Other instruments measure ozone using its absorption of infrared or visible radiation or its emission of microwave or infrared radiation at different altitudes in the atmosphere, thereby obtaining information on the vertical distribution of ozone. Emission measurements have the advantage of providing remote ozone measurements at night, which is particularly valuable for sampling polar regions during winter, when there is continuous darkness.
The first instrument for routinely monitoring total ozone was developed by Gordon M. Dobson in the United Kingdom in the s. A small network of instruments distributed around Europe allowed Dobson to make important discoveries about how total ozone varies with location and time.
In the s a new instrument was developed by Dobson, now called a Dobson spectrophotometer, which precisely measures the intensity of sunlight at two UV wavelengths: one that is strongly absorbed by ozone and one that is weakly absorbed. The difference in light intensity at the two wavelengths provides a measure of total ozone above the instrument location.
The Brewer spectrophotometer was introduced into the global network starting in Whereas the original Dobson instrument measured atmospheric ozone based on observations of UV light at only two wavelengths, the more advanced Brewer instruments rely on observations at multiple wavelengths. The accuracy of these observations is maintained by regular instrument calibrations and intercomparisons. At many of the stations, observations of total ozone are augmented by measurements of the vertical distribution of ozone obtained either by routine launches of ozonesondes or the deployment of lidar instruments.
Numerous stations also quantify the atmospheric abundances of a wide variety of related compounds, taking advantage of the unique optical properties of atmospheric gases. Data from the network have been essential for understanding the effects of chlorofluorocarbons and other ozone-depleting substances on the global ozone layer, starting before the launch of space-based ozone-measuring instruments and continuing to the present day.
Ground-based instruments with excellent long-term stability and accuracy are now routinely used to help calibrate space-based observations of total ozone. Most of these gases accumulate in the lower atmosphere because they are relatively unreactive and do not dissolve readily in rain or snow.
Natural air motions transport these accumulated gases to the stratosphere, where they are converted to more reactive gases. Some of these gases then participate in reactions that destroy ozone. Emission, accumulation, and transport. The principal steps in stratospheric ozone depletion caused by human activities are shown in Figure Q The halogen source gases, often referred to as ozone-depleting substances ODSs , include manufactured chemicals released to the atmosphere in a variety of applications, such as refrigeration, air conditioning, and foam blowing.
Chlorofluorocarbons CFCs are an important example of a chlorine-containing source gas. Emitted source gases accumulate in the lower atmosphere troposphere and are transported to the stratosphere by natural air motions. The accumulation occurs because most source gases are highly unreactive in the lower atmosphere. Small amounts of these gases dissolve in ocean waters. The low reactivity of these manufactured halogenated gases is one property that made them well suited for specialized applications such as refrigeration.
Some halogen gases are emitted in substantial quantities from natural sources see Q6. These emissions also accumulate in the troposphere, are transported to the stratosphere, and participate in ozone destruction reactions.
These naturally emitted gases are part of the natural balance of ozone production and destruction that predates the large release of manufactured halogenated gases. Conversion, reaction, and removal. Halogen source gases do not react directly with ozone. Once in the stratosphere, halogen source gases are chemically converted to reactive halogen gases by ultraviolet radiation from the Sun see Q7.
The rate of conversion is related to the atmospheric lifetime of a gas see Q6. Gases with longer lifetimes have slower conversion rates and survive longer in the atmosphere after emission. Emitted gas molecules with atmospheric lifetimes greater than a few years circulate between the troposphere and stratosphere multiple times, on average, before conversion occurs. The reactive gases formed from halogen source gases react chemically to destroy ozone in the stratosphere see Q8.
The average depletion of total ozone attributed to reactive gases is smallest in the tropics and largest at high latitudes see Q In polar regions, surface reactions that occur at low temperatures on polar stratospheric clouds greatly increase the abundance of the most reactive chlorine gas, chlorine monoxide ClO see Q9. After a few years, air in the stratosphere returns to the troposphere, bringing along reactive halogen gases.
This removal brings to an end the destruction of ozone by chlorine and bromine atoms that were first released to the atmosphere as components of halogen source gas molecules. Tropospheric conversion. Halogen source gases with short lifetimes less than 1 year undergo significant chemical conversion in the troposphere, producing reactive halogen gases and other compounds. Source gas molecules that are not converted are transported to the stratosphere.
Only small portions of reactive halogen gases produced in the troposphere are transported to the stratosphere because most are removed by precipitation. Important examples of halogen gases that undergo some tropospheric removal are the hydrochlorofluorocarbons HCFCs , methyl bromide CH 3 Br , methyl chloride CH 3 Cl , and gases containing iodine see Q6. Principal steps in stratospheric ozone depletion.
The stratospheric ozone depletion process begins with the emission of halogen source gases by human activities and natural processes. These compounds have at least one carbon and one halogen atom, causing them to be chemically stable and leading to common use of the term halocarbon, an abbreviation for halogen and carbon.
Many halocarbon gases emitted by human activities are also called ozone-depleting substances ODSs ; all ODSs contain at least one chlorine or bromine atom see Q7. These compounds undergo little or no chemical loss within the troposphere, the lowest region of the atmosphere, and accumulate until transported to the stratosphere.
Subsequent steps are conversion of ODSs to reactive halogen gases see Q8 , chemical reactions that remove ozone see Q8 , and eventual removal of the reactive halogen gases.
Ozone depletion by halogen source gases occurs globally see Q Large seasonal ozone losses occur in the polar regions as a result of reactions involving polar stratospheric clouds see Q7 and Q9. Our understanding of stratospheric ozone depletion has been obtained through a combination of laboratory studies, computer models, and atmospheric observations.
The wide variety of chemical reactions that occur in the stratosphere have been discovered and investigated in laboratory studies. Chemical reactions between two gases follow well-defined physical rules.
Some of these reactions occur on the surfaces of polar stratospheric clouds formed in the winter stratosphere. Reactions have been studied that involve many different molecules containing chlorine, bromine, fluorine, and iodine and other atmospheric constituents such as carbon, oxygen, nitrogen, and hydrogen.
These studies have shown that several reactions involving chlorine and bromine directly or indirectly destroy ozone in the stratosphere. Computer models have been used to examine the combined effect of the large group of known reactions that occur in the stratosphere. These models simulate the stratosphere by including representative chemical abundances, winds, air temperatures, and the daily and seasonal changes in sunlight.
These analyses show that under certain conditions chlorine and bromine react in catalytic cycles in which one chlorine or bromine atom destroys many thousands of ozone molecules. Models are also used to simulate ozone amounts observed in previous years as a strong test of our understanding of atmospheric processes and to evaluate the importance of new reactions found in laboratory studies.
The response of ozone to possible future changes in the abundances of trace gases, temperatures, and other atmospheric parameters have been extensively explored with specialized computer models see Q Atmospheric observations have shown what gases are present in different regions of the stratosphere and how their abundances vary with respect to time and location.
Gas and particle abundances have been monitored over time periods spanning a daily cycle to decades. Observations show that halogen source gases and reactive halogen gases are present in the stratosphere at the amounts required to cause observed ozone depletion see Q7.
Ozone and chlorine monoxide ClO , for example, have been observed extensively with a variety of instruments. ClO is a highly reactive gas that is involved in catalytic ozone destruction cycles throughout the stratosphere see Q8.
Instruments on the ground and on satellites, balloons, and aircraft now routinely measure the abundance of ozone and ClO remotely using optical and microwave signals. High-altitude aircraft and balloon instruments are also used to measure both gases locally in the stratosphere see Q4.
Observations of ozone and reactive gases made in past decades are used extensively in comparisons with computer models to increase confidence in our understanding of stratospheric ozone depletion. Certain industrial processes and consumer products result in the emission of ozone-depleting substances ODSs to the atmosphere. ODSs are manufactured halogen source gases that are controlled worldwide by the Montreal Protocol.
These gases bring chlorine and bromine atoms to the stratosphere, where they destroy ozone in chemical reactions. Important examples are the chlorofluorocarbons CFCs , once used in almost all refrigeration and air conditioning systems, and the halons, which were used as fire extinguishing agents.
Current ODS abundances in the atmosphere are known directly from air sample measurements. Halogen source gases versus ozone-depleting substances ODSs. Those halogen source gases emitted by human activities and controlled by the Montreal Protocol are referred to as ODSs within the Montreal Protocol, by the media, and in the scientific literature. Halogen source gases such as methyl chloride CH 3 Cl that have predominantly natural sources are not classified as ODSs. The contributions of ODSs and natural halogen source gases to the total amount of chlorine and bromine entering the stratosphere, which peaked in and , respectively, are shown in Figure Q The difference in the timing of the peaks is a result of different phaseout schedules specified by the Montreal Protocol, atmospheric lifetimes, and the time delays between production and emissions of the various source gases.
Ozone-depleting substances ODSs. ODSs are manufactured for specific industrial uses or consumer products, most of which result in the eventual emission of these gases to the atmosphere. Total ODS emissions increased substantially from the middle to the late 20th century, reached a peak in the late s, and are now in decline see Figure Q A large fraction of the emitted ODSs reach the stratosphere, where they are converted to reactive gases containing chlorine and bromine that lead to ozone depletion.
ODSs containing only carbon, chlorine, and fluorine are called chlorofluorocarbons, usually abbreviated as CFCs. CFCs, along with carbon tetrachloride CCl 4 and methyl chloroform CH 3 CCl 3 , historically have been the most important chlorine-containing halogen source gases emitted by human activities.
These and other chlorine-containing ODSs have been used in many applications, including refrigeration, air conditioning, foam blowing, spray can propellants, and cleaning of metals and electronic components.
As a result of the Montreal Protocol controls, the abundances of most of these chlorine source gases have decreased since see Figure Q With restrictions on global production in place since , the atmospheric abundances of HCFCs are expected to peak between and Another category of ODSs contains bromine.
The most important of these gases are the halons and methyl bromide CH 3 Br. Halons are a group of industrial compounds that contain at least one bromine and one carbon atom; halons may or may not contain a chlorine atom. Halons were originally developed to extinguish fires and were widely used to protect large computer installations, military hardware, and commercial aircraft engines. As a consequence, upon use halons are released directly into the atmosphere.
Halon and halon are the most abundant halons emitted by human activities. Methyl bromide is used primarily as a fumigant for pest control in agriculture and disinfection of export shipping goods, and also has significant natural sources.
Halon reached peak concentration in and has been decreasing ever since, reaching an abundance in that was 8. Changes in halogen source gases entering the stratosphere. A variety of halogen source gases emitted by human activities and natural processes transport chlorine and bromine into the stratosphere. Ozone-depleting substances ODSs are the subset of these gases emitted by human activities that are controlled by the Montreal Protocol.
These partitioned columns show the abundances of chlorine- and bromine-containing gases entering the stratosphere in and , when their total amounts peaked, respectively, and in The overall reductions in the total amounts of chlorine and bromine entering the stratosphere and the changes observed for each source gas are also indicated.
The amounts are derived from tropospheric observations of each gas. Note the large difference in the vertical scales: total chlorine entering the stratosphere is about times more abundant than total bromine. Both, however, are important because bromine is about 60 times more effective on a per-atom basis than chlorine at destroying ozone. Human activities are the largest source of chlorine reaching the stratosphere and CFCs are the most abundant chlorine-containing gases.
Methyl chloride is the primary natural source of chlorine. The largest decreases between and are seen in methyl chloroform, carbon tetrachloride, and CFC The HCFCs, which are substitute gases for CFCs and also controlled under the Montreal Protocol, have risen substantially since and are now approaching expected peak atmospheric abundances see Figure Q The abundance of chlorine-containing very short-lived gases entering the stratosphere has risen substantially since ; these compounds originate primarily from human activity, undergo chemical loss within the troposphere, and are not controlled by the Montreal Protocol.
For bromine entering the stratosphere, halons and methyl bromide are the largest contributors. The largest decrease between and is seen in the abundance of methyl bromide attributed to human activities, because of the success of the Montreal Protocol.
Only halon shows an increasing abundance relative to Methyl bromide also has a natural source, which is now substantially greater than the human source. Natural sources make a much larger fractional contribution to bromine entering the stratosphere than occurs for chlorine, and they are thought to have remained fairly constant in the recent past.
Natural sources of chlorine and bromine. There are a few halogen source gases present in the stratosphere that have large natural sources.
These include methyl chloride CH 3 Cl and methyl bromide CH 3 Br , both of which are emitted by oceanic and terrestrial ecosystems. In addition, very short-lived source gases containing bromine such as bromoform CHBr3 and dibromomethane CH 2 Br 2 are also released to the atmosphere, primarily from biological activity in the oceans. Only a fraction of the emissions of very short-lived source gases reaches the stratosphere because these gases are efficiently removed in the lower atmosphere.
Volcanoes provide an episodic source of reactive halogen gases that sometimes reach the stratosphere in appreciable quantities. Other natural sources of halogens include reactive chlorine and bromine produced by evaporation of ocean spray. These reactive chemicals readily dissolve in water and are removed in the troposphere. The amount of chlorine and bromine entering the stratosphere from natural sources is fairly constant over time and, therefore, cannot be the cause of the ozone depletion observed since the s.
Other human activities that are sources of chlorine and bromine gases. Other chlorine- and bromine-containing gases are released to the atmosphere from human activities. Common examples are the use of chlorine-containing solvents and industrial chemicals, and the use of chlorine gases in paper production and disinfection of potable and industrial water supplies including swimming pools.
Most of these gases are very short-lived and only a small fraction of their emissions reaches the stratosphere. The Montreal Protocol does not control the production and consumption of very short-lived chlorine source gases, although the atmospheric abundances of some notably dichloromethane, CH 2 Cl 2 have increased substantially in recent years.
Solid rocket engines, such as those used to propel payloads into orbit, release reactive chlorine gases directly into the troposphere and stratosphere. The quantities of chlorine emitted globally by rockets is currently small in comparison with halogen emissions from other human activities.
Lifetimes and emissions. Estimates of global emissions in for a selected set of halogen source gases are given in Table Q Emission from banks refers to the atmospheric release of halocarbons from existing equipment, chemical stockpiles, foams, and other products. In the global emission of the refrigerant HCFC CHF 2 Cl constituted the largest annual release, by mass, of a halocarbon from human activities.
The emission of methyl chloride CH 3 Cl is primarily from natural sources such as the ocean biosphere, terrestrial plants, salt marshes and fungi. The human source of methyl chloride is small relative to the total natural source see Q After emission, halogen source gases are either naturally removed from the atmosphere or undergo chemical conversion in the troposphere or stratosphere.
Lifetimes vary from less than 1 year to years for the principal chlorine- and bromine-containing gases see Table Q The long-lived gases are converted to other gases primarily in the stratosphere and essentially all of their original halogen content becomes available to participate in the destruction of stratospheric ozone.
Gases with short lifetimes such as HCFCs, methyl bromide, and methyl chloride are effectively converted to other gases in the troposphere, which are then removed by rain and snow. Therefore, only a fraction of their halogen content potentially contributes to ozone depletion in the stratosphere.
The amount of an emitted gas that is present in the atmosphere represents a balance between its emission and removal rates. A wide range of current emission rates and atmospheric lifetimes are derived for the various source gases see Table Q The atmospheric abundances of most of the principal CFCs and halons have decreased since in response to smaller emission rates, while those of the leading substitute gases, the HCFCs, continue to increase under the provisions of the Montreal Protocol see Q In the past few years, the rate of the increase of the atmospheric abundance of HCFCs has slowed down.
In the coming decades, the emissions and atmospheric abundances of all controlled gases are expected to decrease under these provisions. Emissions of halogen source gases are compared in their effectiveness to destroy stratospheric ozone based upon their ODPs, as listed in Table Q see Q The calculations, which require the use of computer models that simulate the atmosphere, use as the basis of comparison the ozone depletion from an equal mass of each gas emitted to the atmosphere.
Halon and halon have ODPs significantly larger than that of CFC and most other chlorinated gases because bromine is much more effective about 60 times on a per-atom basis than chlorine in chemical reactions that destroy ozone. The gases with smaller values of ODP generally have shorter atmospheric lifetimes or contain fewer chlorine and bromine atoms. Fluorine and iodine.
Fluorine and iodine are also halogens. Many of the source gases in Figure Q also contain fluorine in addition to chlorine or bromine.
After the source gases undergo conversion in the stratosphere see Q5 , the fluorine content of these gases is left in chemical forms that do not cause ozone depletion. As a consequence, halogen source gases that contain fluorine and no other halogens are not classified as ODSs.
Iodine is a component of several gases that are naturally emitted from the oceans and some human activities. Although iodine can participate in ozone destruction reactions, iodine-containing source gases all have very short lifetimes. The importance for stratospheric ozone of very short-lived iodine containing source gases is an area of active research.
Other non-halogen gases. Other non-halogen gases that influence stratospheric ozone abundances have also increased in the stratosphere as a result of emissions from human activities see Q Important examples are methane CH 4 and nitrous oxide N 2 O , which react in the stratosphere to form water vapor and reactive hydrogen, and nitrogen oxides, respectively.
These reactive products participate in the destruction of stratospheric ozone see Q1. Increased levels of atmospheric carbon dioxide CO 2 alter stratospheric temperature and winds, which also affect the abundance of stratospheric ozone. Should future atmospheric abundances of CO 2 , CH 4 and N 2 O increase significantly relative to present day values, these increases will affect future levels of stratospheric ozone through combined effects on temperature, winds, and chemistry see Figure Q Efforts are underway to reduce the emissions of these gases under the Paris Agreement of the United Nations Framework Convention on Climate Change because they cause surface warming see Q18 and Q Although past emissions of ODSs still dominate global ozone depletion today, future emissions of N 2 O from human activities are expected to become relatively more important for ozone depletion as future abundances of ODSs decline see Q Table Q The chlorine- and bromine-containing gases that enter the stratosphere arise from both human activities and natural processes.
When exposed to ultraviolet radiation from the Sun, these halogen source gases are converted to more reactive gases that also contain chlorine and bromine. Some reactive gases act as chemical reservoirs which can then be converted into the most reactive gases, namely ClO and BrO. These most reactive gases participate in catalytic reactions that efficiently destroy ozone. Halogen-containing gases present in the stratosphere can be divided into two groups: halogen source gases and reactive halogen gases see Figure Q Once in the stratosphere, the halogen source gases chemically convert at different rates to form the reactive halogen gases.
The conversion occurs in the stratosphere instead of the troposphere for most gases because solar ultraviolet radiation a component of sunlight is more intense in the stratosphere see Q2. Reactive gases containing the halogens chlorine and bromine lead to the chemical destruction of stratospheric ozone.
Reactive halogen gases. The chemical conversion of halogen source gases, which involves solar ultraviolet radiation and other chemical reactions, produces a number of reactive halogen gases. These reactive gases contain all of the chlorine and bromine atoms originally present in the source gases.
The most important reactive chlorine- and bromine-containing gases that form in the stratosphere are shown in Figure Q These two gases are considered important reservoir gases because, while they do not react directly with ozone, they can be converted to the most reactive forms that do chemically destroy ozone. The most reactive forms are chlorine monoxide ClO and bromine monoxide BrO , and chlorine and bromine atoms Cl and Br. A large fraction of total reactive bromine is generally in the form of BrO, whereas usually only a small fraction of total reactive chlorine is in the form of ClO.
Conversion of halogen source gases. Halogen source gases containing chlorine and bromine are chemically converted to reactive halogen gases, primarily in the stratosphere. Most of the halogen source gases are ozone-depleting substances.
The conversion requires solar ultraviolet radiation and a few chemical reactions. The shorter-lived gases undergo partial conversion in the troposphere.
The reactive halogen gases contain all the chlorine and bromine originally present in the source gases before conversion. The reactive gases can be grouped into the reservoir gases, which do not directly destroy ozone, and the most reactive gases, which participate in ozone destruction cycles see Q8. Reactive chlorine at midlatitudes. Reactive chlorine gases have been observed extensively in the stratosphere using both local and remote measurement techniques.
Schools may be advised to cancel outdoor sports activities. To further reduce outdoor exposure, individuals may consider wearing a face mask that can effectively scavenge ozone. Face masks rated N95 or higher can filter out PM 2. However, few models of face masks are designed to remove ozone. Making ozone forecasting available to the general public will enhance the effectiveness of such personal protection methods to reduce ozone exposure A wealth of data from animal studies and human studies are available in the literature to help understand pathophysiologic mechanisms by which ozone affects the lung.
Relatively less is known to understand how ozone affects the cardiovascular health outcomes, although the immune-inflammatory responses initiated in the lung are thought to be the key in more downstream systemic effects. The mechanistic understanding appears to be sufficient to support the use of antioxidants or ozone scavenger to alleviate the ozone effects. For example, rodent studies confirmed that the use of N-acetylcysteine and sulfide salt can help prevent or recover the lung impairment caused by ozone 82 , Limited studies in humans have shown promising results.
A randomized trial found that a daily supplement of vitamins C and E might provide some protection against acute nasal inflammatory response to ozone in asthmatic children In a control human exposure study 2-h exposure to ppb ozone vs. Cohort and population-based interventional trials should be conducted in real-world settings to develop more targeted preventive or therapeutic strategies especially in vulnerable populations and individuals.
This should be part of the overall strategy, along with air pollution control polices, to combat ozone pollution, a lasting worldwide health hazard. JZ and YW: conception. All authors gave final approval for publishing. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Ambient air pollution exposure and full-term birth weight in California. Environ Health. This November your donation goes even further to improve lung health and defeat lung cancer. Double Your Gift. Your tax-deductible donation funds lung disease and lung cancer research, new treatments, lung health education, and more. Join over , people who receive the latest news about lung health, including COVID, research, air quality, inspiring stories and resources. Thank you! You will now receive email updates from the American Lung Association.
Select your location to view local American Lung Association events and news near you. Our service is free and we are here to help you. Ozone Ozone also called smog is one of the most dangerous and widespread pollutants in the U. Section Menu. Ozone is one of the six common air pollutants identified in the Clean Air Act. These standards apply to the concentration of a pollutant in outdoor air. After working with the states and tribes and considering the information from air quality monitors, EPA "designates" an area as attainment or nonattainment with national ambient air quality standards.
If the air quality in a geographic area meets or does better than the national standard, it is called an attainment area; areas that don't meet the national standard are called nonattainment areas. Learn more about ozone air quality designations. In order to improve air quality, states must draft a plan known as a state implementation plan SIP to improve the air quality in nonattainment areas. The plan outlines the measures that the state will take in order to improve air quality.
Once a nonattainment area meets the standards, EPA will designate the area as a "maintenance area. Actions include vehicle and transportation standards, regional haze and visibility rules, and regular reviews of the NAAQS. Learn more about ozone standards.
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