# Chapter 10. The Role of Troposphe ric Ozone in Climate Change and UV -B Effects

10.1. In tro d u c tio n

1 Atmospheric O 3 plays an important role in the Earth's energy budget by interacting with



### 2 incoming solar radiation and outgoing infrared radiation. Tropospheric O 3 makes up only a small



### 3 portion of the total column of O 3 , but it has important incremental effects on the overall radiation



### 4 budget. This chapter assesses the specific role that tropospheric O 3 plays in the earth's radiation



### 5 budget and how perturbations in tropospheric O 3 might affect climate through its role as a



### 6 greenhouse gas (Section 10.2 ), and health, ecology and welfare through its role in shielding the



### 7 earth’s surface from solar ultraviolet radiation (Section 10.3 ).



# 10.2. Effects of Tr opos pheric Ozone on Climate

10.2.1. Background

8 Tropospheric O 3 is a major greenhouse gas, and increases in its abundance may contribute to



### 9 climate change (I PCC, 2007, 092980 )

. Models calculate that the global burden of tropospheric O 3 

### 10 has doubled since the preindustrial era (Gauss et al., 2006, 630275 ) , while observations indicate that



### 11 in some regions O 3 may have increased by factors as great as 4 or 5 (Marenco et al., 1994, 047733 ;



### 12 Staehelin et al., 1994, 055369 )

. The se increase s are tied to the rise in emissions of O 3 precursors


### 13 from human activity, mainly fossil fuel consumption and agricultural processes

. The impact on



### 14 climate of the O 3 cha nge since preindustrial times has been estimated to be about 25

- 40% of



### 15 anthropogenic CO 2 impact and about 75% of anthropogenic CH 4 impact (IPCC, 200 7, 092980 )

, 

### 16 ranking it third in importance of the greenhouse gases. In the 21 st century as the Earth’s population



### 17 continues to grow and energy technology spr eads to developing countries, a further rise in the global



### 18 burden of tropospheric O 3 is possible , with consequences for future climate.



### 19 To examine the science of a changing climate and to provide balanced and rigorous



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



### 21 Nations Environment Program me (UNEP) formed the Intergovernmental Panel on Climate Change



### 22 (IPCC) in 1988

. The IPCC supports the work of the Conference of Parties (COP) to the United



### 23 Nations Framework Conve ntion on Climate Change (UNFCCC)

. The IPCC periodically brings

Note: Hyperlinks to the reference citations throughout this document will take you to the NCEA HERO database (Health and Environmental Research Online) at http://epa .gov/hero . HERO is a database of scientific literature used by U.S. EPA in the process of developing science assessments such as the Integrated Science Assessments (ISAs) and the Integrated Risk Information System ( IRIS).

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### 1 together climate scientists from member countries of WMO and the United Nations to review



### 2 knowledge of the physical climate system, past and future climate change, and evidence of human



### 3 induced climate change

. IPCC climate assessment reports are issued every 5 to 7 years

. 

### 4 This chapter draws in part on the fourth IPCC Assessment Report (AR4) (IPCC, 2007,



### 5 092980 ) , as well as other peer

- reviewed published research

. Section 10.2.2 reviews the physics and



### 6 chemistry of c limate change and radiative forcing, together with evidence of climate change in the



### 7 recent past and projections of future climate change. It also offers a brief description of tropospheric



### 8 O 3 as compared to other greenhouse gases. Section 10.2.3 describes factors that influence the



### 9 magnitude of O 3 effects of climate. Section 10.2.4 considers the competing effects of O 3 precursors



### 10 on climate

. Sections 10.2.5 and 10.2.6 describe the effects of changing tropospheric O 3 on present



### 11 day and future climate , respectively. Finally, Section 10.2.7 presents a summary of the effects of



### 12 tropospheric O 3 on climate.



## 10.2.2. Ph ys ic s a nd Ch e mis try o f Clima te Cha n ge an d Radia tive Fo rcing

10.2.2.1. Phys ics of Greenhous e Gas es

13 The Earth’s climate depends upon the flux of energy from the sun and its redistribution in the



### 14 earth

- atmosphere

- ocean system

. Radiant energy from the sun enters the atmosphere in a range of



### 15 wavelengths, but peaks strongly in the shortwave (visible) part of the spectrum

. Most solar energy at



### 16 very short wavelengths (e.g., ultraviolet) is absorbed at high altitudes by gases such as stratospheric



### 17 O 3

. About 30% of incoming solar radiation is reflected back to space, mainly by clouds or surfaces



### 18 with high albedo (reflectivity), such as snow

, ice , and desert sand. In the troposphere, gases and



### 19 particles can interact with a fraction of the incoming solar radiation, but for the most part the



### 20 troposphere is transparent to shortwave radiation

. Thus about 70% of shortwave solar radiation



### 21 penetrates to the Earth’s surface and is absorbed

. About one

- third of the absorbed energy is then re



### 22 emitted in the longwave (infrared) portion of the spectrum

. The rest goes into evaporating water or



### 23 soil moisture or emerges as sensible heat.



### 24 The troposphere is opaque to the outgoing longwave radiation

. Polyatomic gases such as CO 2 , 

### 25 CH 4 , and O 3 absorb and re

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



### 26 efficiency with which that energy returns to space

. In effect, these gases act as a blanket warming the



### 27 Earth's surface

. This phenomenon, known as the “Greenhouse Effect,” was first quantified in the 19 th



### 28 century (Arrhenius, 1896, 043125 ) , and gives rise to the term “greenhouse gas”

.



## 10.2.2.2. Clima te Change in the Recent Pas t

29 From the end of the Last Ice Age 12,000 years ago until the mid

- 1800s, observations fro m ice



### 30 cores show that concentrations of the long

- lived greenhouse gases CO 2 , CH 4 , and N 2 O have been



### 31 relatively stable

. Unlike these greenhouse gases, O 3 is not preserved in ice, and no record of it before



### 32 the late 1800s exists

. Models

, however

, suggest tha t it

, too

, has remained relatively constant during

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1 this time period (Thompson, 1992, 043463 ; Thompson et al., 1993, 029510 )

. The stable mix of



### 2 greenhouse gases in the atmosphere has kept the global mean temperature of the Earth close to 15oC

. 

### 3 Without the presence of greenhouse gases in the at mosphere, the Earth’s temperature would be about



### 4 30oC cooler, or - 15oC

. Since the start of the Industrial Revolution, human activity has led to

significant increases of greenhouse gases in the atmosphere, mainly through fossil fuel combustion

. 

### 6 According to the IPCC AR4 (IPCC, 2007, 092980 ) , we now have “ very high confidence” that the net



### 7 effect of anthropogenic greenhouse gas emissions since 1750 has led to warming

, and it is “very



### 8 likely” that human a ctivity contributed to the 0.76 oC rise in global mean temperature observed over



### 9 the last century. The inc rease of tropospheric O 3 may have contributed 0.1 -0.3 oC warming to the

global climate during this time period (Hansen et al., 2005, 190596 ; Mickley et al., 2004, 057416 )

. 

### 11 Global cooling due to anthropogenic aerosols (IPCC, 2007

, 092980 ) has likely masked the full



### 12 warming effect of the anthropogenic greenhouse gases.



## 10.2.2.3. Proje c tions of Fu ture Clima te Cha nge

13 The IPCC AR4 projects a warmi ng of

~ 0.2oC per decade for the remainder of the 21 st century



### 14 (IPCC, 2007, 092980 )

. Even at constant concentrations of greenhouse gases in the atmosphere,

temperatures are expected to increase by about 0.1oC per decade, due to the slow response of oceans



### 16 to the warming applied so far

. It is likely that t he Earth will experience longer and more frequent



### 17 heat waves in the 21 st century, together with more frequent droughts and/or heavy precipitation



### 18 events in some regions, due to perturbations in the hydrological cycle that result from changing



### 19 temperatures (IPCC, 2007, 092980 )

. Sea levels could increase by 0.3

- 0.8 m by 2300 due to thermal

expansion of the oceans

. The extent of Arctic sea ice is expected to decline, and contraction of the



### 21 Greenland ice sheet could further contribute to the sea level rise (IPCC, 2007, 092980 ).



### 22 Projections of future climate change are all associated with some degree of unce rtainty . A



### 23 major uncertainty involves future trends in the anthropogenic emissions of greenhouse gases or their



### 24 precursors. For the IPCC AR4 climate projections, a set of distinct “storylines” or emission pathways

was developed (IPCC, 2000, 080704 )

. Each storyline took into account factors such as population



### 26 growth, mix of energy technologies, and the sharing of technology between developed and



### 27 developing nations, and each resulted in a different scenario for anthropogenic emissions

. When



### 28 these trends in emissions are applied to models, these scenarios yield a broad range of possible



### 29 climate trajectories for the 21 st century.
A second factor bringing large uncertainty to model projections of future climate is the



### 31 representation of climate and, especially, climate feedbacks

. A rise in surface temperatures would



### 32 perturb a suite of other processes in the earth -a tmosphere

- ocean system, which may in turn either



### 33 amplify the temperature increase (positive feedback) or diminish it (negative feedback)

. One



### 34 important feedback involves the increase of water vapor content of the atmosphere that would

accompany higher temp eratures (Bony et al., 2006, 630272 )

. Water va por is a potent greenhouse gas;



### 36 accounting for the water vapor feedback may increase the climate sensitivity to a doubling of CO 2 by



### 37 nearly a factor of two (Held and Soden, 2000, 630279 )

. The ice

- albedo feedback is also strongly

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1 positive ; a decline in snow cover and sea ice extent would diminish the Earth’s albedo, allowing



### 2 more solar energy to be deposited to the surface (Holland and Bitz, 2003, 630280 ; Rind et al., 1995,



### 3 630285 )

. A final e xample of a climate feedback involves the effects of changing cloud cover in a



### 4 warming atmosphere

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

surface temperatures (Soden and Held, 2006, 631183 ).



## 10.2.2.4. Me tric s of Po te n tia l Clima te Cha nge

6 Two different metrics are frequently used to estimate the potential climate impact of some



### 7 perturbation such as a change in greenhouse gas concentration: ( 1) global warming potential (GWP);



### 8 and (2) radiative forcing (RF).



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

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



### 11 magn itude of this forcing relative to that of CO 2

. GWP is most useful for comparing the potential



### 12 climate impacts of long

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

. Since tropospheric O 3 has a lifetime on the



### 13 order of weeks to months, GWP is not seen as a valuable metri c for quantifying the importance of O 3 

### 14 on climate (Forster et al., 2007, 092936 ).

Radiative forcing is a change in the radiative balance at a particular level of the atmosphere or



### 16 at the surface when a perturbation is introduced in the earth

- atmosphere

- ocean system. In the global



### 17 mean, r adiative forcing of greenhouse gases at the tropopause (top of the troposphere) is roughly



### 18 proportional to the surface temperature response



### 19 (Committee on Radiative Forcing Effects on Climate; Climate Research Committee; National

Research Council et al., 2005, 057409 ; Hansen et al., 2005, 190596 ) . It thus provides a useful metric



### 21 for policymakers for assessing the response of the earth's surface temperature to a given change in



### 22 the concentration of greenhouse gas. Positive values of radiative forcing indicate warming in a test



### 23 case relative to the control ; negative values indicate cooling . The u nits of radiative forcing are



### 24 energy flux per area, or W /m 2

. Radiative forcing requires just a few model years to calculate, and it shows consistency from



### 26 model to model

. However, radiative forcing does not take into account the climate feedbacks that



### 27 could amplify or dampen the actual surface temperature response, depending on region (Section



### 28 10.2.2.3)

. Quantifying the change in surface temperature requires a climate simulation in which all



### 29 important feedbacks are accounted for. As these proces ses are not well understood, the surface

temperature response to a given radiative forcing is highly uncertain and can vary greatly among



### 31 models and even from region to region within the same model

.



## 10.2.2.5. Tro pos pheric Ozone as a Greenhouse Gas

32 Tropospheric O 3 di ffers in important ways from other greenhouse gases . It is not emitted



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



### 34 organic compounds (VOCs) in the presence of nitrogen oxide radicals ( NO X = NO + NO 2 ; see

Se ction 3.2 ). It is also supplied by vertical transport from the stratosphere

. The lifetime of O 3 in the

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



### 2 seasonally ; the distribution of the long

- lived greenhouse ga ses like CO 2 and CH 4 are much more



### 3 uniform. The longwave radiative forcing by O 3 is mainly d ue to absorption in the 9.6 μm window,



### 4 where absorption by water vapor is weak

. It is therefore less sensitive to local humidity than the



### 5 radiative forcing by CO 2 or CH 4 , for which there is much more overlap with the water absorption



### 6 bands (Lenoble, 1993, 630969 )

. And unlike other major greenhouse gases, O 3 absorbs in the



### 7 shortwave as well as the longwave part of the spectrum

. 

### 8 Figure 10 -1 shows the main steps involved in the influence of t ropospheric O 3 on climate. An



### 9 increase in the emissions of O 3 precursors leads to an increase in the burden of tropospheric O 3 . The



### 10 added O 3 then perturbs the radiative balance of the atmosphere, an effect quantified by the radiative



### 11 forcing metric. This f orcing results in climate change, usually expressed as a change in surface



### 12 temperature. Climate change can also perturb tropospheric O 3 , as will be discussed in Section



### 13 10.2.6.3 . As shown in Figure 10 -2 , the IPCC (IPCC, 2007, 092980 ) reports a radiative forcing of



### 14 0.35 W /m 2 for the change in tropospheric O 3 since the preindustrial era, ranking it third in



### 15 importance after the greenhouse gases CO 2 (1.66 W /m 2 ) and CH 4 (0.48 W /m 2 ). The error bars



### 16 encompassing the tropospheric O 3 radiative forcing estimate range from 0.25 to 0.65 W/m 2 , making



### 17 it relatively more uncertain than the long

- lived greenhouse gases.



### Emissions of the ozone ﷒ Figure 10 -1

. Flow chart for the effects of tropospheric ozone on climate

. precursors CO, VOCs, CH 4 

### , and NO X 

### lead to production of tropospheric ozone

. A change in the burden of tropospheric ozone perturbs the radiative balance of the atmosphere, leading to radiative forcing. The earth

- atmosphere

- ocean system responds to the forcing with a change in climate. Climate change, in turn, can affect the abundance of tropospheric ozone through multiple mechanisms. Units shown are those typical for each quantity, with the climate response expressed as a change in surface temperature.

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### Figure 10 -2

. Global average radiative forcing (RF) estimates and ranges in 2005 fo r anthropogenic CO 2 

### , CH 4 

### , ozone and other important agents and mechanisms

, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU)

.
The net anthropogenic radiative forcing and its range are also shown

. These require summing asymmetric uncertainty estimates from the component terms, and cannot be obtained by simple addition

. Additional forcing factors not included here are considered to have a very low LOSU.



## 10.2.3. Fa c to rs th at Influe n ce th e Effec t of Tro p os ph e ric O zone on Climate

1 This section describes the main factors that influence the magnitude of the climate response to



### 2 changes in tropospheric O 3

. They include: (1) trends in the burden of tropospheric O 3 ; (2) the effect



### 3 of surface albedo on O 3 forcing ; (3) the effect of vertical distribution on O 3 forcing ; (4) feedback



### 4 factors that can alter the climate response to O 3 forcing; and (5) the indirect effects of tropospheric



### 5 O 3 on the carbon cy cle. Trends in stratospheric O 3 may also affect temperatures at the Earth’s



### 6 surface, but that topic is beyond the scope of this assessment.

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## 10.2.3.1. Tre n ds in the Burde n of Tro p os p he ric O zone

1 To first order, the effect of tropospheric O 3 on climate is proportional to the change in O 3 

### 2 burden. The earth's s urface temperatures are most sensitive to O 3 perturbations in the mid to upper



### 3 troposphere

. This section therefore focuses mainly on observed O 3 trends in the free troposphere or



### 4 in regions far from O 3 sources, where a change in O 3 concentrations may indicate change throughout

the troposphere . Data from ozonesondes, mountaintops, and remote surface sites are discussed, as



### 6 well as satellite data.



### Observed Trends in O zone Since the Preindustrial Era

7 Measurements of O 3 at two European mountain sites dating from the late 1800s to early 1900s



### 8 show values at about 10 ppb , about one

- fifth the values observed today at similar sites (Marenco et



### 9 al., 1994, 047733 ; Pavelin et al., 1999, 087296 )

. The accuracy of these early measurements is

questionable however, in part because they exhibit O 3 concentrations equivalent to or only a couple



### 11 of parts per billion greater than those observed at nearby low

- altitude sites during the same time



### 12 period (Mickley et al., 2001, 080134 ; Volz and Kley, 1988, 041650 )

. A larger vertical gradient in



### 13 tro pospheric O 3 would be expected because of its stratospheric source and its longer lifetime aloft

. In



### 14 another study, Staehelin et al

. (1994, 055369 ) revisited observations made in the Swiss mountains

during the 1950s and found a doubling in O 3 concentrations from that era to 1989

- 1991.



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



### 17 borne ozoneso ndes, but even this record is sparse

. Trends from ozonesondes have been highly



### 18 variable and dependent on region (Logan et al., 1999, 631175 )

. Over most sites in the U.S.,



### 19 ozonesondes reveal little trend

. Over Canada, observations show a decline in O 3 between 1980 and

1990, then a rebound in the following decade (Tarasick et al., 2005, 631184 )

. Ozonesondes over



### 21 Europe give a mixed picture, with Hohenpeissenberg in Germany showing declines through the



### 22 troposphere in recent decades, while Zugspi tze, also in Germany

, exhibiting small increases



### 23 (Oltmans et al., 2006, 180188 )

. Over Japan, O 3 in the lower troposphere increased about 0.2

­ 

### 24 0.4 ppb / y during the 1990s (Naja and Akimoto, 2004, 631178 ).

Ground- based measurements in remote regions provide a record of background trop ospheric



### 26 O 3 extending as far back as the 1980s or, for ship measurements, the late 1970s. S pringtime O 3 

### 27 observations from several mountain sites in the western U.S

. show a positive trend of about of 0.5

­ 

### 28 0.7 ppb / y since the 1980s (Cooper et al., 2010, 380093 ; Jaffe et al., 2003, 052229 ) . S hip

- borne O 3 

### 29 measurements for the time period 1977 to 2002 indicate increases of 0.1 -0.7 ppb /y over the tropical

and South Atlantic

, but no significant change over the North Atlantic (Lelieveld et al., 2004,



### 31 630578 ) . The lack of trend for the North Atlantic would seem at odds with O 3 observations at Mace



### 32 Head on the west coast of Ireland , which show a significant positive trend of about 0.5 ppb / y from



### 33 1987 to 2003 (Simmonds et al., 2004, 631182 )

. Over Japan, O 3 at a remote mountain site has



### 34 increased 1 ppb / y from 1998 to 2003 (Tanimoto, 2009, 620751 ) , a rate more than double that

recorded by ozonesondes in the lower troposphere over Japan during the 1990s (Naja an d Akimoto,



### 36 2004, 631178 ).

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1 The satellite record is now approaching a length that can be useful for diagnosing trends in



### 2 total column amounts of tropospheric O 3

. In contrast to the surface data from ships, tropospheric O 3 

### 3 columns from the Total Ozone Mapping Spectrometer (TOMS) show no trend over the tropical



### 4 Atlantic for the period 1980

- 1990 (Thompson and Hudson, 1999, 631185 )

. Over the Pacific, a

longer, 25 year record of TOMS data again reve als no trend over the tropics, but shows increases in



### 6 tropospheric column O 3 of about 2

- 3 Dobson Units (DU [1 DU = 2.69 × 10 16 molecules of O 3 /cm 2 ] )



### 7 at midlatitudes in both hemispheres (Ziemke et al., 2005, 631193 ) ; for comparison, t he tropospheric



### 8 O 3 burden averages about 34 DU

. 

### 9 Interpreting th ese recent trends in tropospheric O 3 is challenging . The first difficulty is

reconciling apparently contradictory trends in the observations, e.g., over tropical oceans. A second



### 11 difficulty is that the O 3 trends depend on several factors, not all of which can be well characterized.



### 12 These factors include (1) trends in emissions of O 3 precursors, ( 2 ) variation in the stratospheric



### 13 source of O 3 , ( 3 ) changes in solar radiation resulting from stratospheric O 3 depletion, and ( 4 ) trends



### 14 in tropospheric temperature s (Fusco and Logan, 2003, 051229 )

. The positive trends in the western

U.S

. and over Japan are consistent with the rapid increase in emissions of O 3 precursors from



### 16 mainland Asia and transport of pollution across the Pacific (Cooper et al., 2010, 380093 ; Tanimoto,



### 17 2009, 620751 )

. The satellite trends over the north ern mid

- latitudes are consistent with this picture as



### 18 well (Z iemke et al., 2005, 631193 )

. Increases in tropospheric O 3 in the Southern Hemisphere are also



### 19 likely due to increased anthropogenic NO X emissions, especiall y from biomass burning

. The declines

in O 3 over Europe can be at least partly explained by decreases in O 3 precursor emissions there



### 21 (Jonson et al., 2005, 630282 ), though recent O 3 depletion in the lower stratosphere may also



### 22 contribute to the decreases by reducing stratospheric input to the troposphere (Fusco and Logan,



### 23 2003, 051229 ).



### Calculation of O zone Trends for the Recent Past

24 Attempts to simulate trends in tropospheric O 3 allow us to test current knowledge of O 3 processes and to predict with greater confidence trends in future O 3 concentrations. Time

- dependent



### 26 emission inventories of O 3 precursors have also been developed (e.g., Lamarque et al., 2010, 630289



### 27 for 1850

- 2000; Van Aardenne et al., 2001, 055564 for 1890

- 1990)

. These inventories allow for the



### 28 calculation of changing O 3 burden over time.



### 29 One recent multi

- mo del study calculated an increase in the O 3 burden since preindustrial times

of 8

- 14 DU , or about 30

- 70% (Gauss et al., 2006, 630275 )

. The large spread in model ed est imates



### 31 reveals our limited knowledge of processes in the pristine atmosphere

. Models typically overestimate



### 32 the late nineteenth and early twentieth century observations available in surface air and at mountain



### 33 sites by 50

- 100% (Kiehl et al., 19 99, 047917 ) (Lamarque et al., 2005, 630287 ; Mickley et al., 2001,



### 34 080134 ; Shindell et al., 2003, 057417 )

. Reconciling the differences between models and

measurements will require more accurate simulation of the natural sources of O 3 (Mickley et al.,



### 36 2001, 080134 ) and/ or implementation of novel sinks such as bromine radicals, which may reduce



### 37 background O 3 in the pristine atmosphere by as much as 30% (Parrella et al., In Press, 664506 ).

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### 1 For the more recent past (si nce 1970), application of time

- dependent emissions reveals an



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



### 3 industrialization of societies at low

- latitudes (Berntsen et al., 2000, 047916 ; Lamarque et al., 2005,



### 4 630287 )

. By constraining a model with historical (1950s

- 2000) observations, Shindell et al. (2002,



### 5 080130 ) calculated a large increase of 8.2 DU in tropospheric O 3 over polluted continental regions



### 6 since 1950

. Their result appears consistent with the large change in tropospheric O 3 since



### 7 preindustrial times implied by the observations from the late 1800s (Marenco et al., 1994, 047733 ;


### 8 Pavelin et al., 1999, 087296 ).



## 10.2.3.2. The Ef fe c t o f Su rfa c e Albe do on Ozone Forcing

9 The Earth's surface albedo plays a role in O 3 forcing

. Through most of the troposphere,



### 10 absorption of incoming shortwave solar radiation by O 3 is small relative to its absorption of outgoing



### 11 longwave terrestrial radi ation

. However, over surface s characterized by high albedo ( e .g., over snow,



### 12 ice, or desert sand), incoming radiation is more likely to be reflected than over darker surfaces, and



### 13 the probability that O 3 will absorb shortwave solar energy is therefore larg er

. In other words, energy



### 14 that would otherwise return to space may instead be deposited in the atmosphere

. Several studies



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

- latitudes leads to radiative forcing estimates



### 16 greater than 1.0 W/ m 2 in the r egion, especially in summer (Liao et al., 2004, 057414 ; Mickley et al.,



### 17 1999, 047918 ; Shindell et al., 2006, 631181 )

. Because the Arctic is especially s ensitive to radiative



### 18 forcing through the ice

- albedo feedback, the large contribution in the shortwave to the total radiative



### 19 forcing in the region may be important.



## 10.2.3.3. The Effec t of Vertical Dis tribution on Ozone Forcing

20 In the absence of feedbacks, O 3 incre ments added near the tropopause produce the largest



### 21 increases in surface temperature (Lacis et al., 1990, 037834 ; Wang et al., 1980, 674821 ) . This is a



### 22 result of the colder temperature of the tropopause relative to the rest of the troposphere and



### 23 stratosphere. Since radiation emitted by the atmosphere is approxi mately proportional to the fourth



### 24 power of its temperature 1

, the colder the added O 3 is relative to the earth’s surface, the weaker the



### 25 radiation emitted and the greater the “trapping” of longwave radiation in the troposphere.



## 10.2.3.4. Fe e dba c k Fa c tors tha t Alte r t he Climate Res pons e to Cha nges in Ozone Forcing

26 Estimates of radiative forcing provide a first

- order assessment of the effect of tropospheric O 3 

### 27 on climate. In the real atmosphere, climate feedbacks and transport of heat alter the sensitivity of



### 28 Earth’s su rface temperature to addition of tropospheric O 3 . Assessment of the full climate response to



### 29 increases in tropospheric O 3 requires use of a climate model to simulate these interactions.

1 As described by the Stefan

- Bolt zmann law, an ideal blackbody

-- which the atmosphere approximates

-- absorbs at all wavelengths and re

radiates proportional to the fourth power of its temperature.

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36 Due to its short lifetime, O 3 is heterogeneously distributed through t he troposphere. Sharp

horizontal gradients exist in the radiative forcing of O 3 , with the greatest radiative forcing since

preindustrial times occurring over the north ern mid

- latitudes (more on this in Section 10.2.5 ). If

clima te feedbacks are particularly powerful

, they may obscure or even erase the correlation between

regional radiative forcing and climate response (Boer and Yu, 2003, 630271 ; Harvey, 2004, 190598 )

. For example, several model studies have reported that the horizontal pattern of surface temperature

response from 2000

- 2100 trends in short

- lived species ( including O 3 ) closely matches the pattern

from the trends in the long

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

631174 ; Shindell et al., 2007, 521350 ; Shindell et al., 2008, 190393 ) . This correspondence occurs

even though the patterns of radiative forcing for the s hort- lived and long

- lived species differ

significantly . In a separate paper, Shindell (2007, 521350 ) found that Arctic temperatures are

especially sensitive to the mid

- latitude radiative forcing from tropospheric O 3

. Other studies have found that the signature of warming d ue to tropospheric O 3 does show

some consistency with the O 3 forcing . For example, Mickley et al. (2004, 057416 ) examined the

change in O 3 since preindustrial times and found greater warming in the Northern Hemisphere than

in the Southern Hemisphere (+0.4oC versus +0. 2oC), as well as higher surface temperatures

downwind of Europe and A sia and over the North American interior in summer

. For an array of

short- lived species including O 3 , Shindell and Faluvegi (2009, 631180 ) found that radiative forcing

applied over north ern mid

- latitudes yield more localized responses due to local cloud, water vapor,

and albedo feedbacks than radiative forcing applied over the tropics.

Climate feedbacks can also alter the sensitivity of surface temperature to the vertic al

distribution of tropospheric O 3 . The previous section (Section 10.2.3.3 ) described the greater impact

of O 3 added to the upper troposphere (near the tropopause) on radiative forcing, relative to additions

in the mid

- to lowe r troposphere. However, w arming induced by increased O 3 in the upper

troposphere could stabilize the atmosphere to some extent, limiting the transport of heat to the

Earth’s surface and mitigating the impact of the added O 3 on surface temperature (Christiansen,

1999, 047920 ; Joshi et al., 2003, 193752 )

. Hansen et al. (1997, 043104 ) determined that allowing

cloud feedbacks in a climate model meant that O 3 enhancements in the mid

- troposphere had the

greatest effect on s urface temperature.

Finally, climate feedbacks can amplify or diminish the climate response of one greenhouse gas

relative to another. For example, Mickley et al. (2004, 057416 ) found a greater temperature response

to CO 2 forcing than to an O 3 forcing of similar global mean magnitude , due in part to the relatively

weak ice

- albedo feedb ack for O 3

. Since CO 2 absorbs in the same bands as water vapor, CO 2 forcing

saturates in the middle troposphere and is also shifted toward the drier poles

. A poleward shift in

radiative forcing amplifies the ice

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

troposphere radiative forcing allows for greater surface temperature response, relative to that for O 3

.

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## 10.2.3.5. Indire c t Effe c ts of Tro pos phe ric Ozone on the Carbon Cycle

1 A proposed indirect effect of tropospheric O 3 on climate involves the carbon cycl e

. By directly



### 2 damaging plant life in ways discussed in Chapter 9 , increases in tropospheric O 3 may depress the



### 3 land

- carbon sink of CO 2 , leading to accumulation of CO 2 in the atmosphere and ultimately warming



### 4 of the Earth's surface. Sitch et al. (2007, 093294 ) calculated that this indirect warming effect of O 3 on climate has about the same magnitude as the O 3 direct effect

. Their results suggest a doubled



### 6 sensitivity of surface temperatures to O 3 forcing, compared to current model estimates.



## 10.2.4. Co mp e tin g Effe cts o f Ozone Precurs ors on Climate

7 Changes in O 3 p recursors affect not just O 3 concentrations, but also other species that have



### 8 importa nce to the radiative balance of the earth's climate system. For example, an increase in CO or



### 9 VOCs would lead to a decrease in hydroxyl (OH) concentrations. Since OH is a major sink of the

greenhouse gas CH 4 , a decline in OH would lengthen the CH 4 lifetime , enhance the CH 4 burden, and



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



### 12 OH in certain locations, shortening the CH 4 lifetime and leading to surface cooling (Fuglestvedt et



### 13 al., 1999, 047431 ).



### 14 Analyzing the net radiative forcing per unit emission for a su ite of O 3 precursors, Shindell and

Faluvegi (2009, 631180 ) calculated positive (+0.25 W/m 2 ) radiative forcing from the increase in



### 16 anthropogenic emissions of CO and VO Cs since preindustrial times, as well as for CH 4 (+1 W/m 2 ).



### 17 In contrast, they found negative (- 0.29 W/m 2 ) radiative forcing from anthropogenic emissions of



### 18 NO X due mainly to the link between NO X and CH 4 . O ther studies have found a near cancellation of



### 19 the positive O 3 forcing and the negative CH 4 forcing that arise from an incremental change in

anthropogenic NO X emissions (Fiore et al., 2002, 051221 ; Fuglestvedt et al., 1999, 047431 ; Naik et



### 21 al., 2005, 193194 ) . In addition, Wild et al. (2001, 193196 ) found that an increase in surface NO



### 22 emissions would lead to net cooling, while an increase in aircraft NO emissions would lead to net



### 23 warming.



### 24 These results point out the need for careful assessment of net radiative forcing involving

multiple pollut ants in developing climate change policy (Unger et al., 2008, 631186 ) . Naik et al.



### 26 (2005, 193194 ) has calculated that a carefully combined reduction of CO, VOCs, and NO X emissions



### 27 could lead to net cooling. In addition, several s tudies point to CH 4 as an attractive target for



### 28 emissions control since CH 4 is itself a n important precursor of O 3 (Fiore et al., 2002, 051221 ; West



### 29 et al., 2007, 622733 ) . Fiore et al. (20 02, 051221 ) found that reducing anthropogenic CH 4 emissions

by 50% would lead to a global radiative cooling of -0.37 W/m 2 , mostly from CH 4

.



## 10.2.5. Calculating Ra diative Fo rcin g a nd Clima te Re s po n s e to Pa s t Tren d s in Tro po s ph e ric O zone

31 The magnitude of the radiative forcing from the change in tropospheric O 3 since the



### 32 preindustrial era is uncertain

. Th is uncertainty derives in part from the scarcity of early

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23

24

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26

27

28

29

30

31

32

33

34

35

36 measureme nts and in part from our limited knowledge regarding processes in the natural

atmosphere . As noted previously, the IPCC AR4 reports a radiative forcing of 0.3 5 W/ m 2 from the

change in tropospheric O 3 since 1750 (Forster et al., 2007, 092936 ) , ranking it third in importance

among greenhouse gases after CO 2 and CH 4

. The O 3 forcing could , in fact, be as large as 0.7 W/ m 2

, if reconstructions of preindustrial and mid

- 20 th century O 3 based on the measurement record are

valid (Mickley et al., 2001, 080134 ; Shindell and Faluvegi, 2002, 080130 ) . In any event, Unger et al.

(2010, 387104 ) showed that present -day O 3 forcin g can be attributed to emissions from across many

economic sectors, including on

- road vehicles, household biofuel, power generation, and biomass

burning

. As much as one

- third of the radiative forcing from the 1890 to 1990 change in tropospheric

O 3 could be due to increased biomass burning (Ito et al., 2007, 608803 ).

These calculated rad iative forcing estimates can be compared to those obtained from satellite

data

. Using data from TOM S , Worden et al. (2008, 631188 ) estimated a reduction in clear -sky

outgoi ng longwave radiation of 0.48 W/ m 2 by O 3 in the upper troposphere over oceans in 2006

. This

radiative forcing includes contributions from both anthropogenic and natural O 3

. Assuming that the

burden of O 3 has roug hly doubled since preindustrial times (Gauss et al., 2006, 630275 ) , the total O 3 forcing estimated with TOMS is consistent with that obtained from models estimating just the

anthropogenic contribution.

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

complexity of feedbacks, as mentioned in Sections 10.2.2.3 and 10.2.3.4 . In their model study

, Mickley et al. (2004, 057416 ) reported a global mean increase of 0.28oC since preindustrial times,

with values as large as 0.8oC in continental interiors

. For the time period since 1870, Hansen et al.

(2005, 190596 ) estimated a much smaller increase in global mean surface temperature (0.11oC), but

they implemented 1880s anthropogenic emissions in their base simulation and also took into account

trends in both stratospheric and tropospheric O 3 ; the model ed decline of lower stratospheric O 3 , especially over polar regions, cooled surface temperatures in th is study, counteracting the warming

effect of increasing tropospheric O 3

. Figure 10 -3 shows the Hansen et al. (2005, 190596 ) results as reported in Shindell et al. (2006,

631181 ) . In that figure, summertime O 3 has the largest radiative imp act over the continental interiors

of the Northern Hemisphere

. In winter, the impact of tropospheric O 3 is greatest over the snow and

ice regions of the Arctic, where the probability of O 3 absorption of shortwave radiation is high

(Section 10.2.3.2 )

. Shindell et al. (2006, 631181 ) estimated that the change in tropospheric O 3 over

the 20 th century could have contributed about 0.3 oC to annual mean Arctic warming and as much as

0.4-0.5 oC during winter and spring

. Over eastern China, Chang et al. (2009, 630273 ) calculated a

surface temperature increase of 0.4oC to the 1970

- 2000 change in tropospheric O 3

. It is not clear,

however, to what degree regional changes in O 3 burden influenced this response, as opposed to more

global changes.

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### Figure 10 -3

. Ensemble average 1900

- 2000 surface temperature trends ( oC per century) in response to tropospheric ozone changes and the input radiative 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 radiative 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 where 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.



## 10.2.6. Calculating the Radiative Forcing and Climate Res pons e to Future Tre nd s in Trop o s p h eric O zone

1 Future trends in tropospheric O 3 concentrations depend in large part on what pathways in



### 2 energy technology the world's societies will follow in coming decades. The t rends in O 3 will also



### 3 depend on the changes in a suite of climate

- sensitive factors , such as the water vapor content of the



### 4 atmosphere. This section describes the following issues: (1) p rojected trends in the anthropogenic



### 5 emissions of O 3 precursors; (2) the effects of these emissions on the tropospheric O 3 burden; (3) the

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1 effects of changing climate on tropospheric O 3 ; and (4) radiative forcing and climate response to 21 st



### 2 century trends in tropospheric O 3

.



## 10.2.6.1. Emis s io ns o f Anthro po ge nic O zone Precursors a cross the 21 st Century

3 The IPCC SRES effort devised scenarios for short -lived O 3 precursors as well as the well



### 4 mixed greenhouse gases including NO X , CO, and VOCs (IPCC, 2000, 080704 )

. Using the IMAGE

socioeconomic model, Streets et al. (2004, 190423 ) provided speciation for NO X and VOCs and



### 6 allocated the trends in emissions over 17 regions and 8 economic sectors for the 2000

- 2050 time



### 7 period. The worst

- case IPCC scenario, A2, features continued dependence on fossil f uels, rapid



### 8 population growth, and little sharing of technology between developed and developing nations

. By



### 9 2100 in this scenario, global NO X emissions increase by a factor of 3.4 and CO emissions and CH 4 by ~2.7, relative to 2000 (IPCC, 2000, 080704 )

. Most of these increases in emissions occur over



### 11 developing countries

. For example over Asia, NO X emission s in the A2 scenario increase by more



### 12 than a factor of 4 by 2100. The more moderate A1B scenario has global NO X and CO emissions



### 13 increasing by 25% and 90%, respectively by 2100, but global CH 4 emissions decreasing by 10%. In



### 14 the B1 scenario , with its empha sis on clean and efficient technologies, global emissions of NO X , CO,

and CH 4 all decrease by 2100 relative to the present day (

- 40%,

- 60%, and

- 30%, respectively).



### 16 Other emissions scenarios have been recently developed to describe trends in the short

- term



### 17 (up to 2030)

. The Current Legislation (CLE) scenario provides trends consistent with existing air



### 18 quality regulations; the Maximum Feasible Reduction (MFR) scenario seeks to reduce emissions of



### 19 O 3 precursors to the maximum extent possible

. Emission source changes relative to the present day

for CLE, MFR, and A2 are given in Stevenson et al. (2006, 089222 ).



### 21 For the Fift h Assessment Report (IPCC AR5), a new set of scenarios has been developed: the



### 22 Representative Concentration Pathways (RCPs) (Moss et al., 2010, 664501 )

. The RCPs will explore



### 23 fo r the first time approaches to climate change mitigation

. The scenarios are designed to achieve



### 24 radiative forcing targets of 2.6, 4.5, 6.0 and 8.5 W/m 2 by 2100, and have been designated RCP 2.6,

RCP 4.5, RCP 6.0, and RCP 8.5 (RCP 2.6 is also known as RCP3

- PD.) In all scenarios, global



### 26 anthropogenic NO X emissions decline 30

- 50% during the 21 st century, though RCP 8.5 shows a peak



### 27 during the 2020s at a value ~15% greater than that of 2000. Global anthropogenic VOC and CO



### 28 emissions are relatively flat during t he 2000

- 2050 time range, and then decline by 30

- 50% by the



### 29 end of the century. For CH 4 , global mean emission trends for the four RCP scenarios differ

significantly across the 21 st century, with RCP 8.5 showing a tripling of emissions by 2100, and



### 31 RCP 2.6 s howing the emissions cut by half in this time range. All these global trends, however,



### 32 contain some regional variation. For example, Asian emissions of both NO X and VOCs show



### 33 significant increases in the near term (2030s to 2050s). Plots of the RCP trends can be found at



### 34 http://iiasa.ac.at/web

- apps/tnt/RcpDb/dsd?Action=htmlpage&page=about (RCP, 2009, 677552 ).

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## 10.2.6.2. Impa c t o f 21 st C en tury Tren ds in Emis s io ns o n Trop os p heric O zone

1 Due to its short lifetime, tropospheric O 3 will respond readily to changes in anthropogenic



### 2 emissions of its precursors. As shown in Table 10 -1 , a recent multi

- mo del study found increases in



### 3 the tropospheric O 3 burden of 15% and 6% for the IPCC A2 and CLE scenarios respectively for the



### 4 2000

- 2030 time period, and a decrease for the MFR scenario of 5% (Stevenson et al., 2006, 089222 )

. 

### 5 These results indicate that the growth in tropospheric O 3 between 2000 and 2030 could be reduced or



### 6 even reversed, depending on emission controls

. For the relatively moderate A1B emissions sc enario



### 7 over the 2000

- 2050 time period, Wu et al. (2008, 190039 ) calculated a change in O 3 burden of about



### 8 20 %. Looking further into the 21 st century, Gauss et al. (2003, 094204 ) reported O 3 burden changes



### 9 of 30

- 60% in response to application of the A2p anthropogenic emissions over the 2000

- 2100 time



### 10 period (t he A2p scenario was a prelimi nary version of the A2 scenario). U sing the A2 scenar io for



### 11 the same 100 year time period , Pyle et al. (2007, 630284 ) projected a 50% increase in the O 3 burden

, 

### 12 consistent with Gauss et a l. (2003, 094204 ) and with Liao et al. (2006, 664500 ) who calculated an O 3 

### 13 change of 60% for the same conditions. Given the large (+40 ppb ) monthly mean increases in



### 14 surface O 3 that the A2 or A2p scenario s would yield over A sia and elsewhere by the end of the 21 st



### 15 century (Prather et al., 2003, 047879 ) , these 100 -yr projections of the O 3 burden would lead to



### 16 extremely unhealthy air quality.



### 17 As noted above, the RCP scenarios of AR5 show long

- term declines in the global mean



### 18 emissions of O 3 precursors, with some regional increases in the near

- term. As of this writing, no



### 19 model study has reported the response of the tropospheric O 3 burden to any of the IPCC AR5



### 20 scenarios.



### Table 10 -1

. 2000

- 2030 changes in anthropogenic emissions, and CH 4 

### and tropospheric ozone burdens, and the associated tropospheric ozone forcing for three scenarios ; v alues are ensemble

means

Current Legislation Maximum Feasible Reduction Scenario IPCC A2 (CLE) (MFR)

Percent change in NO X emissions +96% +18%

- 53%

Percent change in CO emissions +62%

- 16%

- 53%

Percent change in CH 4 burden +23% +19% 0%

Percent change in tropospheric O 3 burden +15% +6%

- 5%

Radiative forcing due t o O 3 change a (W/ m 2 ) 0.3 0.18

- 0.05

a Includes radiative forcing due to corresponding CH 4 change.

Source: A dapted from Stevenson et al. (2006, 089222 ).

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## 10.2.6.3. Impa c t o f 21 st Cen tury Clima te o n Trop os p he ric O zone

1 For the time period from the 1800s to the present

- day, most of the increase in the burden of



### 2 tropospheric O 3 can be traced to changing emissions. Model studies show th at climate change so far



### 3 has likely had little impact on the tropospheric O 3 (e.g., Grenfell et al., 2001, 664496 ) . In the future,



### 4 however, climate change is expected to bring large changes in a suite of variables that could affect

O 3 production, loss, and transport. For example

, increased water vapor in a warming atmosphere i s



### 6 expected to enhance OH concentrations, which i n remote, NO X - poor regions will accelerate O 3 loss



### 7 rates (Johnson et al., 1999, 052390 ).



### 8 In the 2050s A1B climate, Wu et al

. (2008, 629684 ) calculated a 5 ppb decre ase in surface O 3 

### 9 over oceans

. A rise in temperatures will also likely promote emissions of isoprene, an important

biogenic precursor of O 3

. Model studies have calculated 21 st

- century increases in isoprene emissions



### 11 ranging from 25

- 50%, depending on climat e scenario and time horizon (Wu et al., 2008, 190039 , and



### 12 references therein)

. These studies however did not take into account the effects of changing climate



### 13 and CO 2 burden on vegetation extent, which could have large consequences for biogenic emissions



### 14 (Heald et al., 2008, 191617 ; Sanderson et al., 2003, 630286 ) . In any event, enhanced isoprene

emissions will increase O 3 concentrations in VOC

- limited regions, but decre ase O 3 in NO X - limited



### 16 regions (Pyle et al., 2007, 630284 ; Sanderson et al., 2003, 630286 ; Wu et al., 2008, 190039 )

. 

### 17 Convection f requencies and lightning flash rates w ill also likely change in a changing climate, with



### 18 consequences for lightning NO X emissions and O 3 concentrations in the upper troposphere (Price and



### 19 Rind, 1994, 630283 ; Sinha and Toumi, 199 7, 047932 ) . While Wu et al. (2008, 190039 ) calculated an

increase in lightning NO X by 2050 due to enhanced deep convection, Jacobson and Streets (2009,



### 21 630281 ) projected a decrease in lightning NO X due to a declining cloud ice in their future



### 22 atmosphere

. Finally, changes in transport processes w ill almost certainly accompany global climate



### 23 change

. For the 2050 A1B climate, Wu et al. (2008, 629684 ) showed that flattening of the meridional



### 24 temperature gradient in a warming world would lead to slower intercontinental transport of

tropospheric O 3

. For the A2 climate in 2100, Zeng and Pyle (2003, 047492 ) projected an 80%



### 26 i ncrease in the flux of stratospheric O 3 into the troposphere, relative to the present

- day.



### 27 Taken together, these climate

- driven processes could have significant effects on the burden



### 28 and distribution of tropospheric O 3

. As shown in Wu et al. (2008, 629684 ) , model projections of the



### 29 change in O 3 burden due solely to future climate change range from

- 12% to +3%, depending on the

model, scenario , and time horizon.



## 10.2.6.4. Ra dia tive Forc in g a n d Clima te Re s p ons e fro m 21 st Ce ntury Tre nds in Tro pos pheric O zone

31 In the near term (2000

- 2030), Stevenson et al. (2006, 089222 ) estimated an O 3 forcing of near


### 32 zero for MFR, 0.18 W/m 2 for CLE, and +0.3 W/ m 2 for the A2 scenario ( Table 10 -1 ). Menon et al.



### 33 (2008, 613861 ), follow ing the moderate A1B scenario, calculat ed a radiative forcing of 0.12 W/ m 2



### 34 from the 2000

- 2030 change in troposphe ric O 3 , about the same as that derived by Stevenson et al.

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### 1 (2006, 089222 ) for the CLE scenario . Over the longer term (2000 to 2100) for the A1B scenario,



### 2 Gauss et al. (2003, 094204 ) reported larg e positive radiative forcing ( 0.40 to 0.78 W/ m 2 ) due to the



### 3 change in tropospheric O 3 , as shown in Figure 10 -4

. Normalized radiative forcing for these model



### 4 calculations fell within a relatively narrow range, 0.032 to 0.040 W/ m 2 / DU, indicating that the



### 5 lar gest uncertainty lies in the model

- calculated changes in O 3 burden . Applying the A2 scenario

, 

### 6 Chen et al. (2007, 630274 ) estimated a global mean radiative forcing of 0.65 W/ m 2 from



### 7 tropospheric O 3 by 2100 , consistent with the Gauss et al. (2003, 094204 ) results. These studies took



### 8 into account only the impact of changing emissions on tropospheric O 3

. In their calculations of the



### 9 2000

- 2100 radiative forcing from O 3 in the A2 scenario, Liao et al. (2006, 664500 ) found that



### 10 inclusion of climate effects on tropospheric O 3 reduced their radiative forcing estimate by 20%.

Source: Used with permission from American Geophysical Union, Gauss et al. (2003, 094204 )



### Figure 10 -4

. Global mean radiative forcing estimates calculated by a set of models for the 2000

­ Shown are the compone nts of radiative forcing ﷒ 2100 change in tropospheric ozone

. in W/m 2 

### . SW = shortwave component; LW = longwave component; Net = total f orcing; t = tropospheric ozone changes only; and t + s = both tropospheric and stratospheric changes.

11 Several studies have included tropospheric O 3 in their investigations of the response in the



### 12 future atmosphere to a suite of short

- lived species (e.g., Levy H et al., 2008, 631174 ; Shindell et al.,

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1 2007, 521350 ; Shindell et al., 2008, 190393 ) . Few studies, however, have calculated the climate



### 2 response to changes in tropospheric O 3 alone in the future atmosphere. For the A2 atmosphere, Chen



### 3 et al. (2007, 630274 ) estimated a global mean surface temperature increase of +0.34oC by 2100 in



### 4 response to the change in O 3

. The largest temperature increases in this study , as much as 5oC,

occurred over the populous regions of Asia and the Middle East and downwind of biomass burning



### 6 regions in South Africa and South America

.



## 10.2.7. Su mma ry of the Effe cts o f Tro p os p he ric Ozone on Climate

7 Tropospheric O 3 is a major greenhouse gas, third in importance after CO 2 and CH 4

. While the



### 8 developed world has successfully reduced emissions of O 3 precursors in recent decades, many



### 9 developing countries have experienced large increases in precursor emissions and these trends are

expected to continue, at least in the near term

. Projections of radiative forcing due to changing O 3 

### 11 over the 21 st century show wide variation, due in large part to the uncertainty of future emissions of



### 12 sourc e gases . In the near

- term (2000

- 2030), projections of O 3 radiative forcing range from near zero



### 13 to +0.3 W/m 2 , depending on the emissions scenario (Stevenson et al., 2006, 089222 ) . Reduction of



### 14 tropospheric O 3 concentrations could therefore provide an important means to slow climate change

in addition to the added benefit improving surface air quality.



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



### 17 have competing effects on the greenhouse gas CH 4 , complicating emissions reduction strategies. A



### 18 decrease in CO or VOC emissions would enhance OH concentrations, shortening th e lifetime of



### 19 CH 4 , while a decrease in NO X emissions could depress OH concentrations in certain regions and

lengthen the CH 4 lifetime. Recent research, however, has shown that a carefully combined reduction



### 21 of CO, VOCs, and NO X emissions could lead to net cooling (Naik et al., 2005, 193194 ) . In addition

, 

### 22 abatement of CH 4 emissions would prov ide a straightforward means to address climate change since



### 23 CH 4 is itself an important precursor of background O 3 (Fiore et al., 2002, 051221 ; West et al., 2006,



### 24 196558 ; West et al., 2007, 622733 ).

Important uncerta i nties remain regarding the impact of O 3 on future climate change

. To



### 26 address these uncerta i nties, further research is needed to : (1) enhance our knowledge of the natural



### 27 atmosphere ; (2) int erpret observed trends of O 3 in the free troposphere and remote regions ; (3)



### 28 understand the relationship between regional O 3 forcing and regional climate change ; and (4)



### 29 determine the optimal mix of emissions reductions that would act to limit future clima te change.



# 10.3. UV

- B Re la te d Effe c ts a n d Tro p os p h e ric O zone

10.3.1. Background

Ultraviolet (UV) radiation emitted from the Sun contains sufficient energy when it reaches the



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



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

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### 1 UV radiation at the Earth's surface. S tratospheric O 3 is responsible for the majority of this shielding



### 2 effect, as approximately 90% of total atmospheric O 3 i s located there over mid

- latitudes (Crist et al.,



### 3 1994, 668881 ; Kar et al., 2010, 670423 ) . Investigation of the supplemental shielding of UV radiation



### 4 provided by tropospheric O 3 is important for quantifying UV exposure and the incidence of related



### 5 human health effects, ecosystem effects, and materials damage. The role of tropospheric O 3 in



### 6 shielding of UV radiation is discussed in this section.



## 10.3.2. Ph ys ic s o f UV Radia tio n a nd Flu x

7 Solar UV radiation is subdivided into classes based on wavelen gth: UV -A refers to wavelengths from



### 8 400

- 315 nm ; UV

- B from 315

- 280 nm ; and UV

- C from 280

- 10 0 nm . Since t he energy possessed by a



### 9 photon is inversely proportional to its wavelength , UV

- A radiation is the least energetic and UV

- C is



### 10 the most energetic, with UV

- B falling in

- between. The wavelength determines how the photons



### 11 interact with the complex mixture of gases, clouds and particles present in the atmosphere (see



### 12 Figure 10 -5 ). UV

- A radiation can be scattered but is not absorbed to any meaningful degree by



### 13 atmospheric gases including O 3 . UV

- B radiation is absorbed and scattered in part within the



### 14 atmosphere

. UV

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



### 15 in photoionization and photodissociation processes . Since UV

- A i s less energetic and does not



### 16 interact with O 3 and UV

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

- B is the most



### 17 important band of UV radiation to consider in relation to tropospheric O 3 shielding.

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### 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17 Source: 2006 O 3 AQCD (2006, 088089 ).



### Figure 10 -5 . Diagram of the factors that determine human exposure to ultraviolet radia tion.

Solar flux has a temporal dependence, while radiative scattering and absorption have strong

wavelength, path length, and/or particle concentration dependencies

. These combine to create

nonlinear effects on UV flux at the E arth’s surface

. Thus, carefu l quantification of atmospheric

absorbers and scatterers, along with a well

- resolved description of the physics of these interactions,

is necessary for predicting the impact of ground

- level O 3 on UV flux . Chapter 10 of the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) describes in detail several key factors that influence the

spatiotem poral distribution of ground

- level UV radiation flux, including: (1) long

- term solar activity

including sunspot cycle ; (2) solar rotation; (3) the position of the Earth in its orbit around the sun ; (4)

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

absorption and scattering by stratospheric and tropospheric clouds; and (6) surface albedo. T he

efficiencies of absorption and scattering are highly dependent on the concentration of the scattering

medium , partic le size (for aerosols and clouds) , and the altitude at which these processes are

occurring. T hese properties are sensitive to meteorology, which introduces additional element s of

temporal dependency in ground

- level UV radiation flux. As seen in data collected by the Global

Ozone Monitoring by Occultation of Stars (GOMOS) instrument onboard the European Space

Agency’s ENVISAT satellite (Figure 10 -6)

, atmospheric O 3 density undergoes wide natural variation

on relatively short timescales , particularly at mid -l atitudes (Kyrola et al., 2010, 667819 ).

March 20 11 10

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### Figu re 10 -6

. Monthly s tratospheric ozone number density ( scaled by 1 × 10 12 

### molecules / cm 3 

### ), in 3 latitude belts as a function of time (August 2002

- December 2008) and altitude (15

- 40 km) from the Global Ozone Monitoring by Occultation of Stars (GOMOS) instrum ent onboard the European Space Agency’s ENVISAT satellite.

Latitude belts: 30o N- 50oN (top), 10oS

- 10oN (middle), 30oS

- 50o S (bottom). White space in the panels means that there are not enough data available.

1 The lower atmospheric pressure in the stratosphere mean s fewer gas molecules are present that



### 2 can absorb or scatter radiation

. Stratospheric clouds and aerosols are also thinner and more dispersed



### 3 than those in the troposphere

. In the language of the radiative transfer literature, these conditions



### 4 make the stratosphere a “single scattering” regime for UV radiation

. The troposphere, due to its high



### 5 gas and particle concentrations is referred to as a “multiple scattering” regime

. In practical terms,



### 6 UV radiation traverses the stratosphere with a substantially lower probability of encountering a gas



### 7 molecule , cloud, or aerosol particle than it would in the troposphere

. The multiple scattering of UV



### 8 radiation in the troposphere accounts for the “disproportionate” role that tropospheric O 3 is said to



### 9 play in a bsorbing UV radiation versus stratospheric O 3 on a molecule per molecule basis (Balis et al.,

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1 2002, 055023 ; Bruhl and Crutzen, 1989, 012518 ; Crist et al., 1994, 668881 ; Zerefos et al., 2002,



### 2 055169 ).



### 3 Latitude and altitude are primary variables in defining UV

- B flux at the Earth’s surface,



### 4 immediately followed in importance by clouds, surface albedo, PM concentration and composition,

and t hen by gas phase pollution

. Of all these variables , only latitude and altitude can be defined with



### 6 small uncertainty in any effort to develop a UV climatology for use in a public health benefits



### 7 analysis relevant to the areas not presently attaining the NA AQS for O 3

. Cloud cover, and its effect



### 8 on surface UV flux, continues to be extremely difficult to define and predict. Particulate matter and



### 9 gas- phase tropospheric pollutants are subject to similarly high degrees of uncertainty in predicting

their relativ e concentration distributions , but recent advancements have been made (e.g., Bais et al.,



### 11 2005, 669135 ; Bergstrom et al., 2004, 669158 ; Goering et al., 2005, 669164 )

. Land cover and,



### 12 consequently, surface albedo is highly variable at the geographic scales relevant to NAAQS



### 13 attainment

. 

### 14 The 2006 WMO assessment (WMO, 2006, 669178 ) reported that global average total column

O 3 had decl ined by 3.5% from pre

- 1980 concentrations due to the presence of anthropogenic O 3 ­ 

### 16 depleting substances in the atmosphere . In the period 2002

- 2005, no additional declines were found



### 17 in the global average due to bans on and reduced emissions of O 3 - deplet ing substances. The report



### 18 found that O 3 depletion has a strong latitude and seasonal dependence: total column O 3 declined by



### 19 ~3% in the Northern Hemisphere, declined by ~6% in the Southern Hemisphere

, and remained

essentially unchanged over the tropics re lative to pre

- 1980 total column O 3 abundances. Polar



### 21 stratospheric O 3 depletion is more complex and exhibits large interannual variations driven by



### 22 changes in meteorology.



## 10.3.3. Hu ma n Exp o s u re a nd Su s ce p tibility to Ultraviole t Radia tio n

23 The factors that potentially influence UV radiation exposure were discussed in detail in



### 24 Chapter 10 of the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) and are summarized here. These factors

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



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



### 27 hours, the predominant factor for exposure to solar UV radiation. Participation in outdoor sports



### 28 (e.g., basketball, soccer, golf, swimming, cycling) significantly increased UV radiation exposure



### 29 (Moehrle, 2001, 057502 ; Moehrle et al., 2000, 057503 ; Thieden et al., 2004, 057558 ; Thieden et al.,

2004, 057557 ) . Occupations that substantially increased exposure to UV radiation included farming



### 31 (Airey et al., 1997, 057458 ; Schenker et al., 2002, 057522 ) , fish ing (Rosenthal et al., 1988, 057513 )

, 

### 32 landscap ing (Rosenthal et al., 1988, 057513 ) , construction (Gies and Wright, 2003, 057477 )

, 

### 33 physical education (Vishvakarman et al., 2001, 057614 ) , mail delivery (Vishvakarman et al., 2001,



### 34 057614 ) , and various other occupations that require workers to spend the majority of their day

outdoors du ring peak UV radiation hours

. 

### 36 Age and gender were found to be factors that influence human exposure to UV radiation,



### 37 particularly by influencing other factors of exposure such as outdoor activity and risk behavior.

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### 1 Studies indicated that females generally spen t less time outdoors and, consequently, ha d lower UV



### 2 radiation exposure compared to males (Gies et al., 1998, 057478 ; Godar et al., 2001, 057165 ;



### 3 Shoveller et al., 1998, 057524 ) . T he lowest exposure to UV radiation among Americans in the Godar



### 4 et al. (2001, 057165 ) study was received in females during their child raising years (age



### 5 22

- 40 years ); t he highest exposure was observed in males aged 41

- 59 years. A similar Canadian



### 6 survey found that younger adult males had the greatest exposures to UV radiation (Shoveller et al.,



### 7 1998, 057524 ).



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



### 9 radiation. In the U.S. study by Godar et al. (2001, 057165 )

, north erners and south erners were found



### 10 to spend a n equal amount of time outdoors; however, the higher solar flux at lower latitudes



### 11 significantly increased the annual UV radiation dose for south erners. The annual UV radiation doses



### 12 in south erners were 25 and 40% higher in females and males, respectively, compared to north erners.



### 13 Other studies also have shown that altitude and latitude influence personal exposure to UV radiation



### 14 (e.g., Kimlin et al., 1998, 057491 ; Rigel et al., 1999, 057511 ).



### 15 Protective behaviors such as using sunscreen (e.g., Nole and Johnson, 2004, 057505 ) , wearing



### 16 protective clothing (e.g., Rosenthal et al., 1988, 057513 ) , and spending time in shaded areas (e.g.,



### 17 Moise et al., 1999, 057504 ) were shown to reduce exposure to UV radiation

. In one study, the use of



### 18 sunscreen was associated with extended intentional UV radiation exposure (Autier et al., 1999,



### 19 057459 ) ; however, a follow

- up study indicated that sunscreen use increased duration of exposures to



### 20 doses of UV radiation that were below the threshold level for erythema (Autier et al., 2000, 057069 ).



### 21 Given these and other factors that potentially influence UV radiation exposure, the 2006 O 3 

### 22 AQCD (U.S. EPA, 2006, 088089 ) listed the following subpopulations potentially at risk for hi gher



### 23 exposures to UV radiation:



### 24  Individuals who engage in high -r isk behavior (e.g., sunbathing);



### 25  Individuals who participate in outdoor sport s and activities;



### 26  Individuals who work outdoors with inadequate shade (e.g., farmers, construction



### 27 workers

, etc.); and



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



### 29 ( e.g., Honolulu, HI) and higher alti tudes ( e.g., Denver, CO)

.



### 30 The risks associated with all these factors are, of course, highly dependent on season and region



### 31 (Sliney and Wengraitis, 2006, 651896 ).

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## 10.3.4. Hu ma n Health Effe cts d u e to UV

- B Radiation

Chapter 10 of the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) covered in detail the human

health effects associated with solar UV

- B radiation exposure. These effects include erythema, skin

cancer, ocular damage, and immune system suppression. These adverse effects, along with pro tective

effects of UV radiation through increased production of vitamin D are summarized in this section.

For additional details, the reader is referred to Chapter 10 of the 2006 O 3 AQCD (U.S. EPA, 2006,

088089 ) and references therein.

The most conspicuous and well

- recognized acute response to UV radiation is erythema, or the

r eddening of the skin . Erythema is likely caused by direct damage to DNA by UV radiation

(Matsumura and Ananthaswamy, 2004, 057184 ) . Many studies discussed in the 2006 O 3 AQCD

(U.S. EPA, 2006, 088089 ) found skin type to be a significant risk factor for erythema. Additional

risk factors include atopic dermatitis (ten Berge et al., 2009, 651897 ).

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

radiation is considered to be a major risk factor for all forms of skin cancer (Diepgen and Mahler,

2002, 093593 ; Gloster and Brodland, 1996, 057479 )

. Ultraviolet radiation is especially effective in

inducing genetic mutations and acts as both a tumor initiator and promoter

. Keratinocytes have

evolved DNA repair mechanisms to correct the damage induced by UV; however, mutations can

occur, leading to skin cancers that are appeari ng with increasing frequency (Hildesheim and Fornace,

2004, 057168 )

. The relationship between skin cancer and chronic exposure to UV radiation is further

explored in Chapter 10 of the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ).

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

associated epithelial and conjunctival tissues. The region of the eye effected by exposure to UV

radiation depends on the wavelength of the incident UV radiation. Depending on wavelength,

common health effects associated with UV radiation include photokeratitis (snow blindness; short

wavelengths) and cataracts (opacity of the lens; long wavelengths).

Experimental studies have suggested that exposure to UV radiation may suppress local and

systemic immune responses to a variety of antigens (Clydesdale et al., 2001, 057105 ; Garssen and

Van Loveren, 2001, 057161 ; Selgrade et al., 1997, 036165 )

. In rodent models, these effects have

been shown to worsen the course and outcome of some infectious diseases and cancers (Granstein

and Matsui, 2004, 057747 ; Norval et al., 1999, 036089 ) . R esults from human clinical studies suggest

that immune suppression induced by UV radiation may be a risk factor contributing to skin cancer

inducti on (Caforio et al., 2000, 080058 ; Lindelof et al., 2000, 080084 ; Ullrich, 2005, 095635 )

. There

is also evidence that UV radiation has indirect involvement in viral oncogenesis through the human

papillomavirus (Pfister, 2003, 057515 ) , dermatomyositis (Okada et al., 2003, 057769 )

, human

immunodeficiency virus (Breuer

- McHam et al., 2001, 057736 ) and other forms of

immunosuppression (Selgrade M

- JK; Smith et al., 2001, 057454 ).

A potential health benefit of increased UV

- B exposure relates to the production of vitamin D

in humans

. Most humans depend on sun exposure to satisfy their requirements for vitamin D

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1 (Holick, 2004, 057691 )

. Vitamin D deficiency can cause metabolic bone disease among children and



### 2 adults, and also may increase the risk of many common chronic diseases, including type I diabetes



### 3 mellitus and rhe umatoid arthritis (Holick, 2004, 057691 )

. Substantial in vitro and toxicological



### 4 evidence also support a role for vitamin D activity against the incidence or progression of various

fo rms of cancer (Freedman et al., 2002, 035530 ; Garland et al., 1990, 025242 ; Giovannucci, 2005,



### 6 074099 ; Gorham et al., 1990, 000682 ; Grant, 2002, 034981 ; Grant, 2002, 025244 ; Grant and



### 7 Garland, 2004, 075093 ; Hanchette and Schwartz, 1992, 025257 ; Hughes et al., 2004, 074101 ; John



### 8 et al., 1999, 057489 ; John et al., 2005, 670422 ; Lefkowitz and Garland, 1994, 025253 ; Smedby et



### 9 al., 2005, 669175 ; Studzinski and Moore, 1995, 057554 ) . In some studies, UV

- B related production

of vitamin D had potential beneficial immunomodulato ry effects on multiple sclerosis, insulin



### 11 dependent diabetes mellitus, and rheumatoid arthritis (Ponsonby et al., (2002, 080061 ) ; Cantorna,



### 12 (2000, 080060 ) ). More details on UV

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

### 13 AQCD (U.S. EPA, 2006, 088089 ).



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

on Non

- Ionizing Radiation Protection ( ICNIRP ) agreed that some low

- level exposure to UV



### 16 radiation has health benefits (ICNIRP, 2004, 057187 )

. However, th e adverse health effects of higher



### 17 UV exposures necessitated the development of exposure limits for UV radiation

. The ICNIRP



### 18 recognized the challenge in establishing exposure limits that would achieve a realistic balance



### 19 between beneficial and adverse heal th effects . As concluded by ICNIRP (2004, 057187 ) , "[t]he

present understanding of injury mechanisms and long

- term effects of exposure to [UV radiation] is



### 21 incomplete, and awaits further research".



## 10.3.5. Ec o s ys te m a nd Ma te rials Da ma g e Effe cts Due to UV

- B Radiation

22 A 2009 progress report on the environmental effects of O 3 depletion from the UNE P,



### 23 Environmental Effects Assessment Panel (UNEP, 2009, 669084 ) lists many ecosystem and materials



### 24 damage effects from UV

- B rad iation. An in

- depth assessment of the global ecosystem and materials

damage effects from UV

- B radiation per se is out of the scope of this assessment. However, a brief



### 26 summary of some mid

- latitude effects is provided in this section to provide context for UV

- B related



### 27 issues pertaining to tropospheric O 3 . The reader is referred to the UNEP report (UNEP, 2009,



### 28 669084 ) and referenc es therein for further details. All of these UV

- B related ecosystem and materials



### 29 effects can also be influenced by climate change through temperature and other meteorological

alterations, making quantifiable predictions of UV

- B effects difficult.



### 31 Terrestr ial ecosystem effects from increased UV

- B radiation include reduced plant productivity



### 32 and plant cover, changes in biodiversity, susceptibility to infection, and increases in natural UV



### 33 protective responses. In general, however, these effects are small for moderate UV

- B increases at



### 34 mid

- latitudes . A field study on wheat in southern Chile found no substantial changes in crop yield

with moderate increases in UV

- B radiation (Calderini et al., 2008, 668893 ) . Similarly, field studies



### 36 on silver birch ( Betula pendula ) in Finland found no significant effects in photosynthe tic function



### 37 with increases in UV

- B radiation (Aphalo et al., 2009, 668923 ) . Subtle, but important, changes in

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### 1 habitat and biodiversity have also been linked to increases in UV

- B radiation (Maz za et al., 2010,



### 2 668911 ; Obara et al., 2008, 668913 ; Wahl, 2008, 668921 ) . Some plants have natural coping



### 3 mechanisms for dealing with changes in UV

- B radiation (Brown and Jenkins, 2008, 668892 ; Favory



### 4 et al., 2009, 668897 ; Ioki et al., 2008, 668904 ; Jenkins, 2009, 66890 5 ) , but these defenses may have



### 5 costs in terms of reduced growth (Clarke and Robinson, 2008, 668928 ; Phoenix et al., 2000, 668915 ;



### 6 Semerdjieva et al., 2003, 668919 ; Snell et al., 2009, 668920 ).



### 7 Aquatic ecosystem effects from increased UV

- B radiation include sensitivity in growth,



### 8 immune response, and behavioral patterns of aquatic organisms. One study looking at



### 9 coccolithophores, a n abundant phytoplankton group, found a 25% reduction in cellular growth with



### 10 UV

- B exposure (Gao et al., 2009, 668899 ) . E xposure to relevant levels of UV

- B radiation has been



### 11 shown to modify immune response, blood chemistry, and behavior in certain species of fish (Holtby



### 12 and Bothwell, 2008, 668903 ; Jokinen et al., 2008, 668906 ; Markkula et al., 2009, 613291 ) . Adverse



### 13 effects on growth and development from UV

- B radiation have also been observed for amphibians,



### 14 sea urchins, mollusks, corals, and zooplankton (Croteau et al., 2008, 668894 ; Croteau et al., 2008,



### 15 603785 ; Garcia et al., 2009, 668935 ; Marquis and Miaud, 2008, 668908 ; Marquis et al., 2008,



### 16 668909 ; Oromi et al., 2008, 668914 ; Romansic et al., 2009, 515730 ).



### 17 Biogeochemical cycles

, particularly the carbon cycle, can also be influenced by increased UV



### 18 B radiation . A study on high latitude wetlands found UV

- induced increases in CO 2 uptake through



### 19 soil respiration (Haapala et al., 2009, 607260 ) while studies on arid terrestrial ecosystems found



### 20 evidence for UV

- induced release of CO 2 through photodegradati on of above

- ground plant litter



### 21 (Brandt et al., 2009, 668891 ; Caldwell et al., 2007, 668927 ; Henry et al., 2008, 668902 ; Zepp et al.,



### 22 2007, 668896 ) . Changes in solar UV radiation may also have effects on carbon cycling and CO 2 

### 23 uptake in the oceans (Brewer and Peltzer, 2009, 669197 ; Fritz et al., 2008, 668898 ; Hader et al.,



### 24 2007, 668901 ; Meador et al., 2009, 668912 ; Zepp et al., 2008, 668922 ) as well as release of



### 25 dissolved organic matter from sediment and algae (Mayer et al., 2009, 668910 ; Riggsbee et al., 2008,



### 26 668917 ) . Additional studies showing effects on these and additional biogeochemical cycles including



### 27 the water cycle and halocarbon cycle can be found in the UNEP report (UNEP, 2009, 669084 ) and



### 28 references therein.



### 29 Materials damage fro m increased UV

- B radiation include UV

- induced photodegradation of



### 30 wood (Kataoka et al., 2007, 670425 ) and plastics (Pickett et al., 2008, 668916 ) . These studies and



### 31 others summarizing photo

- resistant coatings and materials designed to reduce photodegradation of



### 32 materials are summarized in the UNEP report (UNEP, 2009, 669084 ) and references therein

.



## 10.3.6. UV

- B Rela te d Effec ts As s ociate d with Cha ng e s in Trop o s p he ric Ozone Concentrations

33 T here are multiple complex ities in attempting to quantify the relationship between changes in



### 34 tropospheric O 3 concentrations and UV radiation exposure, as described above. Furthermore,



### 35 q uantifying the relationship between UV radiation and health or welfare effects is complicated by the



### 36 uncertainties involved in the selection of an action spectrum and appropriate characterization of dose

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39 (e.g., peak or cumulative levels of exposure, timing of exposures , etc.) The lack of published studies

that critically examine these issues together

-- that is the incremental health or welfare effects

attributable specifically to UV

- B changes resulting from reductions in tropospheric O 3
concentrations

-- reflects the significant challenges in this field.

As reported in the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) , one analysis by Lutter and Wolz

(1997, 082672 ) attempted to estimate the effects of a nationwide 10 ppb reduction in seasonal

average tropospheric O 3 on the incidence of nonmelanoma and melanoma skin cancers and cataracts

in humans. Their estimate, however, depended upon several simplifying assumptions, ranging from

an assumed generalized 10

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

rates for the two types of skin cancer, and simple, linear biological amplification factor s

. Specifically, the decrease of 10 ppb v in seasonally averaged O 3 concentrations is likely an

overestimate since it doesn't account for the influence of background O 3 coming from the global

accumulation or generation of regional chemistry (Adamowicz et al., 2004, 670421 )

. Further, the

methodologies used in this analysis have ignored area

- specific factors that are important in

estimating the extent to which small, variable chang es in ground

- level O 3 mediate long

- ter m exposures to UV

- B radiation.

A handful of studies have addressed the relationship between changes in tropospheric pollutant

concentrations and UV

- B radiation exposure, providing some additional insight. A study by Pa lancar

and Toselli (2002, 057207 ) looked at changes in measured UV

- B radiation in relation to ground

- level

air pollutants during several air pollution episodes in Cordoba, Argentina. They found that changes

in aerosol concentrations explained the majority of UV

- B radiation fluctuations, and that changes in

tropospheric O 3 and SO 2 had little effect. Repapis et al. (1998, 038015 ) performed a similar study on

UV

- B exposures during high and low air pollution days in Athens, Greece. They found cloud cover

and aerosols to be the major factors in observed UV

- B exposures reductions. Studies by Acosta and

Evans (2000, 670420 ) in Mexico City and Koronakis et al. (2002, 129938 ) in Athens, Greece both

found significant reductions in surface

- level UV exposures during pollution episodes. Both thes e studies include tropospheric O 3 as a potential driver for the reductions, but neither study was able to

quantify the influence of individual atmospheric components involved in the observed attenuation in

UV

- B radiation.

In the absence of studies specifi cally addressing UV -B related health effects from a reduction

in tropospheric O 3 , inferences were made in the 2006 O 3 AQCD (U.S. EPA, 2006, 088089 ) on the

basis of studies focused on stratospheric O 3 depletion . Studies included in that review examined the

potential effect of stratospheric O 3 depletion on the risk of erythema (Longstreth et al., 1998,

001200 )

, skin cancer (De Gruijl, 1995, 057471 ; Longstreth et al., 1995, 055174 ; Madronich and

De G ruijl, 1993, 055183 ; Slaper et al., 1996, 055128 ; Urbach, 19 97, 086255 ) , nonmelanoma skin

cancer (Longstreth et al., 1995, 055174 ; Slaper et al., 1996, 055128 ) , and cataracts (Longstreth et al.,

1995, 055174 )

. Note that several of the concerns expressed above in relation to the Lutter and Wolz

(1997, 082672 ) analysis are relevant to these analyses as well. Furthermore, these studies have a

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

- response

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32 relationships . As a result, c aution is advised when assessing and interpreting the quantitative results

of health risks due to stratospheric O 3 depletion in the co ntext of tropospheric O 3 shielding.

Although the UV

- B related health effect s attributed to marginal reductions in tropospheric or

ground- level O 3 that would result from attainment of the O 3 NAAQS have not been directly

assessed, t hey would be expected to b e small or nonexistent given the above findings and the fact

that tropospheric O 3 makes up only ~ 10% of the total atmospheric O 3 column at mid

- latitudes (Kar

et al., 2010, 670423 ) . Furthermore, O 3 present in the planetary boundary layer makes up only ~10%

of tropospheric O 3 (Thompson et al., 2007, 090796 ) and the NAAQS has only a fractional influence

on those ground

- level O 3 concentrations (i.e., it is not the intent of the NAAQS to entirely eliminate

ground- level O 3 ). The net result is a very small influence on total column O 3 through attainment of

the O 3 standard. In addition, the health benefits of UV

- B in the production of vitamin D suggests that

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

- B radiation exposure may be offset by

the benefits of enhanced vitamin D production

. However, as with other impacts of UV

- B on human

health, this beneficial effect of UV

- B has not been studied in sufficient detail to allow for a cred ible

health benefits assessment . Hence, the above mentioned health and welfare effects associated with

UV

- B exposures resulting from changes in ground

- level O 3 concentrations would likely be small or

nonexistent based on current information.

M ore reasonabl e estimates of the human health impacts of enhanced UV

- B penetration

following reduced ground

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

multiple factors that define the extent of human exposure to UV -B, and (b) well

- defined and

quantifiable links between hu man disease and UV

- B exposure. Within the uncertain context of

presently available information on UV

- B surface fluxes, a risk assessment of UV -B- related health

effects would need to factor in human habits (e.g., daily activities, recreation, dress, and skin care) in

order to adequately estimate UV

- B exposure levels

. Little is known about the impact of variability in

these human factors on individual exposure to UV radiation

. Furthermore, detailed information does

not exist regar ding the relevant type (e.g., peak or cumulative) and time period (e.g., childhood,

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

individual variability in UV resistance

. In conclusion, the effect of changes in surface

- level O 3
concentrations on UV

- induced health outcomes cannot yet be critically assessed within reasonable

uncertainty

. The reader is referred to the U.S. EPA 2002 Final Response to Court Remand (2003,

015702 ) for detailed discussions of the data and scientific issues associated with the determination of

public health benefits resulting from the attenuation of UV

- B by surface

- level O 3

.

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# References

A lis t of all references considered for inclusion in this chapter can be found at http://hero.epa.gov/index.cfm?action=search.do&submit=Search&group_id=478

Acosta, L. R.; Evans, W. F. J. (2000). Design of the Mexico City UV monitoring network: UV

- B measurements at ground level in the urban environment. J Geophys Res, 105: 5017

- 5026. http://dx.doi.org/10.1029/1999JD900250 670420

Adamowicz, V.; Dales, R.; Hale, B. A.; Hrudey, S. E.; Krupnick, A.; Lippman, M.; McConnell, J.; Renzi, P. (2004). Report of an expert panel to review the socio

- economic models and relat ed components supporting the development of

Canada- Wide Standards (CWS) for particulate matter (PM) and ozone to the Royal Society of Canada. J Toxicol Environ Health B Crit Rev, 7: 147

- 266. http://dx.doi.org/10.1080/10937400490253238 670421

Airey, D. K.; Wong, J. C. F.; Fleming, R. A.; Meldrum, L. R. (1997). An estimate of the UV

- B exposure for outdoor workers during a south

- east Queensland summer. Health Phy s, 72: 544

- 549. 057458

Aphalo, P. J.; Vapaavuori, E. M.; de la Rosa, T. M.; Lehto, T. (2009). Does supplemental UV

- B radiation affect gas exchange and RuBisCO activity of Betula pendula Roth. seedlings grown in forest soil under greenhouse conditions? Plant Ecol Divers, 2: 37

- 43. http://dx.doi.org/10.1080/17550870902780299 668923

Arrhenius, S. (1896). On the influence of carbonic acid in the air upon the temperature of the ground. Philos Mag, 41: 237

­ 276. http://www.rsc.org/images/Arrhenius1896_tcm18

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