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## 10 THE ROLE OF TROPOSPH ERIC OZONE IN CLIMATE CHANGE AND U V - B EFFECTS

10.1 Introduction Atmospheric O 3 plays an important role in the Earth ’ s energy budget by interacting with 

### 1 incoming solar radiation and outgoing infrared radiation. Over mid

- latitudes, 

### 2 approximately 90% of the total atmospheric O 3 column is located in the stratosphere ( Kar 

### 3 et al., 2010 ; Crist et al., 1994 ) . Therefore, tropospheric O 3 makes up a relatively small 

### 4 portion (~10%) of the total column of O 3 over mid

- latitudes, but it does play an 

### 5 important role in the overall radiation budget. The next sectio n (Section 10.2) briefly 

### 6 describes the physics of the earth ’ s radiation budget, providing background material for 

### 7 the subsequent two sections assessing how perturbations in tropospheric O 3 might affect 

### 8 (1) climate through its role as a greenhouse gas (Sect ion 10.3), and (2) health, ecology 

### 9 and welfare through its role in shielding the earth ’ s surface from solar ultraviolet 

### 10 radiation (Section 10.4). 

### 11


## 10.2 Physics of the Earth ’ s Radiation Budget Radiant energy from the sun enters the atmosphere in a range of wavele ngths, but peaks 

### 12 strongly in the visible (400 nm up to 750 nm) part of the spectrum. Longer wavelength 

### 13 infrared (750 nm up to ~1 mm) and shorter wavelength ultraviolet (400 nm down to 

### 14 100 nm) radiation are also present in the solar electromagnetic spectrum . Since the 

### 15 energy possessed by a photon is inversely proportional to its wavelength, infrared (IR) 

### 16 radiation carries the least energy per photon, and ultraviolet (UV) radiation carries the 

### 17 most energy per photon. UV radiation is further subdivided into cl asses based on 

### 18 wavelength: UV

- A refers to wavelengths from 400

- 315 nm; UV

- B from 315

- 280 nm; and 

### 19 UV

- C from 280

- 100 nm. By the same argument above describing the relationship 

### 20 between photon wavelength and energy, UV

- A radiation is the least energetic and UV

- C 

### 21 is the most energetic band in the UV spectrum. 

### 22 The wavelength of radiation also determines how the photons interact with the complex 

### 23 mixture of gases, clouds, and particles present in the atmosphere (see Figure 10

- 1). UV

- A 

### 24 radiation can be scattered but is not absorbed to any meaningful degree by atmospheric 

### 25 gases including O 3 . UV

- B radiation is absorbed and scattered in part within the 

### 26 atmosphere. UV

- C is almost entirely blocked by the Earth ’ s upper atmosphere, where it 

### 27Draft

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- 2 September 2011 participates in photoionization a nd photodissociation processes including absorption by 

### 1 stratospheric O 3 . 

### 2

Source : 2006 O 3 AQCD. 

### Figure 10 - 1 Diagram of the factors that determine human exposure to ultraviolet radiation.

Since UV

- A radiation is less energetic and does not interact with O 3 in the troposphere or 

### 3 the stratosphere and UV

- C radiation is almost entirely blocked by stratospheric O 3 , UV



### 4 B radiation is the most important band to consider in relation to tropospheric O 3 

### 5 shielding. Furthermore, tropospheric O 3 plays a “ disproportionate ” role in absorbing UV



### 6 B radiation compared with stratospheric O 3 on a molecule per molecule basis ( Balis et 

### 7 al., 2002 ; Zerefos et al., 2002 ; Crist et al., 1994 ; Bruhl and Crutzen, 1989 ) . This effect 

### 8 results from the higher atmospheric pressure present in the troposphere, resulting in 

### 9 higher concentrations of gas molecules present that can absorb or scatte r radiation. For 

### 10 this reason, the troposphere is referred to as a “ multiple scattering ” regime for UV 

### 11 absorption, compared to the “ single scattering ” regime in the stratosphere. Thus, careful 

### 12 quantification of atmospheric absorbers and scatterers, along wi th a well

- resolved 

### 13Draft

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- 3 September 2011 description of the physics of these interactions, is necessary for predicting the impact of 

### 1 tropospheric O 3 on UV

- B flux at the surface. 

### 2 Solar flux at all wavelengths has a temporal dependence, while radiative scattering and 

### 3 absorption h ave strong wavelength, path length, and gas/particle concentration 

### 4 dependencies. These combine to create nonlinear effects on UV flux at the Earth ’ s 

### 5 surface. Chapter 10 of the 2006 O 3 AQCD ( U.S. EPA, 2006b ) describes in detail several 

### 6 key factors that influence the spatiotemporal distribution of ground

- level UV radiation 

### 7 flux, including: (1) long

- term solar activity including sunspot cycle; (2) solar rotation; (3) 

### 8 the position of the Earth in its orbit around the sun; (4) atmospheric absor ption and 

### 9 scattering of UV radiation by gas molecules and aerosol particles; (5) absorption and 

### 10 scattering by stratospheric and tropospheric clouds; and (6) surface albedo. The 

### 11 efficiencies of absorption and scattering are highly dependent on the concentra tion of the 

### 12 scattering medium, particle size (for aerosols and clouds), and the altitude at which these 

### 13 processes are occurring. These properties are sensitive to meteorology, which introduces 

### 14 additional elements of temporal dependency in ground

- level UV r adiation flux. 

### 15 About 30% of incoming solar radiation is directly reflected back to space, mainly by 

### 16 clouds or surfaces with high albedo (reflectivity), such as snow, ice, and desert sand. 

### 17 Radiation that does penetrate to the Earth ’ s surface and is absorbed can be re

- emitted in 

### 18 the longwave (infrared) portion of the spectrum (750 nm up to ~1 mm); the rest goes into 

### 19 evaporating water or soil moisture or emerges as sensible heat. The troposphere is opaque 

### 20 to the outgoing longwave radiation. Polyatomic gases su ch as CO 2 , CH 4 , and O 3 absorb 

### 21 and re

- emit the radiation upwelling from the Earth ’ s surface, reducing the efficiency with 

### 22 which that energy returns to space. In effect, these gases act as a blanket warming the 

### 23 Earth ’ s surface. This phenomenon, known as the “ Greenhouse Effect, ” was first 

### 24 quantified in the 19 th century ( Arrhenius, 1896 ) , and gives ri se to the term “ greenhouse 

### 25 gas. ” 

### 26


## 10.3 Effect s of Tropospheric Ozone on Climate
Background As a result of its interaction with incoming solar radiation and outgoing longwave 

### 27 radiation, tropospheric O 3 is a major greenhouse gas, and increases in its abundance may 

### 28 contribute to climate change ( IPCC, 2007b ) . Models estimate that the global average 

### 29 concentration of O 3 in the troposphere has doubled since the preindustrial era ( Gauss et 

### 30 al., 2006 ) , while observations indicate that in some regions tropospheric O 3 may have 

### 31Draft

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- 4 September 2011 increased by factors as great as 4 or 5 ( Marenco et al., 1994 ; Staehelin et al., 1994 ) . These 

### 1 increases are tied to the rise in emissions of O 3 precursors from human activity, mainly 

### 2 fossil fuel consumption and agricultural processes. 

### 3 The impact on climate of the tropospheric O 3 change since preindustrial times has been 

### 4 estimated to be about 25

- 40% of anthropogenic CO 2 impact and about 75% of 

### 5 anthropogenic CH 4 impact ( IPCC, 2007b ) , ranking it third in importance of the 

### 6 greenhouse gases. In the 21 st century as the Earth ’ s population continues to grow and 

### 7 energy technology spreads to develo ping countries, a further rise in the global 

### 8 concentration of tropospheric O 3 is likely, with associated consequences for human 

### 9 health and ecosystems relating to climate change. 

### 10 To examine the science of a changing climate and to provide balanced and rigor ous 

### 11 information to policy makers, the World Meteorological Organization (WMO) and the 

### 12 United Nations Environment Programme (UNEP) formed the Intergovernmental Panel on 

### 13 Climate Change (IPCC) in 1988. The IPCC supports the work of the Conference of 

### 14 Parties ( COP) to the United Nations Framework Convention on Climate Change 

### 15 (UNFCCC). The IPCC periodically brings together climate scientists from member 

### 16 countries of WMO and the United Nations to review knowledge of the physical climate 

### 17 system, past and future cli mate change, and evidence of human

- induced climate change. 

### 18 IPCC climate assessment reports are issued every five to seven years. 

### 19 This section draws in part on the fourth IPCC Assessment Report (AR4) ( IPCC, 2007b ) , 

### 20 as well as other peer

- reviewed published research. Section 10.3.1 reviews evidence of 

### 21 clima te change in the recent past and projections of future climate change. It also offers a 

### 22 brief comparison of tropospheric O 3 relative to other greenhouse gases. Section 10.3.2 

### 23 describes factors that influence the magnitude of tropospheric O 3 effects on climate. 

### 24 Section 10.3.3 considers the compe ting effects of O 3 precursors on climate. Finally, 

### 25 Section 10.3.4 describes the effects of changing tropospheric O 3 concentrations on 

### 26 present

- day c limate. Downstream effects resulting from climate change, such as 

### 27 ecosystem responses, are outside the scope of this assessment, which focuses on the 

### 28 direct effects of tropospheric O 3 on climate. 

### 29

10.3.1 Climate Change Evidence and the Influence of Tropospheric Oz one

10.3.1.1 Climate Change in the Recent Past From the end of the Last Ice Age 12,000 years ago until the mid

- 1800s, observations 

### 30 from ice cores show that concentrations of the long

- lived greenhouse gases CO 2 , CH 4 , 

### 31Draft

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- 5 September 2011 and N 2 O have been relatively stable. Unlike these greenhouse gases, O 3 is not preserved 

### 1 in ice, and no record of it before the late 1800s exists. Models, however, suggest that it, 

### 2 too, has remained relatively constant during this time period ( Thompson et al., 1993 ; 

### 3 Thompson, 1992 ) . The stable mix of greenhouse gases in the atmosphere has kept the 

### 4 global mean temperature of the Earth close to 15oC. Without the presence of greenhouse 

### 5 gases in the atmospher e, the Earth ’ s temperature would be about 30oC cooler, or

- 15oC. 

### 6 Since the start of the Industrial Revolution, human activity has led to significant increases 

### 7 of greenhouse gases in the atmosphere, mainly through fossil fuel combustion. According 

### 8 to the IP CC AR4 ( IPCC, 2007b ) , there is now “ very high confidence ” that the net effect 

### 9 of anthropogenic greenhouse gas emissions since 1750 has led to warming, and it is “ very 

### 10 likely ” that human activity contributed to the 0.76oC rise in global mean temperature 

### 11 observed over the last century. The increase of tropospheric O 3 may have contributed 

### 12 0.1

- 0.3oC warming to the global climate during this time period ( Hansen et al., 2005 ; 

### 13 Mickley et al., 2004 ) . Global cooling due to anthropogenic aerosols ( IPCC, 2007b ) has 

### 14 likely masked the full warming effect of the anthropogenic greenhouse gases. Emissions 

### 15 of aerosols and their precursors in the United States and other developed countries are 

### 16 presently decreasing rapidly due to regulatory policies. The consequences of such 

### 17 decreases on regional climate could be large, as indicated by observations (e.g., Philipona 

### 18 et al., 2009 ; Ruckstuhl et al., 2008 ) and models (e.g., Kloster et al., 2009 ; Mick ley et al., 

### 19 In Press )

. 

### 20

10.3.1.2 Projections of Future Climate Change The IPCC AR4 projects a warming of ~0.2oC per decade for the remainder of the 21 st 

### 21 century ( IPCC, 2007b ) . Even at constant concentrations of greenhouse gases in the 

### 22 atmosphere, temperatures are expected to increase by about 0.1oC per decade, due to the 

### 23 slow resp onse of oceans to the warming applied so far. It is likely that the Earth will 

### 24 experience longer and more frequent heat waves in the 21 st century, together with more 

### 25 frequent droughts and/or heavy precipitation events in some regions, due to perturbations 

### 26 in the hydrological cycle that result from changing temperatures ( IPCC, 2007b ) . Sea 

### 27 levels could increase by 0.3

- 0.8 m by 2300 due to thermal expansion of the oceans. The 

### 28 extent of Arctic sea ice is expected to decline, and contraction of the Greenland ice sheet 

### 29 could further contribute to the sea level rise ( IPCC, 2007b )

. 

### 30 Projections of future climate change are all associated with some degree of uncertainty. A 

### 31 major uncertainty involves future trends in the anthropogenic emissions of greenhouse 

### 32 gases or their precursors. For the IPCC AR4 climate projections, a set of disti nct 

### 33

“ storylines ” or emission pathways was developed ( IPCC, 2000 ) . Each storyline took into 

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- 6 September 2011 account factors such as population growth, mix of energy technologies, and the sharing of 

### 1 technology between developed and developing nations, and each resulted in a different 

### 2 scenario for anthropogenic emissions. When these trends in emissions are applied to 

### 3 models, these scenarios yield a broad range of possible climate trajectories for the 21 st 

### 4 century. 

### 5 A second factor bringing large uncertainty to model projections of future climate is the 

### 6 representation of climate and, especially, climate feedbacks. A rise in surface 

### 7 temperatures would perturb a suite of other processes in the earth

- atmosphere

- ocean 

### 8 system, which may in turn either amplify the temperature increase (positive feedback) or 

### 9 diminish it (negative feedback). One important feedback involves the increase of water 

### 10 vapor content of the atmosphere that would accompany higher temperatures ( Bony et al., 

### 11 2006 ) . Water vapor is a potent greenhouse gas; accounting for the water vapor feedback 

### 12 may increas e the climate sensitivity to a doubling of CO 2 by nearly a factor of two ( Held 

### 13 and Soden, 2000 ) . The ice

- albedo feedback is also strongly positive; a decline in snow 

### 14 cover and sea ice extent would diminish the Earth ’ s albedo, allowing more sola r energy 

### 15 to be deposited to the surface ( Holland and Bitz, 2003 ; Rind et al., 1995 ) . A final 

### 16 example of a climate feedback involves the effects of changing cloud cover in a warming 

### 17 atmosphere. Models disagree on the magnitude and even the sign of this feedback on 

### 18 surface temperatures ( Soden and Held, 2006 )

. 

### 19

10.3.1.3 Metrics of Potential Climate Change Two different metrics are frequently used to estimate the potential climate imp act of 

### 20 some perturbation such as a change in greenhouse gas concentration: (1) radiative 

### 21 forcing; and (2) global warming potential (GWP). 

### 22 Radiative forcing is a change in the radiative balance at a particular level of the 

### 23 atmosphere or at the surface when a perturbation is introduced in the earth

- atmosphere



### 24 ocean system. In the global mean, radiative forcing of greenhouse gases at the tropopause 

### 25 (top of the troposphere) is roughly proportional to the surface temperature response 

### 26 ( Hansen et al., 2005 ; NRC, 2005 ) . It thus provides a useful metric for policymakers for 

### 27 assessing the response of the earth ’ s surface temperature to a given change in the 

### 28 concentration of a greenhouse gas. Positive values of radiative forcing indicate warming 

### 29 in a test case relative to the control; negative values indicate cooling. The units of 

### 30 radiative forcing are energy flux per area, or W/m 2

. 

### 31 Radiative forcing requires just a few model years to calculate, and it shows c onsistency 

### 32 from model to model. However, radiative forcing does not take into account the climate 

### 33 feedbacks that could amplify or dampen the actual surface temperature response, 

### 34Draft

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- 7 September 2011 depending on region. Quantifying the change in surface temperature requires a climate 

### 1 simulation in which all important feedbacks are accounted for. As these processes are not 

### 2 well understood, the surface temperature response to a given radiative forcing is highly 

### 3 uncertain and can vary greatly among models and even from region to r egion within the 

### 4 same model. 

### 5 GWP indicates the integrated radiative forcing over a specified period (usually 100 years) 

### 6 from a unit mass pulse emission of a greenhouse gas or its precursor, and are reported as 

### 7 the magnitude of this radiative forcing relati ve to that of CO 2 . GWP is most useful for 

### 8 comparing the potential climate impacts of long

- lived gases, such as N 2 O or CH 4 . Since 

### 9 tropospheric O 3 has a lifetime on the order of weeks to months, GWP is not seen as a 

### 10 valuable metric for quantifying the import ance of O 3 on climate ( Forster et al. , 2007 )

. 

### 11

10.3.1.4 Tropospheric Ozone as a Greenhouse Gas Tropospheric O 3 differs in important ways from other greenhouse gases. It is not emitted 

### 12 directly, but is produced through photochemical oxidation of CO, CH 4 , and nonmethane 

### 13 volatile organic compounds (VOCs) in the presence of nitrogen oxide radicals ( NO X = 

### 14 NO + NO 2 ; see Section 3.2 for further details on the chemistry of O 3 formation). It is also 

### 15 supplied by vertical transport from the stratosphere. The lifetime of O 3 in the troposphere 

### 16 is typically a few weeks, resulting in an inhomogeneous distribution that varies 

### 17 seasonally; the distribution of the long

- lived greenhouse gases like CO 2 and CH 4 are 

### 18 much more uniform. The longwave radiative forcing by O 3 is mainly due to absorption in 

### 19 the 9.6 μm window, where absorption by water vapor is weak. It is therefore less 

### 20 sensitive to local humidity than the radiative forcing by CO 2 or CH 4 , for which there is 

### 21 much more overlap with the water absorption bands ( Lenoble, 1993 ) . And unlike other 

### 22 major greenhouse gases, O 3 absorbs in the shortwave as well as the longwave part of the 

### 23 spectrum. 

### 24 Figure 10

- 2 shows the main steps involved in the influence of tropospheric O 3 on 

### 25 climate. Emissions of O 3 p recursors including CO, VOCs, CH 4 , and NO X lead to 

### 26 production of tropospheric O 3 . A change in the abundance of tropospheric O 3 perturbs 

### 27 the radiative balance of the atmosphere, an effect quantified by the radiative forcing 

### 28 metric. The earth

- atmosphere

- ocea n system responds to the radiative forcing with a 

### 29 climate response, typically expressed as a change in surface temperature. Finally, the 

### 30 climate response causes downstream climate

- related health and ecosystem impacts, such 

### 31 as redistribution of diseases or ecosystem characteristics due to temperature changes. 

### 32 Feedbacks from both the climate response and downstream impacts can, in turn, affect 

### 33 the abundance of tropospheric O 3 and O 3 precursors through multiple feedback 

### 34Draft

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- 8 September 2011 mechanisms. Direct feedbacks are discuss ed further in Section 10.3.3.4; the downstream 

### 1 climate impacts and their feedbacks are extremely complex and outside the scope of this 

### 2 assessment. 

### 3

Figure 10 - 2 Schematic illustrating the effects of tropospheric ozone on climate. Figure includes the relationship between precursor emissions, tropospheric ozone abundance, radiative forcing, climate response, and climate impacts. Units shown are those typical for each quantity illustrated. Feedbacks from both the climate response and climate impacts can, in turn, affect the abundance of tropospheric ozone and ozone precursors through multiple feedback mechanisms. Climate impacts are deemphasized in the figure since these downstream effects are extremely complex and outside the scope o f this assessment.

The IPCC ( 2007b ) reported a radiative forcing of 0.35 W/m 2 for the change in 

### 4 tropospheric O 3 since the preindustrial era, ranking it third in importance after the 

### 5 greenhouse gases CO 2 (1.66 W/m 2 ) and CH 4 (0.48 W/m 2 ). Figure 10

- 3 shows the global 

### 6 average radiative forcing estimates and uncertainty ranges in 2005 for anthropogenic 

### 7 CO 2 , CH 4 , O 3 and other important agents and mechanisms . The error bars encompassing 

### 8Draft

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- 9 September 2011 the tropospheric O 3 radiative forcing estimate in the figure range from 0.25 to 0.65 W/m 2 , 

### 1 making it relatively more uncertain than the long

- lived greenhouse gases. 

### 2

Source: Used with permission from Cambridge University Pres s, IPCC ( 2007b ) 

### Figure 10 - 3 Global average radiative forcing (RF) estimates and uncertainty ranges in 2005 for anthropogenic CO 2 

### , CH 4 

### , ozone and other important agents and mechanisms
. Figure shows the typical geographical extent (spatial scale) of the radiative forcing and the assessed level of scientific understanding (LOSU). The net anthropogenic radiative forcing and its range are also shown. T hese require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition. Additional radiative forcing factors not included here are considered to have a very low LOSU.Draft

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- 10 September 2011 

### 10.3.2 Factors that Influe nce the Effect of T ropospheric Ozone on Climate This section describes the main factors that influence the magnitude of the climate 

### 1 response to changes in tropospheric O 3 . They include: (1) trends in the concentration of 

### 2 tropospheric O 3 ; (2) the effect of surface albedo on O 3 radiative forcing; (3) the effect of 

### 3 vertical distribution on O 3 radiative forcing; (4) feedback factors that can alter the climate 

### 4 response to O 3 radiative forcing; and (5) the indirect effects of tropospheric O 3 on the 

### 5 carbon cycle. Trends in stratosph eric O 3 may also affect temperatures at the Earth ’ s 

### 6 surface, but aside from issues relating STE discussed in Chapter 3, Section 3.4.2, 

### 7 stratospheric O 3 assessment is beyond the scope of this document. 

### 8

10.3.2.1 Trends in the Concentration of Tropospheric Ozone
To fi rst order, the effect of tropospheric O 3 on climate is proportional to the change in 

### 9 tropospheric O 3 concentration. The earth ’ s surface temperatures are most sensitive to O 3 

### 10 perturbations in the mid to upper troposphere. This section therefore focuses main ly on 

### 11 observed O 3 trends in the free troposphere or in regions far from O 3 sources, where a 

### 12 change in O 3 concentrations may indicate change throughout the troposphere. Data from 

### 13 ozonesondes, mountaintops, and remote surface sites are discussed, as well as satellite 

### 14 data. 

### 15

Observed Trends in Ozone Since the Preindustrial Era Measurements of O 3 at two European mountain sites dating from the late 1800s to early 

### 16 1900s show values at about 10 ppb, about one

- fifth the values observed today at similar 

### 17 sites ( Pavelin et al., 1999 ; Marenco et al., 1994 ) . The accuracy of these early 

### 18 measurements is questionable however, in part because they exhibit O 3 concentrations 

### 19 equivalent to or only a couple of parts per billion greater than those observed at nearby 

### 20 low

- altitude sites during the same time period ( Mickley et al., 2001 ; Volz and Kley, 

### 21 1988 ) . A larger vertical gradient in tropospheric O 3 would be expected because of its 

### 22 stratospheric source and its longer lifetime aloft. In another study, Staehelin et al. ( 1994 ) 

### 23 revisited observations made in the Swiss mountains during the 1950s and found a 

### 24 doubling in O 3 concentrations from that era to 1989

- 1991. 

### 25 Routine observations of O 3 in the troposphere began in the 1970s with the use of balloon



### 26 borne ozonesondes, but even this record is sparse. Trends from ozonesondes have been 

### 27 highly variable and depe ndent on region ( Logan et al., 1999 ) . Over most sites in the U.S., 

### 28 ozonesondes reveal little trend. Over Canada, observations show a dec line in O 3 between 

### 29 1980 and 1990, then a rebound in the following decade ( Tarasick et al., 2005 )

. 

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- 11 September 2011 Ozonesondes over Europe give a mixed picture. European ozonesondes showed 

### 1 significant increases in the 1970s and 1980s, with smaller increases or even declines 

### 2 since then ( Oltmans et al., 2006 ; Logan et al., 1999 ) . Over Japan, O 3 in the lower 

### 3 troposph ere increased about 0.2

- 0.4 ppb/y during the 1990s ( Naja and Akimoto, 2004 )

. 

### 4 Ground

- based measurements in remote regions provide a record of tropospheric O 3 

### 5 extending as far back as the late 1960s or, for ship measurements, the late 1970s. A long



### 6 term record of O 3 in the San Bernardino Mountains of California reveals that the number 

### 7 of high O 3 days (defined as days with daily maximum O 3 levels above 95 ppb) rose from 

### 8 about 100 per summer in 1969 to over 160 in 1978 ( Lee et al., 2003a ) . Ov er the next 20 

### 9 years, the number of high O 3 days dropped slowly, to well below 100 per summer by the 

### 10 end of the record in 1999. Springtime O 3 observations from several other mountain sites 

### 11 in the western U.S. show a positive trend of about of 0.5

- 0.7 ppb/y since the 1980s 

### 12 ( Cooper et al., 2010 ; Jaffe et al., 2003 ) . Ship

- borne O 3 m easurements for the time period 

### 13 1977 to 2002 indicate increases of 0.1

- 0.7 ppb/y over the tropical and South Atlantic, but 

### 14 no significant change over the North Atlantic ( Lel ieveld et al., 2004 ) . The lack of trend 

### 15 for the North Atlantic would seem at odds with O 3 observations at Mace Head on the 

### 16 west coast of Ireland, which show a significant positive trend of about 0.5 ppb/y from 

### 17 1987 to 2003 ( Simmonds et al., 2004 ) . Over Japan, O 3 at a remote mountain site has 

### 18 increased 1 ppb/y from 1998 to 2003 ( Tanimoto, 2009 ) , a rate more than double that 

### 19 recorded by ozonesondes in the lower troposphere over Japan during the 1990s ( Naja and 

### 20 Akimoto, 2004 ) . At Zugspitze, a mountain site in Germany, O 3 increased by 12% per 

### 21 decade during the 1970s and 1980s, consistent with European ozonesondes ( Oltmans et 

### 22 al., 2006 ) . Since then, O 3 continues to increase at Zugspitze, but more slowly. What little 

### 23 data exist for the Southern Hemisphere point to significant increases in tropospheric O 3 

### 24 in recent decades, as much a s ~15% at Cape Grim in the 1989

- 2004 time period ( Oltmans 

### 25 et al., 2006 )

. 

### 26 The satellite record is now approaching a length that can be useful for diagno sing trends 

### 27 in the total tropospheric O 3 column (details on the use of satellites to measure 

### 28 tropospheric O 3 are covered in Section 3.5.5.5). In contrast to the surface data from ships, 

### 29 tropospheric O 3 columns from the Total Ozone Mapping Spectrometer (TOM S) show no 

### 30 trend over the tropical Atlantic for the period 1980

- 1990 ( Thompson and Hudson, 1999 )

. 

### 31 Over the Pacific, a longer, 25 year record of TOMS data again reveals no trend over the 

### 32 tropics, but shows increases in tropospheric column O 3 of about 2

- 3 Dobson Units (DU) 1 

### 33 at midlatitudes in b oth hemispheres ( Ziemke et al., 2005 )

. 

### 34

1 The Dobson Unit is a typical unit of measure for the total O 3 in a vertical column above t he Earth’s surface

. One DU is equivalent to the amount of O 3 that would exist in a 1 μm ( 10 – 5 m ) thick layer of pure O 3 at standard temperature (0 oC ) and pressure (1 atm), and corresponds to a column of O 3 containing 2.69 x 10 20 molecules /m 2 . A typical value for the amount of ozone in a column of the Earth’s atmosphere, although highly variable, is 300 DU and approximately 10% (30 DU) of that exists in the troposphere at mid latitudes

.Draft

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- 12 September 2011 Interpreting these recent trends in tropospheric O 3 is challenging. The first difficulty is 

### 1 reconciling apparently contradictory trends in the observations, e.g., over tropical oceans. 

### 2 A second difficulty is that the O 3 tr ends depend on several factors, not all of which can be 

### 3 well characterized. These factors include (1) trends in emissions of O 3 precursors, (2) 

### 4 variation in the stratospheric source of O 3 , (3) changes in solar radiation resulting from 

### 5 stratospheric O 3 depl etion, and (4) trends in tropospheric temperatures ( Fusco and Logan, 

### 6 2003 ) . The trends in O 3 in the San Bernardino Mountains reported by Lee et al

. ( 2003a ) 

### 7 likely reflects regional increases in population and motor vehicles usage, and sub sequent 

### 8 implementation of more stringent motor vehicle emissions controls. More recent positive 

### 9 trends in the western U.S. and over Japan are consistent with the rapid increase in 

### 10 emissions of O 3 precursors from mainland Asia and transport of pollution acr oss the 

### 11 Pacific ( Cooper et al., 2010 ; Tanimoto, 2009 ) . The satellite trends over the northern mid



### 12 latitude s are consistent with this picture as well ( Ziemke et al., 2005 ) . Increases in 

### 13 tropospheric O 3 in the Southern Hemisphere are also likely due to increased 

### 14 anthropogenic NO X emissions, especially from biomass burning ( Fishman et al., 1991 )

. 

### 15 Recent declines in summertime O 3 over Europe can be partly explained by decreases in 

### 16 O 3 precursor emissions there ( Jonson et al., 2005 ) , while springtime increases at some 

### 17 European sites are likely linked to changes in stratospheric dynamics ( Ordonez et al., 

### 18 2007 )

. Over Canada, Fusco and Logan ( 2003 ) found that O 3 depletion in the lowermost 

### 19 stratosphere may have reduced the stratospheric flux of O 3 into the troposphere by as 

### 20 much as 30% f rom the early 1970s to the mid 1990s, consistent with the trends in 

### 21 ozonesondes there. 

### 22

Calculation of Ozone Trends for the Recent Past Attempts to simulate trends in tropospheric O 3 allow us to test current knowledge of O 3 

### 23 processes and to predict with gre ater confidence trends in future O 3 concentrations. 

### 24 Time

- dependent emission inventories of O 3 precursors have also been developed ( for 

### 25 1850

- 2000, Lamarque e t al., 2010 ; for 1890

- 1990, Van Aardenne et al., 2001 ) . These 

### 26 inventories allow for the calculation of changing O 3 concentration over time. 

### 27 One recent multi

- model study calculated an increase in the O 3 concentration since 

### 28 preindustrial times of 8

- 14 DU, or about 30

- 70% ( Gauss et al., 2006 ) . The large spread in 

### 29 modeled estimates reveals our limited knowledge of processes i n the pristine atmosphere. 

### 30 Models typically overestimate the late nineteenth and early twentieth century 

### 31 observations available in surface air and at mountain sites by 50

- 100% ( Lamarque et al., 

### 32 2005 ; Shindell et al., 2003 ; Mickley et al., 2001 ; Kiehl et al., 1999 ) . Reconciling the 

### 33 differences between models and measurements will require more accurate simulation of 

### 34 the natural sources of O 3 ( Mickley et al., 2001 ) and/or implementation of novel sinks 

### 35Draft

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- 13 September 2011 such as bromine radicals, which may reduce background O 3 in the pristine atmosphere by 

### 1 as much as 30% ( Yang et al., 2005c )

. 

### 2 For the more recent past (since 1970), application of time

- dependent emiss ions reveals an 

### 3 equatorward shift in the distribution of tropospheric O 3 in the Northern Hemisphere due 

### 4 to the industrialization of societies at low

- latitudes ( Lamarque et al., 2005 ; Berntsen et 

### 5 al., 2000 )

. By constraining a model with historical (1950s

- 2000) observations, Shindell et 

### 6 al. ( 2002 ) calculated a large increase of 8.2 DU in tropospheric O 3 over polluted 

### 7 continental regions since 1950. Their result appears consistent with the large change in 

### 8 tropospheric O 3 since preindustrial times implied by th e observations from the late 1800s 

### 9 ( Pavelin et al., 1999 ; Marenco et al., 1994 )

. 

### 10

10.3.2.2 The Effect of Surface Albedo on Ozone Radiative Forcing
The Earth ’ s surface albedo plays a role in O 3 radiative forcing. Through most of the 

### 11 troposphere, absorption of incoming shortwave solar radiation by O 3 is small relative to 

### 12 its absorption of outgoing longwave terrestrial radiation. Howev er, over surfaces 

### 13 characterized by high albedo (e.g., over snow, ice, or desert sand), incoming radiation is 

### 14 more likely to be reflected than over darker surfaces, and the probability that O 3 will 

### 15 absorb shortwave solar radiation is therefore larger. In ot her words, energy that would 

### 16 otherwise return to space may instead be deposited in the atmosphere. Several studies 

### 17 have shown that transport of O 3 to the Arctic from mid

- latitudes leads to radiative forcing 

### 18 estimates greater than 1.0 W/m 2 in the region, es pecially in summer ( Shindell et al., 2006 ; 

### 19 Liao et al., 2004b ; Mickley et al., 1999 ) . Because the Arctic is especially sensitive to 

### 20 radiative forcing through the ice

- albedo feedback, the large contribution in the shortwave 

### 21 solar spectrum to the total radiative forcing in the region may be important. 

### 22

10.3.2.3 The Effect of Vertical Distribution on Ozone Radiative Forcing In the absence of feedbacks, O 3 increments near the tropopause produce the largest 

### 23 increases in surface temperature ( Lacis et al., 1990 ; Wang et al., 1980 ) . This is a result of 

### 24 the colder temperature of the tropopause relative to the rest of the troposphere and 

### 25 stratosphere. Since radiation emitted by the atmosphere is approximately proportional t o 

### 26 the fourth power of its temperature 2 , the colder the added O 3 is relative to the earth ’ s 

### 27

2 As described by the Stefan
- Boltzman n law, an ideal blackbody
-- which the atmosphere approximates
-- absorbs at all wavelengths and re
- radiates proportional to the fourth power of its temperature.Draft

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- 14 September 2011 surface, the weaker the radiation emitted and the greater the “ trapping ” of longwave 

### 1 radiation in the troposphere. 

### 2

10.3.2.4 Feedback Factors that Alter the Climate Response t o Changes in Ozone Radiative Forcing Estimates of radiative forcing provide a first

- order assessment of the effect of 

### 3 tropospheric O 3 on climate. In the atmosphere, climate feedbacks and transport of heat 

### 4 alter the sensitivity of Earth ’ s surface temperatur e to addition of tropospheric O 3 . 

### 5 Assessment of the full climate response to increases in tropospheric O 3 requires use of a 

### 6 climate model to simulate these interactions. 

### 7 Due to its short lifetime, O 3 is heterogeneously distributed through the troposphere. 

### 8 Sharp horizontal gradients exist in the radiative forcing of O 3 , with the greatest radiative 

### 9 forcing since preindustrial times occurring over the northern mid

- latitudes (more on this 

### 10 in Section 10.3.4 ). If climate feedbacks are particularly powerful, they may obscure or 

### 11 even erase the correlation between regional radiative forcing and climate response 

### 12 ( Harvey, 2004 ; Boer and Yu, 2003 ) . For example, several model studies have reported 

### 13 that the horizontal pattern of surface temperature response from 2000

- 2100 trends in 

### 14 predicted short

- lived species (including O 3 ) closely matches the pattern from t he trends 

### 15 in the long

- lived greenhouse gases over the same time period ( Levy et al., 2008 ; Shindell 

### 16 et al., 2008 ; Shindell et al., 2007 ) . This correspondence occurs even though the patterns 

### 17 of radiative forcing for the short

- lived and long

- lived species differ significantly. In a 

### 18 separate paper, Shindell ( 2007 ) found that Arctic temperatures are especially sensitive to 

### 19 the mid

- latitude radiative forcing from tropospheric O 3 . 

### 20 Other studies have found that the signature of warming due to tropospheric O 3 does show 

### 21 some consistency with the O 3 radiative forc ing. For example, Mickley et al. ( 2004 ) 

### 22 examined the change in O 3 since preindustr ial times and found greater warming in the 

### 23 Northern Hemisphere than in the Southern Hemisphere (+0.4oC versus +0.2oC), as well 

### 24 as higher surface temperatures downwind of Europe and Asia and over the North 

### 25 American interior in summer. For an array of short

- lived species including O 3 , Shindell 

### 26 and Faluvegi ( 2009 ) found that radiative forcing applied over northern mid

- latitudes yield 

### 27 more localized responses due to local cloud, water vapor, and albedo feedbacks than 

### 28 radiative forcing applied over the tropics. 

### 29 Climate feedbacks can also alter the sensitivity of surface temperature to the vertical 

### 30 distribution of tropospheric O 3 . The previous section (Section 10.3.2.3 ) described the 

### 31 greater impact of O 3 added to the upper troposphere (near the tropopause) on radiative 

### 32 forcing, relative to additions in the mid

- to lower troposphere. However, warming 

### 33Draft

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- 15 September 2011 induced by increased O 3 in the upper troposphere could stabilize the atmosphere to some 

### 1 extent, limiting the transport of heat to the Earth ’ s surface and mitigating the impact of 

### 2 the added O 3 on surface temperature ( Joshi et al., 2003 ; Christiansen, 1999 ) . Hansen et 

### 3 al. ( 1997 ) determined that allowing cloud feedbacks in a climate model meant that O 3 

### 4 enhanceme nts in the mid

- troposphere had the greatest effect on surface temperature. 

### 5 Finally, climate feedbacks can amplify or diminish the climate response of one 

### 6 greenhouse gas relative to another. For example, Mickley et al. ( 2004 ) found a greater 

### 7 temperature response to CO 2 radiative forcing than to an O 3 radiative forcing of similar 

### 8 global mean magnitude, due in part to the relatively weak ice

- albedo feedback for O 3 . 

### 9 Since CO 2 absorbs in the same bands as water vapor, CO 2 radiative forcing saturates in 

### 10 the middle troposphere and is also shifted toward the drier poles. A poleward shift in 

### 11 rad iative forcing amplifies the ice

- albedo feedback in the case of CO 2 , and the greater 

### 12 mid

- troposphere radiative forcing allows for greater surface temperature response, 

### 13 relative to that for O 3 . 

### 14

10.3.2.5 Indirect Effects of Tropospheric Ozone on the Carbon Cycle A pr oposed indirect effect of tropospheric O 3 on climate involves the carbon cycle. By 

### 15 directly damaging plant life in ways discussed in Chapter 9, increases in tropospheric O 3 

### 16 may depress the land

- carbon sink of CO 2 , leading to accumulation of CO 2 in the 

### 17 atmo sphere and ultimately warming of the Earth ’ s surface. Sitch et al. ( 2007 ) calculated 

### 18 that this indirect warming effect of O 3 on climate has about the same magnitude as the 

### 19 O 3 direct effect. Their results su ggest a doubled sensitivity of surface temperatures to O 3 

### 20 radiative forcing, compared to current model estimates. 

### 21

10.3.3 Competing Effects of Ozone Precursors on Climate Changes in O 3 precursors affect not just O 3 concentrations, but also other species that 

### 22 have importance to the radiative balance of the earth ’ s climate system. More specifically, 

### 23 O 3 and its precursors exert a strong control on the oxidizing capacity of the troposphere 

### 24 ( Derwent et al., 2001 ) . For example, an increase in CO or VOCs would lead to a decrease 

### 25 in hydroxyl (OH) concentrations . Since OH is a major sink of the greenhouse gas CH 4 , a 

### 26 decline in OH would lengthen the CH 4 lifetime, enhance the CH 4 concentration, and 

### 27 amplify surface warming. A rise in NO X emissions, on the other hand, could lead to an 

### 28 increase in OH in certain locati ons, shortening the CH 4 lifetime and leading to surface 

### 29 cooling ( Fuglestvedt et al., 1999 ) . O 3 can itself generate OH through (1) photolysis 

### 30Draft

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- 16 September 2011 leading to excited oxygen atoms followed by reaction with water vapor and (2) reaction 

### 1 with HO 2 . 

### 2 Analyzing the net radiati ve forcing per unit emission for a suite of O 3 precursors, 

### 3 Shindell and Faluvegi ( 2009 ) calculated positive (+0.25 W/m 2 ) radiative forcing from the 

### 4 increase in anthropogenic emissions of CO and VOCs since preindustrial times, as well 

### 5 as for CH 4 (+1 W/m 2 ). These species also contribute to warming via their eventual 

### 6 contribution to CO 2 . In contrast, Shindell and Faluvegi ( 2009 ) found negative (

- 

### 7 0.29 W/m 2 ) radiative forcing from anthropogenic emissions of NO X due mainly to the 

### 8 link between NO X and CH 4 . These results are consistent with those of Forster et al. 

### 9 ( 2007 ) who reported a net warming of +0.27 W/m 2 for combined anthropogenic CO and 

### 10 VOCs emissions and a net cooling of

- 0.21 W/m 2 for anthropogenic NO X emissions. 

### 11 Other studies have found a near cancellation of the positive O 3 radiative forcing and the 

### 12 negative CH 4 radiative forcing that arise from an incremental increase in anthropogenic 

### 13 NO X emissions ( Naik et al., 2005 ; Fiore et al., 2002 ; Fuglestvedt et al., 1999 )

. 

### 14 The net effect of aircraft NO X on climate is complex. While Isaksen et al. ( 2001 ) reported 

### 15 that the net radiative forcing effect of aircraft NO emissions is near zero, Wild et al. 

### 16 ( 2001 ) calculated a net warming due to increased O 3 production efficiency in the upper 

### 17 troposphere. More recently, Stevenson et al. ( 2004 ) completed a detailed analysis of the 

### 18 OH budget in the years following a pulse of aircraft NO X emissions. They calculated that 

### 19 while such a pulse leads initially to warming through O 3 production over a few months, 

### 20 th e long

- term effect is cooling through the effects on CH 4 . Both aircraft NO X and the O 3 

### 21 it generates enhance OH concentrations, with the longer

- lived O 3 responsible for 

### 22 transferring the oxidizing effects of aircraft emissions away from flight corridors. 

### 23 Fi nally, OH production from O 3 precursors can affect regional sulfate air quality and 

### 24 climate forcing by increasing gas

- phase oxidation rates of SO 2 . Using the A1B scenario 

### 25 in the IPCC AR4, Unger et al. ( 2006 ) reported that at 2030, enhanced OH from the A1B 

### 26 O 3 precursors increased surface sulfate aerosol concentrations by up to 20% over India 

### 27 and China, relative to the present

- day, with a corresponding increase in radiative cooling 

### 28 over these regions. In this way, O 3 precursors may impose an indirect cooling via sulfate 

### 29 ( Unger, 2006 )

. 

### 30 Taken together, these results point out the need for careful assessment of net radiative 

### 31 forcing involving mul tiple pollutants in developing climate change policy ( Unger et al., 

### 32 2008 ) . Naik et al. ( 2005 ) calculated that a carefully combined reduction of CO, VOC s, 

### 33 and NO X emissions could lead to net cooling, especially over the tropics. Several studies 

### 34 point to CH 4 as a particularly attractive target for emissions control since CH 4 is itself an 

### 35 important precursor of O 3 ( West et al., 2007 ; Fiore et al., 2002 ) . Shindell et al. ( 2005 ) 

### 36 calculated that the emissions

- based radiative forcing of anthropogenic CH 4 , which 

### 37Draft

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- 17 September 2011 includes both its own radiative forcing and that of CH 4 - generated O 3 , is 0.8

- 0.9 W/m 2 , 

### 1 about double that of the CH 4 abundance

- based radiative forcing. Fiore et al. ( 2002 ) found 

### 2 that reducing anthropogenic CH 4 emissions by 50% would lead to a global negative (

- 

### 3 0.37 W/m 2 ) radiative forcing, mostly from CH 4 . In later research, Fiore et al. ( 2008 ) 

### 4 reported that CH 4 reductions would most strongly affect tropospheric O 3 column 

### 5 amounts in a zonal band centered around 30 N, a region of strong downwelling and NO X 

### 6 saturated conditions near the surface. 

### 7

10.3.4 Calculating Radiative Forcing and Climate Response to Past Trends in Tropospheric Ozone The magnitude of the radiative forcin g from the change in tropospheric O 3 since the 

### 8 preindustrial era is uncertain. This uncertainty derives in part from the scarcity of early 

### 9 measurements and in part from our limited knowledge regarding processes in the natural 

### 10 atmosphere. As noted previousl y, the IPCC AR4 reports a radiative forcing of 0.35 W/m 2 

### 11 from the change in tropospheric O 3 since 1750 ( Forster et al., 2007 ) , ranking it third in 

### 12 importance among greenhouse gases after CO 2 and CH 4 . The O 3 radiative forcing could, 

### 13 in fact, be as large as 0.7 W/m 2 , if reconstructions of pr eindustrial and mid

- 20 th century 

### 14 O 3 based on the measurement record are valid ( Shindell and Faluvegi, 2002 ; Mickley et 

### 15 al., 2001 )

. In any event, Unger et al. ( 2010 ) showed that present

- day O 3 radiative forcing 

### 16 can be attributed to emissions from many economic sectors, including on

- road vehicles, 

### 17 household biofuel, power generation, and biomass burning. As much as one

- third of the 

### 18 radi ative forcing from the 1890 to 1990 change in tropospheric O 3 could be due to 

### 19 increased biomass burning ( Ito et al., 2007a )

. 

### 20 These calculated radiative forcing estimates can be compared to those obtained from 

### 21 satellite data. Using data from TOMS, Worden et al. ( 2008 ) estimated a reduction in 

### 22 clear

- sky outgoing longwave radiation of 0.48 W/m 2 by O 3 in the upper troposphere over 

### 23 oceans in 2006. This radiative forcing includes contributions from both anthro pogenic 

### 24 and natural O 3 . Assuming that the concentration of O 3 has roughly doubled since 

### 25 preindustrial times ( Gauss et al., 2006 ) , the total O 3 radiative forcing estimated with 

### 26 TOMS is consistent with that obtained from models estimating just the anthropogenic 

### 27 contribution. 

### 28 Calculation of the climate response to the O 3 radiative forcing is challenging due to 

### 29 complexity of feedbacks, as mentioned in Sections 10.3.1.2 and 10.3.2.4 . In their 

### 30 modeling study, Mickley et al. ( 2004 ) reported a global mean increase of 0.28oC since 

### 31 preindustrial times, with value s as large as 0.8oC in continental interiors. For the time 

### 32 period since 1870, Hansen et al. ( 2005 ) estimated a much smaller increase in global mean 

### 33Draft

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- 18 September 2011 surface temperature (0.11oC), but the y implemented 1880s anthropogenic emissions in 

### 1 their base simulation and also took into account trends in both stratospheric and 

### 2 tropospheric O 3 ; the modeled decline of lower stratospheric O 3 , especially over polar 

### 3 regions, cooled surface temperatures in t his study, counteracting the warming effect of 

### 4 increasing tropospheric O 3 . 

### 5 Figure 10

- 4 shows the Hansen et al. ( 2005 ) results as reported in Shindell et al. ( 2006 ) . In 

### 6 that figure, summertime O 3 has the largest radiative impact over the continental interiors 

### 7 of the Northern Hemisphere. Shindell et al. ( 2006 ) estimated that the change in 

### 8 tropospheric O 3 over the 20 th century could have contributed about 0.3 oC to annual mean 

### 9 Arctic warming and as much as 0.4

- 0.5 oC during winter and spring. Over eastern China, 

### 10 Chang et al. ( 2009 ) calc ulated a surface temperature increase of 0.4oC to the 1970

- 2000 

### 11 change in tropospheric O 3 . It is not clear, however, to what degree regional changes in 

### 12 O 3 concentration influenced this response, as opposed to more global changes. 

### 13Draft

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- 19 September 2011 Source: Used with permis sion from American Geophysical Union ( Shindell et al., 2006 ) Figure includes the input radiativ e forcing (W / m 2 ), as computed by the NASA GISS chemistry
- climate model. Values are surface temperature trends for the annual average (top left), June

– August (bottom left), and December
- February (bottom right) and annual average tropopause instantaneous rad iative forcing from 1880 to 1990 (top right). Temperature trends greater than about 0.1 oC are significant over the oceans, while values greater than 0.3 oC are typically significant over land, except for northern middle and high latitudes during winter wher e values in excess of about 0.5 oC are significant. Values in the top right corner give area
- weighted global averages in the same units as the plots. 

### Figure 10 - 4 Ensemble average 1900 - 2000 surface temperature trends ( oC per century) in response to tropospheric ozone changes.


## 10.4 UV - B Related Effects and Tropospheric Ozone

10.4.1 Background UV radiation emitted from the Sun contains sufficient energy when it reaches the Earth to 

### 1 break (photolyze) chemical bonds in molecules, there by leading to damaging effects on 

### 2 living organisms and materials. Atmospheric O 3 plays a crucial role in reducing exposure 

### 3 to solar UV radiation at the Earth ’ s surface. Stratospheric O 3 is responsible for the 

### 4 majority of this shielding effect, as approxima tely 90% of total atmospheric O 3 is located 

### 5 there over mid

- latitudes ( Kar et al., 2010 ; Crist et al., 1994 ) . Investigation of the 

### 6Draft

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- 20 September 2011 supplemental shielding of UV

- B radiation provided by tropospheric O 3 is necessary for 

### 1 quantifying UV

- B exposure and the incidence of related human health effects, ecosystem 

### 2 effects, and material s damage. The role of tropospheric O 3 in shielding of UV

- B radiation 

### 3 is discussed in this section. 

### 4

10.4.2 Human Exposure and Susceptibility to Ultraviolet Radiation The factors that potentially influence UV radiation exposure were discussed in detail in 

### 5 Chapter 1 0 of the 2006 O 3 AQCD and are summarized here. These factors included 

### 6 outdoor activity, occupation, age, gender, geography, and protective behavior. Outdoor 

### 7 activity and occupation both influenced the amount of time people spend outdoors during 

### 8 daylight ho urs, the predominant factor for exposure to solar UV radiation. Participation in 

### 9 outdoor sports (e.g., basketball, soccer, golf, swimming, cycling) significantly increased 

### 10 UV radiation exposure ( Thieden et al., 2004a ; Thieden et al., 2004b ; Moehrle, 2001 ; 

### 11 Moehrle et al., 2000 ) . Occupations that substantially increased exposure to UV radiation 

### 12 included farming ( Schenker et al., 2002 ; Airey et al., 1997 ) , fishing ( Rosenthal et al., 

### 13 1988 ) , landscaping ( Rosenthal et al., 1988 ) , construction ( Gies and Wright, 2003 ) , 

### 14 physical education ( Vishvakarman et al., 2001 ) , mail delivery ( Vishvakarman et al., 

### 15 2001 ) , and various other occupations that require workers to spend the majority of their 

### 16 day outdoors during peak UV radiation hours. 

### 17 Age and gender were found to b e factors that influence human exposure to UV radiation, 

### 18 particularly by influencing other factors of exposure such as outdoor activity and risk 

### 19 behavior. Studies indicated that females generally spent less time outdoors and, 

### 20 consequently, had lower UV rad iation exposure compared to males ( Godar et al., 2001 ; 

### 21 Gies et al., 1998 ; Shoveller et al., 1998 ) . The lowest exposure to UV radiation among 

### 22 Americans in the Godar et al. ( 2001 ) study was received in females during their child 

### 23 raising years (age 22

- 40 years); the highest exposure was observed in males aged 

### 24 41

- 59 years. A similar Canadian survey found that yo unger adult males had the greatest 

### 25 exposures to UV radiation ( Shoveller et al., 1998 )

. 

### 26 Geography influences the degree of solar UV flux to the surface, and hence exposure to 

### 27 UV radiation. In the U.S. study by Godar et al. ( 2001 ) , northerners and southerners were 

### 28 found to spend an equal amount of time o utdoors; however, the higher solar flux at lower 

### 29 latitudes significantly increased the annual UV radiation dose for southerners. The annual 

### 30 UV radiation doses in southerners were 25 and 40% higher in females and males, 

### 31 respectively, compared to northerners . Other studies also have shown that altitude and 

### 32 latitude influence personal exposure to UV radiation ( Rigel et al., 1999 ; Kimlin et al., 

### 33 1998 )

. 

### 34Draft

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- 21 September 2011 Protective behaviors such as using sunscreen ( Nole and Johnson, 2004 ) , wearing 

### 1 protective clothing ( Rosenthal et al., 1988 ) , and spe nding time in shaded areas ( Moise et 

### 2 al., 1999 ) were shown to reduce expos ure to UV radiation. In one study, the use of 

### 3 sunscreen was associated with extended intentional UV radiation exposure ( Autier et al., 

### 4 1999 ) ; however, a follow

- up study indicated that sunscreen use increased duration of 

### 5 exposures to doses of UV radiation that were below the threshold level for erythema 

### 6 ( Autier et al., 2000 )

. 

### 7 Given these and other factors that potentially influence UV radiation exposure, the 2006 

### 8 O 3 AQCD l isted the following subpopulations potentially at risk for higher exposures to 

### 9 UV radiation: 

### 10

 Individuals who engage in high

- risk behavior (e.g., sunbathing); 

### 11

 Individuals who participate in outdoor sports and activities; 

### 12

 Individuals who work outdoors with i nadequate shade (e.g., farmers, 

### 13 construction workers, etc.); and 

### 14

 Individuals living in geographic areas with higher solar flux including lower 

### 15 latitudes (e.g., Honolulu, HI) and higher altitudes (e.g., Denver, CO). 

### 16 The risks associated with all these facto rs are, of course, highly dependent on season and 

### 17 region ( Sliney and Wengraiti s, 2006 )

. 

### 18

10.4.3 Human Health Effects due to UV - B Radiation Chapter 10 of the 2006 O 3 AQCD covered in detail the human health effects associated 

### 19 with solar UV

- B radiation exposure. These effects include erythema, skin cancer, ocular 

### 20 damage, and immune system s uppression. These adverse effects, along with protective 

### 21 effects of UV radiation through increased production of vitamin D are summarized in this 

### 22 section. For additional details, the reader is referred to Chapter 10 of the 2006 O 3 AQCD 

### 23 ( U.S. EPA, 2006b ) and references therein. 

### 24 The most conspicuous and well

- recognized acute response to UV radiation is erythema, 

### 25 or the reddening of the skin. Erythema is likely caused by direct damage to DNA by UV 

### 26 radiation ( Matsumura and Ananthaswamy, 2004 ) . Many studies discussed in the 2006 O 3 

### 27 AQCD found skin type to be a significant risk factor for erythema. Additional risk factors 

### 28 include atopic dermatitis ( ten Berge et al., 2009 )

. 

### 29 Skin cancer is another prevalent health effect associated with UV radiation. Exposure to 

### 30 UV radiation is considered to be a major risk factor for all forms of skin cancer ( Diepgen 

### 31 and Mahler, 2002 ; Gloster and Brodland, 1996 ) . Ultraviolet radiation is especially 

### 32Draft

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- 22 September 2011 effective in inducing genetic mutations and acts as both a tumor initiator and promoter. 

### 1 Keratinocytes have evolved DNA repair mechanisms to correct the damage induced by 

### 2 UV; however, mutations can occur , leading to skin cancers that are appearing with 

### 3 increasing frequency ( Hildesheim and Fornace, 2004 ) . The relations hip between skin 

### 4 cancer and chronic exposure to UV radiation is further explored in Chapter 10 of the 

### 5 2006 O 3 AQCD ( U.S. EPA, 2006b )

. 

### 6 Ocular damage from UV radiation exposure includes effects on the cornea, lens, iris, and 

### 7 associated epi thelial and conjunctival tissues. The region of the eye affected by exposure 

### 8 to UV radiation depends on the wavelength of the incident UV radiation. Depending on 

### 9 wavelength, common health effects associated with UV radiation include photokeratitis 

### 10 (snow bl indness; short wavelengths) and cataracts (opacity of the lens; long 

### 11 wavelengths). 

### 12 Experimental studies have shown that exposure to UV radiation may suppress local and 

### 13 systemic immune responses to a variety of antigens ( Clydesdale et al., 2001 ; Garssen and 

### 14 Van Loveren, 2001 ; Selgrade et al., 1997 ) . In rodent models, these effects have been 

### 15 shown to worsen the course and outcome of some infectious diseases and cancers 

### 16 ( Granstein and Matsui, 2004 ; Norval et al., 1999 ) . Results from human clinical studies 

### 17 suggest that immune suppression induced by UV radiation may be a risk factor 

### 18 contributing to skin cancer induction ( Ullrich, 2005 ; Caforio et al., 2000 ; Lindelof et al., 

### 19 2000 ) . There is also evidence that UV radiation has indirect involvement in viral 

### 20 oncogenesis through the human papillomavirus ( Pfi ster, 2003 ) , dermatomyositis ( Okada 

### 21 et al., 2003 ) , human immunodeficiency virus ( Breuer

- McHam et al., 2001 ) and other 

### 22 forms of immunosuppression ( Selgrade et al., 2001 )

. 

### 23 A potential health benefit of increased UV

- B exposure relates to the production of 

### 24 vitamin D in humans. Most humans depend on s un exposure to satisfy their requirements 

### 25 for vitamin D ( Ho lick, 2004 ) . Vitamin D deficiency can cause metabolic bone disease 

### 26 among children and adults, and also may increase the risk of many common chronic 

### 27 diseases, including type I diabetes mellitus and rheumatoid arthritis ( Holick, 2004 )

. 

### 28 Substantial in vitro and toxicological evidence also support a role for vit amin D activity 

### 29 against the incidence or progression of various forms of cancer ( Giovannucci, 2005 ; John 

### 30 et al., 2005 ; Smedby et al., 2005 ; Grant and Garland, 2004 ; Hughes et al., 2004 ; 

### 31 Freedman et al., 2002 ; Grant, 2002a , b ; John et al., 1999 ; Studzinski and Moore, 1995 ; 

### 32 Lefkowitz and Garland, 1994 ; Hanchette and Schwartz, 1992 ; Garland et al., 1990 ; 

### 33 Gorham et al., 1990 ) . In some studies, UV

- B related production of vitamin D had 

### 34 potential beneficial immunomodulatory effects on multiple sclerosis, insulin

- dependent 

### 35 diabetes mellitus, and rheumatoid arthritis ( Ponsonby et al., 2002 ; Cantorna, 2000 ) . More 

### 36Draft

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- 23 September 2011 details on UV

- B protective studies are provided in Chapter 10 of the 2006 O 3 AQCD 

### 1 ( U.S. EPA, 2006b )

. 

### 2 In establishing guidelines on limits of exposure to UV radiation, the International 

### 3 commission on Non

- Ionizing Radiation Protection (ICNIRP) agreed that some low

- level 

### 4 exposure to UV radiation has health benefits ( ICNIRP, 2004 ) . However, the adverse 

### 5 health effects of higher UV exposures necessitated the development of exposure limits 

### 6 for UV radiation. The ICNIRP recognized the challen ge in establishing exposure limits 

### 7 that would achieve a realistic balance between beneficial and adverse health effects. As 

### 8 concluded by ICNIRP ( 2004 ) , “ [t]he present understanding of injury mechanisms and 

### 9 long

- term effects of exposure to [UV radiation] is incomplete, and awaits further 

### 10 research. ” 

### 11

10.4.4 Ec osystem and Materials Damage Effects Due to UV - B Radiation A 2009 progress report on the environmental effects of O 3 depletion from the UNEP, 

### 12 Environmental Effects Assessment Panel ( UNEP, 2009 ) lists many ecosystem and 

### 13 materials damage effects from UV

- B radiation. An in

- depth assessment of the global 

### 14 ecosystem and materials damage effects from UV

- B radiation per se is o ut of the scope of 

### 15 this assessment. However, a brief summary of some mid

- latitude effects is provided in 

### 16 this section to provide context for UV

- B related issues pertaining to tropospheric O 3 . The 

### 17 reader is referred to the UNEP report ( UNEP, 2009 ) and references therein for further 

### 18 details. All of these UV

- B related ecosystem and materials effects can also be influenced 

### 19 by climate change through temperature and other meteorological alterations, making 

### 20 quantifiable predictions of UV

- B effects difficult. 

### 21 Terrestrial ecosystem effects from increased UV

- B radiation include reduced plant 

### 22 productivity and plant cover, changes i n biodiversity, susceptibility to infection, and 

### 23 increases in natural UV protective responses. In general, however, these effects are small 

### 24 for moderate UV

- B increases at mid

- latitudes. A field study on wheat in southern Chile 

### 25 found no substantial changes in crop yield with moderate increases in UV

- B radiation 

### 26 ( Calderini et al., 2008 ) . Similarly, field studies on silver birch ( Betula pendula ) in 

### 27 Finland found no significant effects in photosynthetic function with increases in UV

- B 

### 28 radiation ( Aphalo et al., 2009 ) . Subtle, but important, changes in habitat and biodiversity 

### 29 have also been linked to increases in UV

- B radiation ( Mazza et al., 2010 ; Obara et al., 

### 30 2008 ; Wahl, 2008 ) . Some plants have natural coping mechanisms for dealing with 

### 31 changes in UV

- B radiation ( Favory et al., 2009 ; Jen kins, 2009 ; Brown and Jenkins, 2008 ; 

### 32 Ioki et al., 2008 ) , but these defenses may have costs in terms of reduced growth ( Snell et 

### 33 al., 2009 ; Clarke and Robinson, 2008 ; Se merdjieva et al., 2003 ; Phoenix et al., 2000 )

. 

### 34Draft

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- 24 September 2011 Aquatic ecosystem effects from increased UV

- B radiation include sensitivity in 

### 1 growth, immune response, and behavioral patterns of aquatic organisms. One study 

### 2 looking at coccolithophores, an abundant phytoplankton group, found a 25% reduction in 

### 3 cellular growth with UV

- B exposure ( Gao et al., 2009a ) . E xposure to relevant levels of 

### 4 UV

- B radiation has been shown to modify immune response, blood chemistry, and 

### 5 behavior in certain species of fish ( Markkula et al., 2009 ; Holtby and Bothwell, 2008 ; 

### 6 Jokinen et al., 2008 ) . Adverse effects on growth and development from UV

- B radiation 

### 7 have also been observed for amphibians, sea urchins, mollusks, corals, and zooplankton 

### 8 ( Garcia et al., 2009 ; Romansic et al., 2009 ; Croteau et al., 2008a ; Croteau et al., 2008b ; 

### 9 Marquis et al., 2008 ; Marquis and Miaud, 2008 ; Oromi et al., 2008 ) . Increases in the flux 

### 10 of UV

- B radiation may also result in an increase in the catalysis of trace metals including 

### 11 mercury, particularly in clear oligotrophic lakes with low levels of dissolved organic 

### 12 carbon to sto p the penetration of UV

- B radiation ( Schindler et al., 1996 ) . This could then 

### 13 alter the mobility of trace metals including the potential for increased mercury 

### 14 volatilization and transport within and among ecosystems. 

### 15 Biogeochemical cycles , particularly the carbon cycle, can also be influenced by 

### 16 increased UV

- B radiation. A study on high latitude wetlands found UV

- induced increases 

### 17 in CO 2 uptake through soil respiration ( Haapala et al., 2009 ) while studies on arid 

### 18 terrestrial ecosystems found evidence for UV

- induced release of CO 2 through 

### 19 photodegradation of above

- ground plant litter ( Brandt et al., 2009 ; Henry et al., 2008 ; 

### 20 Caldwell et al., 2007 ; Zepp et al., 2007 ) . Changes in solar UV radiation may also have 

### 21 effects on carbon cycling and CO 2 uptake in the oceans ( Brewer and Peltzer, 2009 ; 

### 22 Meador et al., 2 009 ; Fritz et al., 2008 ; Zepp et al., 2008 ; Hader et al., 2007 ) as well as 

### 23 release of dissolved organic matter from sediment and algae ( Mayer et al., 2009 ; 

### 24 Riggsbee et al., 2008 ) . Additional studies showing effects on these and additional 

### 25 biogeochemical cycles including the water cycle and halocarbon cycle can be found in 

### 26 the UNEP report ( UNEP, 2009 ) and references therein. 

### 27 Materials damage from increased UV

- B radiation include UV

- induced 

### 28 photodegradation of wood ( Kataoka et al., 2007 ) and plastics ( Pickett et al., 2008 ) . These 

### 29 studies and others summarizing photo

- resistant coatings and materials designed to reduce 

### 30 photodegradation of materials are summarized in th e UNEP report ( UNEP, 2009 ) and 

### 31 references therein. 

### 32Draft

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- 25 September 2011 

### 10.4.5 UV - B Related Effects Associated with Changes in Tropospheric Ozon e Concentrations There are multiple complexities in attempting to quantify the relationship between 

### 1 changes in tropospheric O 3 concentrations and UV radiation exposure. Quantifying the 

### 2 relationship between UV radiation and health or welfare effects is com plicated by the 

### 3 uncertainties involved in the selection of an action spectrum and appropriate 

### 4 characterization of dose (e.g., peak or cumulative levels of exposure, timing of exposures, 

### 5 etc.) The lack of published studies that critically examine these issu es together

-- that is the 

### 6 incremental health or welfare effects attributable specifically to UV

- B changes resulting 

### 7 from reductions in tropospheric O 3 concentrations

-- reflects the significant challenges in 

### 8 this field. 

### 9 As reported in the 2006 O 3 AQCD, one an alysis by Lutter and Wolz ( 1997 ) attempted to 

### 10 estimate the effects of a nationwide 10 ppb reduction in seasonal average tropospheric O 3 

### 11 on the incidence of nonmelanoma and melanoma skin cancers and cataracts in humans. 

### 12 Their estimate, however, depended upon several simpl ifying assumptions, ranging from 

### 13 an assumed generalized 10

- ppb reduction in O 3 column density, national annual average 

### 14 incidence rates for the two types of skin cancer, and simple, linear biological 

### 15 amplification factors. Specifically, the decrease of 10 p pbv in seasonally averaged O 3 

### 16 concentrations is likely an overestimate since it doesn ’ t account for the influence of 

### 17 background O 3 coming from the global accumulation or generation of regional chemistry 

### 18 ( Adamowicz et al., 2004 ) . Further, the methodologies used in this analysis have ignored 

### 19 area - specific factors that are important in estimating the extent to which small, va riable 

### 20 changes in ground

- level O 3 mediate long

- term exposures to UV

- B radiation. 

### 21 A more recent study by Madronich et al. ( 2011 ) used CMAQ to estimate UV radiation 

### 22 response to changes in tropospheric O 3 under different control scenarios projected out to 

### 23 2020. This study focused on southeastern U.S. and accounted for spatial and temporal 

### 24 variation in tropospheric O 3 reductions, an important consideration since most controls 

### 25 are focused on reducing O 3 in populated urban areas. The contrasting control strategies 

### 26 considered in this study included a historical scenario designed to meet an 84 ppb 8

- h 

### 27 daily max standard and a reduced scenario designed to bring areas predicted to exceed a 

### 28 similarly designed 70 ppb standard into attainment. A biologically effective irradiance 

### 29 was estimated by multiplying the modeled UV irradiance by a sensitivity function (action 

### 30 spectrum) for the induction of nonmelanoma skin cancer in mice corrected for human 

### 31 skin transmission, then integrating over UV wavelengths. The average relative change in 

### 32 skin cancer

- weighted surf ace UV radiation between the two scenarios was about 0.11% 

### 33 over June, July and August. Weighting by population, this estimate increased to 0.19%. 

### 34 Madronich et al. ( 2011 ) report that their estimated UV radiation increment is an order of 

### 35Draft

– Do Not Cite or Quote 10
- 26 September 2011 magnitude less than that by Lutter and Wolz ( 1997 ) with the main reason for the 

### 1 discrepancy coming from the unre alistic uniform 10 ppb reduction in O 3 assumed in the 

### 2 former study. Madronich et al. ( 2011 ) did not attempt to link their predicted increase in 

### 3 UV radiation to a predicted increase in skin cancer incidence, however, due to several 

### 4 remaining and substantial uncertainties. 

### 5 A handful of additional studies have addressed the relationsh ip between changes in 

### 6 tropospheric pollutant concentrations and UV

- B radiation exposure, providing some 

### 7 additional insight. A study by Palancar and Toselli ( 2002 ) looked at changes in measured 

### 8 UV

- B radiation in relation to ground

- level air pollutants during several air pollution 

### 9 episodes in Cordoba, Argentina. They found that changes in aerosol concentrations 

### 10 explained the majority of UV

- B radiation fluctuations, and that changes in tropospheric 

### 11 O 3 and SO 2 had little effect. Repapis et al. ( 1998 ) performed a similar study on UV

- B 

### 12 exposures during high and low air pollution days in Athens, Greece. They foun d cloud 

### 13 cover and aerosols to be the major factors in observed UV

- B exposures reductions. 

### 14 Studies by Acosta and Evans ( 2000 ) in Mexico City and Koronakis et al. ( 2002 ) in 

### 15 Athens, Greece both found significant reductions in surface

- level UV exposures during 

### 16 pollution episodes. Both these studies include tropospheric O 3 as a potential driver for the 

### 17 reductions, but neither study was able to quantify the influence of individual atmospheric 

### 18 components involved in the observed attenuation in UV

- B radiation. 

### 19 In the absence of reliable studies specifically addressing UV

- B re lated health effects from 

### 20 a reduction in tropospheric O 3 , inferences were made in the 2006 O 3 AQCD on the basis 

### 21 of studies focused on stratospheric O 3 depletion. Studies included in that review 

### 22 examined the potential effect of stratospheric O 3 depletion on the risk of erythema 

### 23 ( Longstreth et al., 1998 ) , skin ca ncer ( Urbach, 1997 ; Slaper et al., 1996 ; De Gruijl, 1995 ; 

### 24 Longstreth et al., 1995 ; Madronich and De Gruijl, 1993 ) , nonmelanoma skin cancer 

### 25 ( Slaper et al., 1996 ; Longstreth et al., 1995 ) , and cataracts ( Longstreth et al., 1995 ) . Note 

### 26 that several of the concerns expressed above in relation to the Lutter and Wolz ( 1997 ) 

### 27 analysis are relevant to these analyses as well. Furthermore, these studies have a high 

### 28 degree of uncertainty due to inadequate information on the action spectrum and dose



### 29 response relationships. As a result, caution is advised when assessing and interpreting the 

### 30 quantitative results of health risks due to stratospheric O 3 depletion in the context of 

### 31 tropospheric O 3 shielding. 

### 32 Although the UV

- B related health effects attributed to marginal reductio ns in 

### 33 tropospheric or ground

- level O 3 that would result from reductions in O 3 concentrations 

### 34 have not been directly assessed, they would be expected to be small given the above 

### 35 findings and the fact that tropospheric O 3 makes up only ~10% of the total atmo spheric 

### 36 O 3 column at mid

- latitudes ( Kar et al., 2010 ) . Furthermore, O 3 present in the planetary 

### 37Draft

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- 27 September 2011 boundary layer makes up only ~10% of tropospheric O 3 ( Thomps on et al., 2007 ) and the 

### 1 NAAQS has only a fractional influence on those ground

- level O 3 concentrations. The net 

### 2 result is a very small influence on total column O 3 through attainment of the O 3 standard. 

### 3 In addition, the health benefits of UV

- B in the pr oduction of vitamin D suggests that 

### 4 increased risks of human disease due to a slight excess in UV

- B radiation exposure may 

### 5 be offset by the benefits of enhanced vitamin D production. However, as with other 

### 6 impacts of UV

- B on human health, this beneficial e ffect of UV

- B has not been studied in 

### 7 sufficient detail to allow for a credible health benefits assessment. Hence, the above 

### 8 mentioned health and welfare effects associated with UV

- B exposures resulting from 

### 9 changes in ground

- level O 3 concentrations would likely be small or nonexistent based on 

### 10 current information. 

### 11 More reasonable estimates of the human health impacts of enhanced UV

- B penetration 

### 12 following reduced ground

- level O 3 concentrations require both (a) a solid understanding 

### 13 of the multiple factors that define the extent of human exposure to UV

- B, and (b) well



### 14 defined and quantifiable links between human disease and UV

- B exposure. Within the 

### 15 uncertain context of presently available information on UV

- B surface fluxes, a risk 

### 16 assessment of UV

- B

- related health effects would need to factor in human habits (e.g., 

### 17 daily activities, recreation, dress, and skin care) in order to adequately estimate UV

- B 

### 18 exposure levels. Little is known about the impact of variability in these human factors on 

### 19 individual expos ure to UV radiation. Furthermore, detailed information does not exist 

### 20 regarding the relevant type (e.g., peak or cumulative) and time period (e.g., childhood, 

### 21 lifetime, or current) of exposure, wavelength dependency of biological responses, and 

### 22 inter

- indiv idual variability in UV resistance. In conclusion, the effect of changes in 

### 23 surface

- level O 3 concentrations on UV

- induced health outcomes cannot yet be critically 

### 24 assessed within reasonable uncertainty. The reader is referred to the U.S. EPA 2002 Final 

### 25 Res ponse to Court Remand ( U.S. EPA, 2003 ) for detailed discussions of the data and 

### 26 scientific issues associated with the determination of public health benefits resulting from 

### 27 the attenuation of UV

- B by surface

- level O 3 . 

### 28


## 10.5 Summary

10.5.1 Summary of the Effects of Tropospheric Ozone on Climate Tropospheric O 3 is a major greenhouse gas, third in importance after CO 2 and CH 4 . 

### 29 While the developed world has successfully reduced emissions of O 3 precursors in recent 

### 30 decades, many developing countries have experienced large increases in precursor 

### 31 emissions and these trends are expected to continue, at least in the near term. Projections 

### 32Draft

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- 28 September 2011 o f radiative forcing due to changing O 3 over the 2 1 st century show wide variation, due in 

### 1 large part to the uncertainty of future emissions of source gases. In the near

- term (2000

- 

### 2 2030), projections of O 3 radiative forcing range from near zero to +0.3 W/m 2 , depending 

### 3 on the emissions scenario ( Stevenson et al., 2006 ) . Reduction of tropospheric O 3 

### 4 concentrations could therefore provide an important means to slow climate change in 

### 5 addition to the added benefit improving surface air quality. 

### 6 It is clear that increases in tropospheric O 3 lead to warming. However the precursors of 

### 7 O 3 also have competing effects on the greenhouse gas CH 4 , complicating emissions 

### 8 reduction strategies. A decrease in CO or VOC emissions would enhance OH 

### 9 concentrations, shortenin g the lifetime of CH 4 , while a decrease in NO X emissions could 

### 10 depress OH concentrations in certain regions and lengthen the CH 4 lifetime. Recent 

### 11 research, however, has indicated that a carefully combined reduction of CO, VOCs, and 

### 12 NO X emissions could lead to net cooling ( Naik et al., 2005 ) . They calculate that such 

### 13 r eductions would have the greatest impact for developing countries in tropical regions. 

### 14 Abatement of CH 4 emissions would likely provide the most straightforward means to 

### 15 address climate change since CH 4 is itself an important precursor of background O 3 

### 16 ( West et al., 2007 ; West et al., 2006 ; Fiore et al., 2002 ) . A reduction of CH 4 emissions 

### 17 would also improve air quality on its own right. A set of global abatement measures 

### 18 identified by West and Fio re ( 2005 ) could reduce CH 4 emissions by 10% at a cost 

### 19 savings, decrease background O 3 by about 1 ppb in the Northern Hemisphere summer, 

### 20 and lead to a global net cooling of 0.12 W/m 2 . Unlike measures to reduce NO X , which 

### 21 would have immediate impacts on surface O 3 but little net radia tive forcing, the cooling 

### 22 effects of CH 4 controls would be realized gradually, over ~12 years. West et al. ( 2007 ) 

### 23 explored further the benefits of CH 4 abatement, finding that a 20% reduction in global 

### 24 CH 4 emissions would lead to significantly greater cooling per unit reduction in surface 

### 25 O 3 , compare d to 20% reductions in VOCs or CO. 

### 26 Important uncertainties remain regarding the impact of tropospheric O 3 on future climate 

### 27 change. To address these uncertainties, further research is needed to: (1) enhance our 

### 28 knowledge of the natural atmosphere; (2) inte rpret observed trends of O 3 in the free 

### 29 troposphere and remote regions; (3) improve our understanding of the CH 4 budget, 

### 30 especially emissions from wetlands and agricultural sources, (4) understand the 

### 31 relationship between regional O 3 radiative forcing and regional climate change; and (5) 

### 32 determine the optimal mix of emissions reductions that would act to limit future climate 

### 33 change. 

### 34Draft

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- 29 September 2011 

### 10.5.2 Summary of UV - B Related Effects on Human Health, Ecosystems, and Materials Relating to Changes in Tropospheric Ozone Concentra tions UV radiation emitted from the Sun contains sufficient energy when it reaches the Earth to 

### 1 break (photolyze) chemical bonds in molecules, thereby leading to damaging effects on 

### 2 living organisms and materials. Atmospheric O 3 plays a crucial role in red ucing exposure 

### 3 to solar UV radiation at the Earth ’ s surface. Ozone in the stratosphere is responsible for 

### 4 the majority of this shielding effect, as approximately 90% of total atmospheric O 3 is 

### 5 located there over mid

- latitudes. Ozone in the troposphere provides supplemental 

### 6 shielding of radiation in the wavelength band from 280

- 315 nm, referred to as UV

- B 

### 7 radiation. UV

- B radiation has important effects on human health and ecosystems, and is 

### 8 a ssociated with materials damage. 

### 9 Adverse human health effects associated with solar UV

- B radiation exposure include 

### 10 erythema, skin cancer, ocular damage, and immune system suppression. A potential 

### 11 human health benefit of increased UV

- B exposure involves th e UV

- induced production 

### 12 of vitamin D which may help reduce the risk of metabolic bone disease, type I diabetes, 

### 13 mellitus, and rheumatoid arthritis, and may provide beneficial immunomodulatory effects 

### 14 on multiple sclerosis, insulin

- dependent diabetes mellit us, and rheumatoid arthritis. 

### 15 Adverse ecosystem and materials damage effects associated with solar UV

- B radiation 

### 16 exposure include terrestrial and aquatic ecosystem impacts, alteration of biogeochemical 

### 17 cycles, and degradation of man

- made materials. Terrestrial ecosystem effects from 

### 18 increased UV

- B radiation include reduced plant productivity and plant cover, changes in 

### 19 biodiversity, susceptibility to infection, and increases in natural UV protective responses. 

### 20 In general, however, these effects are s mall for moderate UV

- B increases at mid



### 21 latitudes. Aquatic ecosystem effects from increased UV

- B radiation include sensitivity in 

### 22 growth, immune response, and behavioral patterns of aquatic organisms and the potential 

### 23 for increased catalysis and mobility o f trace metals. Biogeochemical cycles, particularly 

### 24 the carbon cycle, can also be influenced by increased UV

- B radiation with effects ranging 

### 25 from UV

- induced increases in CO 2 uptake through soil respiration to UV

- induced release 

### 26 of CO 2 through photodegrada tion of above

- ground plant litter. Changes in solar UV 

### 27 radiation may also have effects on carbon cycling and CO 2 uptake in the oceans as well 

### 28 as release of dissolved organic matter from sediment and algae. Finally, materials damage 

### 29 from increased UV

- B radi ation includes UV

- induced photodegradation of wood and 

### 30 plastic. 

### 31 There is a lack of published studies that critically examine the incremental health or 

### 32 welfare effects (adverse or beneficial) attributable specifically to changes in UV

- B 

### 33 exposure resulting f rom perturbations in tropospheric O 3 concentrations. While the 

### 34Draft

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- 30 September 2011 effects are expected to be small, they cannot yet be critically assessed within reasonable 

### 1 uncertainty. 

### 2


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