Ecosystem Science Reports

Atmospheric Sciences

Project Title: A Review of Surface Ozone Background Levels and Trends

Publication Date: Atmospheric Environment, in press, accepted March 12, 2004

Investigator: Roxanne Vingarzan, Aquatic and Atmospheric Sciences Division, Environment Canada, Pacific and Yukon Region

Study Overview

The control of ground level ozone has been an important aspect of airshed management over the past two decades. This priority is reflected in the government of Canada's recent promulgation of the Canada Wide Standard for Ozone and associated provisions for Continuous Improvement and Keeping Clean Areas Clean. In managing ground level ozone, it is important to distinguish between the portion that can be controlled through local emission reductions and the background level, which cannot be controlled at the local scale. Background ozone, defined as the fraction of ozone present in a given area that is not attributed to anthropogenic sources of local origin, can exert a significant influence on ozone levels in an airshed. The portion attributable to background not only contributes to exceedences of the air quality standard and objectives in some areas of Canada, but it also contributes to the overall health risk due to cumulative ozone exposure. Understanding the magnitude of background ozone and how it changes over time is an important component of air quality management. This paper: 1) reviews 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, 2) presents plausible explanations for observed trends and 3) explores projections of future ozone levels for the 21^st century.

Key Points for Decision-Makers

Key Scientific Findings

Key Points for Decision-Makers

• Background ozone levels over the midlatitudes of the Northern Hemisphere have continued to rise over the past three decades; this rise slowed down in the 1990s in response to the introduction of ozone abatement measures.
• Ozone concentrations at Canadian background stations are similar to those at low elevation background sites in the US and around the world.
• Increases in NOx emissions since the 1970s account for most of the increase in background ozone. Rising methane levels from industry and agriculture have also contributed to the increase, but to a smaller extent.
• Asian pollution has been documented to contribute to the background level in the western United States during the spring. A continued rise in anthropogenic emissions from Asia is expected to increase the background level even further.
• The projected rise in global emissions over the 21^st century is expected to cause a continuing rise in surface ozone levels around the world. This will have negative effects on human health, crops and vegetation.
• Current gaps in our understanding of background ozone require an increase in the number of monitoring stations, especially in Asia and the Southern Hemisphere. An enhanced network would be invaluable in assessing impacts of intercontinental transport and emission changes on background levels.
  

Key Scientific Findings

• The annual cycle of ozone at background sites in the Northern Hemisphere is characterized by a spring maximum peaking during the month of May. Sites which are affected to some extent by local ozone production exhibit a broad summer maximum. There is no overarching consensus as to the origin of the spring maximum, as evidence supports both enhanced photochemistry in the free troposphere and stratospheric-tropospheric exchange.
• Modern day annual average background ozone concentrations over the midlatitudes of the Northern Hemisphere range between approximately 20-45 ppb, with variability being a function of geographic location, elevation and extent of anthropogenic influence.
• Annual average ozone concentrations at Canadian background stations fall between 23-34 ppb, a range similar to low elevation background sites in the US and around the world.
• Background ozone levels over the midlatitudes of the Northern Hemisphere have approximately doubled from those measured over a century ago with the greatest increase having occurred since the 1950s.
• Background ozone levels have continued to rise over the past three decades, and this rise has been in the range of approximately 0.5-2% per year. Rising trends have not been uniform, however, as the relatively steep trends of the 1970s and 1980s have given way to more modest trends throughout 1990s. The slower rate of increase, or in some cases lack of an increase, over the past decade is believed to reflect recent declines in ozone precursor emissions in North America and Europe.
• Modelling studies 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.
• 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. A continued rise in anthropogenic emissions from Asia is expected to increase the background level even further.
• 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.

References

The paper will be published in the journal Atmospheric Environment in 2004. It may also be obtained directly from the author at roxanne.vingarzan@ec.gc.ca .

The correct citation of the paper is:

Vingarzan, R. 2004. A review of surface ozone background levels and trends. Atmospheric Environment, in press.