What is the best way to measure ozone?

Measurement of equivalent thickness of atmospheric trace gas

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The Dobson unit (DU) is a unit of measurement of the amount of a trace gas in a vertical column through the Earth's atmosphere. It originated by, and continues to be primarily used in respect to, the study of atmospheric ozone, whose total column amount, usually termed "total ozone", and sometimes "column abundance", is dominated by the high concentrations of ozone in the stratospheric ozone layer.

The Dobson unit is defined as the thickness (in units of 10 μm) of that layer of pure gas which would be formed by the total column amount at standard conditions for temperature and pressure (STP).[1][2] This is sometimes referred to as a 'milli-atmo-centimeter'. A typical column amount of 300 DU of atmospheric ozone therefore would form a 3 mm layer of pure gas at the surface of the Earth if its temperature and pressure conformed to STP.

The Dobson unit is named after Gordon Dobson, a researcher at the University of Oxford who in the s built the first instrument to measure total ozone from the ground, making use of a double prism monochromator to measure the differential absorption of different bands of solar ultraviolet radiation by the ozone layer. This instrument, called the Dobson ozone spectrophotometer, has formed the backbone of the global network for monitoring atmospheric ozone[3] and was the source of the discovery in of the Antarctic ozone hole.[4]

Ozone

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NASA uses a baseline value of 220 DU for ozone. This was chosen as the starting point for observations of the Antarctic ozone hole, since values of less than 220 Dobson units were not found before . Also, from direct measurements over Antarctica, a column ozone level of less than 220 Dobson units is a result of the ozone loss from chlorine and bromine compounds.[5]

Sulfur dioxide

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In addition, Dobson units are often used to describe total column densities of sulfur dioxide, which occurs in the atmosphere in small amounts due to the combustion of fossil fuels, from biological processes releasing dimethyl sulfide, or by natural combustion such as forest fires. Large amounts of sulfur dioxide may be released into the atmosphere as well by volcanic eruptions. The Dobson unit is used to describe total column amounts of sulfur dioxide because it appeared in the early days of ozone remote sensing on ultraviolet satellite instruments (such as TOMS).

Derivation

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The Dobson unit arises from the ideal gas law

P V = n R T , {\displaystyle PV=nRT,}

where P and V are pressure and volume respectively, and n, R and T are the number of moles of gas, the gas constant (8.314 J/(mol·K)), and T is temperature in kelvins (K).

The number density of air is the number of molecules or atoms per unit volume:

n air = A a v n V , {\displaystyle n_{\text{air}}={\frac {A_{av}n}{V}},}

and when plugged into the real gas law, the number density of air is found by using pressure, temperature and the real gas constant:

n air = A a v P R T . {\displaystyle n_{\text{air}}={\frac {A_{av}P}{RT}}.}

The number density (molecules/volume) of air at standard temperature and pressure (T = 273 K and P =  Pa) is, by using this equation,

n air = A a v P R T = ( 6.02 × 10 23 molecules mol ) &#; (   Pa ) 8.314 J mol K &#; 273   K . {\displaystyle n_{\text{air}}={\frac {A_{av}P}{RT}}={\frac {(6.02\times 10^{23}\,{\frac {\text{molecules}}{\text{mol}}})\cdot (~{\text{Pa}})}{8.314{\frac {\text{J}}{\text{mol K}}}\cdot 273~{\text{K}}}}.}

With some unit conversions of joules to pascal cubic meters, the equation for molecules/volume is

( 6.02 × 10 23   molecules mol ) &#; (   Pa ) 8.314   Pa m 3 mol K &#; 273   K = 2.69 × 10 25   molecules &#; m &#; 3 . {\displaystyle {\frac {(6.02\times 10^{23}~{\frac {\text{molecules}}{\text{mol}}})\cdot (~{\text{Pa}})}{8.314~{\frac {{\text{Pa}}\,{\text{m}}^{3}}{\text{mol K}}}\cdot 273~{\text{K}}}}=2.69\times 10^{25}~{\text{molecules}}\cdot {\text{m}}^{-3}.}

A Dobson unit is the total amount of a trace gas per unit area. In atmospheric sciences, this is referred to as a column density. How, though, are units of molecules per cubic meter, a volume, to be converted to molecules per square centimeter, an area? This must be done by integration. To get the column density, integrate the total column over a height. Per the definition of Dobson units, 1 DU = 0.01 mm of trace gas when compressed down to sea level at standard temperature and pressure. So integrating the number density of air from 0 to 0.01 mm, it becomes equal to 1 DU:

&#; 0   mm 0.01   mm ( 2.69 × 10 25   molecules &#; m &#; 3 ) d x = 2.69 × 10 25   molecules &#; m &#; 3 &#; 0.01   mm &#; 2.69 × 10 25   molecules &#; m &#; 3 &#; 0   mm {\displaystyle \int _{0~{\text{mm}}}^{0.01~{\text{mm}}}(2.69\times 10^{25}~{\text{molecules}}\cdot {\text{m}}^{-3})\,\mathrm {d} x=2.69\times 10^{25}~{\text{molecules}}\cdot {\text{m}}^{-3}\cdot 0.01~{\text{mm}}-2.69\times 10^{25}~{\text{molecules}}\cdot {\text{m}}^{-3}\cdot 0~{\text{mm}}}

= 2.69 × 10 25   molecules &#; m &#; 3 &#; 10 &#; 5   m = 2.69 × 10 20   molecules &#; m &#; 2 . {\displaystyle {}=2.69\times 10^{25}~{\text{molecules}}\cdot {\text{m}}^{-3}\cdot 10^{-5}~{\text{m}}=2.69\times 10^{20}~{\text{molecules}}\cdot {\text{m}}^{-2}.}

Thus the value of 1 DU is 2.69× molecules per meter squared.

References

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TL;DR &#; Ozone acts as a pollutant at the ground level, and accounts for a wide range of negative environmental and public health impacts. While the highest concentrations of ground-level ozone tends to be found in urban areas in warmer climates, there are large gaps across the United States (US) and the world where ozone is not measured, which limits our ability to understand ozone pollution trends and health effects. While lower-cost ozone sensor technologies exist, they can be subject to considerable error, and the preferred method of measuring ozone uses the fundamental principle of light absorption. Fortunately, ozone monitors have become more affordable and portable in recent years, opening up a wide range of potential applications for FEM-capable ozone instruments. 

If you want to learn more about ozone and the applications for ozone monitoring, register for our upcoming ASIC workshop on ozone monitoring: Reliable Ozone Measurement at Scale: Applications of the Clarity Ozone Module for Air Quality Research, Modeling, and Management.

Guest Author: Jessa Ellenburg
Director of Educational Outreach, 2B Technologies

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Ground-level ozone air pollution

Ozone, or O3, occurs both in the upper atmosphere and at the ground level. While it is a helpful substance in the upper atmosphere &#; forming a layer around the Earth to protect the planet&#;s surface from the sun&#;s ultraviolet rays &#; ozone acts as a pollutant at the ground level, causing negative effects to human and environmental health. Also known as ambient or tropospheric ozone, ground-level ozone is the primary component of the type of air pollution commonly referred to as smog. A good way to remember ozone&#;s role in the stratosphere vs. troposphere is &#;ozone is good up high and bad nearby&#;.

Ozone composition, formation, and sources

Ozone is a colorless gas that is composed of three atoms of oxygen.

Ozone, or O3, is composed of 3 oxygen atoms. While ozone in the upper atmosphere forms a protective layer around the Earth and protects from the sun&#;s UV rays, ozone at the ground level is an air pollutant.

Ground-level ozone forms just above the Earth&#;s surface, up to two miles above the ground, and is not emitted directly into the air but rather formed through photochemical reactions that occur when sunlight interacts with nitrous oxides (NOx) and volatile organic compounds (VOCs).

Volatile organic compounds may be emitted from sources such as motor vehicles, factories, chemical plants, and other sources. Nitrous oxides, such as those released from cars and power plants, react in the presence of sunlight. To learn more about nitrous oxide air pollutants and their sources, you can read Clarity&#;s recent post on nitrogen dioxide. 

Ozone that occurs in the upper layers of the atmosphere, known as the stratosphere, acts as a protective shield around the Earth. Lower down in the troposphere, ozone results from reactions between sunlight and air pollutants, like nitrous oxides and volatile organic compounds, and acts as an air pollutant.
(Image source: Climate Central)

Human health impacts of ground-level ozone exposure

Ozone is known to have a variety of negative health impacts, especially for those already affected by respiratory conditions like asthma. Respiratory effects, such as difficulty breathing, aggravation of lung disease, and more frequent asthma attacks can result from immediate exposure to ground-level ozone. Worsened allergies, premature death, and nervous system and reproductive harm are also traceable to ozone exposure.

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Studies have shown that in addition to increasing the risk for premature death, long-term ozone exposure is linked to lower birth weight, decreased lung function, and increased risk for asthma development in newborns.

Keeping in mind that ground-level ozone pollution occurs as a result of the presence of other harmful air pollutants, it&#;s important to note the compounding effects that exposure to these various pollutants can have on human health. There is no safe level of exposure to ozone pollution, as exhibited by a study showing that the risk of premature death remained even when ozone pollution was well below the national standard.

How ozone air pollution affects the environment

High ozone concentrations can affect sensitive ecosystems and vegetation. In plants, ground-level ozone exposure can reduce photosynthesis, slow the plant&#;s growth, and increase the plant&#;s sensitivity to potentially damaging factors, including disease, insect damage, weather damage, and even the negative effects of other air pollutants. 

Given its impacts on vegetation, ground-level ozone pollution accounts for significant damage to crop yields. One study found between 2% and 14% decreases in yields for maize, wheat, and soybean globally attributable to ground-level ozone pollution. In , damage to 23 crops in Europe created a loss of 6.7 billion Euros. In the same year, global crop production losses were the equivalent of $11 billion to $18 billion (USD) lost. Because of increasing trends in global ozone levels &#; and the fact that climate change will likely increase ozone formation in certain areas &#; the study predicts that global losses in will reach up to $35 billion.

Ozone exposure can even have a visible effect on plants, where some plants&#; leaves show marks under certain conditions of high ozone concentration. The National Center for Atmospheric Research has leveraged these bioindicator species to create a number of &#;ozone gardens&#; that help visitors visualize the impacts of ozone pollution and determine what ozone concentrations cause damage to plants. 

Where ground-level ozone pollution occurs

Ground-level ozone tends to be found in higher concentrations in urban areas, particularly on sunny days. High ozone concentrations are common where there is hot, stagnant air &#; studies have found that California cities such as Palm Springs, Los Angeles, and Bakersfield exhibit unusually high numbers of days with unhealthy ozone levels. Using EPA data, this study found that Palm Springs reported 450 unhealthy ozone days over a four-year period &#; nearly a third of the days covered by the study.

While ground-level pollution is typically the worst in urban areas, rural areas can still experience high levels of ozone pollution, especially if wind brings ozone or its precursors from industrial operations or urban areas. Wind can play an influential role in how air pollution is distributed &#; for more on this topic you can refer to Clarity&#;s blog detailing wind&#;s effects on air pollution here.

While the majority of research on ground-level ozone has been conducted in urban environments, certain studies have also found high levels of ozone pollution in environments not typically considered high-risk. A wintertime ozone phenomenon was discovered in Wyoming, for example, with high concentrations of ground-level ozone being formed by a specific set of weather and geographical conditions coupled with oil and gas drilling activity.  To assist K-12 students observe this wintertime ozone phenomenon, the GO3 Project and 2B Technologies installed ozone monitors at 112 schools around the world, including four schools in Wyoming. The students were able to observe ozone concentrations in their rural Wyoming communities on winter days that were higher than those observed in Los Angeles.

It&#;s important to recognize that ozone pollution can occur in a variety of conditions, and that many factors that can contribute to a system of high pollution. There are likely many ozone &#;hotspots&#; around the world that we are not yet aware of due to the extremely limited number of ozone measurement points currently available to researchers and policymakers.

How ground-level ozone is regulated

Due to the many health impacts associated with ozone exposure, it is regulated by many countries around the world.

Globally, the World Health Organization (WHO) sets recommended levels of maximum ground-level ozone concentration. The WHO&#;s report states that they recommend a maximum ozone level of 100 µg/m3 based on 8-hour averaging times, or the equivalent of roughly 47 parts per billion (ppb)*.

* To compare observations or metrics reported in units of ppb or µg/m3 we use the 2B Technologies ozone units conversion calculator (1 ppb is equal to approximately 2 µg/m3, depending on temperature and pressure.)

In the United States, ozone is one of the six criteria air pollutants identified by the Clean Air Act and regulated under the National Ambient Air Quality Standards (NAAQS) with an upper limit of 70 ppb, as the fourth-highest daily maximum eight-hour concentration, averaged across three consecutive years.  This standard sits at a significant level above the benchmark level recommended by the WHO. Other regulatory and legislative actions like vehicle and transportation standards and regional haze and visibility rules are also in place to help decrease ground-level ozone pollution and its negative effects.

Reductions of nitrous oxides and volatile organic compounds seen in the United States between and have led to a consequent 32% reduction in ozone at 200 monitoring stations across the United States, as demonstrated by this study. Despite these general improvements, certain hotspots of ozone pollution exist in cities such as Los Angeles, California and Atlanta, Georgia. The same study reports that just over 100 million people nationwide &#; or about one in three people &#; still live in counties where ozone pollution exceeds the NAAQS standards.

In the European Union, ground-level ozone is also regulated as a major pollutant. The European Commission set ground-level ozone levels at 120 ug/m3 (equivalent to roughly 56 ppb) over a maximum daily 8-hour mean period. In comparison to the World Health Organization, both the US and European standards exceed both and recommended levels. 

The regional and global prevalence of ground-level ozone pollution

Globally, ozone is still poorly measured in many countries around the world &#; and even where it is measured, ozone pollution often exceeds the recommended levels set forth by the World Health Organization.

The number of ozone measurement points available globally is quite limited compared to other criteria pollutants. One study found that the TOAR-Surface Ozone Database recognizes ozone metrics from only 4,800 ozone monitoring sites worldwide for the 5-year period of to , while global air quality databases like OpenAQ and the World Air Quality Index list nearly 30,000 measurement points available for particulate matter in . Further, as illustrated by the map below, there are large gaps in the measurement and characterization of ground-level ozone concentrations across Africa, the Middle East, South and Southeast Asia, and South America. 

This map demonstrates a significant skew toward the Northern hemisphere when it comes to ozone monitoring locations &#; of the 4,800 measurement points included in this study, 1,470 (30.6%) are in North America, 1,935 (40.3%) are in Europe, 1,239 (25.8%) are in South, Southeast, and East Asia, and only 176 (3.7%) are located in other regions of the world.
(Image source: Fleming et al., )

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Globally, ground-level ozone pollution exposure has increased significantly in some countries, while others have experienced a decline when looking at population-weighted average seasonal 8-hour maximum ozone concentrations. Over the course of to , countries such as Ethiopia and Brazil experienced more than a 20% increase in their level of exposure. These exposure numbers were estimated by the Global Burden of Disease Project by the Institute for Health Metrics and Evaluation, which incorporates a variety of factors affecting pollution exposure at a small scale.
(Image source: State of Global Air)

In the United States, although ground-level ozone is regulated as a criteria pollutant, there are still many locations classified as nonattainment areas that have excessive ozone concentrations. For example, in Los Angeles &#; a city known for its frequent smog &#; at least 80 unhealthy ozone days were noted every year from to , with this number sometimes rising to more than 100 days for the year. 

In , 24 European countries, 19 of which were European Union Member States, exceeded the ground-level ozone standard of 120 µg/m3, and all European countries exceeded the World Health Organization benchmark of 100 µg/m3 for an 8-hour mean.

Concentrations of O3 in in relation to the EU target value

As of , 29% of the reporting stations in the European Union exceeded the EU ground-level ozone pollution maximum level of 120 µg/m3, while 97% of reporting stations reported levels that exceed the World Health Organization&#;s recommended maximum level of 100 µg/m3. The highest levels of ground-level ozone tend to be found in southern Europe in the spring and summer due to meteorological conditions, such as high levels of sunlight, which favor increased ozone production.
(Image source: European Environmental Agency)

According to the European Environment Agency Technical Report entitled &#;Assessment of ground-level ozone in EEA member countries, with a focus on long-term trends&#;, the European Union has experienced a steady decline in precursor pollutants to ozone, particularly nitrous oxides and volatile organic compounds, especially those coming from anthropogenic sources &#; yet the vast majority of European countries are still not meeting EU standards for ozone. 

Air quality monitoring in Africa is currently sparse, with only seven of 54 countries having real-time air pollution monitors. Ground-level ozone monitoring is especially absent, with research reporting that there are only a few active and passive monitoring sites present in the southern Africa region. Despite growing concern about the health impacts of ground-level ozone, the pollutant is often left out of air quality monitoring in low- and middle-income countries across Africa.

In Asia, many countries continue to deal with the impacts of high air pollution. China and India, in particular, have seen ozone concentrations in major cities increase in recent years. With the combined presence of other air pollutants at high levels &#; fine particulate matter being a major one &#; this region continues to see the negative health and environmental impacts of high pollution environments. 

How to measure ground-level ozone

Ground-level ozone pollution is typically expressed in parts per billion (ppb) and micrograms per cubic meter (μg/m3).

An ozone analyzer is one tool that is commonly used to measure real-time ozone concentration. This technology exposes air to ultraviolet light, and a detector measures the intensity of light that passes through the air. Because ozone blocks light at wavelengths of approximately 254 nanometers &#; the wavelength of ultraviolet light &#; the instrument can measure the intensity of light that passes through the air to determine the concentration of ozone present.

View the video below from 2B Technologies to learn more about how their reference-grade ozone monitor functions.

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The technology described above is an ozone monitor; however, ozone sensors also exist, such as electrochemical sensors and heated metal oxide semiconductor (HMOS) sensors. While these sensor technologies exist, they can be subject to considerable error. 

Sensors can make it more difficult to arrive at an accurate measure of a pollutant&#;s concentration for a variety of reasons. In cases where electrochemically similar compounds exist, the sensor can respond to a variety of chemical substances commonly found in air, rather than solely the one it is intended to measure. Ozone sensors can also be affected by temperature, relative humidity, and pressure and can experience a drift in their sensitivity and their baseline measurements over time. Ozone monitors, on the other hand, use the fundamental principle of light absorption to measure ozone, which is well-established and has been used for decades.  Additionally, ozone monitors are subject to relatively few interfering compounds, compounds that are generally not present in high concentrations in outdoor air.  

The multitude of use cases for measuring ozone pollution

Ground-level ozone is measured by a variety of organizations. In addition to regulatory bodies such as the US EPA (Environmental Protection Agency) and EU EEA (European Environment Agency) which collect measurements for compliance purposes, other organizations across a range of industries also monitor ground-level ozone air pollution. For example, the United States National Park Service measures ozone pollution in approximately 100 national parks because this pollutant affects environmental health and causes haze, impacting citizens&#; ability to enjoy the natural beauty of these parks.

Another reason for measuring ground-level ozone is to determine the efficacy of policies that look to reduce vehicle-related pollution, such as changing traffic patterns and reducing the number of cars on the road. The reduction in smog and increase in visibility, which reflect ozone pollution levels, can be used as further evidence to support these efforts.

Ozone monitors are also useful in building ozone monitoring capacity where none exists. In many developing countries, ozone monitoring equipment is sparse, preventing the opportunity to provide a complete picture of air pollution in the area. Even if the equipment is not operated precisely in accordance with the strict operating procedures required for regulatory data, which can be obtained with equipment that is less expensive than federal reference-grade equipment, non-regulatory data from reference-grade instruments can provide an indication of ozone pollution and drive action to reduce harmful exposure.

Parallel monitoring allows air quality managers to confirm their attainment or nonattainment of regulatory standards and determine the need for investment in additional air quality monitoring equipment. By monitoring ambient air quality at an existing monitoring site and a potential replacement site, air quality districts can gather sufficient monitoring data at both monitoring sites to determine whether a new reference monitoring site is needed or if an existing site should be relocated.  

Ground-level ozone pollution measurements are also useful in improving the accuracy of atmospheric models. While NASA works to model ground-level ozone levels by satellite, ozone is not directly measurable from space because of the significant concentrations of ozone higher in the atmosphere that obscures measurements of tropospheric ozone. Enhanced spatial coverage of ground-level ozone measurements would help to offset the difficulties that come with measuring ground-level ozone by satellite and improve the accuracy of these atmospheric models.  Additionally, many agencies publish ozone forecasts to warn residents of upcoming bad ozone days. These forecasts are generated by model predictions based on weather patterns and measurements at ozone monitoring sites. With expanded ozone monitoring at more sites in different locations, these forecasts can become more accurate and serve a larger number of people.

The GO3 Project (directed by the team here at 2B Technologies) used modern, portable ozone analyzers to both educate and engage members of the community on their local air quality and to raise awareness of the harmful effects of ozone pollution. A study covering the project &#; which installed ozone and weather monitoring stations at more than 100 schools around the world &#;revealed that about 43% of the middle and high school students surveyed did not have any previous knowledge of ground-level ozone, but by the end of the project, 98% of students reported they were familiar with the pollutant. Raising awareness about the impacts of ozone and other air pollutants is an important step in working towards positive environmental change.

As a criteria pollutant in the United States, and one that is regulated by many organizations worldwide, monitoring ozone is essential to ensuring compliance with air quality standards. Ground-level ozone monitoring is highly important in determining ground-level ozone trends and, thus, implementing action to reduce its harmful effects and informing emission reduction policies. 

Include ozone measurements in your Clarity network

Our partners at Clarity recently introduced their Ozone Module, based on best-in-class technology from 2B Technologies. The Ozone Module provides accurate measurements of ozone in the air over a wide dynamic range extending from a few parts per billion by volume (ppb) to an upper limit of 100 parts per million (ppm).

The ozone monitoring technology used by the Clarity Ozone Module is based on the well-established technique of absorption of ultraviolet light at 254 nm and has been approved by the U.S. Environmental Protection Agency as a modification of the Federal Equivalent Method (FEM): EQOA--218. You can visit the Ozone Module product page on Clarity&#;s website to download a specification sheet or request a quote. 

If you want to learn more about ozone and the applications for ozone monitoring, register for our upcoming ASIC workshop on ozone monitoring: Reliable Ozone Measurement at Scale: Applications of the Clarity Ozone Module for Air Quality Research, Modeling, and Management.