Supplementary Material for Chapter 33

Don’t Blame the Trees: Using Data to Examine how Trees Contribute to Air Pollution

This chapter is published as:

Crumrine PW. 2016. Don’t Blame the Trees: Using Data to Examine how Trees Contribute to Air Pollution. In: Byrne L (ed) Learner-Centered Teaching Activities for Environmental and Sustainability Studies. Springer, New York. DOI 10.1007/978-3-319-28543-6_33

Patrick W. CrumrineDepartments of Geography & Environment and Biological Sciences, Rowan University, Glassboro, NJ USACrumrine@rowan.edu

This file contains the following supplementary material: A: Key terms and concepts … beginning on p. 1 B: Figures and tables to distribute to students … beginning on p. 5 C: Annotated instructors guide for questions … beginning on p. 13 D: Sample quiz …beginning on p. 16 F: Possible responses to follow-up questions … beginning on p. 18

This chapter also has the following supplementary material, available on the chapter’s website:

E: Student worksheet

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Supplementary Material A: Key terms and concepts

This activity focuses on the contribution of plant emissions to the production of ozone in the troposphere (ground-level), a secondary air pollutant that can harm human health, reduces plant productivity, and contributes to global warming. Students should be familiar with the “Air pollution” and “Ozone” points before starting the activity. These points are all contained within the reading assigned to students prior to the activity. The points outlined under, “Plant emissions and ozone” and "Future considerations" will be useful for the instructor to review before the activity. A list of references is provided at the end which instructors may also find useful.

Air pollution

Criteria air pollutants are commonly found air pollutants listed under the Clean Air Act (CAA) The CAA was passed by the U.S. Congress in 1970 and updated in 1977 and 1990. The criteria air

pollutants include: ozone, particulate matter, carbon monoxide, nitrogen oxides, sulfur dioxide, and lead.

Primary air pollutants are chemicals emitted into the air directly from a source that cause harm to humans and/or the environment

Secondary air pollutants are chemicals formed through reactions between primary air pollutants and components of the atmosphere

Ozone

Ozone is a pale blue gas with the chemical formula, O3. Ozone in the stratosphere is often termed "good" ozone and screens harmful UV radiation. Ozone in the troposphere (ground-level) is termed "bad" ozone. It poses human health risks, damages crops and vegetation, and contributes to global climate change. Ground-level ozone is a secondary air pollutant formed as a result of chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight and is a key component of smog.

NOx includes NO and NO2 and are produced from the reaction between nitrogen and oxygen gasses during fuel combustion. Some natural sources exist, but the majority of NOx are anthropogenic. In urban areas with significant vehicular use, NOx are particularly abundant.

VOCs are compounds with relatively low boiling points (i.e. they change from the liquid to gas phase at temperatures commonly found in indoor and outdoor environments) that are emitted from organic solids or liquids. VOCs are emitted from solvents, paints, petrochemicals, transportation and natural sources such as plants. VOC's are sometimes differentiated from non-methane volatile organic compounds (NMVOCs) which are similar to VOCs but do not include methane.

The reactions that form ozone at the ground-level are dependent on sunlight and high temperatures. Ozone pollution is particularly problematic in urban areas with ample NOx and VOCs during the summer, but can also occur in rural regions downwind from urban centers.

Ozone levels have increased from historical background concentrations of ~10 ppb to 30-40 ppb in parts of the northern hemisphere. Levels above 75 ppb, the current U.S. EPA standard, are not uncommon in many regions during the summer and extreme ozone pollution events with levels exceeding 200 ppb have also been reported (Hartmann et al. 2013).

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Plant emissions and ozone

Some plants produce VOCs in the form of isoprene and monoterpenes. Isoprene is a particularly reactive VOC and plant emissions contribute approximately 75 – 80% of the VOCs emitted in the U.S. annually (Lerdau and Slobodkin 2002, EPA 2013).

o Isoprene emission by plants increases at high temperature and isoprene may provide protection against heat stress.

In 1980, U.S. President Ronald Reagan stated that “approximately 80% of our air pollution stems from hydrocarbons released by vegetation, so let’s not go overboard in setting and enforcing tough emission standards from man-made sources” (Pope 1980). This statement was made, most likely, in reference to the large quantities of isoprene emitted by trees in the U.S. This statement is an oversimplification and exaggeration because isoprene does not directly cause air pollution. Ozone is a secondary air pollutant and without vast quantities of NOx from fossil fuel combustion, these biogenic VOCs would not lead to significantly elevated ground-level ozone (Purves et al. 2004).

o In fact, at low levels of NOx VOCs actually reduce the amount of ozone. Anthropogenic emissions of ozone precursors (NOx and VOCs) have actually decreased in the last 20-30

years due to pollution control legislation in the developed world but ozone pollution continues to be a serious problem (Hartmann et al. 2013).

The contribution of isoprene to the production of ground-level ozone is highly significant in some regions. In the eastern U.S., modeling studies predict significant increases in ozone levels when plant VOCs are taken into account along with anthropogenic VOCs (Purves et al. 2004).

Atlanta, Georgia provides a particularly relevant case-study for this phenomenon. Here, significant vehicular usage contributes to high NOx. Atlanta lies in a region with relatively high forest coverage that includes many isoprene emitting trees (Lerdau et al. 1997). To minimize ground-level ozone pollution in this region, strategies must consider emissions from both anthropogenic and plant sources. Some have suggested that since the emissions of biogenic VOCs are so high, control strategies should focus on reducing NOx .

It is particularly important to focus on plant VOCs in the production of ground-level ozone because studies indicate that the rate of plant VOC emission is increasing in some regions, particularly the eastern U.S. This region has ample NOx which could result in high ground-level ozone concentrations despite reductions in anthropogenic VOCs (Purves et al. 2004).

It is also important to consider the contribution of plant VOCs to ground level ozone production because several of the species being considered for large-scale biomass production in Europe and other parts of the world (poplar, willow, eucalyptus) are high isoprene emitting species. Using conservative estimates, studies suggest that large scale planting of these species could lead to noticeable increases in ground-level ozone production in Europe which would result in further reductions in crop production and greater risks to human health (Ashworth et al. 2013).

Isoprene emission is species-specific and some species emit significantly more isoprene than others. It is particularly important that isoprene emission of trees is considered for large scale tree planting programs in cities and urban regions. These regions often have high concentrations of NOx such that adding VOCs from trees can lead to additional ground-level ozone pollution. Urban regions also suffer from the heat island effect and isoprene emission increases with temperature adding to the importance of considering isoprene emission potential of trees in urban regions (Calfapietra et al. 2013).

Although anthropogenic emissions of ozone precursors are abundant in urban centers, ozone and its precursors can drift downwind towards suburban and rural areas. In addition, NOx from urban areas can be transported to more rural areas with VOCs from plants to produce ground-level ozone.

Future considerationsPage 2

The basic chemistry involved in the production of ground-level ozone is well-established and the contribution of plant VOCs to the production of ground-level ozone has been recognized for many years. We have become increasingly aware that isoprene emission from plants plays a significant role in the chemistry of our atmosphere.

Initial research in this area illuminated the adaptive value of isoprene, factors that influence emission rates, species-specific emission potential, contribution of plant VOCs relative to anthropogenic VOCs, and the importance of considering isoprene emission potential when selecting trees for large scale planting.

Current research is further illuminating how land-use change, changes in species composition, and climate change may influence isoprene emission and ground-level ozone production.

In the future, this activity can be updated by consulting the latest reports from the IPCC on the concentration of ground-level ozone and its precursors. Similarly, data on anthropogenic emissions of NOx and VOCs in the U.S. can be obtained from updated reports from the U.S. EPA's National Emissions Inventory.

Useful References

Ainsworth EA, Yendrek CR, Sitch S, Collins WJ, Emberson LD (2012) The effects of tropospheric ozone on net primary productivity and implications for climate change. Annu Rev Plant Biol 63:637-661

Although the main focus of this review paper is on the effects of ozone on plant productivity, the introduction clearly describes the effects of ground-level ozone, the basic chemistry of ozone formation, and the importance of NOx concentration relative to VOC concentration for ozone production.

Lerdau M, Slobodkin L (2002) Trace gas emissions and species-dependent ecosystem services. Trends Ecol Evol 17:309-312

This opinion piece provides a concise and straightforward description of the basic biology of VOCs and conservation implications of VOC emission by plants.

Purves DW, Caspersen JP, Moorcroft PR, Hurtt GC, Pacala SW (2004) Human-induced changes in US biogenic volatile organic compound emissions: evidence from long-term forest inventory data. Global Change Biol 10:1737–1755

This paper describes how changes in land-use and species composition have resulted in increased VOC emissions in the eastern U.S. in recent years. The introduction of the paper clearly outlines many of the basic concepts encompassed in this activity.

Calfapietra C, Fares S, Manes F, Morani A, Sgrigna G, Loreto F (2013) Role of biogenic volatile organic compounds (BVOC) emitted by urban trees on ozone concentration in cities: A review. Environ Pollut 183:71-80

This review paper considers the role of trees in the production of ground-level ozone in urban environments. It also describes many of the factors that influence the production of isoprene such as temperature. Finally, the paper touches on the importance of considering VOC emissions by plants in urban environments because of the abundance of NOx.

Churkina G, Grote R, Butler TM, Lawrence M (2015) Natural selection? Picking the right trees for urban greening. Environ Sci Policy 47:12-17

Karlik JF, Pittenger DR (2012) Urban trees and ozone formation: A consideration for large-scale plantings. University of California Agriculture and Natural Resources 9, http://anrcatalog.ucdavis.edu/pdf/8484.pdf

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Both of these papers touch on some of the basic concepts associated with ground-level ozone pollution but focus more on the differences in isoprene emission between species. This information is then discussed in the context of tree planting programs.

Additional references cited in this supplementary file:

Ashworth K, Wild O, Hewitt CN (2013) Impacts of biofuel cultivation on mortality and crop yields. Nature Clim Change 3:492-496

Hartmann DL, Klein AMG, Tank M et al (2013) Observations: atmosphere and surface. In: Stocker TF, Qin D, Plattner GK et al (eds) Climate change 2013: the physical science basis. Contribution of working group I to the fifth assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, p 159–254

Lerdau M, Guenther A, Monson R (1997) Plant production and emission of volatile organic compounds. Bioscience: 373-383

US EPA (2013) Overview of the air pollutant emissions in the 2011 national emissions inventory version 1.01. http://www.epa.gov/ttn/chief/net/lite_finalversion_ver10.pdf. Accessed 12 Jan 2015

Pope C (1980) The candidates and the issues. Sierra 65:15-17

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Supplementary Material B: Figures and tables to distribute to students

Figure 1. Annual average surface ozone concentrations from regionally representative ozone monitoring sites around the world. (a) Europe. (b) Asia and North America. (c) Remote sites in the Northern and Southern Hemispheres. The station name in the legend is followed by its latitude and elevation. Time series include data from all times of day and trend lines are linear regressions following the method of Parrish et al. (2012). Trend lines are fit through the full time series at each location, except for Jungfraujoch, Zugspitze, Arosa and Hohenpeissenberg where the linear trends end in 2000 (indicated by the dashed vertical line in (a)). Twelve of these 19 sites have significant positive ozone trends (i.e., a trend of zero lies outside the 95% confidence interval); the seven sites with non-significant trends are: Japanese MBL (marine boundary layer), Summit (Greenland), Barrow (Alaska), Storhofdi (Iceland), Samoa (tropical South Pacific Ocean), Cape Point (South Africa) and South Pole (Antarctica).

Figure 2.7 in: Hartmann DL, Klein AMG, Tank M et al (2013) Observations: atmosphere and surface. In: Stocker TF, Qin D, Plattner GK et al (eds) Climate change 2013: the physical science basis. Contribution of working group I to the fifth assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, p 159–254. Reprinted with permission from the Intergovernmental Panel on Climate Change.

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Figure 2. Historical trends in human-driven emissions of O3 precursors from Lamarque et al. (2010), in black; RETRO (Schultz et al., 2008; in red) and EDGAR-HYDE (Olivier and Berdowski, 2001; van Aardenne et al., 2001) in green. Open biomass burning is not included.

Figure 2.6 in: Integrated Assessment of Black Carbon and Tropospheric Ozone. United Nations Environment Programme, 2011. Retrieved May 1, 2015 from http://www.unep.org/dewa/Portals/67/pdf/BlackCarbon_report.pdf.Reprinted with permission from the United Nations Environment Programme.

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Table 1. Anthropogenic and natural emissions for the year 2005 (Mt/yr).NOx

a NMVOCAnthropogenic

Large-scale combustion 34.1 1.2Industrial processes 2.4 8.0Residential-commercial combustion 5.0 37.9

Transport 71.5 38.5Fossil-fuel extraction and distribution 1.4 36.4

Solvents N/A 23.4Waste/landfill 0.12 1.1Agriculture b 0.26 4.0Total Anthropogenic 115 150Natural c 54-60 470-549 d

Global Total 169-175 620-699a Reported as NO2.b Includes the burning of agricultural residues.c Includes the open burning of all biomass other than agricultural residues.d Isoprene from trees.

Adapted from Table 2.1 in: Integrated Assessment of Black Carbon and Tropospheric Ozone. United Nations Environment Programme, 2011. Retrieved May 1, 2015 from http://www.unep.org/dewa/Portals/67/pdf/BlackCarbon_report.pdf.

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Table 2. Isoprene emission rates of U.S. trees. N is the number of emission rate measurements and N sp is the number of species represented by these measurements. Area is the percent of the total 3.8 x 106 km2 of U.S. woodlands where the genus is a dominant component of the landscape. Emission rates (ug C g -1 h-1) are representative of a leaf temperature of 30oC and a leaf-level PAR flux of 1000 umol m-2 s-`1.Family Genus Example N Nsp Area IsopreneAceraceae Acer Maple 89 2 25% <0.1Betulaceae Betula Birch 1 1 22% <0.1Fagaceae Fagus Beech 2 1 16% <0.1Hamamelidaceae Liquidambar Sweetgum 110 1 10% 70Pinaceae Picea Spruce 27 2 4% 14Picea Pinus Pine 359 10 62% <0.1Platanaceae Plaranus Sycamore 112 2 1% 35Salicaceae Populus Poplar 38 3 3% 70Fagaceae Quercus Oak 220 17 64% 70Pinaceae Tsuga Hemlock 1 1 3% <0.1

Adapted from Table 1 in: Guenther A, Zimmerman P, Wildermuth M (1994) Natural volatile organic compound emission rate estimates for U.S. woodland landscapes. Atmos Environ 28:1197–1210.

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Figure 3. Landscape average isoprene emission rate factors (mg C m-2 h-1 at 30°C and an above canopyPAR flux of 1000 umol m-2 s-1) for U.S. woodlands.

Adapted from Figure 1 in: Guenther A, Zimmerman P, Wildermuth M (1994) Natural volatile organic compound emission rate estimates for U.S. woodland landscapes. Atmos Environ 28:1197–1210. Reprinted with permission from Elsevier.

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Figure 4. Estimated decadal change in heatwave emission rate mid-1980s to mid-1990s (mg m -2 h-1, per decade) for isoprene and monoterpenes, compared with decadal change in anthropogenic volatile organic compounds (VOC) emissions. Change estimate given by model B2 (Methods) driven separately with mid-1980s and mid-1990s USDA Forest Service inventory data (FIA). Anthropogenic emissions taken from the EPA AIRS data. Note nonlinear scale. Insets give percentage changes (scale from -30% to +30% decadal change). Average change in emission rate over all grid cells is given in parentheses above each map.

Figure 3 in: Purves DW, Caspersen JP, Moorcroft PR, Hurtt GC, Pacala SW (2004) Human-induced changes in US biogenic volatile organic compound emissions: evidence from long-term forest inventory data. Global Change Biol 10:1737–1755. Reprinted with permission from John Wiley and Sons.

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Figure 5. 2011 NEI CAP Emissions Density Maps (Tons/Sq.Mi.), no prescribed burning or wildfires.

Figure 7 in: US EPA (2013) Overview of the air pollutant emissions in the 2011 national emissions inventory version 1.01. http://www.epa.gov/ttn/chief/net/lite_finalversion_ver10.pdf. Accessed 12 Jan 2015 Reprinted with permission from U.S. EPA.

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Figure 6. Seasonal mean ambient ozone concentration between 09:00 and 16:00 h local time over the continental U.S. from 1 July to 31 September, 2005.

Figure 1 in: Tong D, Mathur R, Schere K, Kang D, Yu S (2007) The use of air quality forecasts to assess impacts of air pollution on crops: methodology and case study. Atmos Environ 41:8772–8784. Reprinted with permission from Elsevier.

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Supplementary Material C: Annotated instructors guide for questions

Figure 1. Annual average surface ozone concentrations from regionally representative ozone monitoring sites around the world.

Describe the general trend for surface (ground-level) ozone levels at these monitoring stations.o These data were published in the 2013 IPCC report. Students should recognize that ozone

concentration in the troposphere (surface/ground-level) has increased at many reporting locations and is higher than historical background levels (~10 ppb). They should also read the caption and identify that this trend is significant in 12 of 19 locations.

What factors might contribute to these patterns and what are the environmental consequences?o The assigned reading should provide students with the knowledge to answer this question.

Students should know that ground-level ozone is a secondary air pollutant formed through chemical reactions between NOx gasses and VOCs. They should hypothesize that if ozone levels have increased, the emission of precursor chemicals must have also increased. Most students will attribute this pattern to the increase in anthropogenic emissions of NOx gasses and VOCs. The environmental consequences of elevated ground-level ozone include climate warming, decreased plant productivity, and respiratory distress.

Figure 2.7 from Hartmann et al., 2013

Figure 2. Historical trends in human-driven emissions of ozone precursors. Describe the general trend for anthropogenic NOx and NMVOC emissions.

o These data appeared in the 2011 United Nations Environment Programme publication, Integrated Assessment of Black Carbon and Tropospheric Ozone. Students should recognize that anthropogenic emission of ozone precursors, particularly NOx and non-methane VOCs (NMVOCs) increased markedly from 1860 through 1990 but have actually declined in more recent years.

What factors might contribute to these patterns?o The assigned reading should provide students with the knowledge to answer this question.

Students should know that NOx gasses are produced during fuel combustion and VOCs are emitted from solvents, vehicles, and industrial processes. These activities and the resulting emission of NOx gasses and VOCs have increased considerably during this time period. Students should also attribute the recent decrease in emissions to pollution control legislation in the developed world.

Figure 2.6 from Integrated Assessment of Black Carbon and Tropospheric Ozone

Table 1. Anthropogenic and natural emissions for the year 2005. Compare the total anthropogenic and natural emissions of NMVOC. Which is larger and what chemical

is primarily responsible for this?o Students should identify that the natural emissions of NMVOC (470-549Mt/yr) are several

times greater than the anthropogenic natural emissions (150 Mt/yr). They should also see the footnote that isoprene from trees is the chemical that represents the natural emissions.

Do you think that natural sources of NMVOCs play a major role in the formation of ground-level ozone, explain?

o At this point, students should begin to think about the relative contribution of natural and anthropogenic VOCs to the production of ground-level ozone. They should also recognize that it is not only VOCs that are required for the production of ground-level ozone but NOx gasses as well. Even if there are large emissions of VOCs from natural sources, NOx gasses must also be present to form ground-level ozone. This is an important relationship to highlight because it is the key point that is oversimplified in Regan’s statements. Based on their own experience

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and familiarity with ozone pollution in urban areas, students may also suggest that plants do not play a major role in the production of ground-level ozone. This is true in many urban areas of the world where there are significant anthropogenic sources of NOx and VOCs.

Table 2.1 from Integrated Assessment of Black Carbon and Tropospheric Ozone

Table 2. Isoprene and monoterpene emission rates of U.S. trees. What trees emit the most isoprene and do these species cover a considerable amount of area?

o This table is adapted from a paper synthesizing data from many studies on the emission of isoprene from trees in the U.S. Students should recognize that many common trees that cover a considerable amount of area do not emit much isoprene (e.g. maples) while others do (e.g. oaks). Some trees in this table are being grown for pulpwood and as an energy crop (e.g. poplars).

Table 1 from Guenther et al., 1994.

Figure 3. Landscape average isoprene and monoterpene emission for U.S. woodlands. Describe the spatial distribution of isoprene emissions in the U.S.

o Students should recognize that considerable isoprene emission occurs in the southeastern and southcentral U.S. and along the west coast of the U.S. Emission of isoprene by trees in the southeastern U.S. has had a major impact on ground-level ozone production in some major cities including Atlanta, Georgia. Here, motor vehicles and other combustion sources contribute to elevated NOx levels. Isoprene produced by trees then reacts with these NOx gasses to produce ground-level ozone.

Figure 1 from Guenther et al., 1994.

Figure 4. Change in VOC emission rates in the eastern U.S. from the mid 1980’s to the mid 1990’s. How has isoprene emission from trees changed during this time period relative to anthropogenic

sources of VOCs?o Students should recognize that anthropogenic VOC emissions have actually decreased in the

eastern U.S. over this time period. Students might also wonder why ground-level ozone continues to be a problem in this region. Part of the explanation is that isoprene emission by trees has actually increased.

What factors might contribute to the change in isoprene emissions during this time period? o Encourage students to consult table 2 when answering this question. Part of the answer to this

question lies in the change in forest area in this region and the change in species composition. There has been an increase in the area covered by trees that emit isoprene in this region. Increases in sweetgum have had a significant impact.

Figure 3 from Purves et al., 2004

Figure 5. Criteria Air Pollutant emissions in the U.S. Describe the spatial distribution of NOx and anthropogenic VOC emissions in the U.S.

o These data were produced by the U.S. EPA. Students should recognize that anthropogenic NOx and VOC emissions are quite high in the eastern half of the U.S., particularly in the highly-urban northeastern U.S.

Based on the spatial distribution of NOx, anthropogenic VOC, and isoprene emissions in the U.S., where might you predict ground-level ozone pollution to be a problem?

o Students should hypothesize that ozone pollution events will occur where both NOx and VOCs are emitted at high levels. One region where this is true is the northeastern U.S. Based on

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isoprene emission data from Figures 3 and 4 students may also suggest that the southeastern U.S. is a region with the potential for ground-level ozone pollution.

Figure 7 from the Overview of the 2011 National Air Emissions Inventory

Figure 6. Seasonal mean ambient ozone concentration between 09:00 and 16:00 h local time over the continental U.S. from 1 July to 31 September, 2005.

Describe the spatial distribution of ground-level ozone pollution in the U.S.o Students should identify that ground-level ozone pollution can occur in urban regions such as

the northeastern U.S. and in cities such as Atlanta, Georgia, but also in areas that are less urban such as the Rocky Mountains in Colorado.

Does this pattern match your prediction from the previous question, explain?o Students should be able to identify the more obvious regions where ground-level ozone

pollution is a problem such as the northeastern U.S. but may be surprised to learn that ground-level pollution is so extensive. This is partly due to the fact that ozone precursors can be transported some distance in the atmosphere.

Figure 1 from Tong et al., 2007

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Supplementary Material D: Sample quiz 1. List four of the six commonly found air pollutants listed as criteria air pollutants under the U.S. Clean

Air Act.

_______________________________________________

_______________________________________________

_______________________________________________

_______________________________________________

2. Ground-level ozone is formed by the reaction of what two types of chemicals in the presence of sunlight?

a. Water and carbon dioxideb. Water and methanec. Nitrogen oxides (NOx) and volatile organic compounds (VOCs)d. Nitrogen oxides (NOx) and watere. Hydrocarbons and volatile organic compounds (VOCs)

3. State 2 negative effects of ground-level ozone.

4. Briefly describe the difference between “good” ozone and “bad” ozone.

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5. More than half of anthropogenic nitrogen oxides (NOx) in the U.S. are emitted by:a. Natural gas combustionb. Industrial processesc. Wood combustiond. Solvent usee. Motor vehicles

6. Air pollution generally remains in close proximity to the emission source.a. Trueb. False

Briefly explain your answer to question #6.

7. Stratospheric ozone has been depleted by manmade chemicals called:a. Chlorofluorocarbons (CFCs) b. Volatile organic compounds (VOCs)c. Polychlorinated biphenyls (PCBs)d. Carbon monoxidee. Diesel fuel

8. Some toxic chemicals emitted into the air can travel thousands of miles away from the emission source and maintain their chemical structure and toxicity. This characteristic is termed:

a. Bioaccumulationb. Biomagnificationc. Contaminationd. Persistencee. Incineration

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Supplementary Material F: Possible responses to follow-up questions

Follow-up Engagement These prompts could be used for extended discussion or assessment questions. Students will benefit

from seeking additional references when answering these questions.o Summarize the role of anthropogenic and biogenic emissions in the formation of ground-level

ozone.o Ground-level ozone is produced through chemical reactions between NOx and VOCs in

the presence of sunlight. Ground-level ozone and its precursor chemicals have increased over time resulting in ground-level ozone pollution. VOCs have both natural and anthropogenic sources while the majority of NOx are from anthropogenic sources. In many urban regions there are sufficient NOx and VOCs from anthropogenic sources to produce ground-level ozone pollution. Natural sources of VOCs can further exacerbate the production of ground-level ozone when sufficient NOx are present. Although pollution control legislation has resulted in a reduction in anthropogenic VOC emissions, natural emissions have increased in some parts of the world such as the southeastern U.S. Ultimately, control of ground-level ozone pollution must take into account natural and anthropogenic sources of VOCs as well as NOx gasses

o In urban regions, ozone production tends to be limited by the availability of VOC’s, while in rural regions it is limited by the availability of NOx gasses. Why do you think this is the case?

o The concentration of NOx relative to the concentration of VOCs plays a critical role in the formation of ground-level ozone. When NOx levels are high relative to VOCs, as is the case in many urban regions, adding additional NOx does not result in more ground-level ozone. The availability of VOC’s has a greater influence on the production of ground-level ozone in this situation. In rural areas, NOx levels tend to be lower and the production of ground-level ozone is promoted by additional NOx.

o Suppose you were responsible for coordinating a large scale tree-planting program in an urban region such as New York City. What species would you select to plant and why?

o Planners take into account many factors when making these types of decisions including aesthetics and various horticultural considerations. Planners in urban regions where NOx are abundant and ground-level ozone pollution is a problem are increasingly being asked to consider the isoprene emission potential of trees when a significant number of trees are to be planted. Ideally, low isoprene emitting species that also meet other desired characteristics should be selected. Calfapietra et al. (2013) provides a good review on this topic.

An interesting way to follow-up on this activity is to have students explore the impact of chestnut blight on forest composition in North America, how this may have influenced isoprene emission from these forests, and ultimately its effects on ground-level ozone concentration (Lerdau et al., 1997).

o American chestnut was once a dominant tree species in eastern North American forests but the chestnut blight resulted in the near-extinction of this species. American chestnuts do not emit isoprene whereas the species they were replaced by, oaks, do emit isoprene. This change in species composition very likely led to an increase in isoprene emission from forests in the eastern U.S.

A similar exercise would be for students to explore how different forest management practices influence isoprene emission and ground level ozone production. For example, planting high isoprene emitting tree species for biofuel production has the potential to influence tropospheric ozone production (Ashworth et al., 2013).

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References cited in Supplementary Material F:

Ashworth K, Wild O, Hewitt CN (2013) Impacts of biofuel cultivation on mortality and crop yields. Nature Clim Change 3:492-496

Calfapietra C, Fares S, Manes F, Morani A, Sgrigna G, Loreto F (2013) Role of biogenic volatile organic compounds (BVOC) emitted by urban trees on ozone concentration in cities: A review. Environ Pollut 183:71-80

Lerdau M, Guenther A, Monson R (1997) Plant production and emission of volatile organic compounds. Bioscience: 373-383

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