Ozone

Monitoring data

Administrative data

Endpoint:

monitoring data

Type of information:

experimental study

Adequacy of study:

other information

Study period:

Information on air ozone levels from 19th century until 2003

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Peer reviewed publication by Environment Canada (governmental department).

Data source

Reference

Reference Type:

review article or handbook

Title:

A review of surface ozone background levels and trends

Author:

Vingarzan R

Year:

2004

Bibliographic source:

Atmospheric Environment 38: 3431–3442

Materials and methods

Test guideline

Qualifier:

no guideline required

Principles of method if other than guideline:

A survey of the literature was conducted to review historical and current surface ozone data from background stations in Canada, United States and around the world for the purpose of characterizing background levels and trends, present plausible explanations for observed trends and explore projections of future ozone levels.

GLP compliance:

no

Type of measurement:

background concentration

Media:

air

Test material

Test material information

Constituent 1

Reference substance name:

Ozone

EC Number:

233-069-2

EC Name:

Ozone

Cas Number:

10028-15-6

Molecular formula:

O3

IUPAC Name:

trioxygen

Test material form:

gas

Specific details on test material used for the study:

- Test material: Background ozone

Study design

Details on sampling:

Not applicable, this survey only mentions the results.
For further details please refer to the section "Any other information on materials and methods incl. tables".

Results and discussion

Concentration

Key result

Country:

other: worldwide

Substance or metabolite:

substance

Remarks:

ozone

Conc.:

ca. 0.02 - ca. 0.045 ppm

Remarks on result:

other: Annual average background ozone concentrations

Any other information on results incl. tables

Seasonal cycles

- The background level of ozone is not static, but has been shown to exhibit pronounced seasonal cycles that have different shapes at different latitudes and altitudes. These cycles are controlled by a number of processes including photochemistry, deposition, and transport, acting at local, regional and global scales.

- Surface ozone measurements from the late 19th and early 20th centuries show a spring to early summer ozone maximum.

- This spring maximum is well documented in current surface observations from background sites in the Northern Hemisphere. However, at some background sites, a summer maximum is more evident, due to the influence of local photochemical ozone production from precursor emissions.

- The spring maximum is reported to peak in May in the latitudinal range 10–60, and increase in concentration in a south to north direction. North of 60 latitude it declines in concentration and shifts to later months.

- Longitudinal gradients in the seasonal cycle have also been reported, with concentrations increasing in a west to east direction in continental United States and in a northwest to southeast direction in Europe, where the peak also shifts to late summer.

- In a recent review of the available literature, it was concluded that the spring maximum is a Northern Hemispheric phenomenon, and that the magnitude of the spring ozone maximum at ground level seems to have increased over the last century.

- It appears that photochemical production resulting from increased solar radiation acting upon a pool of accumulated NOx and hydrocarbons built up during the winter period is the major cause of the spring time increase in ozone.

- A factor more recently linked to the annual ozone cycle is the intercontinental transport of pollution. The same atmospheric mechanism responsible for the intercontinental transport of Asian desert dust during the spring has been shown to transport primary emissions and ozone at least as far as North America.

 

Monitoring Data

- Pre-industrial levels: European measurements between the 1850s and 1900 were mostly in the range of approximately 17–23 ppb.

- Mid-20th century: Surface ozone data collected at the alpine location of Arosa, Switzerland (elevation of 1800 m) during the 1950s, provide valuable insight in the progression of the rise in ozone concentrations over the past century. Based on measurements taken between June 1950 and May 1951, a median annual ozone concentration of approximately 18 ppb was reported. Measurements taken at the same location between 1989–1991, indicate an approximate doubling of the median ozone concentration over a period of three decades.

- Current levels: See tables 1, 2, 3 and also Figure 1 below.

Trends

- Comparisons of ozone background levels with those measured in the late 19th–early 20th centuries indicate that current ozone levels have risen by approximately two times.

- The fact that this rise has occurred in parallel with industrial development indicates that present day background ozone includes a substantial anthropogenic component.

- There seems to be fairly good indication that background ozone levels in the Northern Hemisphere have continued to rise over the past three decades. However, the evidence for increasing trends in surface ozone is not global in nature and is not always consistent among monitoring sites.

- For background stations which have reported increasing ozone trends over the past three decades, trends ranged between 0.06 and 2.6% per year with some of the largest increasing trends seen at stations in Europe and Japan.

- At the same time there is a growing number of studies reporting declining trends in ozone concentrations at urban sites or sites downwind of urban centers at locations in North America and Europe. This trend is strongest for concentrations at the high end of the distribution, while increasing trends are often seen for values at the mid to low end of the distribution.

Possible reasons for observed trends

- Changes in emissions of ozone precursors are commonly believed to have affected background ozone levels. According to model results, increased surface emissions of NOx from fossil-fuel combustion have had the largest effect on ozone in the lower troposphere since 1970. These increased emissions are estimated to be responsible for more than a 10% increase in year round ozone over Canada, Europe and Japan and as much as a 20% increase over Europe and Japan in the summer. In contrast to NOx emissions, increases in hydrocarbon emissions from fossil-fuel combustion have been more modest. Although long-term changes in hydrocarbon emissions have affected the strength of regional high ozone episodes, model results indicate that they have not contributed significantly to mean tropospheric ozone trends.

- The global rise in methane levels, which occurred primarily from the late 1970s to the late 1980s, may also have contributed to the observed increase in tropospheric ozone. Model results estimate that this rise in methane levels is responsible for roughly one-fifth of the anthropogenically induced increase in tropospheric ozone at northern mid latitudes and to a global ozone increase of 3–4% over the past quarter century.

- Modeling studies indicate that stratospheric ozone sources are estimated to exert a small but significant influence on ozone levels in the lower troposphere during the winter and spring at northern mid latitudes, where approximately 10% of ozone is estimated to be of stratospheric origin. During the summer the stratospheric ozone influence is believed to be negligible. It is estimated that due to ozone

depletion in the lowermost stratosphere during the winter and spring, the stratospheric flux of ozone into the troposphere may have declined by as much as 30% from the early 1970s to the mid-1990s.

- A consequence of declining stratospheric ozone levels is an increase in UV radiation reaching the lower troposphere. This effect would have a potential impact on both photochemical production and loss of ozone. Model results indicate that increases in UV radiation due to stratospheric ozone depletion do not appear to have significantly reduced tropospheric ozone, except at mid latitudes in the Southern Hemisphere following the breakup of the ozone hole.

- Intercontinental transport appears to be an important factor that may explain observed ozone trends. Recent studies have indicated that trans-Pacific transport of Asian pollution affects North America, trans-Atlantic transport of North American pollution affects Europe and trans-Eurasian transport of European pollution affects Asia.

- Studies focusing on trans-Pacific transport have quantified an Asian pollution influence of about 3–10 ppb on background ozone levels in the western United States. This effect is most pronounced during the spring, when storm and frontal activity in Asia is most prevalent and westerly transport of Asian air across the North Pacific is strongest. The last two decades have seen increasing NOx emissions from East Asia in the range of 4–6% per year.

Future background ozone levels

- Studies using coupled climate-tropospheric chemistry models indicate that surface ozone concentrations are expected to rise significantly throughout the 21st century (IPCC, 2001). This rise is expected to occur primarily as a result of the projected rise in ozone precursor emissions over the next century, with about half the rise being due to the rise in methane emissions and half due to the rise in NOx emissions (IPCC, 2001). Of the range of global emission scenarios studied under the most recent IPCC assessment (Nakicenovic, 2000), all but one projected increases in global tropospheric ozone during the 21st century.

- IPCC ozone projections for the 21st century are illustrated in Fig. 1, along with historic and current surface ozone concentrations reported in the various studies referenced in this paper

- Although there is significant uncertainty in the projected concentrations, as indicated by the vertical error bars, all projections show a rise in surface ozone over the 21st century. Note that projected concentrations for the 21st century exceed internationally accepted environmental criteria, ranging around 40–50 ppb to protect human health, crops and vegetation (WHO, 1987).

- Given that these values represent background conditions, any additional ozone production associated with smog episodes would make it very difficult to achieve a clean air standard of < 80 ppb over most-populated regions (IPCC, 2001).

- Fig. 1 also illustrates the broad range of modern day ozone concentrations. In the lower part of the range are relatively un-impacted sites such as American Samoa, US Virgin Islands and Mt. Rainier National Park. Most low-elevation stations fall in the middle range.

Applicant's summary and conclusion

Conclusions:

This Canadian governmental survey of the literature was conducted around 2003 to review historical and current surface ozone data (Canada, United States and worldwide) for the purpose of characterizing background levels and trends, present plausible explanations for observed trends and explore projections of future ozone levels. The main result for our risk assessment is that annual average background ozone concentrations range between approximately 20 and 45 ppb (including Europe). This is with the understanding that the level varies as a function of geographic location, elevation and extent of anthropogenic influence. Another important point is that there is mostly a rising ozone trend, which is predicted to continue in the years after 2003, and as a result, the gap between air quality standards and background ozone levels is expected to narrow further. This has implications for the settings of the ozone gas monitoring and warning devices installed at ozone generation plants. An alarm level set at 0.1 ppm would give frequent false alarms, because it will also measures ozone that is already present in the outdoor air, and that has entered the indoor space via windows or ventilation.
This study was evaluated as scientifically sound, given that it had to work with available pre-industrial data for trend analysis. Indeed, in the 19th century only a few stations observed ozone continuously for more than a few years, and hence long-term data are limited to these stations. Data obtained from these early observations is only semi-quantitative in nature because of difficulties related to the method’s sensitivity to humidity and antioxidants in the air. In spite of these uncertainties, these data give a general indication of what the natural background level of ozone would be in the absence of significant anthropogenic influences. The fact that the rise in ozone level in the 20th century has occurred in parallel with industrial development indicates that present day background ozone includes a substantial anthropogenic component. Although there is good evidence for an increase in the global background level of ozone over the past century, there is less certainty regarding trends in the past few decades. Part of the difficulty in determining a global trend is due to the relatively small number of background stations and the difficulty in identifying monitoring sites which are representative of background conditions. Some sites designated as background are subject to anthropogenic pollution under the influence of specific synoptic patterns. Others are affected by intercontinental long range transport of pollutants. These anthropogenic influences complicate the interpretation of data from many background stations. Additional difficulties arise in comparing reported trends among sites due to differences in the time period or season chosen for analysis and choice of reported statistic. In spite of these difficulties, there seems to be fairly good indication that background ozone levels in the Northern Hemisphere have continued to rise over the past three decades. However, the evidence for increasing trends in surface ozone is not global in nature and is not always consistent among monitoring sites.

Executive summary:

A survey of the literature was conducted by the Canadian government (Environment Canada) to review historical and current surface ozone data from background stations in Canada, United States and around the world for the purpose of characterizing background levels and trends, present plausible explanations for observed trends and explore projections of future ozone levels.

The main results were:

- Seasonal: The annual cycle of ozone at background sites in the Northern Hemisphere is characterized by a spring maximum peaki during the month of May. Sites which are affected to some extent by local ozone production exhibit a broad summer maximum.

- Trend until 2004: Present day ozone levels appear to have approximately doubled, with the greatest increase having occurred since the 1950s.

- Current levels: Modern day (2004) annual average background ozone concentrations range between approximately 20 and 45 ppb with variability being a function of geographic location, elevation and extent of anthropogenic influence.

- Rise per year: There is some indication that background ozone levels over the mid latitudes of the Northern Hemisphere have continued to rise over the past three decades and that this rise has been in the range of approximately 0.5–2 % per year.

- Reasons: Model results indicate that increases in NOx emissions since the 1970s account for a 10–20% increase in background ozone over certain areas of the globe. Rising methane levels from industry and agriculture are believed to have increased global ozone levels by 3–4%. Countering this, are estimates of declines in the ozone flux from the stratosphere to the troposphere, resulting from stratospheric ozone depletion. Intercontinental transport of ozone: Recent global chemical transport model studies indicate that Asian pollution contributes about 3–10 ppb to background ozone levels in the western United States during the spring.

- Projected rise: The projected rise in global emissions over the 21st century is expected to have a profound effect on surface ozone levels around the world. Using five of the less conservative IPCC emission scenarios, the average global surface ozone concentration is expected to be in the range of 35–48 ppb by 2040, 38–71 ppb by 2060, 41–87 by 2080 and 42–84 ppb by 2100. Such increases would exceed internationally accepted environmental criteria and have negative implications on human health, crops and vegetation.

As the gap between air quality standards and objectives and background levels narrows, it becomes increasingly important to have a good understanding of the anthropogenic enhancement to the background.