The Responses of Ozone to the Solar Eclipse on the 21st of June 2020 in the Mesosphere and Upper Stratosphere

Abstract

Microwave Limb Sounder (MLS) observations showed an obvious variation of ozone concentration during the annular solar eclipse on 21 June 2020 in the mesosphere and upper stratosphere. Ozone concentration slightly reduced near 40 km in the regions of 24°N–36°N, and increased in low latitudes at 40 km. In the heights of 45–60 km, the increase in ozone concentration in most of the regions was obvious. The ozone increases and decreases were more obvious between 60–65 km, where enhancement took the leading role. The nighttime ozone variation was weaker than the daytime in most of the heights of 30–65 km. The variation of HO₂ and CO is investigated to study the photochemical and dynamical causes of ozone variation. As HO₂ decreased at 1 hPa and increased at 60–65 km, ozone variation shows a mostly reversed relationship to HO₂ variation. CO increased at 32–39 km and decreased at 52–60 km, which was related to the upwelling at these heights. The dynamic processes also contributed to the decrease in ozone concentration at 40 km and increase at 50–60 km.

1. Introduction

Ozone constitutes a minor fraction of the atmosphere, serving as a trace component. Despite its limited presence, it exerts a significant influence on ecology, climate, and the environment. Kerr and McElroy [1] identified a correlation between the increase in light intensity and the decline in ozone in Toronto since 1989. They proposed that ozone plays a crucial role in absorbing harmful radiation, a conclusion consistent with the findings of Lubin and Jensen [2]. Furthermore, Waugh et al. [3] established a link between the size of the ozone hole and the Antarctic stratosphere temperature, influencing zonal-mean ozone in the Southern Hemisphere and subsequently impacting tropospheric circulation. Recent studies by Wang et al. [4,5] suggest that variations in Arctic ozone levels may have predictive capabilities for ocean currents and could be associated with sea surface temperatures in the North Pacific. Therefore, understanding and monitoring ozone are critical for safeguarding the environment and human health.

A solar eclipse occurs when the moon passes between the sun and the earth, casting its shadow upon the earth. During this phenomenon, regions affected by the eclipse witness a rapid shift from dusk to dawn due to the moon’s shadow significantly decreasing solar radiation. This decrease in radiation has a sensitive impact on the formation and destruction of ozone [6], leading to changes in the vertical distribution of ozone concentration [7]. Accordingly, the solar eclipse presents an opportunity to study the change in the vertical distribution of ozone concentration.

Tian et al. [8] utilized surface ozone observations in China and discovered a decrease in ozone concentration during the annual solar eclipse on 21 June 2020. They attributed this decrease to variations in one of the reaction precursors of ozone production, namely NO₂, in conjunction with meteorological factors. Dutta et al. [9] reported that in the tropics, the ozone concentration and temperature decrease below the tropopause and rise above it, which may be attributed to the dynamic changes of gravity wave in that region. Also, Gerasopoulos et al. [10] observed a gradual drop of total ozone near the eclipse in the stratosphere on 29 March 2006. Manchanda et al. [2] reported a decrease in ozone during the maximum phase of the eclipse on 15 January in the upper troposphere and lower stratosphere (20–40 km). Then, it increased until the end of this solar eclipse. They suggested that meteorological parameters like temperature, relative humidity, and wind speed indicated the alteration of gravity waves. These changes in gravity waves resulted in the oscillatory behavior of ozone. Kumar et al. [11] studied the same eclipse event and reported both reduction and enhancement in ozone in the stratosphere. They demonstrated that the depletion of ozone was associated with the decrease in photolysis of molecular oxygen, which is the main source of ozone in the stratosphere. However, the increase in ozone is dependent on the decrease in photodissociation of ozone owing to reduced solar radiation. Therefore, the variations of ozone depend on the balance between these two processes. Besides this, the sudden decrease in solar radiation can also cause great changes in meteorological parameters and then disturb the heat balance. During the solar eclipse on 15 January 2010, Ratnam et al. [12] showed an increase in ozone concentration and temperature in the stratosphere. They suggested that the changes may be attributed to contraction between the middle and lower troposphere and horizontal advection. In the mesosphere, Kulikov et al. [13] observed a 40% increase in ozone density at the height of 60 km. However, using observations from SMILES, Imai et al. [14] found ozone decreases in the mesosphere during a solar eclipse. They suggested that the changes in HO₂ and OH were responsible for the loss of ozone in the mesosphere using statistical analysis. Most of the previous studies focus on the troposphere and stratosphere, and the ozone variations during an eclipse in the mesosphere are minor. Aside from that, the physical mechanisms causing the ozone variations during an eclipse are different.

Ozone is of great importance in protecting the earth from the damage of UV radiation. The changes in ozone concentration at all heights are closely related to the sudden cutoff of photochemical reactions due to rapid changes in solar radiation during a solar eclipse. Therefore, it is necessary to investigate the variations of ozone during a solar eclipse and the mechanism of the changes in ozone. In this study, Microwave Limb Sounder (MLS) observations are utilized to investigate the variations of ozone concentration in the upper stratosphere and mesosphere during the solar eclipse on 21 June 2020. In Section 2, we will introduce the solar eclipse and the data. The results are in Section 3 and the discussion is in Section 4. Finally, the conclusion is in Section 5.

2. Solar Eclipse Event and Data

2.1. Description of This Solar Eclipse

The annular solar eclipse, which took place on 21 June 2020, had a long path from Africa to Asian countries. It started from the Democratic Republic of the Congo at 04:48 UT (Universal Time), moved eastwards through Asian countries, and ended on the east side of the Pacific Ocean at 08:32 UT. This solar eclipse is suitable for researching the changes in ozone as it happened on the summer solstice, on which day the daytime is the longest in the whole year. The path and regions affected by this solar eclipse are shown in Figure 1.

2.2. Data from MLS

MLS is one of the instruments in Aura, which was launched on 15 July 2004. With the help of natural thermal radiation from the edge of Earth’s atmosphere, it measures the vertical profiles of gases, temperature, and pressure on the mesosphere, stratosphere, and upper troposphere. It observed the daily mean ozone data between −180°E–180°E and −82°N–82°N. Also, the vertical range is from 261 hPa to 0.0215 hPa. The horizontal resolution is 5° × 4° (latitude x longitude) and the vertical resolution is 3–6 km. Aside from that, the daily zonal mean HO₂ data cover −180°E–180°E and −85°N–85°N, and the horizontal resolution is 10°. The vertical height range is between 21.5 to 0.0464 hPa and the vertical resolution is 3–6 km. The daytime in this paper is defined when the Solar Zenith Angle (SZA) is smaller than 70 degrees, which contains the whole process of the solar eclipse. The nighttime represents when the SZA is over 110 degrees.

3. Results

3.1. The Variation of the Vertical Distribution of Ozone

Figure 2 shows the variation percentage of ozone concentration from 18th June to 24th June and 10 hPa to 0.1 hPa (approximately 30 km–65 km). The ozone concentration used in this figure represents the average value of ozone concentration in the area within 5 degrees of latitude and longitude near the solar eclipse track at each height. It contains the whole process from the day before the solar eclipse to the recovery from the solar eclipse. The variation percentage of ozone is defined by (daily O₃ concentration—quiet-time daily O₃ means)/quiet-time daily O₃ means. Here, the quiet-time daily O₃ mean represents the mean of daily ozone from 18 to 20 June. The impacts of the solar eclipse on ozone concentration began on 21 June and returned to normal on 23 June. The increase in ozone concentration is clear to be seen from 21 June in the heights of about 45 km–65 km. From 60 km to 65 km, the variation of O₃ reached its maximum value of over 13% on 21 June. Also, the ozone concentration increased by about 3–5% owing to the effect of the solar eclipse below 60 km. However, the increases below 45 km are minor, even with a slight decrease of 3–5% in the heights of 30 km–35 km. As a result, compared with the variations of ozone concentration in quiet time, the O₃ changes during the solar eclipse are obvious in most of the stratosphere and the mesosphere. Also, it can be easily observed that the ozone concentration began to recover from the impact of the solar eclipse on 22 June, which indicates that the influence of the solar eclipse lasted for about two days.

To show the detailed variation of ozone concentration with height and latitude between the daytime and nighttime, Figure 3 is given. The variation percentage in Figure 3 is similar to Figure 2. The longitude mean between 22.5°E–137.5°E is selected in Figure 3, which is the longitude range of the solar eclipse path. In the daytime, the response of ozone to the solar eclipse was the most obvious shown in Figure 3a. At the height of 40 km, there is a slight O₃ decrease of about −2–−4% in the range of 22°N–32°N, and O₃ increases of about 2–6% below 16°N. In the heights of 45 km–50 km, the variation of ozone dropped −2–−6% below 16°N and enhanced by 2–8% above 16°N. In the heights of 50 km–60 km, the ozone concentration increased in most regions, and it reached a maximum of 20% at the height of 60 km. Then, from 60 km to 65 km, the ozone concentration varied dramatically, decreasing and increasing. In the range of 16°N–28°N, the ozone concentration increased with the maximum value of over 20% in 63 km. Then at that height, the ozone concentration also decreased to −20% in low latitudes and 32°N–36°N. The duration of nighttime contains the periods before the happening of the solar eclipse and the recovery of the ozone after the solar eclipse. As a result, the nighttime variation is smaller than in the daytime, especially in the mesosphere (55 km–65 km). The maximum decrease in the mesosphere has decreased to −6–−8%. The increase in ozone was about 2–8%, which was smaller than in the daytime in most of the region. Also, the maximum decrease in the ozone concentration was about 10% in the heights of 55 km −60 km at 30°N, while in the same location in Figure 3a, the ozone concentration increased by 8–18%, which indicated that the ozone concentration had decreased a lot. Aside from that, the decrease in ozone in 1 hPa height had recovered and was gradually replaced by an increase in ozone.

According to the whole day mean value of the ozone variation (Figure 3c), the increases and decreases in ozone were all obvious, especially in the heights of 60 km–65 km. The increase reached its maximum value of over 20% in the low latitudes of heights of 60 km–65 km, while there was a reduction of −2–−14% at 32°N–40°N. At that range of heights, ozone concentration also rose with the range of 2–10% at 24°N–32°N. The ozone in most regions from 50 km to 60 km increased, with a maximum value of about 18% at 8°N at the height of 57 km. However, there were also some slight reductions in these heights, with the range of −6–−2%. At the height of 40 km, the variations of ozone were slight. In low latitudes, ozone concentration raised slightly, and at 32°N–40°N it reduced by about −2–−4%.

3.2. The Spatial Distribution of Ozone Variations

As shown in Figure 3c, there was both an increase and decrease in ozone concentration near the height of 40 km, and near 50 km, the ozone concentration increased in most of the regions, while there was a great decrease in the height of 60–65 km. Consequently, the heights of 3.16 hPa (~40 km), 1 hPa (~46 km), and 0.14 hPa (~62 km) are picked as typical heights to research the detailed ozone spatial variation.

At the height of 3.16 hPa (~40 km), the percentage of variation is calculated similarly to that in Figure 2. It is illustrated that the ozone changes before the solar eclipse are minor and less than 4% on 20 June in Figure 4a, while on 21 June (Figure 4b) it dropped with the minimum value of −12% near the path of the solar eclipse. The decrease in ozone was concentrated in the region of 25°N–35°N. However, in low latitudes, there is a slight increase of about 4% in the ozone concentration. As a result, the effects of the solar eclipse on ozone in this height and middle latitudes were to decrease its concentration, but in some regions near the equator, the solar eclipse can cause an increase in ozone. The results are similar to Figure 3, but the O₃ changes are more significant than those in Figure 3. The small changes away from the solar eclipse path are responsible for these differences.

Compared to the O₃ variation at the height of 3.16 hPa, the O₃ variation at the height of 1 hPa (~46 km) was contrary to that at 3.16 hPa. Compared to the quiet-time variation on 20th June (Figure 5a), ozone increased obviously during the solar eclipse in most regions near the solar eclipse on 21 June (Figure 5b). The maximum value of the increase was about 20%, and the increase was roughly distributed on the two sides of the path. Aside from the increase above, there were still some regions where the ozone concentration decreased with a minimum value of about −10%.

At the height of 0.14 hPa (~62 km), the variation is much more dramatic than at low altitudes. In the quiet time (Figure 6a), the variation of ozone was in the range of −30–35%. During the solar eclipse on 21 June (Figure 6b), the ozone still increased dramatically near the path of the solar eclipse, with a maximum value of over 60%, and there was also an obvious decrease in ozone concentration near the path of the solar eclipse, with a minimum value of −30%. The O₃ decreases were generally accompanied by O₃ increases.

3.3. Formatting of Mathematical Components

The photochemical chain reactions related to hydrogen (OH, HO₂, H) are quite important for O₃ changes [14,15]. To understand the reason for the variation of ozone, the variation percentage of HO₂ in the heights of 30–65 km in the daytime is shown in Figure 7. It is a pity that the HO₂ data from MLS are not available in the nighttime and the whole day during the solar eclipse. The HO₂ variation percentage is defined as (daily HO₂ concentration—quiet-time daily HO₂ means)/quiet-time daily HO₂ means. Here, the quiet-time daily HO₂ mean represents the mean of daily HO₂ from the 18 to the 20 of June.

Compared to Figure 3a, the variation of O₃ is approximately inverse to that of HO₂ change. Near the height of about 37 km, there is an obvious decrease with a maximum value of −80% in the regions of 12°N–40°N. Then, the HO₂ rose with a variation from 10% to 130% in the heights of 35 km–45 km, while in Figure 3a, there was a slight ozone decrease at the same place. In addition, the HO₂ decreased slightly in the region of 0°N–4°N at 40 km, where the ozone concentration increased slightly. At the height of 45 km, there is a reduction with its maximum value of −80% in the region of 10°N–28°N, whereas ozone increased in that region. Aside from that, in the heights of 40 km–50 km, HO₂ increased within the range of 10–40% in the low latitudes, corresponding to a slight decrease in ozone concentration. From 50 km to 60 km, HO₂ increased in most of the regions, with a slight reduction in low latitudes near the height of 50 km. This reduction was related to the slight increase in ozone at the same height. This inverse relationship between O₃ and HO₂ was more obvious at higher altitudes. In the heights of 60–65 km, the minimum decrease in ozone had been over 20%. On the contrary, the concentration of HO₂ increased by 90%. Furthermore, the rapid decrease in ozone in 32°N–36°N at that height corresponded with the obvious increase in HO₂. As a result, the relation between the variations of HO₂ and O₃ was roughly inverse, and the ozone decrease was related to the rapid increase in HO₂ below the height of 65 km, similar to the previous research [14].

3.4. The Vertical Distributions of CO Concentration and Variation

With the characteristics of a long photochemical lifetime, CO can be used as a tracer of dynamic transport [16]. To understand the dynamic causes of ozone variation, the CO concentration and variation percentage in the heights of 30–65 km have been shown in Figure 8. The data at the heights of 43–48 km are not available. Figure 8a,b describe the vertical distribution of CO concentration in the whole day on 20 June and 21 June (solar eclipse), respectively. The CO concentration decreased with height in the heights of 30–38 km, from over 0.2 vmr to 0.05 vmr in the height of 38 km, and the concentration in the range of 28°N–40°N is lower than the low latitudes. Then, in the heights of 40–42 km, the CO concentration increased with the height in the range of 0.01–0.07 vmr. From 49 km to 65 km, it increased with height and reached 0.05 vmr at 65 km.

Figure 8c describes the variation percentage of CO in the heights of 30–65 km. The CO variation percentage is defined as (daily CO concentration—quiet-time daily CO means)/quiet-time daily CO means. Here, the quiet-time daily CO mean represents the mean of daily CO from 18 to 20 June. In the heights of 32–39 km, CO concentration increased with the maximum value of 90% in the regions of 20°N–36°N. It can also be observed in Figure 8a to Figure 8b that CO increased from less than 0.01 vmr to 0.03–0.04 vmr in the same region, which indicates the upwelling motion from below 39 km. Below the CO increases, CO concentration slightly decreased in the region of 24°N–32°N with a minimum value of −20%. This may be attributed to the southward motion at this height. The CO concentration increased in most of the regions around 50 km with a maximum value of over 100%. The increase in the region of 32°N–40°N is related to the northward and downward motion, and the increase in other regions may be connected to the motion from below 50 km. In the height of 52–60 km, CO concentration reduced in most of the regions of 12°N–36°N with the range of −10–−50%, while it increased with the maximum value of 100% in the regions of 24°N–32°N in the height of around 57 km. As is shown in Figure 8a,b, this reduction might be attributed to the upwelling transportation, where the CO concentration was lower than 0.01 vmr. Moreover, in the heights of 60–65 km, CO concentration enhanced to over 100% in most of the regions of 32°N–40°N. This increase might be due to the downwelling transport.

4. Discussion

The variation of ozone during a solar eclipse can be attributed to two aspects: the photochemistry and dynamics, and the effect of photochemistry is more important over 10 hPa [11,17,18]. As solar radiation decreases suddenly owing to the solar eclipse, the change of photochemical chain reactions related to ozone can lead to a variation in ozone concentration. During the occurrence of the solar eclipse, the variation of HO₂ is nearly reversed to ozone variation (Figure 7). The concentration of HO₂ increased dramatically to over 150% around the height of 40 km, where ozone concentration decreased slightly. Apart from that, the HO₂ concentration decreased from 70% to 90% at the height of 1 hPa (~46 km), in which height the ozone concentration increased 2–6%. Then, it increased from 10% to 90% in most of the regions from 60 km to 65 km, while there was an obvious decrease in ozone concentration at these heights. Accordingly, the variation in HO₂ concentration is mostly reversed to the variation in ozone concentration in Figure 3a, which indicates that the ozone variation may be attributed to the change in HO₂. This relationship between ozone and HO₂ is consistent with previous research. Pallister and Tuck [19] reported that the ozone variation below 42 km was related mostly to the reaction of NO_x, and the HO_x reaction was more important at the height over 42 km. Meanwhile, Imai et al. [14] reported that the reaction of HO₂ was dominant to the decrease in O₃ during the solar eclipse in the stratosphere and lower mesosphere. Therefore, the variation of ozone concentration during a solar eclipse is mostly dependent on photochemical reactions related to HO₂ in the height of 30–65 km.

As the ozone concentration below 35 km was much lower than over 35 km, the upwelling motion in the heights of 32–39 km can lead to a decrease near 40 km in Figure 3c. Moreover, the upwelling motion in the heights of 52–60 km in most of the region of 12°N–36°N can cause an increase in ozone concentration in the same region, as the ozone concentration decreases with height in this height range. Aside from that, the ozone decreases in the heights of 60–65 km in the regions of 32°N–40°N in Figure 3c can be attributed to the downwelling transport in this region. Consequently, the variation of ozone in some of the regions in the heights of around 40 km, 52–60 km, and 60–65 km may be associated with the upwelling and downwelling motions.

5. Conclusions

In this paper, MLS observations were used to analyze the variation of ozone concentration in the heights of 30–65 km during the solar eclipse on 21 June 2020. MLS observations provide a chance to explore the chemical composition changes and the mechanisms involved during the solar eclipse in the mesosphere and upper stratosphere. The main conclusions are summarized as follows:

• Ozone concentration obviously increased during the solar eclipse on 21 June 2020 at the heights of 45–65 km compared to the variation in quiet time, and it reduced slightly by −3–−5% from 30 km to 35 km. Also, the variation becomes stronger with height increases. This variation recovered after 22nd June.
• Ozone concentration decreased around 40 km in the region of 24°N–36°N during the solar eclipse, and it increased in low latitudes at this height. Then, it increased in most of the regions at the heights of 45–60 km. Between 60 and 65 km, the variation of ozone concentration was obvious both for increase and decrease. The nighttime ozone variation was smaller than that in the daytime, especially in the height of 60–65 km. Aside from that, the decrease in the height of 1 hPa in the daytime changed into an increase in the nighttime in the low latitudes.
• At the height of 40 km, ozone concentration slightly increased by 2–6% in low latitudes and decreased by −2–−4% at 32°N–40°N. As a result, ozone concentration decreased near the track of the solar eclipse with a slight increase near the equator under the impact of the solar eclipse. At the height of 46 km, ozone concentration increased in most of the regions. At the height of 62 km, the increase and decrease were both obvious, with a maximum value of over 30% for both increase and decrease. Also, enhancement plays a leading role in the ozone variation at this height.
• HO₂ increased to over 150% in 40 km, where ozone decreased slightly. HO₂ also decreased by 70–90% at the height of 1 hPa, which corresponded with the increase in ozone concentration at this height. In most of the regions of 60 km–65 km, it increased by 10–90%, where ozone mainly decreased in the middle and high latitudes. The variation of HO₂ was mostly reversed to the variation of ozone concentration during the solar eclipse. Therefore, the variation of ozone during the solar eclipse is mostly due to the photochemical reactions related to HO₂.
• In the heights of 32–39 km, CO concentration increased with the maximum value of 90% in the regions of 20°N–36°N. This indicated the upwelling motion and was related to the decrease in ozone near 40 km. Also, the CO decrease of −10–−50% in 52–60 km indicated the upwelling motion and was related to the ozone increase in this region. From 60 km–65 km, the downward motion in 32°N–40°N caused a decline in ozone, which was associated with the increase in CO concentration by over 100%. Accordingly, ozone variations are also related to dynamic transportation.