1a. When did ground based ozone measurements begin?1b. Referring to Figure 9.01, what is the long-term ozone trend values over Arosa from the mid-1920s to the mid-1970s? From the mid-1970s to the present?
1c. What evidence do we have that the recent Arosa trend does not arise from displacement of ozone from one atmospheric location to another?
2a. What are some natural factors that affect ozone variability?
2b. In this Chapter, what "proxies" are used for the 11-year solar cycle and the QBO in the statistical regression model for ozone? Can you think of other time series that could be used as proxies for these model components?
2c. How does the uncertainty of the trend estimate from the statistical model for ozone change as variables affecting ozone are added to the model. Why is this expected?
Refer to Figures 9.06 a-d, Figure 9.10, and Figure 9.11 to answer the following questions about ozone time series components in each of the latitude bands shown.
3a. In what latitudes does the seasonal cycle component have its largest and smallest magnitude?
3b. In what latitude band is the QBO component the largest?
3c. How is the phase of the QBO in the tropics and midlatitudes related?
3d. Why is there large uncertainty in the contribution of the solar cycle to the ozone model?
3e. How is the phase of the tropical ozone QBO and the midlatitude ozone QBO related?
4a. What part do the ozone measuring instruments play in the estimated uncertainty of the model long-term trend?
4b. Why are satellite data products, such as the Nimbus-7 TOMS data, released in versions?
5a. Consider Figure 9.07. What regions and seasons show the most negative trend in total ozone? Explain.
5b. Consider Figure 9.08. Does the same trend pattern as a function of latitude exist for the average annual ozone trend?
Use Figure 9.09 to address the questions below.
6a. Which altitude ranges show little annual average ozone trend values for most latitudes?
6b. At what altitude and latitude ranges are ozone trends largest?
6c. Why does the altitude axis on Figure 9.09 end at approximately 25 km? How can we acquire data for the lower stratosphere?
Use Figure 9.10 to answer the following questions.
7a. At what altitude is the 30-50°N trend largest? What is the proposed cause?
7b. At what altitude is the trend at a minimum?
7c. A second trend maximum occurs from this data. What is the altitude and proposed cause for this maximum?
8a. What is an action spectrum? How is it employed in assessing UV radiation effects?
8b. What is the difference between an action spectrum and daily UV exposure, as defined in the text?
8c. Based on Figure 9.11, at what latitudes are increases of erythema, plant, and DNA exposure the greatest? The least?
8d. Consider Figure 9.12. Do clouds make a significant difference in the percent change of UV exposure at most latitudes? Explain.
ANSWERS
1a. Ozone measurements from various locations on the ground began as long ago as 1926.
1b. From the beginning of the record until the early 1970s, the ozone varied up and down, but there was little long-term change, or trend, in the data. However, after about 1973, the ozone levels started to decline (dotted line). While the monthly average ozone values were still oscillating up and down about an equilibrium ozone level, this level was declining over this period.
1c. You can look at a time series of global average ozone from satellite data. This is the average of all the ozone at all locations. If the ozone is only moving from one place to another (which we know happens all the time because of natural variations), then the global average ozone amount should remain constant. Figure 9.02a shows the global average time series from the TOMS series of instruments. Even without removing the seasonal cycle, it is clear that the total amount of ozone above the Earth has decreased since the late 1970s.
2a. Qzone variability is dominated by natural factors such as the seasonal cycle, Quasi-Biennial Cycle (QBO), and 11-year solar cycle.
2b. The 10.7 cm solar flux measurements and the QBO in the equatorial zonal wind measured at Singapore are used as proxies fot eh 11-year solar cycle and the QBO, respectively, in the statistical regression model.
2c. Uncertainty of the trend estimate decreases as more variables affecting ozone are added to the model. We expect this since the more variables affecting ozone that are added to the model, the less unexplained variability we have and the more of the calculated trend can be attributed to known factors.
3a. The seasonal cycle is largest in the northern hemisphere midlatitudes. At 30-50E N the seasonal cycle is 80 DU from maximum to minimum, whereas in the southern hemisphere midlatitudes the seasonal cycle is 60 DU from maximum to minimum. In the Equator10EN band, the seasonal cycle is about 30 DU from maximum to minimum. This is much less than what we see in the midlatitudes. This is because there is little variation in the position of the Sun, and therefore in the amount of solar irradiance, during the year in the tropics.
3b. The QBO is driven by tropical waves propagating into the stratosphere. We therefore expect the ozone QBO contribution to be largest in the tropics, and this is indeed what is observed. However, the amplitude of the ozone QBO in the northern and southern hemisphere midlatitudes is almost as large as the tropical QBO.
3c. The phase of the ozone QBO in the tropics and the ozone QBO in the midlatitudes is inversely related. When the amplitude of the ozone QBO signal is high in the tropics, it is low in the midlatutdes, and vice versa. Why the two are oppositely related is given in Chapter 8.
3d. Currently, the solar cycle in ozone is not well quantified, and some of the variability that we attribute to solar cycle may be due to other factors, particularly aerosol contamination. Many researchers are currently studying the solar cycle in ozone, but because the period of the solar cycle is so long (11 years), it will take more data over longer time periods to minimize the uncertainty in the solar cycle estimate.
3e. The seasonal variability is largest and most negative in the northern hemisphere midlatitudes.
4a. After a satellite instrument is launched into space, it is very difficult to physically check how it is performing. Instruments in space slowly degrade over time, causing the data to slowly change, or drift, over time. If the drift is large enough, our trend calculations will be incorrect. If uncorrected, this adds uncertainty to the measurements, hence the model long-term trend results.
4b. To estimate accurately the long-term trend in the time series, we must know the relative calibration of the instruments from the beginning to the end of the time period to within ~1%. Determining the calibration of an instrument requires a lot of analysis over a long time. Therefore, data products from an instrument are generally released in versions, where each new version is an improvement over the previous version.
5a. The largest negative trends occur during winter and spring in the respective hemispheres, with especially large losses in the high latitude southern hemisphere during October (greater than -22% per decade). This is associated with the development of the Antarctic ozone hole.
5b. Yes, the high (pola) latitudes show the greatest change in annual average total ozone.
6a. In the lower stratosphere in the 25-30 km altitude range, the annual average ozone trend value is near zero for most latitudes (except regions south of about 40S).
6b. Trends are largest (most negattive) above 40 km altitude in both hemispheres poleward of about 50°.
6c. The SBUV instrument that provided the data in Figure 9.09 is not designed to see below 25 km altitude in the atmosphere. We can acquire data for the lower stratosphere down to 15 km altitude using SAGE data. From the surface to 15 km, ozonesondes can provide data.
7a. The 30-50°N trend is largest and most negative (-7.5% per decade) around an altitude of 40 km. The trend at this altitude is thought to be the result of gas phase photochemical ozone depletion.
7b. The negative trend is minimum near 30 km. with a value of about -2.5 % per decade.
7c. The second trend maximum occurs near an altitude of 15 km. It is almost as negative, nearly -7.5% per decade, and it is thought to be caused by heterogeneous chemistry on the surfaces of atmospheric aerosols.
8a. An action spectrum characterizes the relative effectiveness of radiation at each wavelength in the UV range in provoking a certain biological response such as erythema, changes in plant growth, or changes in molecular DNA. The action spectrum is employed as a weighting factor to find an effective biological dose for a particular response.
8b. The action spectrum is defined in question 8a. If we multiply the action spectrum for a particular UV response to the spectrum of UV radiation, we get the dose at each wavelength. If we then add all the doses, we get a total dosage. If we then account for the changes in Sun angle during the day, we get the daily total UV dosage for each type of response. It is this daily total UV dosage that we call the daily UV exposure.
8c. The largest increases in daily exposure time for each are in the high latitudes. The smallest increases in daily exposure time for each are in the tropics.
8d. The Herman et al. (1996) study found that despite the fact that clouds can strongly affect surface UV, they had little effect in altering long-term UV trends. This is because there is no significant trend in cloud cover.
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