies, and fabrics. In many cases, there is a growing influence from changes in seasonality and extreme events due to climate change. Additionally, we assess the transmission and environmental effects of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is responsible for the COVID-19 pandemic, in the context of linkages with solar UV radiation and the Montreal Protocol.