### Draft

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- 1 June 2012 

## 10 THE ROLE OF TROPOSPH ERIC OZONE IN CLIMAT E CHANGE AND UV

- 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., 199 4 ) . Therefore, tropospheric O 3 makes up a relatively small 

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

- latitudes, but it d oes play an important 

### 5
role in the overall radiation budget. The next section (Section 10.2 ) briefly describes the 

### 6
physics of the earth’s radiation b udget, providing background material for the subsequent 

### 7
two sections assessing how perturbations in tropospheric O 3 might affect (1) climate 

### 8
through its role as a greenhouse gas (Section 10.3 ), and (2) health, ecology and welfare 

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

### 10

(Section 10.4 ). The concluding section in this chapter (Section 10.5 ) includes a summary 

### 11

of effe cts assessed in this chapter along with their associated causal determinations. 

### 12



## 10.2 Physics of the Earth’s Radiation Budget

Radiant energy from the sun enters the atmosphere in a range of wavelengths, but peaks 

### 13
strongly in the visible (400

- 750 nm) part of the spectrum. Longer wavelength infrared 

### 14

(750 nm

- 1 mm) and shorter wavelength ultraviolet (100

- 400 nm) radiation are also 

### 15
present in the solar electromagnetic spectrum. Since the energy possessed by a photon is 

### 16
inversely proportional to its wavelength, infrare d (IR) radiation carries the least energy 

### 17
per photon, and ultraviolet (UV) radiation carries the most energy per photon. UV 

### 18
radiation is further subdivided into classes based on wavelength: UV

- A refers to 

### 19
wavelengths from 400

- 315 nm; UV

- B from 315

- 280 nm; and UV

- C from 280

- 100 nm. 

### 20
By the same argument above describing the relationship between photon wavelength and 

### 21
energy, UV

- A radiation is the least energetic and UV

- C is the most energetic band in the 

### 22
UV spectrum. 

### 23

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

### 24
mixture of gases, clouds, and particles present in the atmosphere (see Figure 10

- 1 ). UV

- A 

### 25

radiation can be s cattered but is not absorbed to any meaningful degree by atmospheric 

### 26
gases including O 3 . UV

- B radiation is absorbed and scattered in part within the 

### 27
atmosphere. UV

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

### 28

### Draft

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- 2 June 2012 participates in photo ionization and photodissociation processes including absorption by 

### 1
stratospheric O 3
. 

### 2

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

- B 

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

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

- B 

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

### 7

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

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

### 9
concentrations of gas molecules present that can absorb or scatter radiation. For this 

### 10
reason, the troposphere is referred to as a “multi ple scattering” regime for UV absorption, 

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

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

- resolved 

### 13
description of the physics of these interactions, is necessary for predicting the effects of 

### 14
tropospheric O 3 on UV

- B flux at the surface. 

### 15

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

### 16
absorption have strong wavelength, path length, and gas/particle concentration 

### 17
dependencies. These combine to create nonlinear effects on UV flux at the Earth’s 

### 18
surface. Chapter 10 of the 2006 O 3 AQCD ( U.S. EPA, 2006b ) describes in detail several 

### 19
key factors that influence the spatiotemporal distribution of ground

- level UV radiation 

### 20
flux, including: (1) long

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

### 21
the position of the Earth in its orbit around the sun; (4) atmospheric absorption and 

### 22
scattering of UV radiation by gas molecules and aerosol particles; (5) absorption and 

### 23
scattering by stratosph eric and tropospheric clouds; and (6) surface albedo. The 

### 24
efficiencies of absorption and scattering are highly dependent on the concentration of the 

### 25
scattering medium, particle size (for aerosols and clouds), and the altitude at which these 

### 26

processes are o ccurring. These properties are sensitive to meteorology, which introduces 

### 27
additional elements of spatial and temporal dependency in ground

- level UV radiation 

### 28

flux. 

### 29

### Draft

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- 3 June 2012 Source: 2006 O 3 AQCD ( U.S. EPA, 2006b )

.



### Figure 10

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

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

### 1
clouds or surfaces with high albedo (reflectivity), such as snow, ice, and desert sand. 

### 2

Radiation that doe s penetrate to the Earth’s surface and is absorbed can be re

- emitted in 

### 3
the longwave (infrared) portion of the spectrum; the rest goes into evaporating water or 

### 4
soil moisture or emerges as sensible heat. The troposphere is opaque to the outgoing 

### 5
longwave r adiation. Polyatomic gases such as water vapor, CO 2 , CH 4 , and O 3 absorb and 

### 6

re

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

### 7
which that energy returns to space. In effect, these gases act as a blanket warming the 

### 8

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

### 9
quantified in the 19th century ( Arrhenius, 1896 ) , and gives rise to the term “greenhouse 

### 10
gas.” The most important greenhouse gas is water vapor. 

### 11

### Draft

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- 4 June 2012 

## 10.3 Effects of Tropospheric Ozone on Climate

10.3.1 Background

As a result of its interaction with incoming solar radiation and outgoing longwave 

### 1
radiation, tropospheric O 3 plays a major role i n determining climate, and increases in its 

### 2
abundance may contribute to climate change ( IPCC, 2007c ) . Models estimate that the 

### 3
global average concentration of O 3 in the troposphere has increased 30

- 70% since the 

### 4
preindustrial era ( Gauss et al., 2006 ) , while observations indicate that in some regions 

### 5
tropospheric O 3 may have increased by factors as great as 4 or 5 ( Marenco et al., 1994 ; 

### 6

Staehelin et al., 1994 ) . These increases are tied to the rise in emissions of O 3 precursors 

### 7
from human activity, mainly fossil fuel consumption and agricultural processes. 

### 8

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

### 9

estimated to be about 25

- 40% of the anthropogenic CO 2 effect and about 75% o f the 

### 10
anthropogenic CH 4 effect ( IPCC, 2007c ) , ranking it third in importance behind these two 

### 11
major greenhouse gases. In the 21st century, as the Earth’s population continues to grow 

### 12
and energy technology spreads to developing countries, a further rise in the global 

### 13
concentration of tropospheric O 3 is likely, with associated consequences for human health 

### 14
and ecosystems relating to climate change. 

### 15

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

### 16
information to policy makers, the World Meteorological Organization (WMO) and the 

### 17
United Nations Environment Programme (UNEP) formed the Intergovernmental Panel on 

### 18
Climate Change (IPCC) in 1988. The IPCC supports the work of the Conference of 

### 19
Parties (COP) to the United Nations Framework Convention on Climate Change 

### 20
(UNFCCC). The IPCC periodically brings together climate scientists from member 

### 21
countries of WMO and the United Nations to review knowledge of the physical climate 

### 22
system, past and future climate change, and evidence of human

- induced climate change. 

### 23
IPCC climate assessment reports are issued every five to seven years. 

### 24

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

, 

### 25
as well as other peer

- reviewed published research. Section 10.3.2 reviews evidence of 

### 26
climate change in the recent past and projections of future climate change. It also offers a 

### 27
brief comparison of tropospheric O 3 relative to other greenhouse gases. Section 10.3.3 

### 28
describes factors that influence the magnitude of tropospheric O 3 effects on climate. 

### 29

Section 10.3.4 considers the competing effects of O 3 precurs ors on climate. Finally, 

### 30

Section 10.3.5 and Section 10.3.6 describe the effects of changing tropospheric O 3 

### 31
concentrations on past and future climate. Downstream effects resulting from climate 

### 32
change, such as ecosystem responses, are outside the scope of this assessment, which 

### 33

### Draft

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- 5 June 2012 focuses rather o n the effects of changes in tropospheric O 3 concentrations on radiative 

### 1
forcing and climate. 

### 2

10.3.2 Climate Change Evidence and the Influence of Tropospheric Ozone

10.3.2.1 Climate Change in the Recent Past

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

- 1 800s, observations 

### 3
from ice cores show that concentrations of the long

- lived greenhouse gases CO 2 , CH 4 , 

### 4

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

### 5

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

### 6
too, has remained relatively constant during this time period ( Thompson et al., 1993 ; 

### 7
Thompson, 1992 ) . The stable mix of these greenhouse gases in the atmosphere, together 

### 8
with water vapor, has kept the global mean temperature of the Earth close to 15oC. 

### 9
Without the presence of greenhouse gases in the atmosphere, the Earth’s global mean 

### 10
temperature would be ab out 30oC cooler, or

- 15oC. 

### 11

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

### 12
increases of greenhouse gases in the atmosphere, mainly through fossil fuel combustion. 

### 13
According to the IPCC AR4 ( IPCC, 2007c ) , there is now “very high confidence” that the 

### 14
net effect of anthropogenic greenhouse gas emissions since 1750 has led to warming, and 

### 15
it is “very likely” that human activity contributed to the 0.76oC rise in global mean 

### 16

temperatur e observed over the last century. The increase of tropospheric O 3 may have 

### 17

contributed 0.1

- 0.3oC warming to the global climate during this time period ( Hansen et 

### 18

al., 2005 ; Mickley et al., 2004 ) . Global cooling due to anthropogenic aerosols ( IPCC, 

### 19

2007c ) has likely masked the full warming effect of the anthropogenic greenhouse gases 

### 20

on a global scale. 

### 21

10.3.2.2 Projections of Future Climate Change

The IPCC AR4 projects a warming of ~0.2oC per decade for the remainder of the 21st 

### 22

century ( IPCC, 2007c ) . Even at constant concentrations of greenhouse gases i n the 

### 23
atmosphere, temperatures are expected to increase by about 0.1oC per decade, due to the 

### 24
slow response of oceans to the warming applied so far. It is likely that the Earth will 

### 25
experience longer and more frequent heat waves in the 21st century, togeth er with more 

### 26
frequent droughts and/or heavy precipitation events in some regions, due to perturbations 

### 27
in the hydrological cycle that result from changing temperatures. Sea levels could 

### 28

### Draft

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- 6 June 2012 increase by 0.3

- 0.8 meters by 2300 due to thermal expansion of the oce ans. The extent of 

### 1
Arctic sea ice is expected to decline, and contraction of the Greenland ice sheet could 

### 2

further contribute to the sea level rise ( IPCC, 2007c )

. 

### 3

Projections of future climate change are all asso ciated with some degree of uncertainty. A 

### 4
major uncertainty involves future trends in the anthropogenic emissions of greenhouse 

### 5
gases or their precursors. For the IPCC AR4 climate projections, a set of distinct 

### 6
“storylines” or emission pathways was develop ed ( IPCC, 2000 ) . Each storyline took into 

### 7
account factors such as population growth, mix of energy technologies, and the sharing of 

### 8
technology between developed and developing nations, and each resulted in a different 

### 9
scenario for anthropogenic emissions. When these tre nds in emissions are applied to 

### 10
models, these scenarios yield a broad range of possible climate trajectories for the 21st 

### 11

century. 

### 12

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

### 13
representation of climate and, espec ially, climate feedbacks. A rise in surface 

### 14
temperatures would perturb a suite of other processes in the earth

- atmosphere

- ocean 

### 15
system, which may in turn either amplify the temperature increase (positive feedback) or 

### 16
diminish it (negative feedback). One im portant feedback involves the increase of water 

### 17
vapor content of the atmosphere that would accompany higher temperatures ( Bony et al., 

### 18

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

### 19
may increase the climate sensitivity to a doubling of CO 2 by ne arly a factor of two ( Held 

### 20

and Soden, 2000 ) . The ice

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

### 21
cover and sea ice extent would diminish the Earth’s albedo, allowing more solar energy 

### 22

to be retained at the surface ( Holland and Bitz, 2003 ; Rind et al., 1995 ) . A final ex ample 

### 23
of a climate feedback involves the effects of changing cloud cover in a warming 

### 24
atmosphere. Models disagree on the magnitude and even the sign of this feedback on 

### 25
surface temperatures ( Soden and Held, 2006 )

. 

### 26

10.3.2.3 Metrics of Potential Climate Change

Two metrics frequently used to estimate the potential climate effect of some perturbation 

### 27
such as a change in greenhouse gas concentr ation are: (1) radiative forcing; and (2) global 

### 28
warming potential (GWP). These metrics differ in a fundamental way as described below. 

### 29

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

### 30
atmosphere or at the surface when a perturbation is introduced in the earth

- atmosphere



### 31
ocean system. In the global mean, radiative forcing of greenhouse gases at the tropopause 

### 32
(top of the troposphere) is roughly proportional to the surface temperature response 

### 33

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

### 34

### Draft

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- 7 June 2012 assessing the response of the ea rth’s surface temperature to a given change in the 

### 1
concentration of a greenhouse gas. Positive values of radiative forcing indicate warming 

### 2
in a test case relative to the control; negative values indicate cooling. The units of 

### 3
radiative forcing are energy flux per area, or W/m 2

. 

### 4

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

### 5
from model to model. However, radiative forcing does not take into account the climate 

### 6
feedbacks that could amplify or dampen the actual surfac e temperature response, 

### 7
depending on region. Quantifying the change in surface temperature requires a climate 

### 8
simulation in which all important feedbacks are accounted for. As some of these 

### 9
processes are not well understood, the surface temperature respons e to a given radiative 

### 10
forcing can be highly uncertain and can vary greatly among models and even from region 

### 11
to region within the same model. 

### 12

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

### 13
from a unit mass pulse emission of a greenhouse gas or its precursor, and is reported as 

### 14
the magnitude of this radiative forcing relative to that of CO 2
. GWP is most useful for 

### 15
comparing the potential climate effects of long

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

### 16
tropospheric O 3 has a lifetime on the order of weeks to months, GWP is not seen as a 

### 17
valuable metric for quantifying the importance of O 3 on climate ( Forster et al., 2007 )

. 

### 18
Thus, this assessment focuses on radiative forcing as the metric of climate influence 

### 19
resulting from changes in tropospheric O 3
. 

### 20

10.3.2.4 Tropospheric Ozone as a Greenhouse Gas

Tropospheric O 3 differs in important ways from other greenhouse gases. It is not emitted 

### 21
directly, but is produced through photochemical oxidation of CO, CH 4 , and nonmethane 

### 22
volatile organic compounds (VOCs) in the presence of nitrogen oxide radicals 

### 23

( NO X = NO + NO 2 ; see Chapter 3

, Section 3.2 for further details on the chemistry of O 3 

### 24
formation). It is also sup plied by vertical transport from the stratosphere. The lifetime of 

### 25

O 3 in the troposphere is typically a few weeks, resulting in an inhomogeneous 

### 26
distribution that varies seasonally; the distribution of the long

- lived greenhouse gases like 

### 27

CO 2 and CH 4 are m uch more uniform. The longwave radiative forcing by O 3 is mainly 

### 28
due to absorption in the 9.6 μm window, where absorption by water vapor is weak. It is 

### 29
therefore less sensitive to local humidity than the radiative forcing by CO 2 or CH 4 , for 

### 30

which there is much more overlap with the water absorption bands ( Lenoble, 1993 ) . And 

### 31
unlike other major greenhouse gases, O 3 absorbs in the shortwave as well as the 

### 32
longwave part of the spectrum. 

### 33

### Draft

– Do Not Cite or Quot e 10

- 8 June 2012 Figure 10

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

### 1

Emissions of O 3 precursors including CO, VOCs, CH 4 , and NO X lead to production of 

### 2
tropospheric O 3 . A change in the abundance of tropospheric O 3 perturbs the radiative 

### 3
balance of the atmosphere, an effect quantified by the radiative forcing metric. The earth



### 4
atmosphere

- ocean system responds to the radiative forcing with a climate response, 

### 5
typically expressed as a change in surface temperature. Finally, the climate response 

### 6

causes downstream climate

- related health and ecosystem effects, such as redistribution of 

### 7
diseases or ecosystem characteristics due to temperatur e changes. Feedbacks from both 

### 8
the climate response and downstream effects can, in turn, affect the abundance of 

### 9
tropospheric O 3 and O 3 precursors through multiple mechanisms. Direct feedbacks are 

### 10

discussed further in Section 10.3.3.4 ; the downstream climate effects and their feedbacks 

### 11
are extremely complex and outside the scope of this assessment. 

### 12

### Draft

– Do Not Cite or Quot e 10

- 9 June 2012 Note: Figure includes the relationship between prec ursor emissions, changes in tropospheric ozone abundance, radiative forcing, climate response, and climate effects. Units shown are those typical for each quantity illustrated. Feedbacks from both the c limate response and climate effects can, in turn, affe ct the abundance of tropospheric ozone and ozone precursors through multiple feedback mechanisms. Climate effects and their feedbacks are deemphasized in the figure since these downstream effects are extremely complex and outside the scope of this assessme nt.



### Figure 10

- 2 Schematic illustrating the effects of tropospheric ozone on climate.

The IPCC (2007c ) reported a radiative forcing of 0.35 W/m 2 for t he change in 

### 1
tropospheric O 3 since the preindustrial era, ranking it third in importance after the 

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

- 3 shows the global 

### 3
average radiative forcing estimates and uncertainty ranges in 2005 for anthropogenic 

### 4

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

### 5
the tropospheric O 3 radiative forcing estimate in the f igure range from 0.25 to 0.65 W/m 2

, 

### 6
making it relatively more uncertain than the long

- lived greenhouse gases. 

### 7

### Draft

– Do Not Cite or Quot e 10

- 10 June 2012 Note: 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. These 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.

Source: Reprinted with permission of Cambridge University Press ( IPCC, 2007c )

.



### 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.

10.3.3 Factors that Influence the Effect of Tropospheric 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 stratospheric O 3 may also affect tempe ratures at the Earth’s 

### 6
surface, but aside from issues relating to stratospheric

- tropospheric exchange discussed in 

### 7
Chapter 3 , Section 3.4.1.1 , stratospheric O 3 assessment is beyond the scope of this 

### 8

document. 

### 9

### Draft

– Do Not Cite or Quot e 10

- 11 June 2012 10.3.3.1 Trends in the Concentration of Tropospheric Ozone

To first order, the effect of tropos pheric O 3 on global surface temperature is proportional 

### 1
to the change in tropospheric O 3 concentration. The earth’s surface temperatures are most 

### 2

sensitive to O 3 perturbations in the mid to upper troposphere. This section therefore 

### 3
focuses mainly on observ ed O 3 trends in the free troposphere or in regions far from O 3 

### 4
sources, where a change in O 3 concentrations may indicate change throughout the 

### 5
troposphere. Data from ozonesondes, mountaintops, and remote surface sites are 

### 6

discussed, as well as satellite da ta. 

### 7

Observed Trends in Ozone since the Preindustrial Era

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

### 8

1900s show values at about 10 ppb, about one

- fifth the values observed today at similar 

### 9

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

### 10
measurements is questionable however, in part because they exhibit O 3 concentrations 

### 11
equivalent to or only a couple of parts per billion greater than those observed at nearby 

### 12

low

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

### 13

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

### 14
stratospheric source and its longer lifetime aloft. In another study, Staehelin et al. (1994 ) 

### 15
revisited observations made in the Swiss mountains during the 1950s and found a 

### 16
doubling in O 3 concentrations from that era to 1989

- 1991. 

### 17

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



### 18
borne ozonesondes, but even this record is sparse. Trends from ozonesondes have been 

### 19
highly variable and dependent on regio n ( Logan et al., 1999 ) . Over most sites in the U.S., 

### 20

ozonesondes reveal little trend. Over Canada, observations show a decline in O 3 b etween 

### 21
1980 and 1990, then a rebound in the following decade ( Tarasick et a l., 2005 )

. 

### 22
Ozonesondes over Europe give a mixed picture. European ozonesondes showed increases 

### 23

in the 1970s and 1980s, with smaller increases or even declines since then ( Oltmans et 

### 24

al., 2006 ; Logan et al., 1999 ) . Over Japan, O 3 in the lower trop osphere increased about 

### 25

0.2

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

. 

### 26

Ground

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

### 27
like ozonesonde data are sparse before the 1970s. Springtime O 3 observations from 

### 28
several mountain sites in the western U.S. show a positive trend of about of 0.5

- 

### 29

0.7 ppb/year since the 1980s ( Cooper et al., 2010 ; Jaffe et al., 2003 ) . Ship

- borne O 3 

### 30
measurements for the time period 1977 to 2002 indic ate increases of 0.1

- 0.7 ppb/year 

### 31

over much of the Atlantic south of 40oN, but no appreciable change north of 40oN 

### 32

( Lelieveld et al., 2004 ) . The lack of trend for the Nor th Atlantic would seem at odds with 

### 33

### Draft

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- 12 June 2012 O 3 observations at Mace Head (53oN) on the west coast of Ireland, which show a 

### 1
significa nt positive trend of about 0.5 ppb/year from 1987 to 2003 ( Simmonds et al., 

### 2

2004 ) . Over Japan, O 3 at a remote mountain site has increased 1 ppb/year from 1998 to 

### 3

2003 ( Tanimoto, 2009 ) , a rate more than double that recorded by ozonesondes in the 

### 4
lower troposphere over Japan during the 1990s ( Naja and Akimoto, 2004 ) . At Zugspitze, 

### 5
a mountain site in Germany, O 3 increased by 12% per decade during the 1970s and 

### 6
1980s, consistent with European ozonesondes ( Oltmans et al., 2006 ) . Since then, O 3 

### 7
continues to increase at Zugspitze, but more slowly. What little data exist for the 

### 8
Southern Hemisphere point to measurable increases in tropospheric O 3 in recent decades, 

### 9
as much as ~15% at Cape Grim in t he 1989

- 2004 time period ( Oltmans et al., 2006 )

. 

### 10

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

### 11

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

### 12
tropospheric O 3 are covered in Chapter 3 , Section 3.5.5.5 ). In contrast to the surface data 

### 13
from ships, tropospheric O 3 columns from the Total Ozone Mapping Spectrometer 

### 14
(TOMS) show no trend over the tropical Atlantic f or the period 1980

- 1990 ( Thompson 

### 15

and Hudson, 1999 ) . Over the Pacific, a longer, 25 year record of TOMS data again 

### 16
reveals no trend over the tropics, but shows increases in tropospheric column O 3 of about 

### 17

2

- 3 Dobson Units (DU) 1 at mid

- latitudes in both hemispheres ( Ziemke et al., 2005 )

. 

### 18

Interpreting these recent trends in tropospheric O 3 is challenging. The first difficulty is 

### 19
reconciling apparently contradictory trends in the observations, e.g., over tropical oceans. 

### 20
A second difficulty is that the O 3 trends depend on several factors, not all of whic h can be 

### 21

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

### 22

(2) variation in the stratospheric source of O 3 , (3) changes in solar radiation resulting 

### 23
from stratospheric O 3 depletion, and (4) trends in tropospheric temperatu res ( Fusco and 

### 24
Logan, 2003 ) . Recent positive trends in the western U.S. and over Japan are consistent 

### 25
with the rapid increase in emissions of O 3 p recursors from mainland Asia and transport of 

### 26
pollution across the Pacific ( Cooper et al., 2010 ; Tanimot o, 2009 ) . The satellite trends 

### 27

over the norther n mid

- latitudes are consistent with this picture as well ( Ziemke et al., 

### 28

2005 ) . Increases in tropospheric O 3 in the Southern Hemisphere are also likely due to 

### 29
increased anthropogenic NO X emissions, especially from biomass burning ( Fishman et 

### 30

al., 1991 ) . Recent declines in summertime O 3 over Europe can be partly explained by 

### 31

decreases in O 3 precursor emissions there ( Jonson et al., 2005 ) , while springtime 

### 32
increases at some European sites are likely linked to changes in stratospheric dynamics 

### 33

( Ordonez et al., 2007 )

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

### 34

1 T he 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 a tm), and corresponds to a column of O 3 containing 2.69 × 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

– Do Not Cite or Quot e 10

- 13 June 2012 the lowermost stratosphere may have reduced the stratospheric flux of O 3 into the 

### 1
troposph ere by as much as 30% from the early 1970s to the mid 1990s, consistent with 

### 2

the trends in ozonesondes there. 

### 3

Calculation of Ozone Trends for the Recent Past

Simulations of trends in tropospheric O 3 provide a means for test ing current knowledge 

### 4

of O 3 processes and predict ing with greater confidence trends in future O 3 concentrations. 

### 5

Time

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

### 6

1850

- 2000 ( Lamarque et al., 2010 ) and for 1890

- 1990 ( VanAardenne et al., 2001 ) . These 

### 7
inventories allow for the calculation of changing O 3 concentration over time. 

### 8

One recent multi

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

### 9
preindustrial times of 8

- 14 DU, or about 30

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

### 10

modeled estimates reveals the limitations in knowledge of processes in the pristine 

### 11
atmosphere. Models typically overestimate the late ni neteenth and early twentieth century 

### 12
observations available in surface air and at mountain sites by 50

- 100% ( Lamarque et al., 

### 13

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

### 14
differences between models and measurements will require more accurate simulation of 

### 15

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

### 16
such as bromine radicals, which may reduce background O 3 in the pristine atmosphere by 

### 17

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

. 

### 18

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

- dependent emissions reveals an 

### 19
equatorward shift in the distribution of tropospheric O 3 in the Northern Hemisphere due 

### 20

to the industrialization of societies at low

- latitudes ( Lamarque et al., 2005 ; Berntse n et 

### 21

al., 2000 ) . By constraining a model with historical (1950s

- 2000) observations, Shindell 

### 22
and Faluvegi (2002 ) calculated a large increase of 8.2 DU in tropospheric O 3 over 

### 23
polluted continental regions since 1950. This trend is not captured in standard chemistry 

### 24
models, but is consistent with the change in tropospheric O 3 since preindustrial times 

### 25
implied by the observations from the late 1800s ( Pavelin et al., 1999 ; Marenco et al., 

### 26

1994 )

. 

### 27

10.3.3.2 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 

### 28
troposphere, a bsorption of incoming shortwave solar radiation by O 3 is small relative to 

### 29
its absorption of outgoing longwave terrestrial radiation. However, over surfaces 

### 30
characterized by high albedo (e.g., over snow, ice, or desert sand), incoming radiation is 

### 31

### Draft

– Do Not Cite or Quot e 10

- 14 June 2012 more lik ely to be reflected than over darker surfaces, and the probability that O 3 will 

### 1
absorb shortwave solar radiation is therefore larger. In other words, energy that would 

### 2
otherwise return to space may instead be retained in the atmosphere. Several studies hav e 

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

- latitudes leads to radiative forcing 

### 4
estimates greater than 1.0 W/m 2 in the region, especially in summer ( Shindell et al., 2006 ; 

### 5

Liao et al., 2004b ; Mickley et al., 1999 ) . Both the high surface albedo of the Arctic and 

### 6

the large solar zenith angles there (which increase the path length of incoming sunlight) 

### 7
lead to strong shortwave forcing in the region. Because the Arctic is especially sensitive 

### 8
to radiative forcing through the ice

- albedo feedback, the large contribution i n the 

### 9
shortwave solar spectrum to the total radiative forcing in the region may be important. 

### 10

10.3.3.3 The Effect of Vertical Distribution on Ozone Radiative Forcing

In the absence of feedbacks, O 3 increments near the tropopause produce the largest 

### 11

increases in sur face temperature ( Lacis et al., 1990 ; Wang et al., 1980 ) . This is a result of 

### 12
the colder temperature of the tropopause relative to the rest of the troposphere and 

### 13
stratosphe re. Since radiation emitted by the atmosphere is approximately proportional to 

### 14
the fourth power of its temperature 1 , the colder the added O 3 is relative to the earth’s 

### 15
surface, the weaker the radiation emitted and the greater the “trapping” of longwave 

### 16

rad iation in the troposphere. 

### 17

10.3.3.4 Feedback Factors that Alter the Climate Response to Changes in Ozone Radiative Forcing

Estimates of radiative forcing provide a first

- order assessment of the effect of 

### 18
tropospheric O 3 on climate. In the atmosphere, climate feedba cks and transport of heat 

### 19
alter the sensitivity of Earth’s surface temperature to addition of tropospheric O 3
. 

### 20
Assessment of the full climate response to increases in tropospheric O 3 requires use of a 

### 21

climate model to simulate these interactions. 

### 22

Due to it s short lifetime, O 3 is heterogeneously distributed through the troposphere. Sharp 

### 23
horizontal gradients exist in the radiative forcing of O 3 , with the greatest radiative forcing 

### 24
since preindustrial times occurring over the northern mid

- latitudes (more on t his in 

### 25

Section 10.3.5 and Section 10.3.6 ). If climate feedbacks are particularly powerful, they 

### 26
may obscure or even erase the correlation between regional radiative forcing and climate 

### 27

1 As described by the Stefan

- Boltzmann law, an ideal blackbody

-- which the atmosphere approximates

-- absorbs at all wavelengths and re

- radiates proportional to the fourth power of its temperature.

### Draft

– Do Not Cite or Quot e 10

- 15 June 2012 response ( Harvey, 2004 ; Boer and Yu, 2003 ) . The transport of heat through the 

### 1
atmosphere, though not technically a feedback, may also weaken the correlation between 

### 2
forcing and climate response. Several model studies have reported that the hori zontal 

### 3
pattern of surface temperature response from 2000

- 2100 trends in predicted short

- lived 

### 4
species (including O 3 ) closely matches the pattern from the trends in the long

- lived 

### 5
greenhouse gases over the same time period ( Levy et al., 2008 ; Shind ell et al., 2008 ; 

### 6

Shindell et al., 2007 ) . This correspondence occurs even though the patterns of radiative 

### 7

fo rcing for the short

- lived and long

- lived species differ substantially . In a separate paper, 

### 8

Shindell et al. (2007 ) found that Arctic temperatures are especially sensitive to the mid



### 9
latitude radiative forcing from tropospheric O 3
. 

### 10

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

### 11
some consistency with the O 3 radiative forcing. For example, Mickley et al. (2004 ) 

### 12
examined the change in O 3 since preindustrial times and found greater warming in the 

### 13
Northern Hemisphere than in the Southern Hemisphere (+0.4oC versus +0.2oC), as well 

### 14
as higher surface temperatures downwind of Europe and Asia and over the North 

### 15
American interior in summer. For an array of short

- lived species including O 3 , Shindell 

### 16
and Faluvegi (2009 ) found that radiative forcing applied over northern mid

- latitudes yield 

### 17
more localized responses due to local cloud, water vapor, and albedo feedbacks than 

### 18
radiative forcing applied over the tropics. 

### 19

Climate f eedbacks can also alter the sensitivity of surface temperature to the vertical 

### 20
distribution of tropospheric O 3 . The previous section (Section 10.3.3.3 ) described the 

### 21
greater effect of O 3 added to the upper troposphere (near the tropopause) on radiative 

### 22
forcing, relative to additions in the mid

- to lower troposphere. However, warming 

### 23
induced by increased O 3 in the upper troposphere could stabilize the atmosphere to some 

### 24
extent, limiting the transport of heat to the Earth’s surface and mitigating the effect of the 

### 25

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

. Hansen et al. 

### 26

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

### 27

enhanceme nts in the mid

- troposphere had the greatest effect on surface temperature. 

### 28

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

### 29
greenhouse gas relative to another. For example, Mickley et al. (2004 ) found a greater 

### 30
temperature response to CO 2 radiative forcing than to an O 3 radi ative forcing of similar 

### 31
global mean magnitude, due in part to the relatively weak ice

- albedo feedback for O 3
. 

### 32

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

### 33
the middle troposphere and is also shifted toward the drie r poles. A poleward shift in 

### 34
radiative forcing amplifies the ice

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

### 35

mid

- troposphere radiative forcing allows for greater surface temperature response, 

### 36

relative to that for O 3
. 

### 37

### Draft

– Do Not Cite or Quot e 10

- 16 June 2012 10.3.3.5 Indirect Effects of Tropospheri c Ozone on the Carbon Cycle

A proposed indirect effect of tropospheric O 3 on climate involves the carbon cycle. By 

### 1
directly damaging plant life in ways discussed in Chapter 9 , increases in tropospheric O 3 

### 2
may depress the land

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

### 3
atmosphere and ultimately warming of the Earth’s surface. Sit ch et al. (2007 ) calculated 

### 4
that this indirect warming effect of O 3 on climate has about the same magnitude as the O 3 

### 5
direct effect. Their results suggest a doubled sensitivity of surface temperatures to O 3 

### 6
radiative forcing, compared to current model estimates. 

### 7

10.3.4 Competing Effects of Ozone Precursors on Climate

Changes in O 3 precursors can affect the radiative balance of the atmosphere through 

### 8
multiple (and sometimes competing) mechanisms. For example, the O 3 precursor CH 4 is 

### 9
itself a powe rful greenhouse gas. O 3 and its other precursors also exert a strong control on 

### 10
the oxidizing capacity of the troposphere, and so can affect the lifetime of gases such as 

### 11

CH 4 ( Derwent et al., 2001 ) . For example, an increase in CO or VOCs would lead to a 

### 12
decrease in hydroxyl (OH) concentrations. Since OH is a major sink for CH 4 , a decline in 

### 13
OH would lengthen the CH 4 lifetime, enhance the CH 4 concentration, and amplify 

### 14
surface warming. A rise in NO X emissions, on the other hand, could lead to an increase in 

### 15
OH in certain locations, shortening the CH 4 lifetime and causing surface cooling 

### 16

( Fuglestvedt et al., 1999 ) . O 3 can itself generate OH through (1) photolysis leading to 

### 17
excited oxygen atoms followed by reaction with water vapor and (2) reaction with HO 2
. 

### 18

Figure 10

- 4 shows the radiative forcing associated with a suite of anthropogenic 

### 19
emissions, including O 3 precursors ( IPCC, 2007b ) . The emission

- based radiative forcing 

### 20

for CH 4 , which includes the CH 4 effect on O 3 production, is +0.9 W/m 2 , or nearly double 

### 21

that of the CH 4 abundance

- based radiative forcing shown in Figure 10

- 3

. Figure 10

- 4 also 

### 22
shows a warming from anthropogenic CO and VOC emissions of +0.27 W/m 2 and a net 

### 23
cooling of

- 0.21 W/m 2 for NO X emissions. The net cooling for NO X occurs mainly due to 

### 24

the links between NO X and CH 4 . Consistent with these results, Shindell and Faluvegi 

### 25

(2009 ) calculated positive (+0.25 W/m 2 ) radia tive forcing from the increase in 

### 26
anthropogenic emissions of CO and VOCs since preindustrial times, as well as for CH 4 

### 27

(+1 W/m 2 ). In contrast, Shindell and Faluvegi (2009 ) found negative (

- 0.29 W/m 2 ) 

### 28
radiative forcing from anth ropogenic emissions of NO X . Other studies have found a near 

### 29
cancellation of the positive O 3 radiative forcing and the negative CH 4 radiative forcing 

### 30
that arise from an incremental increase in anthropogenic NO X emissions ( Naik et al., 

### 31

2005 ; Fiore et al., 2002 ; Fuglestvedt et al., 1999 ) . The net effect of aircraft NO X on 

### 32
climate is especially complex ( Isaksen et al., 2001 ; Wild et al., 2001 )

. Stevenson (2004 ) 

### 33

### Draft

– Do Not Cite or Quot e 10

- 17 June 2012 calculate d that aircraft NO X leads to short

- term net warming via O 3 production in the cool 

### 1
upper troposphere, but long

- term net cooling because of CH 4 loss

. 

### 2

OH production from O 3 precursors can also affect regional sulfate air quality and climate 

### 3
forcing by increasing gas

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

### 4

IPCC AR4, Unger (2006 ) re ported that by 2030, enhanced OH from the A1B O 3 

### 5
precursors may increase surface sulfate aerosol concentrations by up to 20% over India 

### 6
and China, relative to the present

- day, with a corresponding increase in radiative cooling 

### 7
over these regions. In this w ay, O 3 precursors may impose an indirect cooling via sulfate 

### 8

( Unger, 2006 )

. 

### 9

Taken together, these results point out the need for caref ul assessment of net radiative 

### 10
forcing involving multiple pollutants in developing climate change policy ( Unger et al., 

### 11

2008 ) . Many studies point to CH 4 as a particularly attractive target for emissions control 

### 12

since CH 4 is itself an important precursor of O 3 ( West et al., 2007 ; Fiore et al., 2002 )

. 

### 13

Fiore et al. (2002 ) found that reducing anthropogenic CH 4 emissions by 50% would lead 

### 14
to a global negative (

- 0.37 W/m 2 ) radiative forcing, mostly from CH 4 . In later research, 

### 15

Fiore et al. (2008 ) reported that CH 4 re ductions would most strongly affect tropospheric 

### 16

O 3 column amounts in regions of strong downwelling from the upper troposphere 

### 17
(e.g., around 30 o N) and in regions of NO X - saturated conditions. 

### 18

The magnitude of the radiative forcing from the change in troposp heric O 3 since the 

### 19
preindustrial era is uncertain. This uncertainty derives in part from the scarcity of early 

### 20

measurements and in part from limited knowledge regarding processes in the natural 

### 21
atmosphere. As noted previously, the IPCC AR4 reports a radiative forcing of 0.35 W/m 2 

### 22
from the change in tropospheric O 3 since 1750 ( Forster et al., 2007 ) , ranking it third in 

### 23
importance behind the greenhouse gases CO 2 and CH 4 . The O 3 radiative forcing could, in 

### 24
fact, be as large as 0.7 W/m 2 , if reconstructions of preindustrial and mi d

- 20th century O 3 

### 25

based on the measurement record are valid ( Shindell and Faluvegi, 2002 ; Mickley et al., 

### 26

2001 ) . In any event, Unger et al. (2010 ) showed that present

- day O 3 radiative forcing can 

### 27
be attributed to emissions from many economic sectors, including on

- road vehicles, 

### 28
household biofuel, power generation, and biomass burning. As much as one

- third of the 

### 29
radiative forcing from the 1890 to 1990 change in tropospheric O 3 could be due to 

### 30
increased biomass burning ( Ito et al., 2007a )

. 

### 31

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

### 32
satellite data. Using data from TOMS, Worden et al. (2008 ) estimated a reduction in 

### 33

clear

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

### 34
oceans in 2006. This radiative forcing includes contributions from b oth anthropogenic 

### 35

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

### 36
preindustrial times ( Gauss et al., 2006 ) , the total O 3 radiative forcing estima ted with 

### 37

### Draft

– Do Not Cite or Quot e 10

- 18 June 2012 TOMS is consistent with that obtained from models estimating just the anthropogenic 

### 1

contribution. 

### 2

Note: Values represent radiative forcing in 2005 due to emissions and changes since 1750. (S) and (T) next to gas species represent stratospheric a nd tropospheric changes, respectively. Source: Reprinted with permission of Cambridge University Press ( IPCC, 2007b )

.



### Figure 10

- 4 Components of radiative forcing for emissions of principal gases, aerosols, aerosol precursors , and oth er changes.

### Draft

– Do Not Cite or Quot e 10

- 19 June 2012 10.3.5 Calculating Radiative Forcing and Climate Response to Past Trends in Tropospheric Ozone

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

### 1
complexity of feedbacks, as mentioned in Section 10.3.2.2 and Section 10.3.3.4 . In their 

### 2
modeling study, Mickley et al. (2004 ) reported a global mean increase of 0.28oC since 

### 3
preindustrial times, with values as large as 0.8oC in continental interiors. For the time 

### 4
period since 1870, Hansen et al. (2005 ) estimated a much smaller increase in global mean 

### 5
surface temperature (0.11oC), but they implemented 1880s anthropogenic emissions in 

### 6
their base simulation and also took into account trends in both stratospheric and 

### 7
tropospheric O 3 ,. The modeled decline of lower stratospheric O 3 , especially over polar 

### 8
regions, cooled surface temperatures in this study, counteracting the warming effect of 

### 9
increasing tropospheric O 3
. 

### 10

Figure 10

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

### 11
that figure, summertime O 3 has the largest radiative effect over the continental interiors 

### 12
of the Northern Hemisphere. Shindell et al. (2006 ) estimated that th e change in 

### 13
tropospheric O 3 over the 20th century could have contributed about 0.3 oC to annual mean 

### 14
Arctic warming and as much as 0.4

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

### 15
Chang et al. (2009 ) calculated a surface temperature increase of 0.4oC to the 1970

- 2000 

### 16
change in trop ospheric O 3 . It is not clear, however, to what degree regional changes in O 3 

### 17
concentration influenced this response, as opposed to more global changes. 

### 18

### Draft

– Do Not Cite or Quot e 10

- 20 June 2012 Note: Figure includes 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 (to p right). Temperature trends greater than about
0.1 oC are significa nt over the oceans, while values greater than 0.3 oC are typically significa nt over land, except for northern middle and high latitudes during winter where values in excess of about 0.5 oC ar e significa nt. Values in the top right corner give area

weighted global averages in the same units as the plots. Source: Reprinted with permission of American Geophysical Union ( Shindell et al., 2006 )

.



### Figure 10

- 5 Ensemble average 1900

- 2000 radiative forcing and surface temperature trends ( oC per century) in response to tropospheric

ozone changes.

10.3.6 Calculating Radiative Forcing and Climate Response to Future Trends in Tropospheric Ozone

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

### 1
energy technology the wor ld's societies will follow in coming decades. The trends in O 3 

### 2
will also depend on the changes in a suite of climate

- sensitive factors, such as the water 

### 3
vapor content of the atmosphere. This section describes the following issues: 

### 4

(1) projected trends in the anthropogenic emissions of O 3 precursors; (2) the effects of 

### 5
these emissions on the tropospheric O 3 concentrations; (3) the effects of changing climate 

### 6

### Draft

– Do Not Cite or Quot e 10

- 21 June 2012 on tropospheric O 3 ; and (4) radiative forcing and climate response to 21 st century trends 

### 1
in troposp heric O 3
. 

### 2

10.3.6.1 Emissions of Anthropogenic Ozone Precursors Across the
21 st Century

The IPCC SRES effort devised scenarios for short

- lived O 3 precursors as well as the 

### 3

well

- mixed greenhouse gases including NO X , CO, and VOCs ( IPCC, 2000 ) . Using the 

### 4

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

### 5
VOCs and allocated the trends in emissions over 17 regions and 8 economic sectors for 

### 6

the 2000

- 2050 time peri od. The worst

- case IPCC scenario, A2, features continued 

### 7
dependence on fossil fuels, rapid population growth, and little sharing of technology 

### 8
between developed and developing nations. By 2100 in this scenario, global NO X , CO 

### 9

and CH 4 emissions increase by a factor of 3.5, 2.6, and 2.9, respectively, relative to 2000 

### 10

( IPCC, 2000 ) . Most of these increases in emissions occur over developing countries. For 

### 11
example over Asia, NO X emissions in the A2 scenario increase by more than a factor of 

### 12
four by 2100. The more moderate A 1B scenario has global NO X and CO emissions 

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

### 14
by 10%. In the B1 scenario, with its emphasis on clean and efficient technologies, global 

### 15

emissions of NO X , CO, and CH 4 all dec rease by 2100 relative to the present day (

- 40%, 

### 16

- 60%, and

- 30%, respectively). 

### 17

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



### 18
term (up to 2030). The Current Legislation (CLE) scenario provides trends consistent 

### 19

with existing air quality regulations; the Maximum Feasible Reduction (MFR) scenario 

### 20

seeks to reduce emissions of O 3 precursors to the maximum extent possible. Emission 

### 21
source changes relative to the present day for CLE, MFR, and A2 are given in Stevenson 

### 22

et al. (2006 )

. 

### 23

For the Fifth Assessment Report (IPCC AR5), a new set of climate futures has been 

### 24
developed: the Representative Concentration Pathways (RCPs) ( Moss et al., 2010 ) . The 

### 25
RCPs will explore for the first time approaches to climate change mitigation. The RC Ps 

### 26
are designed to achieve radiative forcing targets of 2.6, 4.5, 6.0 and 8.5 W/m 2 by 2100, 

### 27
and have been designated RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5, respectively (RCP 

### 28

2.6 is also known as RCP3

- PD.) The trends in O 3 precursors for the RCP scenarios were 

### 29
determined by climate policies implicit in each scenario and by plausible assumptions 

### 30
regarding future air quality regulations. These scenarios were chosen to map the wide 

### 31
range of climate outcomes presented in the literature and repres ent only four of many 

### 32
possible scenarios that would lead to the specific radiative forcing targets; a wide range 

### 33

### Draft

– Do Not Cite or Quot e 10

- 22 June 2012 of socioeconomic conditions could be consistent with each forcing pathway ( Moss et al., 

### 1

2010 ) . Therefore, they should not be interpreted as forecasts of future conditions, but 

### 2
rather as plausible climate and socio

- economic futures. 

### 3

Plots and comparisons of the RCP trends are available on the RCP website ( RCP, 2009 )

. 

### 4

In all RCPs, global anthropogenic NO X emissions decline 30

- 50% during the 21 st century, 

### 5
though RCP 8.5 shows a peak during the 2020s at a value ~15% greater than that of 

### 6
2000. Global anthropogenic VOC and CO emissions are relatively flat during the 2000

- 

### 7

2050 time range, and then decline by 30

- 50% by the end of the century. For CH 4 , global 

### 8
mean emission trends for the four RCP projections differ substantially across the 21 st 

### 9
century, with RCP 8.5 showing a tripling of emissions by 2100, and RCP 2.6 showing the 

### 10

emis sions cut by half in this time range. RCP 4.5 and 6.0 show a peak in CH 4 emissions 

### 11
in the middle of the century before dropping by the end of the century to just below 2000 

### 12
emission levels. All these global trends, however, contain some regional variation. For 

### 13
example, Asian emissions of both NO X and VOCs show large increases in the near term 

### 14

(2030s to 2050s). 

### 15

10.3.6.2 Impact of 21 st Century Trends in Emissions on Tropospheric Ozone

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

### 16
anthropogenic emissions of its precursors. As shown in Table 10

- 1 , a recent multi

- model 

### 17
study found increases in the tropospheric O 3 concen tration of 15% and 6% for the IPCC 

### 18
A2 and CLE scenarios respectively for the 2000

- 2030 time period, and a decrease for the 

### 19

MFR scenario of 5% ( Stevenson et al., 2006 ) . These results indicate that the growth in 

### 20
tropospheric O 3 between 2000 and 2030 could be reduced or even reversed, depending on 

### 21
emission controls. For the relatively moderate A1B emissions scenario over the 2000

- 

### 22
2050 time period, Wu et al. (2008a ) calculated a change in O 3 concentration of about 

### 23

20%. 

### 24

As noted abo ve, the RCP scenarios show large variations in their future projections of 

### 25
global mean CH 4 emissions, but mainly declines in the emissions of the other O 3 

### 26
precursors across the 21st century. In one of the first efforts to assess the effect of these 

### 27

emissio n trends on global O 3 abundances, Lamarque et al. (2011 ) found that the large 

### 28

CH 4 increase in the RCP 8.5 scenario would drive a 15% enhancement of the 

### 29
tropospheric O 3 burden by 2100, relative to the present

- day, leading to a global mean 

### 30
radiative forcing of +0.2 W/m 2 . By contrast, the global O 3 burden would decrease in the 

### 31
other three RCPs, with declines in forcing ranging from

- 0.07 to

- 0.2 W/m 2

. 

### 32

### Draft

– Do Not Cite or Quot e 10

- 23 June 2012 Table 10

- 1 2000

- 2030 change s in anthropogenic emissions, and CH 4 

### and tropospheric ozone concentrations, and the associated tropospheric ozone forcing for three scenarios.

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

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

- 53%

Percent change in CO emissions +62%

- 16%

- 53%

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

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

- 5%

Radiative forcing due to O 3 change b (W/m 2 ) 0.3 0.18

- 0.05

a Values are ensemble means.

b Includes radiative forcing due to corresponding CH 4 change.

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

.



### 10.3.6.3 Impact of 21 st Century Climate on Tropospheric Ozone

For the time period from the 1800s to the present

- day, most of the increase in the 

### 1
concentration of tropospheric O 3 can be traced to changing emissions. Model studies 

### 2
show that climate change so far has likely had little effect on the tropospheric O 3 ( e.g., 

### 3

Grenfell et al., 2001 ) . In the future, however, climate change is expected to bring large 

### 4
changes in a suite of variables that could affect O 3 production, loss, and transport. For 

### 5
example, increased water vapor in a warming atmosphere is expected to enhance OH 

### 6

concentrations, which in remote, NO X - poor regions will accelerate O 3 loss rates ( Johnson 

### 7

et al., 1999 )

. 

### 8

In the 2050s A1B climate, Wu et al. (2008b ) calculat ed a 5 ppb decrease in surface O 3 

### 9
over oceans. A rise in temperatures will also likely promote emissions of isoprene, an 

### 10
important biogenic precursor of O 3 . Model studies have calculated 21 st

- century increases 

### 11
in isoprene emissions ranging from 25

- 50%, dep ending on climate scenario and time 

### 12

horizon ( Wu et al., 2008a and references therein ) . These studies however did not take 

### 13
into account the effects of changing climate and CO 2 concentrati on on vegetation extent, 

### 14
which could have large consequences for biogenic emissions ( Heald et al., 2008 ; 

### 15

Sanderson et al., 2003 ) . In any event, enhanced isoprene emissions will increase O 3 

### 16

concentrations in VOC

- limited regions, but decrease O 3 in NO X - limited regions ( Wu et 

### 17

al., 2008a ; Pyle et al., 2007 ; Sanderson et al., 2003 ) . Convection frequencies and 

### 18
lightning flash rates will also likely change in a changing climate, with consequences for 

### 19

l ightning NO X emissions and O 3 concentrations in the upper troposphere ( Sinha and 

### 20

Toumi, 1997 ; Price and Rind, 1994 ) . While Wu et al. (2008a ) calculated an increase in 

### 21
lightning NO X by 2050 due to enhanced deep convection, Jacobson and Streets (2009 ) 

### 22

### Draft

– Do Not Cite or Quot e 10

- 24 June 2012 projected a decrease in lightning NO X due to a declining cloud ice in their future 

### 1
atmosphere. Finally, changes in transport processes will almost certainly accompany 

### 2
global climate change. For the 2050 A1B climate, Wu et al. (2008b ) showed that 

### 3
flattening of the meridional temperature gradient in a warming world would lead to 

### 4
slower intercontinental transport of tropospheric O 3 . For the A2 climate in 2100, Zeng 

### 5
and Pyle (2003 ) projected an 80% increase in the flux of stratospheric O 3 into the 

### 6
troposphere, relative to the present

- day. 

### 7

Taken together, these climate

- driven processes could have appreciable effects on the 

### 8
concentration and distribution of tropospheric O 3 . As shown in Wu et al. (2008b ) , model 

### 9
projections of the change in O 3 conc entration due solely to future climate change range 

### 10

from

- 12% to +3%, depending on the model, scenario, and time horizon. 

### 11

10.3.6.4 Radiative Forcing and Climate Response from 21 st Century Trends in Tropospheric Ozone

In the near term (2000

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

### 12

for MFR, 0.18 W/m 2 for CLE, and +0.3 W/m 2 for the A2 scenario ( Table 10

- 1 ). Menon et 

### 13

al. (2008 ) , following the moderate A1B scenario, calculated a radiative forcing of 

### 14

0.12 W/m 2 from the 2000

- 2030 change in tropospheric O 3 , about the same as that derived 

### 15
by Stevenson et al. (2006 ) for the CLE scenario. Over the longer term (2000 to 2100) for 

### 16

the A1B scenario, Gauss et al. (2003 ) reported large positive radiative forcing (0.40 to 

### 17

0.78 W/m 2 ) due to the change in tropospheric O 3 , as shown in Figure 10

- 6 . Normalized 

### 18
radiative forcing for these model calculations fell within a relatively narrow range, 0.032 

### 19

to 0.040 W/m 2 DU, indicating that the lar gest uncertainty lies in the model

- calculated 

### 20
changes in O 3 concentration. Applying the A2 scenario, Chen et al. (2007b ) estimated a 

### 21
global mean radiative forcing of 0.65 W/m 2 from tropospheric O 3 by 2100, consistent 

### 22

with the Gauss et al. (2003 ) results. These studies took into account only the effect of 

### 23
changing emissions on tropospheric O 3 . In their calculations of the 20 00

- 2100 radiative 

### 24
forcing from O 3 in the A2 scenario, Liao et al. (2006 ) found that inclusion of climate 

### 25
effects on tropospheric O 3 reduced th eir radiative forcing estimate by 20%. 

### 26

### Draft

– Do Not Cite or Quot e 10

- 25 June 2012 Note: Shown are the components of radiative forcing in W/m2. SW = shortwave component; LW = longwave component; Net = total forcing; t = tropospheric ozone changes only; and t + s = both tropospheric and stratospher ic changes.
Source: Reprinted from Gauss et al. (2003 ) , American Geophysical Union.



### Figure 10

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

- 2100 change in tropospheric ozone

.

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

### 1
the future atmosphere to a suite of short

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

### 2

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

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

### 4
atmosphere, Chen et al. (2007b ) estimated a global mean surface temperature inc rease of 

### 5
+0.34oC by 2100 in response to the change in O 3 . The largest temperature increases in 

### 6
this study, as much as 5oC, occurred over the populous regions of Asia and the Middle 

### 7
East and downwind of biomass burning regions in South Africa and South Amer ica. 

### 8

### Draft

– Do Not Cite or Quot e 10

- 26 June 2012 

## 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, thereby 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 approximately 90% of total atmospheric O 3 is located 

### 5

there over mid

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

### 6
supplemental shielding of UV

- B radiation provided by tropospheric O 3 is necessary for 

### 7
quantifying UV

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

### 8
effects, and materials damage. The role of t ropospheric O 3 in shielding of UV

- B radiation 

### 9

is discussed in this section. 

### 10

10.4.2 Human Exposure and Susceptibility to Ultraviolet Radiation

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

### 11
Chapter 10 of the 2006 O 3 AQCD ( U.S. EPA, 2006b ) and are summarized here. These 

### 12

factors includ ed outdoor activity, occupation, age, gender, geography, and protective 

### 13
behavior. Outdoor activity and occupation both influenced the amount of time people 

### 14
spend outdoors during daylight hours, the predominant factor for exposure to solar UV 

### 15

radiation. Age and gender were found to be factors that influence human exposure to UV 

### 16
radiation, particularly by influencing other factors of exposure such as outdoor activity 

### 17
and risk behavior. Studies indicated that females generally spent less time outdoors and, 

### 18

con sequently, had lower UV radiation exposure on average compared to males. 

### 19
Geography influences the degree of solar UV flux to the surface, and hence exposure to 

### 20
UV radiation. Higher solar flux at lower latitudes increased the annual UV radiation dose 

### 21
for pe ople living in southern states relative to northern states. Altitude was also found to 

### 22
influence personal exposure to UV radiation. Protective behaviors such as using 

### 23
sunscreen, wearing protective clothing, and spending time in shaded areas were shown to 

### 24

r educe exposure to UV radiation. Given these and other factors that potentially influence 

### 25
UV radiation exposure, the 2006 O 3 AQCD ( U.S. EPA, 2006b ) listed the following 

### 26
subpopulations potentially at risk for higher exposures to UV radiation: 

### 27

 Individuals who engage in high

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

### 28

 Individuals who participate in outdoor sports and activities; 

### 29

### Draft

– Do Not Cite or Quot e 10

- 27 June 2012  Individuals who work outdoors with inadequate shade (e.g., farmers, 

### 1

construction workers, etc.); and 

### 2

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

### 3
latitudes (e.g., Honolulu, HI) and higher altitudes (e.g., Denver, CO). 

### 4

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

### 5
region ( Sliney and Wengraitis, 2006 )

. 

### 6

10.4.3 Human Health Effects due to UV

- B Radiation

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

### 7

effects associated with solar UV

- B radiation exposure. These effects include erythema, 

### 8

skin cancer, ocular damage, and immune system suppression. These adverse effects, 

### 9
along with protective effects of UV radiation through increased production of vitamin D 

### 10

are summarized in this section. For additional details, the reader is referred to Chapter 1 0 

### 11

of the 2006 O 3 AQCD ( U.S. EPA, 2006b ) and references therein

. 

### 12

The most conspicuous and well

- recognized acute response to UV radiation is erythema, 

### 13
or the reddening of the skin. Erythema is likely caused by direct damage to DNA by UV 

### 14
radiation. Many studies discussed in Chapter 10 of the 2006 O 3 AQCD ( U.S. EPA, 

### 15

2006b ) found skin type to be a significa nt risk factor for erythem a. Skin cancer is another 

### 16
prevalent health effect associated with UV radiation. Exposure to UV radiation is 

### 17
considered to be a major risk factor for all forms of skin cancer. Ocular damage from UV 

### 18
radiation exposure includes effects on the cornea, lens, ir is, and associated epithelial and 

### 19
conjunctival tissues. The region of the eye affected by exposure to UV radiation depends 

### 20
on the wavelength of the incident UV radiation. Depending on wavelength, common 

### 21
health effects associated with UV radiation include p hotokeratitis (snow blindness; short 

### 22
wavelengths) and cataracts (opacity of the lens; long wavelengths). 

### 23

Experimental studies reviewed in Chapter 10 of the 2006 O 3 AQCD ( U.S. EPA, 2006b ) 

### 24
have shown that exposure to UV radiation may suppress local and systemic immune 

### 25
responses to a variety of antigens. Results from hu man clinical studies suggest that 

### 26
immune suppression induced by UV radiation may be a risk factor contributing to skin 

### 27

cancer induction. There is also evidence that UV radiation has indirect involvement in 

### 28
viral oncogenesis through the human papillomavirus , dermatomyositis, human 

### 29
immunodeficiency virus and other forms of immunosuppression. 

### 30

A potential health benefit of increased UV

- B exposure relates to the production of 

### 31
vitamin D in humans. Most humans depend on sun exposure to satisfy their requirements 

### 32

f or vitamin D. Vitamin D deficiency can cause metabolic bone disease among children 

### 33

### Draft

– Do Not Cite or Quot e 10

- 28 June 2012 and adults, and also may increase the risk of many common chronic diseases, including 

### 1
type I diabetes mellitus and rheumatoid arthritis. Substantial in vitro and toxicologic al 

### 2
evidence also support a role for vitamin D activity against the incidence or progression of 

### 3

various forms of cancer. In some studies, UV

- B related production of vitamin D had 

### 4
potential beneficial immunomodulatory effects on multiple sclerosis, insulin

- d ependent 

### 5

diabetes mellitus, and rheumatoid arthritis. More details on UV

- B protective studies are 

### 6
provided in Chapter 10 of the 2006 O 3 AQCD ( U.S. EPA, 2006b )

. 

### 7

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

### 8

commission on Non

- ionizing Radiation Protection (ICNIRP) agreed that so me low

- level 

### 9
exposure to UV radiation has health benefits ( ICNIRP, 2004 ) . However, the adverse 

### 10
health effects of higher UV exposures necessitated the development of exposure limits 

### 11
for UV radiation. The ICNIRP recognized the challenge in establishing exposure limits 

### 12

that would achieve a realistic balance betwee n beneficial and adverse health effects. As 

### 13
concluded by ICNIRP (2004 ) , “[t]he present understanding of injury mechanisms and 

### 14
long

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

### 15

research.” 

### 16

10.4.4 Ecosystem and Materials Damage Effects Due to UV

- B Radiation

A 2009 progress re port on the environmental effects of O 3 depletion from the UNEP, 

### 17

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

### 18
materials damage effects from UV

- B radiation. An in

- depth assessment of the global 

### 19
ecosystem and materials damage effects from UV

- B radiation per se is out of the scope of 

### 20
this assessment. However, a brief summary of some mid

- lati tude effects is provided in 

### 21
this section to provide context for UV

- B related issues pertaining to tropospheric O 3 . The 

### 22
reader is referred to the UNEP report ( UNEP, 2009 ) and references therein for further 

### 23

details. All of these UV

- B related ecosystem and materials effects can also be influenced 

### 24
by climate change through temperature and other meteorological alterations, m aking 

### 25
quantifiable predictions of UV

- B effects difficult. 

### 26

Terrestrial ecosystem effects f rom increased UV

- B radiation include reduced plant 

### 27
productivity and plant cover, changes in biodiversity, susceptibility to infection, and 

### 28
increases in natural UV prot ective responses. In general, however, these effects are small 

### 29

for moderate UV

- B increases at mid

- latitudes. A field study on wheat in southern Chile 

### 30
found no substantial changes in crop yield with moderate increases in UV

- B radiation 

### 31

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

### 32

Finland found no measurable effects in photosynthetic function with increases in UV

- B 

### 33

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

### 34

### Draft

– Do Not Cite or Quot e 10

- 29 June 2012 have also been linked to increases in UV

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

### 1

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

### 2
changes in UV

- B radiation ( Favory et al., 2009 ; Jenkins, 2009 ; Brown and J enkins, 2008 ; 

### 3

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

### 4

al., 2009 ; Clarke and Robinson, 2008 ; Semerdjieva et al., 2003 ; Phoenix et al., 2000 )

. 

### 5

Aquatic ecosystem effects from increa sed UV

- B radiation include sensitivity in 

### 6
growth, immune response, and behavioral patterns of aquatic organisms. One study 

### 7
looking at coccolithophores, an abundant phytoplankton group, found a 25% reduction in 

### 8
cellular growth with UV

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

### 9

UV

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

### 10
behavior in certain species of fish ( Markkula et al., 2009 ; Holtby and Bothwell, 2008 ; 

### 11

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

- B radiation 

### 12
have also been observed for amphibians, sea urchins, mollusks, corals, and zooplankton 

### 13

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

### 14
Marquis et al., 2008 ; Marquis and Miaud, 2008 ; Oromi et al., 2008 ) . Increases in the flux 

### 15

of UV

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

### 16

mer cury, particularly in clear oligotrophic lakes with low levels of dissolved organic 

### 17
carbon to stop the penetration of UV

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

### 18

alt er the mobility of trace metals including the potential for increased mercury 

### 19
volatilization and transport within and among ecosystems. 

### 20

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

### 21

increased UV

- B radiation. A study on hig h latitude wetlands found UV

- induced increases 

### 22

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

### 23
terrestrial ecosystems found evidence for UV

- induced release of CO 2 through 

### 24
photodegradation of above

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

### 25

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

### 26
effects on carbon cycling and CO 2 uptake in the oceans ( Brewer and Peltzer, 2009 ; 

### 27

Meador et al., 2009 ; Fritz et al., 2008 ; Zepp et al., 2008 ; Hader et al., 2007 ) as well as 

### 28
release of dissolved organic matter from sediment and algae ( Mayer et al., 2009 ; 

### 29
Riggsbee et al., 2008 ) . Additional studies showing effects on these and additional 

### 30
biogeochemical cycles including the water cycle and halocarbon cycle can be found in 

### 31
the UNEP report ( UNEP, 2009 ) and references therein. 

### 32

Materials damage from increased UV

- B radiation include UV

- induced 

### 33
photodegradation of wood ( Kataoka et al., 2007 ) and plastics ( Pickett et al., 2008 ) . These 

### 34
studies and others summarizing photo

- resistant coatings and materials designed to reduce 

### 35
photodegradation of materials are summarized in the UNEP report ( UNEP, 2009 ) and 

### 36

references therein. 

### 37

### Draft

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- 30 June 2012 10.4.5 UV

- B Related Effects Associated with Changes in Tropospheric Ozone Concentrations

There are multiple complexities in attempting to quantify the relationship between 

### 1
changes in tropospheric O 3 concentrations and UV

- B exposure. The 2006 O 3 AQCD 

### 2

( U.S. EPA, 2006b ) described a handful of studies addressing this relationship, but none 

### 3
reported quantifiable effects of tropospheric O 3 concentration fluctuations on UV

- B 

### 4
exposure at the surface. Further quantifying the relationship between UV

- B exposure and 

### 5
health or welfare effects is complicated by the uncertainties involved in t he selection of 

### 6
an action spectrum and appropriate characterization of dose (e.g., peak or cumulative 

### 7
levels of exposure, timing of exposures, etc.) The lack of published studies that critically 

### 8
examined these issues together

-- that is the incremental healt h or welfare effects 

### 9
attributable specifically to UV

- B changes resulting from changes in tropospheric O 3 

### 10

concentrations

-- lead to the prior conclusion that the effect of changes in surface

- level O 3 

### 11

concentrations on UV

- induced health outcomes could not be c ritically assessed within 

### 12
reasonable uncertainty ( U.S. EPA, 2006b )

. 1 

### 13

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

### 14
response to changes in tropospheric O 3 under different control scenarios projected out to 

### 15
2020. This study focused on southeastern U.S. and accounted for spatial and temporal 

### 16
variation in tropospheric O 3 reductions, an important cons ideration since most controls 

### 17
are focused on reducing O 3 in populated urban areas. The contrasting control strategies 

### 18
considered in this study included a historical scenario designed to meet an 84 ppb 8

- h 

### 19
daily max standard and a reduced scenario designed to bring areas predicted to exceed a 

### 20
similarly designed 70 ppb standard into attainment. A biologically effective irradiance 

### 21
was estimated by multiplying the modeled UV irradiance by a sensitivity function (action 

### 22
spectrum) for the induction of nonmelanoma skin cancer in mice corrected for human 

### 23
skin transmission, then integrating over UV wavelengths. The average relative change in 

### 24

skin cancer

- weighted surface UV radiation between the two scenarios was 0.11 ± 0.03% 

### 25
over June, July and August. Weighting by p opulation, this estimate increased to 

### 26

0.19 ± 0.06%. Madronich et al. (2011 ) report that their estimated UV radiation increment 

### 27
is an order of magnitude less than that reported in an earlier study by Lutter and Wolz 

### 28

(1997 ) with the main reason for the discrepancy coming from the unrealistic uniform 

### 29

10 ppb reduction in O 3 assumed in the former study. Madronich et al. (2011 ) did not 

### 30
attempt to link their predicted increase in UV radiation to a predicted increase in skin 

### 31
cancer incidence, however, due to several remaining and substantial uncertainties. 

### 32

1 The reader is referred to the U .S. EPA 20 03 Final Response to Court Remand ( U.S. EPA, 2003 ) for detailed discussions of the data and scientific issues associa ted with the determination of public health benefits resulting from the attenuation of UV

- B by surface

- level O 3

.

### Draft

– Do Not Cite or Quot e 10

- 31 June 2012 Quantitatively estimating human health and welfare effects directly attributed t o changes 

### 1

in UV

- B penetration resulting from changes in ground

- level O 3 concentrations will 

### 2
require both (a) a solid understanding of the multiple factors that define the extent of 

### 3
exposure to UV

- B, and (b) well

- defined and quantifiable links between UV

- B exposure 

### 4
and human disease and welfare effects. Detailed information does not exist regarding the 

### 5
relevant type (e.g., peak or cumulative) and time period (e.g., developmental, lifetime, or 

### 6
current) of exposure, wavelength dependency of biological response s, and 

### 7

inter

- individual variability in UV resistance. 

### 8

Although the UV

- B related health effects attributed to marginal reductions in 

### 9
tropospheric or ground

- level O 3 have not been directly assessed to date, they would be 

### 10
expected to be small based on current information indicating a negligibly small effect of 

### 11
potential future changes in tropospheric O 3 concentrations on ground

- level UV

- B 

### 12
radiation. In conclusion, the effect of changes in surface

- level O 3 concentrations on 

### 13

UV

- induced health and welfare outcome s cannot yet be critically assessed within 

### 14
reasonable uncertainty. 

### 15



## 10.5 Summary and Causal Determinations

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 . While 

### 16

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

### 17
decades, many developing countries have experienced large increases in precursor 

### 18
emissions and these trends are expected to continue, at least in the near term. Projections 

### 19

of r adiative forcing due to changing O 3 over the 21st century show wide variation, due in 

### 20
large part to the uncertainty of future emissions of source gases. In the near

- term (2000

- 

### 21
2030), projections of O 3 radiative forcing range from near zero to +0.3 W/m 2 , de pending 

### 22

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

### 23
concentrations could therefore provide an important means to slow climate change in 

### 24
addition to the added benefit of improving surface air quality. 

### 25

It is clea r that increases in tropospheric O 3 lead to warming. However the precursors of 

### 26

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

### 27
reduction strategies. A decrease in CO or VOC emissions would enhance OH 

### 28

concentrations, shorten ing the lifetime of CH 4 , while a decrease in NO X emissions could 

### 29
depress OH concentrations in certain regions and lengthen the CH 4 lifetime. 

### 30

### Draft

– Do Not Cite or Quot e 10

- 32 June 2012 Abatement of CH 4 emissions would likely provide the most straightforward means to 

### 1
address climate change since CH 4 is itself an important precursor of background O 3 

### 2

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

### 3

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

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

### 5
savings, decrease background O 3 by about 1 ppb in the Northern Hemisphere summer, 

### 6
and lead to a global net cool ing of 0.12 W/m 2

. West et al. (20 07 ) explored further the 

### 7

benefits of CH 4 abatement, finding that a 20% reduction in global CH 4 emissions would 

### 8
lead to greater cooling per unit reduction in surface O 3 , compared to 20% reductions in 

### 9

VOCs or CO. 

### 10

Important uncertainties remain regarding t he effect of tropospheric O 3 on future climate 

### 11
change. To address these uncertainties, further research is needed to: (1) improve 

### 12
knowledge of the natural atmosphere; (2) interpret observed trends of O 3 in the free 

### 13
troposphere and remote regions; (3) impro ve understanding of the CH 4 budget, especially 

### 14
emissions from wetlands and agricultural sources, (4) understand the relationship 

### 15
between regional O 3 radiative forcing and regional climate change; and (5) determine the 

### 16
optimal mix of emissions reductions th at would act to limit future climate change. 

### 17

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

### 18

estimated to be about 25

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

### 19
anthropogenic CH 4 effect according to the IPCC. There are large uncertainties in the 

### 20
radiative forcing estimate attributed to tropospheric O 3 , making the effect of tropospheric 

### 21

O 3 on climate more uncertain than the effect of the long

- lived greenhouse gases. Overall, 

### 22
the evidence support s a causal relationship between changes in tropospheric O 3 

### 23
concentrations and radiative forcing. 

### 24

Radiative forcing does not take into account the climate feedbacks that could amplify or 

### 25
dampen the actual surface temperature response. Quantifying the change in surfac e 

### 26
temperature requires a complex climate simulation in which all important feedbacks and 

### 27
interactions are accounted for. As these processes are not well understood or easily 

### 28
modeled, the surface temperature response to a given radiative forcing is highly u ncertain 

### 29
and can vary greatly among models and from region to region within the same model. In 

### 30
light of these uncertainties, the evidence indicates that there is likely to be a causal 

### 31
relationship between changes in tropospheric O 3 concentrations and effec ts on 

### 32

climate. 

### 33

### Draft

– Do Not Cite or Quot e 10

- 33 June 2012 10.5.2 Summary of UV

- B Related Effects on Human Health, Ecosystems, and Materials Relating to Changes in Tropospheric Ozone Concentrations

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

### 1
break (photolyze) c hemical bonds in molecules, thereby 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. Ozone in the stratosphere is responsible for 

### 4
the major ity 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
associated with materials damage. 

### 9

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

### 10
welfare effects (adverse or beneficial) attributable specifica lly to changes in UV

- B 

### 11
exposure resulting from perturbations in tropospheric O 3 concentrations. While the 

### 12
effects are expected to be small, they cannot yet be critically assessed within reasonable 

### 13
uncertainty. Overall, the evidence is inadequate to determi ne if a causal relationship 

### 14
exists between changes in tropospheric O 3 concentrations and effects on health 

### 15

and welfare related to UV

- B shielding. 

### 16

10.5.3 Summary of Ozone Causal Determinations

The evidence reviewed in this chapter describes the recent findings reg arding the climate 

### 17

and UV

- B related effects of changes in tropospheric O 3 concentrations. Table 10

- 2 

### 18
provides an overview of the causal determinations for each of the categories evaluated. 

### 19

Table 10

- 2 Summary of ozone causal determinations for climate and UV

- B effects.

Effects Causal Determination

Radiative Forcing Causal relationship

Climate Change Likely to be a causal relationship

Health and Welfare Effects Related to UV

- B Shielding Inadequate to determine if a causal relationship exists

### Draft

– Do Not Cite or Quot e 10

- 34 June 2012 

### References

Aphalo, PJ; Vapaavuori, EM; de la Rosa, TM; 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 Arrhenius, S. (1896). On the influence of carbonic acid in the air upon the temperature of the ground. Philos Mag 41: 237

- 276. Balis, DS; Zerefos, CS; Kourtidis, K; Bais, AF; Hofzumahaus, A; Kraus, A; Schmitt, R; Blumthaler, M; Gobbi, GP. (2002). Measurements and modeling of photolysis rates during the photochemical activity and ultraviolet radiation (PAUR) II campaign. J Geophys Res 107: 8138. http://dx.doi.org/10.1029/2000JD000136 Berntsen, TK; Myhre, G; Stordal, F; Isaksen, ISA. (2000). Time evolution of tropospheric ozone and its
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