Sable Island Station:

Tropospheric Ozone

(September 2003, revised February 2006, December 2007 – Zoe Lucas © 2007))

An atmospheric pollutant of particular interest is tropospheric ozone (O3). O3 is a critical atmospheric species that drives much of the tropospheric photochemistry. It regulates the oxidation capacity of the troposphere, influencing background levels of trace chemicals, and can influence the fate of polluting emissions as well as the amount of chemical deposition to the biosphere. Also, O3 is an important greenhouse gas and thus has the potential to perturb the global climate, especially in the upper troposphere where its radiative effects are most significant. O3 is a key trace gas for both the chemistry and radiative balance of the troposphere, and is the principal pollutant associated with photochemical smog. Its presence in the lower troposphere has significant implications for issues of air quality and human health. Also, O3 within the continental boundary layer directly affects crops and forests. There is evidence that background levels of tropospheric O3 in the northern hemisphere are increasing. 

An ozonesonde launch from the "hydrogen shed" at the Sable Island Station.

Tropospheric O3 has natural and anthropogenic sources. The main natural source is downward flux from the O3-rich stratosphere. The main anthropogenic source is photochemical production from precursors emitted primarily during combustion of fossil fuels by industry and transportation activities. O3 itself is not emitted by human activities, rather it is formed in the atmosphere through physical and chemical interactions amongst precursor gases that are produced by human activities (e.g. combustion products such as oxides of nitrogen, NOx, and volatile organic compounds, VOCs). Thus human activities increase tropospheric O3 levels by producing these precursor gases.

The heavily polluted east coast of North America is a particularly large source of O3 and its precursors. Transport of these to the temperate North Atlantic Ocean may have a major impact on the tropospheric chemistry over the ocean, and even over the downwind continental region of western Europe. There is evidence that the average O3 level at the surface in western Europe has more than doubled since preindustrial times, and that O3 levels have increased throughout the lower troposphere over Europe in the last 50 years. Although Europe is itself a large source of O3, increased levels in Europe may be partially attributable to transport from North America. Such transport has been supported by results of studies conducted on Sable Island. Further, transport from Europe could affect Asia, and transport from Asia could affect North America—an overall result would be a general increase in O3 levels in the temperate latitudes of the Northern Hemisphere. It becomes more complicated when, during summer, the Azores-Bermuda high-pressure system tends to circulate the exported pollution from both eastern North America and western Europe to regions around the North Atlantic basin.

Determining the distribution and transport of tropospheric O3 and identifying processes responsible for climatological trends in O3 levels are priorities in global atmospheric chemistry. In order to understand the O3 budget, and hence the effects of O3, it is necessary to quantify and compare the magnitude of its natural and anthropogenic sources. Sable Island is important in atmospheric research because it represents a transitional location between the polluted continent (with expected net photochemical O3 production within the boundary layer) and the clean marine environment (with expected net O3 destruction within the boundary layer). A program of continuous monitoring of carbon monoxide (CO) and O3 began on Sable Island in the late 1980s and ran until 1997. Initially a summer project, the study was continued when it was found that levels at Sable Island were much higher than expected. Since then the Sable Island Station has participated in four special studies (“summer intensives”)—NARE 93, NARE 97, IONS 2004, and IONS 2006—and presently supports continuous O3 measurements as part of the Airshed Monitoring Program.

Data gathered on Sable Island have been critical in a number of studies. For example, the long-term time series of O3 and CO measurements at Sable Island (combined with similar data collected at Mace Head, Ireland, and Westman Island, Iceland) were used to evaluate transatlantic transport of anthropogenic O3. Results of this analysis indicated that 20% of the violations of the European Council O3 standard would not have occurred in the absence of anthropogenic emissions from North America. In another example, results from the NARE 93 program showed that during the 1993 study period the Sable Island area was characterized by net O3 production. Because photochemical production of O3 depends on elevated levels of nitrous oxides and non-methane hydrocarbons (combustion products), the O3 levels measured at Sable provided evidence for the influence of continental pollution on the chemistry of the marine boundary layer over the North Atlantic extending to Sable Island. 

In summer 2004, five countries—Canada, Germany, France, the UK, and the USA—participated in the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT), a series of coordinated experiments to study emissions, chemical transformation and removal of aerosol and O3 precursors during their transport to, and over, the North Atlantic. North American participation in ICARTT was the IONS 2004 program, and it provided the most extensive set of free tropospheric O3 measurements ever compiled for this region. A huge amount of data was collected by the combined network of ozonesonde sites, aircraft and NASA satellites, all of which was examined in the context of meteorological observations simultaneously collected by thousands of stations. Data acquired by the Sable Island ozonesonde program, along with O3 profiles from 12 other North American sites, indicated that summer 2004 was unusual. Normally July and August are characterized by stalled or slow-moving weather systems with prolonged periods of haze and warm temperatures, but in summer 2004, a persistent low pressure system in Ontario meant that southerly winds were common, and pollution tended to flow more towards the north than to the east or west. These conditions also resulted in frequent exchanges of O3-laden air from the upper atmosphere to the lower. The subsidence of such air masses in high pressure systems or under adiabatic descent, many thousands of kilometres downwind of continental source, may lead to elevated background concentrations of O3 re-entering the boundary layer.

IONS 2006

Although the goal for the IONS 2004 study was to examine pollution influences under stable high-pressure systems and transport associated with "warm conveyor belt" flows, during summer 2004, the study region was dominated by a series of weak frontal systems that mixed aged pollution with stratospheric O3 in the middle troposphere. During the IONS 2006 study, data were gathered under stronger frontal systems more typical of the region. Participants in the 2006 study were NOAA, NASA (Goddard Space Center), the Meteorological Service of Canada (MSC), Mexico, and Barbados. 

Click on thumbnails for more information

Sable’s participation in these studies is another demonstration of the undeniable synergy embodied in Sable Island Station. Because there are ongoing aerology and meteorology programs at the Station, it has been possible to deploy the specialized O3 sensors as add-ons to the daily radiosonde flights, and also to provide relevant ground-based measurements. The Station’s contribution is further enhanced by the expertise of its staff. The island is a stable and secure platform for gathering data on atmospheric pollutants over the Northwest Atlantic. By supporting the Sable Island Station, Canada enhances its ability to participate in international programs of monitoring and research. 

IONS-2006, Sable Island – Acknowledgements

The participation of the Sable Island Station in the IONS 2006 program was enabled by the Friends of the Green Horse Society which purchased the ozonesondes, electronic interfaces and larger balloons. The Friends of the Green Horse Society thanks ExxonMobil Canada for providing the necessary funding. Personnel, extra lifting gas, and the specially modified radiosondes that permitted integration of additional sensors were provided by the Meteorological Service of Canada.

Tropospheric O3 profiles collected at Sable Island during August 2006 can be viewed at

http://croc.gsfc.nasa.gov/intexb/SONDES/ions06_augsept.html