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DOI: 10.1039/C2RP20074K (Paper) Chem. Educ. Res. Pract., 2013, 14, 51-61

College students' understanding of atmospheric ozone formation

Kristen E. Howard , Shane A. Brown , Serena H. Chung , B. Thomas Jobson and Timothy M. VanReken *
Dept. of Civil & Environmental Engineering, Washington State University, Pullman, WA, USA. E-mail: vanreken@wsu.edu

Received 15th June 2012 , Accepted 3rd October 2012

First published on 6th November 2012

Abstract

Research has shown that high school and college students have a lack of conceptual understanding of global warming, ozone, and the greenhouse effect. Most research in this area used survey methodologies and did not include concepts of atmospheric chemistry and ozone formation. This study investigates college students' understandings of atmospheric ozone formation using established clinical interview methodologies and qualitative data analysis techniques. Three prevalent naïve conceptions were identified: (1) that there is only one atmospheric mechanism for ozone formation; (2) that pollutants and gases only react after being transported up high in the atmosphere; and (3) that concerns about ozone in the atmosphere are due primarily to its role as a greenhouse gas. The existence of these naïve conceptions prevented students from forming correct mental models of atmospheric ozone formation. Existing conceptual change approaches may provide pathways for addressing these naïve conceptions, but further research is required before specific solutions are proposed.

Introduction

High graduation rates and high passing rates on standardized professional exams (e.g., the Fundamentals of Engineering exam) suggest that science and engineering students can effectively apply equations, but low performance on assessments of conceptual understanding in various content areas indicate that they often do not understand them (e.g., Lawson and McDermott, 1986; Hake, 1998; Streveler et al.). Similar evidence of low conceptual understanding has been found in students across age groups studying global warming, the stratospheric ozone layer, and the greenhouse effect. For example, Andersson and Wallin (2000) examined students' conceptions of the greenhouse effect and ozone layer depletion in grades 5, 9, and 12, and found that students linked the model of an ozone barrier stopping harmful UV radiation to the greenhouse phenomenon. Specifically, students believed that when ozone depletion occurs (i.e., when the barrier thins), more radiation penetrates and the surface warms as a result; they incorrectly identify this as the greenhouse effect. Subsequent studies have consistently shown that college students frequently exhibit the same naïve conceptions (Jeffries et al., 2001; Gautier et al., 2006). Similarly low conceptual understanding of solar radiation, air pollutant emissions, and the ozone layer has also been observed. In a study by Rye et al. (1997), 75% of students in grades 6–8 believed that global warming is caused by ozone depletion and/or by increased UV radiation. Khalid (2001) studied college students' naïve conceptions regarding the greenhouse effect and ozone depletion, and found that students believed an increased greenhouse effect may cause skin cancer and that there is a causal relationship between ozone depletion and global warming. Groves and Pugh (2002) found that college students believed that UV radiation was both a cause and effect of ozone depletion. These students also conflated the greenhouse effect and ozone depletion, reasoning that among the causes of the ozone depletion problem were elevated CO2 concentrations and the inability of sunlight to escape from the earth's surface. Research also indicates that college students have many naïve conceptions related to fundamental chemistry concepts (Barke et al., 2009). Improved understanding of chemistry concepts and the role of chemistry in the natural world is important not only for practitioners but for the global community. This is especially so because of the crucial role that green chemistry plays in finding sustainable development solutions now and into the future (Vilches and Gil-Pérez, 2011; Burmeister et al., 2012).

While this existing research has identified several naïve conceptions related to chemistry, greenhouse gases, ozone depletion, and associated topics, college students' understanding of atmospheric ozone formation and related chemistry concepts has not yet been studied explicitly. This study uses the clinical interview methodology (Sommers-Flanagan, 1999) to explore college students' conceptual understanding of atmospheric ozone formation. The goal of the work is to provide educators with important information about students' conceptual understanding of this topic, and to lay a foundation for additional future research and curriculum development for atmospheric chemistry education.

Conceptual change and naïve conceptions

Conceptual change research is based on constructivist learning theories (Wadsworth, 1996) that suggest that learning is a process of change, and that people use their life experiences and existing knowledge as a foundation for building new knowledge and understanding of how the world works. Conceptual change research is largely focused on understanding aspects of existing knowledge that are incorrect and very difficult to change (Chi, 2005), on the structure and organization of existing knowledge, and on how these characteristics make some concepts much harder to learn than others.

The focus of this study is on students' conceptual understanding of ozone formation. Conceptual understanding is defined here to be a “learner's internal representations constructed from the external representations of entities…” (Treagust and Duit, 2008, pg. 298). Students' internalized representations of ideas related to a topic and the interconnections among these concepts are commonly referred to as mental models (Norman, 1983). The mental models can be elicited, imperfectly, using clinical interview techniques (Ginsburg, 1997). Data collection in theory-focused conceptual change research relies heavily on clinical interview techniques that elicit in-depth accounts of student conceptual understanding of a particular topic. Such interview methods not only yield information on how students understand concepts in isolation, but also how understanding of one concept connects to other related concepts (Ginsburg, 1997). The incorrect aspects of a student's conceptual understanding are commonly referred to as ‘misconceptions’ or ‘naïve conceptions’ (Scott et al., 1997). The terms are frequently used interchangeably, and the use of each is common in the conceptual change literature within a debate about what exactly it means to be wrong. For example, Chi (2005) argued that students operate in the incorrect ontological categories for particular concepts, while Vosniadou et al. (2008) suggested that being ‘wrong’ is tied to epistemological commitments such as ‘the world as it appears to be’. In this paper, we use the term ‘naïve conceptions’ and consider it to be a concept or idea that students have that conflicts with accepted scientific views of ozone formation. While a full reckoning of the competing conceptual change theories and naïve conceptions is beyond the scope of this paper, contributions to this discussion rely critically on detailed accounts of student understanding; obtaining such detailed accounts is a key motivation for the current study.

An additional, more practical reason for the current work stems from the previous success of theory-based approaches in meeting the difficult challenges of changing students' naïve conceptions. Such success is not guaranteed; when confronted with knowledge that does not agree with existing knowledge, students have been shown to change the newly obtained information rather than alter their existing knowledge (Montfort et al., 2009). If students' preconceived notions are not addressed explicitly in instruction, “they may fail to grasp the new concept and information, or they may learn them for purposes of a test but revert to their preconceptions outside the classroom” (Donovan and Bransford, 2005). Lising and Elby (2005) argue that students have difficulty applying their experiences and real life knowledge to a classroom environment. In response to these challenges, a small set of science education researchers and practitioners have used theory-based approaches to encourage conceptual change (e.g., Slotta and Chi, 2006). This is best demonstrated by the extensive body of work on student understanding of elementary physics concepts, which is almost completely based on detailed interview methods and qualitative analysis (e.g., Lawson and McDermott, 1986). This work has led to internationally available and adopted curriculum focused on developing conceptual understanding (e.g., McDermott, 1996; McDermott and Shaffer, 2001).

Research justification and goals

Investigations into student conceptual understanding of ozone formation at the university level are sparse, and what research there is does not look at the phenomenon using detailed qualitative research techniques. Previous research has focused on larger scale phenomena in atmospheric chemistry, such as ozone depletion and the greenhouse effect. Furthermore, the majority of these investigations focused on students' understanding of the long-term environmental consequences and societal consciousness of these phenomena, and not on the technical aspects of ozone formation and atmospheric chemistry. While these studies have value to the research and teaching communities, they lack the detailed descriptions of students' understandings of technical concepts that are the basis for most research in conceptual change. More detailed interview data form the basis for Chi and Roscoe's (2002) ontological categorization theory of conceptual change and for framework theory of conceptual change described by Vosniadou et al. (2008).

This study applies established methodologies to study how students learn local-scale atmospheric phenomena by probing their understanding of the processes leading to atmospheric ozone formation. Conducting interview-based qualitative research can help develop detailed accounts of students' mental representations of these concepts and how they relate. The goal of this research is to synthesize students' conceptions of ozone formation and its role in the atmosphere and on identifying students’ naïve conceptions associated with ozone formation.

Methods

Research participants were selected from the pool of students enrolled in Introduction to Environmental Engineering, a junior level engineering course in the Department of Civil and Environmental Engineering (CEE) at Washington State University (WSU) during the 2010 spring semester. Very little attrition from CEE occurs during or after the junior year and the course is required of all CEE graduates, so this study population represents an average graduating class from CEE at WSU. The CEE upper division student population is approximately 80% male and 20% female and is approximately 85% white and 15% black, Hispanic, Native American, and Asian students. WSU is a public university with a student population of about 20[thin space (1/6-em)]000.

Each student in the study population was recruited on a voluntary basis for a 40 minute interview. Forty-four of the 55 students enrolled in the course volunteered to be research participants and were provided with either a small monetary or extra-credit incentive. Participants represent the full spectrum of academic performance in the course, with 17 students in the top, 14 students in the middle, and 13 students in the bottom third of the class. The high percentage of participating students combined with the representative sample with respect to course performance ensures that the results are generalizable to the population from which the sample was drawn, and transferable to other comparable populations. Substantial research has shown that students seldom alter their existing conceptual understanding due to instruction, and that naïve conceptions are common across a variety of educational contexts (Dove, 1996; Meadows and Wiesenmayer, 1999; Chi and Roscoe, 2002; Kautz et al., 2005). This suggests that findings from this study are not only representative of the participant population, but also typical of undergraduate college level science students. However, to provide appropriate context, a summary of students' course-based exposure to ozone formation material is provided in Box 1.


Box 1: Ozone and atmospheric chemistry

Ozone is a major atmospheric oxidant that is formed primarily through chemical reactions involving photolysis—the decomposition of a chemical compound by means of light energy or photons. The specific photolytic interactions between atmospheric gases and incoming solar radiation lead to different ozone production mechanisms in two different regions of the atmosphere, the stratosphere and troposphere. Due to a strong temperature inversion between the troposphere and stratosphere, the exchange of air between them is slow and they are thus treated as separate reservoirs. The stratosphere is dry, with slow vertical mixing rates. The troposphere is wet, with rapid vertical mixing rates.

Stratospheric ozone is produced through the photolysis of oxygen molecules by ultraviolet (UV) light (wavelength λ < 240 nm) to produce O atoms that then react with O2 to form O3. The combination of increasing light flux and rapidly decreasing air pressure with height results in a distinct stratospheric ozone concentration layer, with a maximum of 8 ppmv at around 30–35 km. Within the stratosphere, ozone plays a beneficial role by absorbing harmful UV radiation before it reaches the earth's surface. When chemical reactions reduce this layer, as illustrated by the ozone-hole phenomena at the earth's poles, negative human and environmental impacts such as skin cancer can occur.

Ozone formed in the troposphere is driven by separate and largely independent mechanisms from those that operate in the stratosphere. In contrast to the beneficial role of stratospheric ozone, elevated ozone concentrations near the earth's surface can cause severely negative impacts that include aggravation to human cardiorespiratory systems as well as damage to agricultural systems and other plant life. Tropospheric ozone is also an important greenhouse gas. The key step for tropospheric ozone formation is the photolysis of an NO2 molecule, which produces an O atom that then rapidly reacts with O2 to the formation of O3. This photochemical process is driven by photons in the visible and near-UV portions of the electromagnetic spectrum (λ = 325–450 nm). O2 cannot be photolyzed in the troposphere because all UV radiation with wavelengths below 290 nm has been absorbed by O2 and O3 in the stratosphere. The source of NO2 in the troposphere is the oxidation of NO by peroxy radicals such as HO2˙. Such radicals are formed in the oxidation of carbon monoxide (CO) and volatile organic compounds (VOCs). Oxidation of CO and VOCs is initiated by the hydroxyl radical (HO˙), the troposphere's most important oxidizing agent. When concentrations of NO, CO, and organic compounds are high enough, such as in urban areas, then the chemical reaction rates will be fast enough for O3 to increase. Meteorology and the dispersion rates of pollutants can have significant impacts on the resulting surface O3 concentrations. An important point in the ozone formation process is that HO˙ radicals and NO are regenerated in a radical chain reaction mechanism, and thus act as catalysts. One HO˙ and NO molecule can form many O3 molecules. The key reactions can be illustrated by the oxidation of CO:

CO+HO˙ → CO2+H Oxidation of CO

H+O2 → HO2˙ Creation of hydroperoxy radical

HO2˙+NO → NO2+HO˙ Oxidation of NO & Regeneration of HO˙

NO2+sunlight → NO+O Photolysis of NO2 & Regeneration of NO

O+O2 → O3 Formation of Ozone

CO+2O2 → CO2+O3 Net Reaction

Similar reactions occur that involve VOCs instead of CO. NO2 is ultimately removed by reaction with HO˙ radical to form nitric acid, a component of acid rain. The oxidation of VOCs and NO2 in the troposphere is rapid, and thus these compounds are not transported into the stratosphere.

Tropospheric ozone production relies on the interaction of these NOx and HOx catalytic reaction cycles. Because the complex mechanisms depend on the availability of NOx (=NO+NO2) and VOCs, which are in constant competition for HO˙ oxidation, there is not a simple, uncomplicated way to predict the amount of potential ozone production. Computer models are necessary to numerically analyze numerous combinations of NOx and VOC pollution levels in order to assess their potential to form ozone. These results are often plotted as ozone isopleth diagrams, a useful tool for interpreting the model results to aid air quality management decisions. By examining the plots, managers can determine whether ozone could be most effectively controlled by reducing NOx, VOCs, or some combination of the two. Understanding this information is essential in the career preparation of an environmental engineering student, since it is necessary to first comprehend the underlying chemistry in order to truly grasp the technical reasoning and decision making behind government regulations, and execute appropriate managerial decisions for the future of our environment.


Most of the specific content related to this research was presented in the course material on multiple occasions during the semester. Early lectures included information about atmospheric layers and the nature of solar energy input, which laid the groundwork for important concepts linked to ozone chemistry. A two-week module later in the semester presented detailed material on the formation of ozone and its effects in the stratosphere and troposphere. This module, which immediately preceded the student interviews, placed a strong emphasis on the formation of tropospheric ozone, highlighting the nature of radical chain oxidation mechanisms, the role of nitric oxide as a catalyst, and the importance of hydroxyl radicals in initiating atmospheric oxidation reactions. It included assigned reading, lectures, a homework assignment, and exam material. The lectures discussed the interaction of stratospheric and tropospheric ozone with ultraviolet light through photolytic chemical reactions. All pertinent ozone exam questions were qualitative, requiring no calculations or derivations.

Interview protocol

Investigating conceptual understanding requires determining students' knowledge of the concepts and ideas related to a content area. The motive during interviews was not to evaluate the students based on their answers alone, but rather to gain insight into the thought processes and reasoning behind those answers. Science and engineering education researchers have used the clinical interview technique to investigate student conceptual understanding (e.g., Trowbridge and McDermott, 1980; Trowbridge and McDermott, 1981; Montfort et al., 2009; Andrews et al., 2010). The clinical aspect of the interview (Ginsburg, 1997) refers to the goal of obtaining rich descriptions of student thinking and reasoning using flexible lines of questioning that are adaptable to individuals and their unique ways of knowing. This method includes developing real-time hypotheses about student reasoning, and investigating these hypotheses through probing questions. The interview protocol was semi-structured (Patton, 2002), with a set of questions that was asked of every student, and related subsets of probing questions for each primary question that were used as needed.

The goal of the interview protocol was to investigate student understanding of ozone formation, including technical content relating to basic chemical reactions, terminology, and fundamental ozone concepts. Even small features of a problem statement or interview question can significantly change the way students describe their thinking (diSessa, 2007; Ivarsson et al., 2002). This is likely because learners' conceptions are often organized into distinct groupings of knowledge that may be separately activated based on the specific wording of a problem or question (White, 2002; Vosniadou et al., 2008). This risk can be avoided and a fuller representation of students' thinking may be obtained by incorporating multiple contexts into the interview protocol (Brown, 2011). In this study, to obtain a complete view of students' mental representations and provide multiple and diverse opportunities for students to share their knowledge, the interview protocol included questions from multiple contexts.

The protocol, summarized in Table 1, encouraged students to discuss ozone as an atmospheric cycle (Problem 1), via a set of chemical reactions (Problem 2), using standard terminology and definitions (Problems 3 & 4), as part of a hypothetical scenario (Problem 5), and in the context of an isopleth diagram (Problem 6). The order of questions was intended to allow students to start from a broad understanding of atmospheric ozone, and transition to more detailed content regarding its chemical formation. For the first interview problem, students were given a figure representing the major components of the tropospheric ozone formation cycle (Interview Aid #1; from Masters and Ela, 2008). They were asked to explain what they believed the diagram represented, and to discuss any major themes or points of significance. Problem 2 included general chemistry questions about electrons, protons, photolysis, and chemical stability; students were provided with a list of reactions from the course materials pertinent to ozone formation (Interview Aid #2; from Masters and Ela, 2008) to aid them in this task. Problem 3 probed students' understanding of ozone terminology, including hydrocarbons, volatile organic compounds (VOCs), and nitrogen oxides (NOx). Problem 4 required students to define smog, photochemical smog, and ozone, and to discuss any differences or similarities between these terms. Problem 5 probed student understanding in a different context by presenting them with a hypothetical scenario related to ozone formation. For this task, students were first instructed to imagine a small box open to the atmosphere, and then directed to move their box to different locations (e.g., urban or rural areas), or to change its altitude. As these positional changes were proposed, students were asked to discuss whether something significant happened or changed in their box. They were asked questions about the status of their box, how atmospheric constituents were interacting with one another, and whether anything had changed in their box and why. Finally, for Problem 6, students were asked to explain an ozone isopleth diagram representing the relationship between the amount of VOCs and NOx in the atmosphere and the amount of ozone produced (Interview Aid #3; from Masters and Ela, 2008). Students were asked how to interpret the diagram, and who would use the figure and for what purposes.

Table 1 Summary of interview problems and questions
Problem Interview aids Interview questions
1 #1: Figure representing the main cyclic tropospheric ozone formation components Can you explain what is happening in the Figure?
Can you describe what NO, NO2, and HO˙ are, and their significance to our atmosphere?
2 #2 A list of tropospheric ozone formation reactions Can you define a chemical reaction as if explaining to someone with no chemistry background?
Discuss the difference between a stable and unstable atom using the provided list of reactions as a reference.
Discuss electrons and protons in terms of their location and role during a chemical reaction.
Describe some of the reactions from the provided list, and any correlation they have to previous ozone figures.
What does the arrow represent in a chemical reaction?
Can you describe in detail what a radical is, and its purpose?
What is photolysis and a photon, and do any of the provided reactions represent either of them?
3   Can you describe what a hydrocarbon, VOC, and NOx are, including how they are formed?
4   Explain what smog, photochemical smog, and ozone are, as well as any differences between them, if they exist.
5   Describe the different molecular interactions and ozone concentrations inside a hypothetical box at various locations and altitudes.
6 #3: Isopleth diagram relating NOx, hydrocarbons and O3 production Explain the process of predicting ozone concentrations via the provided reactions and the NOx-hydrocarbon isopleth diagram.
End of interview  

Data analysis

The goal of qualitative analysis is to determine patterns in data—in this case, patterns of how students reasoned in the interviews. To accomplish this, audio-recorded interviews were first transcribed and analyzed using a qualitative data analysis software package (AtlasTi, 1993–2011). The qualitative analysis involved an increasingly complex interpretation of quotations using codes (Miles and Huberman, 1994). Codes are interpretations of sections of interview text. For example, a student quote of ‘chemical reactions relating to ozone formation only happen high in the atmosphere’ could be coded as ‘reactions happen up high.’ The complexity of analysis increased, using an iterative process that involved grouping and refining codes, continually assessing the progression for consistency, and comparing results from previous analyses. The first analysis stage involved coding pertinent information considering only student wording, to avoid excessive interpretation early in the analysis (Miles and Huberman, 1994; Patton, 2002). A second stage then linked themed phrases and topics, which involved the majority of researcher interpretation. The final stage entailed searching for evidence that contradicted the findings as a means of validating them (Lindlof and Taylor, 2002).

Student pseudonyms are provided in the interview excerpts below to ensure anonymity of student responses. Our results were not analyzed with respect to student gender and therefore should not be interpreted in this way. The pseudonyms used are random and do not imply gender.

Results

Understanding atmospheric ozone formation requires bringing together numerous interacting scientific concepts, many of them less tangible than is typical in an engineering curriculum. Fig. 1 presents a concept diagram for the formation, interactions, and impacts of atmospheric ozone in the stratosphere and troposphere. The diagram is not meant to be comprehensive, but it does illustrate a way to link the individual pieces of knowledge together to form a correct mental representation. An important feature of Fig. 1 is how often similar content is repeated in different contexts on the diagram (as indicated by the color coding). For example, separate concepts linked to light/radiative interactions (the orange boxes) are shown to indicate (1) the role of solar radiation in driving the stratospheric photolysis of O2 to form ozone; (2) the absorption of infrared light in the troposphere linked to the greenhouse effect; and (3) the absorption of ultraviolet light by the stratospheric ozone layer, which is linked to the prevention of skin cancer. These instances are important, because interview data indicate that such concept overlap may be one of the main obstacles that prevent students from achieving a strong overall understanding of ozone formation.
Example concept map for atmospheric ozone. General ozone concepts are in tan. Concepts related to light/radiation are in orange. Concepts linked to pollutant emissions are in purple, and those linked to chemical processing are in blue. Positive and negative impacts are in green and red, respectively.
Fig. 1 Example concept map for atmospheric ozone. General ozone concepts are in tan. Concepts related to light/radiation are in orange. Concepts linked to pollutant emissions are in purple, and those linked to chemical processing are in blue. Positive and negative impacts are in green and red, respectively.

Our results indicate that students consistently had difficulty with ozone formation concepts and the relations between these concepts. When they encountered concepts that were difficult, or when naïve conceptions were included in their mental model, students frequently articulated incomplete or incorrect links. This outcome is illustrated in Fig. 2, which demonstrates how a missing conceptual link can affect the overall understanding of the ozone formation process. The diagram illustrates an incomplete conception that was observed frequently during the interviews. Forty-one of 44 students in the sample were successful in linking concepts associated with photolysis (sunlight-driven reactions) to tropospheric ozone/smog formation. This is represented in Fig. 2 by a solid line. Most of them were also able to articulate the basic concept that light, NOx, and VOCs are the major drivers of ozone formation (dotted lines in Fig. 2). However, most students failed to grasp the more refined concepts necessary for a full understanding of tropospheric ozone formation. For example, only a few students could correctly describe the role of atmospheric radicals in driving the cyclical reactions that allow ozone to accumulate in the troposphere. The dashed lines in Fig. 2 represent this missing conceptual link. When these gaps in understanding occur, students will compensate by adding flawed knowledge to their mental models, which can inhibit further learning. In our results, this was manifested by students' inability to understand how the behavior predicted by ozone isopleth diagrams and key to ozone management was linked to the chemical formation processes (see Fig. 1).


Students' simplified representation of ozone formation, illustrating their incomplete understanding of key concepts.
Fig. 2 Students' simplified representation of ozone formation, illustrating their incomplete understanding of key concepts.

As illustrated in the examples below, many of the student conceptualizations found during this study were correct in some respects, but were either incomplete or incorrect in important ways. Because their foundations were weak in the concepts needed to understand ozone formation, students would often incorporate incorrect links in their alternate mental models. Detailed analysis of the qualitative interview data resulted in the identification of three common naïve conceptions:

(1) There is only one atmospheric mechanism for ozone formation.

(2) Pollutants and gases only react after being transported up high in the atmosphere.

(3) Concerns about ozone in the atmosphere are due primarily to its role as a greenhouse gas.

Each of these naïve conceptions was exhibited by at least 25% of the study participants during their interviews. The examples that follow illustrate the nature of the naïve conceptions identified during the study, and the role they played in students' articulated models of ozone formation. The presence of the naïve conceptions appears to greatly hinder students' comprehension of how ozone behaves in the atmosphere.

Naïve conception #1: there is only one atmospheric mechanism for ozone formation.

Among the key ideas that students are expected to understand during the introductory environmental engineering course are the dual functions of ozone in the troposphere and stratosphere, and that different chemical processes dominate the formation of ozone in these different atmospheric layers. As described in Box 1, the high abundance of ozone in the troposphere has harmful health and environmental impacts, and ozone accumulates in the troposphere because of a photolytic reaction mechanism involving NOx and VOCs. In the stratosphere, ozone has a protective function due to its ability to absorb harmful ultraviolet radiation. In this higher atmospheric layer, the dominant formation mechanism is not the NOx/VOC pathway, but rather one that involves the photolysis of oxygen (Fig. 1 and Box 1).

In this study, the students in the interview pool were generally able to describe the differences in ozone's function in the different atmospheric layers. However, over half of the students indicated directly or indirectly that the same ozone formation mechanism was dominant in both atmospheric layers. Students remembered that ozone could be found in the stratosphere and the troposphere, and that it has different dominant functions in these layers, but they did not know that ozone formation in each layer is controlled by distinct chemical mechanisms. Most students indicated a belief that pollutants, specifically NOx and VOCs, were dominant contributors to ozone formation in the stratosphere as well as in the troposphere.

Although most students were able to recall the two roles of ozone, in many instances they displayed confusion about formation mechanisms and often intertwined the different atmospheric functions into an incorrect mental model. For example:

Joe: [While looking at Interview Aid #1] They are more concerned with ozone being formed in the troposphere because it absorbs like the radiation a lot more than in the stratosphere where it blocks it, like incoming radiation.

Joe's explanation alludes to ozone's ability to absorb radiation within the troposphere. While this does occur, absorption is not the critical reason why tropospheric ozone is a concern to human health. This line of reasoning conflates the concept of stratospheric ozone absorbing UV radiation, and thus preventing it from reaching the earth surface with the concept of the function and formation of tropospheric ozone. The result is an incorrect mental model.

Many of the missing links of knowledge in students' mental models were revealed when they were asked to justify the reasoning in their initial responses. For example, many students incorporated the perception that ozone is mainly found in high altitudes in the atmosphere, because that is where the stratospheric ozone is located. The following excerpt is representative of students who attempted to incorporate the common conception of a stratospheric ozone layer into their newly-acquired knowledge of how ozone is formed in the troposphere:

Interviewer: Would any of that [loosely indicating chemistry represented in Interview Aid #1] be in the box on the ground?

Terry: There would definitely be oxygen, at least hopefully. Um, probably some NO2, um, I don't think that the reaction itself would be happening to produce ozone. Um, at least not very much. That takes place more in the ozone layer, at least as far as I know.

Interviewer: Okay, and what about the ozone layer makes this stuff react that it wouldn't be reacting in the box down on the ground?

Terry: I could not tell you that.

Terry discussed how some of the molecules involved in tropospheric ozone formation, such as oxygen and NO2, would be present on the ground, but indicated that the reactions to produce ozone would not occur. Her justification for this reasoning was that the reactions to produce ozone from NOx took place primarily in the stratospheric ozone layer. This student's response, like many, illustrates a tendency to take knowledge of the importance of NOx and VOCs to tropospheric ozone formation and incorrectly apply it to the stratosphere as well. Terry clearly understands that NO2 is present in the lower atmosphere, and that it contributes to ozone formation (as she had stated previously in the interview). She also recalls the stratospheric ozone layer; however, due to her flawed mental model, an incorrect conceptual link was formed between these two concepts. Furthermore, Terry was unable to answer when asked to explain why reactions occur in the stratospheric ozone layer and not on the ground in the troposphere; this was a common result in the participant interviews. Students had difficulty in explaining why pollutants could react and form ozone high in the atmosphere but not near the surface, yet they generally maintained their belief that this was so. Students frequently attempted to articulate other reasons to explain their conceptions.

Naïve Conception #2: pollutants and gases only react after being transported up high in the atmosphere

Another common pattern observed in the interviews was for students to explain that reactions occur primarily in the stratosphere and not at the surface; they asserted that this was because pollutants and gases float higher up into the atmosphere before reactions would occur. This belief is in contrast to the fundamental concepts in atmospheric science that chemical processes occur throughout the atmosphere, and that it is the specific environmental conditions and species of gases present that dictate what reactions predominate at a specific location. Students typically incorporated this naïve conception into their flawed argument as a means of explaining the large abundance of ozone in the stratosphere. The incorporation of this naïve conception is a good example of how students create their own incorrect representations for occurrences that they cannot otherwise explain.

The students who exhibited Naïve Conception #2 usually demonstrated a low overall conceptual understanding of the process of a chemical reaction, specifically concerning where and how molecules interact with one another when a chemical reaction takes place. Their responses indicated a belief that while there were pollutant and other gaseous molecules present in the lower atmosphere, the reactions that originate in the gaseous state (especially those involved with ozone formation) generally occurred at higher altitudes. Students exhibiting this naïve conception would argue that molecules float up to the altitude in which they react, but would not acknowledge any reactions occurring during the process of floating up. For example:

Interviewer: Okay, can sunlight get in through the sides of the box?

Ryan: Oh yeah, but I don't think it reacts on like that close to ground, I don't know. The way like I was always explained to, like when they showed a picture of different like spheres, it's always like at the top of the, I think troposphere is the first and then like up above is where other things react. So nothing ever reacted like right on the ground level really.

Interviewer: Okay so how come all this middle stuff [indicating reactions shown on Interview Aid #1] wasn't going on down below?

Ryan: I think there was some, but like not as much as compared to up there, I feel like when the cars emit the exhaust like all the different pollutants are around you, they tend to like go upwards and they don't really just sink to the ground you know, I mean like all the atoms and molecules if they flow upwards automatically then there is a denser population of each of the different molecules, atoms up high where there are like they are there too.

Because students used this naïve conception to discuss general atmospheric chemical reactions (not only for NOx and VOCs but also for other gases), it seems likely that it is linked to students' awareness of the large abundance of ozone found in the stratosphere. For example, in the excerpt below, Parker suggests that NOx and VOCs go up high and become trapped:

Parker: I see the sun and I think about the gases going up and then how do they interact on the atmosphere.

Interviewer: Okay. So what's causing I guess these concentrations of NOx and VOCs to go straight up and get trapped in this layer rather than like reacting in our box lower (stated previously by respondent), does that make sense?

Parker: Oh yeah, you mean like why they are going to up high.

Interviewer: Yeah like why don't they start reacting like down lower at all?

Parker: Because the VOCs and NOx they are gases, like every time they have been made, they just go up to the atmosphere…

From this passage it is clear that, for this student at least, Naïve Conception #2 is linked to the student's pre-existing knowledge of the significance of the stratospheric ozone layer. Similar statements were found in several other interviews, indicating that in general students who exhibited Naïve Conception #2 tended to also believe that ozone was only formed via a single mechanism (Naïve Conception #1). While more general chemistry concepts not specifically related to ozone were discussed during the interviews, students were aware that the overall goal of the interview was to extract information regarding ozone formation. Therefore, students may have applied this naïve conception to atmospheric gases in a general way, but their thinking was already influenced by the context of ozone-related discussion.

Naïve Conception #3: concerns about ozone in the atmosphere are due primarily to its role as a greenhouse gas

In many cases, students' inability to thoroughly explain the processes of ozone formation appeared to arise from intermingling pre-existing concepts with the new material introduced in the course. Sometimes the pre-existing concepts were incorrect; this was the case for Naïve Conceptions #1 and #2 described above. In other cases, a concept exhibited during an interview was technically correct, but only tangentially related to study material. Naïve Conception #3 is an example of the latter circumstance, and is thus considered a context-dependent naïve conception.

Like everyone, students entering the introductory environmental engineering course had pre-existing conceptualizations stemming from their life experiences, and these seemed to frequently be incorporated into their explanations of atmospheric ozone. For topics relating to atmospheric pollution, the foundations of many students' mental models were from their imperfect understanding of climate change and the greenhouse effect. These topics were also covered in the introductory environmental engineering course in weeks prior to the study of atmospheric ozone. Twenty of the 44 study participants strongly linked climate change concepts to their developing mental model for atmospheric ozone.

Such a link is not, strictly speaking, incorrect. Ozone is indeed a greenhouse gas, and does contribute to climate warming as it absorbs infrared energy radiated from earth's surface. Ozone's potential to warm the atmosphere is on par with other non-CO2 greenhouse gases. However, societal concerns with respect to atmospheric ozone do not stem primarily from its greenhouse role, and while students were taught the greenhouse implications of ozone, it was not a major focus of the module. Instead, as summarized in Box 1, the focus of the teaching module was on ozone in the troposphere as a component of photochemical smog that is harmful to human and ecosystem health, and on its presence in the stratosphere, forming a protective shield against harmful cancer-causing UV radiation.

The example below illustrates how students frequently incorporated the greenhouse gas effect into their notion of how ozone is formed, and what effects it has in our atmosphere. Jordan uses the greenhouse effect as an explanation of how ozone has a negative impact in our atmosphere:

Jordan: Ozone is a bad thing because it creates like a layer…it's with the warming of the earth like it makes it harder for the earth to reflect its heat back to space so it warms, it kind of like acts like a lid that's why on earth it's becoming warmer in certain places because there is like a high concentration of this ozone layer then it makes it harder for earth's surface to reflect its heat back to space.

Interviewer: Anything else about ozone that you would tell somebody who didn't know about any of this?

Jordan: There are ways to prevent it I think.

Interviewer: How is that?

Jordan: With this diagram like the NOx and VOC diagrams it helps pinpoint like what exactly you need to do to reduce ozone.

Jordan refers correctly to some components of ozone's atmospheric roles in her response. First, she recognizes that ozone can be bad in the first line, and then continues on to incorporate the concept of the stratospheric ozone layer. While these are both accurate elements of ozone's role in the atmosphere, the student next incorrectly links these roles to each other and to the greenhouse effect. When given the opportunity to justify her reasoning, Jordan incorporates a greenhouse effect concept—the warming of the earth—into her explanation for why ozone is bad. While this statement contains some truth, it is not the appropriate reasoning to describe why ozone is detrimental in the context of tropospheric air quality. Moreover, near the end of the quoted passage, Jordan states that a way to prevent the ozone-induced warming is to use a NOx-VOC-ozone diagram to find the optimal pathway to reduce ozone. Again, if separated from the present context, it is correct to suggest the use of such a diagram to reduce O3. However, this student has incorrectly linked the ozone isopleth diagram to the prevention of greenhouse warming, rather than to correctly connect it to the control of tropospheric ozone pollution to prevent negative health impacts.

In another example, Brook links the large abundance of both ozone and other greenhouse gases to one location—the stratosphere. Then, he confuses concepts regarding the effect of ozone in the stratosphere. Although he correctly recognizes the positive role of ozone in blocking sunlight (shortwave radiation) from reaching the lower atmosphere, he incorrectly links this with the negative human and environmental implications of the effects of greenhouse gases on longwave radiation, which leads to surface warming:

Interviewer: Does ozone [concentrations] ever reach a max or is it like highest like–?

Brook: Right in the stratosphere I think that's where it is at.

Interviewer: What do you mean by the stratosphere?

Brook: That's like the point in the atmosphere where all the greenhouse gases are and–

Interviewer: Why do they remain there?

Brook: Because that's like a point where they are like at equilibrium or they can't escape and they can't release back to the earth.

Interviewer: How come they don't go down or move up?

Brook: I am not sure.

Interviewer: Okay. So they are just like stuck there?

Brook: Not stuck but for the most part they just can't join up there.

Interviewer: Okay. And is that a good place for them to be or?

Brook: Yeah, because they block sunlight from getting to the earth which causes the earth to heat up.

Brook indicates that ozone reaches a maximum concentration in the stratosphere, which he then defines as the point in the atmosphere where all of the greenhouse gases are. This student incorrectly merges into one concept the separate roles of ozone as a greenhouse gas and as a barrier to UV radiation in the stratosphere. He displays further incorrect conceptual links between the greenhouse effect and the role of stratospheric ozone at the end of his response, stating that the gases in the stratosphere, ‘block sunlight from getting to the earth, which causes the earth to heat up.’

Unlike the previous examples, Casey does not simply conflate the effects of ozone's role as a greenhouse gas with its other functions in the troposphere and stratosphere. Instead, this student relies entirely on the concept of the greenhouse gas effect as the foundation for the formation of ozone, incorrectly stating that ozone is actually formed from greenhouse gases:

Casey: Yeah like the ozone is like higher up in the atmosphere and it like reflects, but it's like created because that absorbs things that come off the earth kind of.

Interviewer: The ozone does?

Casey: Yeah or it's like created by things that are absorbed out there, so I think that higher it would go, the more ozone would be there.

Interviewer: Okay. What is ozone? Like how would you explain ozone to somebody?

Casey: I should say it's probably like formed by greenhouse gases that are absorbed from the earth and then reflected back down on to it.

Casey refers to elevated ozone levels ‘higher up in the atmosphere’ in his response, correctly acknowledging the presence of the stratospheric ozone layer. However, he also indicates that higher in the atmosphere ozone reflects and absorbs radiation from the earth's surface. The reference to reflection is incorrect information, and absorption is only relevant with respect to ozone's greenhouse role in the atmosphere. The student is confusing (1) the beneficial role of ozone absorbing solar (shortwave) radiation to limit the amount of this cancer-causing radiation from reaching the earth's surface; and (2) the greenhouse effect resulting from ozone absorbing infrared (longwave) radiation emitted by the earth's surface. Furthermore, the student relies strongly on the idea of greenhouse gases as a crutch in his overall model of ozone, to the point of stating that greenhouse gases actually form ozone.

Students exhibited substantial difficulties in assimilating the course material, and appeared to find it easier to comprehend new ideas by linking them to the greenhouse gas effect and global warming. The existence of overlapping concepts likely facilitated this. Knowledge of the greenhouse effect requires the incorporation of key concepts such as wavelength-dependent radiation, temperature effects, and the ability of ozone to absorb radiation. Many of these same ideas appear within the different ozone formation conceptions, and as a result, students not only conflated the environmental motivations, but also found a relatable, but ultimately incorrect, context within the greenhouse gas framework. Linking the greenhouse gas concept to other ozone subject matter appears to have allowed students to identify with atmospheric content in a familiar context.

Discussion

Students' efforts to integrate and establish relations between existing and new knowledge related to ozone formation often resulted in mental models with fragments of correct and incorrect conceptions. No student interviewed in this study was able to completely explain the process of ozone formation. The majority of the mental models exhibited by students were not only incorrect, but also frequently incoherent. The three most prominent ozone formation naïve conceptions identified in this study are:

(1) There is only one atmospheric mechanism for ozone formation;

(2) Pollutants and gases only react after being transported up high in the atmosphere; and

(3) Concerns about ozone in the atmosphere are due primarily to its role as a greenhouse gas.

Students who exhibited these naïve conceptions were unable to form correct and coherent models regarding ozone formation. They instead described alternate or incorrect links in order to connect missing pieces of knowledge.

Conceptual change theory may begin to explain the presence and persistence of these naïve conceptions. The naïve conceptions identified in this study generally represent the reduction of a complex non-linear system to a relatively simple and linear system and may be a result of a larger belief that ozone formation and other chemical and physical systems are simple and linear (e.g., one mechanism for ozone formation, or a singular role for ozone in the atmosphere). Addressing these types of naïve conceptions may require an ‘ontological shift’ (Chi, 1992) to allow students to develop more correct and robust understandings. Chi suggests that student naïve conceptions arise from miscategorizing concepts into the wrong ontological category, leading students to use the wrong ontological rules to approach a problem or understand a concept. For example, students often believe that heat is a substance and not a process (Meltzer, 2004), prohibiting them from understanding fundamental processes of heat transfer. Students may have similar ontological categories of linear and complex systems, or not have a category for complex systems at all. Examples of ontological rules for linear systems could indicate that if x doubles then so will y, or that adding more of substance x always increases the output of y. Non-linear ontological rules may consider that the effect of an input on an output may be positive or negative, is dependent on the quantities of both and recognize the uncertainty in the reactions of a system to a change in input. Previous work has shown that ontological shifts can improve learning—ontological training to repair the naïve conception wherein heat is placed in the incorrect ontological category, allowed students to develop and understand a new process category (Slotta and Chi, 2006). Similar training for students focusing on the development or enhanced understanding of a cognitive category of complex non-linear systems may have substantial impacts on learning in content where these systems are present, such as in atmospheric chemistry.

The disturbingly low level of college student conceptual understanding for ozone formation revealed in this study suggests that more research is merited to more fully characterize student understanding of atmospheric processes. Improved understanding of chemistry concepts in general, environmental chemistry in particular, and the role of chemistry in society are all important components of education for sustainable development strategies (Burmeister et al., 2012; Vilches and Gil-Pérez, 2011). Future research on this topic is needed in pursuit of those goals. Future work could incorporate more comprehensive methodologies to measure conceptual understanding, including a larger sample size, and providing students the opportunity to visually map out their personal ozone formation mental models during the interview process. Using pre- and post-instruction interviews would help gauge students' conceptual improvement. The knowledge gained from this work is a small first step towards the development of improved curriculum for training future scientists and engineers. Future professionals who have a robust understanding of air quality and climate change topics are crucial to successfully meet the environmental challenges facing our society in a changing world.

Acknowledgements

Support for this work was provided by U.S. National Science Foundation Award #DUE-0837496. The authors thank Dr Shelley N. Pressley for her assistance in developing the interview and evaluation protocols used in this work.

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