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Ground-level ozone: our new environmental priority.

Readers of Canada's Green Plan may be somewhat surprised to learn that "while measures [to reduce sulphur dioxide emissions] have solved most of Canada's air quality problems, ground-level ozone problems persist and have now become the most serious air quality problem remaining in many areas of the country". Certainly, environmentalists and many scientists would challenge to claim that the acid deposition problem has been solved. But few Canadians know that ground-level ozone has replaced acid deposition in the government's air quality priorities. This new priority was set in October 1988 by the Canadian Council of Ministers of the Environment (CCME), following a particularly hot summer, during which record high levels of ground-level ozone were observed. We should distinguish here between ground-level ozone and ozone in he stratosphere -- that region of the atmosphere between 10 and 50 km above the earth's surface. Stratospheric ozone provides the protective shield which prevents biologically harmful UV radiation from penetrating to the surface. Stratospheric ozone has been decreasing -- dramatically, in the case of the much publicised Antarctic 'ozone hole' -- but also over Canada, as a result of chemical reactions involving halocarbons. These observations have led to international agreements such as the Montreal Protocol to curb halocarbon production.

By contrast, ozone in the troposphere -- the region between the ground and the stratosphere -- has been increasing in the northern hemisphere by more than 1% per year (but not enought to compensate for the loss of stratospheric ozone). Increases in ground-level ozone are highly undesirable, as it is a very toxic substance. It can cause serious respiratory problems, particularly among older people, but also to healthy young people during moderate to heavy exercise such as running. According to the Management Plan of the CCME, more than half of all Canadians are routinely exposed to levels of ozone that are known to cause health effects. Ground-level ozone is also a strong phytotoxin, causing significant platn damage. CCME states that crop damage in Ontario alone throughout the summer growing season is estimated to cost up to $70-million per year. Agricultural crops such as beans, tomatoes, potatoes, soybeans and wheat, and trees such as pine, red spruce and sugar maple are particularly susceptible to foliar damage when exposed to ozone. The CCME Plan calls for a target of less than 82 parts per billion by volume (ppbv) ozone in any one-hour period in all areas of Canada by the year 2005. During the last seven years, levels well above this value have occurred with varying frequency in three major areas of Canada -- the lower Fraser Valley (BC), Saint John, NB, and western Nova Scotia, and in the Windsor-Quebec corridor along the Lower Great Lakes and the St. Lawrence Rive. Ozone levels above this limit also occur in the urban area of Edmonton, Calgray, Regina and Winnipeg.

In the lower Fraser Valley, ozone concentrations exceeded the 82 ppbv target as often as 39 hours per year with peak levels as high as 200 ppbv. In the Atlantic region, concentraions up to 110 ppbv have been recorded with the target exceeded 20 hours per year. By far the worst episodes occur in the Windsor-Quebec corridor, where concentrations in excess of 82 ppbv occur as often as 566 hours per year with peak values up to 190 ppbv.

Sources of Ozone

Ozone is not a primary pollutant. It is formed during photo-chemical reactions involved in the oxidation of gases which are emitted to the atmosphere from natural or anthropogenic sources. Most are emitted in chemically reduced forms: carbon, e.g., in the form of [CH.sub.4], nitrogen in the form of [NH.sub.3] and N oxides, and sulphur as [H.sub.2S], dimethyl sulphide, or [SO.sub.2]. Since we live in an oxidising atmosphere, these gases are oxidized to long-lived products such as [CO.sub.2], or to acidic products such as [HNO.sub.3] or [H.sub.2][SO.sub.4], which are removed by deposition to the surface. Without these processes, we would soon suffocate in the accumulated gases continually emitted into our atmosphere, not only by human activity but also from the rest of the natural biosphere. But in the oxidative process, many intermediates are formed which are toxic to life forms.

Oxidation does not occur, however, by direct reaction with the relatively abundant [O.sub.2] molecules; this is an extremely slow process. Instead, it occurs by reaction of oxidants which are present in very small concentrations, such as ozone, but even more effectively byt he OH radical which is present in even smaller concentrations, less than 1 part in [10.sup.13. (This is equivalent to less than one hair in the whole human race.) In fact, it is the high reactivity of the OH radical that results in its low steady state concentrations.

A necessary ingredient for the formation of these oxidants is [NO.sub.2], which is one of the few gases which can be photolysed by the solar radiation which penetrates into the troposphere:

[NO.sub.2] + h[nu] [right arrow] NO + O O + [O.sub.2] + M [right arrow] [O.sub.3] + M NO + [O.sub.3] [right arrow] [NO.sub.2] + [O.sub.2]

NET: Nil

(M represents any other molecule which acts as a collision partner to absorb the energy produced in the reaction.) Thus, only a small amount of ozone can be formed by the action of sunlight with oxygen and nitric oxide. But this small amount of ozone can be photolyzed by sunlight at wavelengths below 310 nm to produce excited [O.(sup.1.D) atoms.

[O.sub.3] + h [nut] [right arrow] [O.(sup.1.D) + [O.sub.2]

Some of these excited O atoms can react with water to produce OH radicals:

O([sup.1.D]) + [H.sub.2] [right arrow] OH + H

This highly reactive free radical can oxidize CO and volatile hydrocarbons (VOC). If we represent at typical hydrocarbon as RH, then the following (oversimplified) set of reactions can occur: RH + OH [right arrow] R + [H.sub.2.O] R + [O.sub.2] + M [right arrow] [RO.sub.2] + M [RO.sub.2] + NO [right arrow] [NO.sub.2] + RO RO + [O.sub.2] [right arrow R'O + [HO.sub.2] [HO.sub.2] + NO [right arrow] [NO.sub.2] + HO 2 [NO.sub.2] + h[nu] [right arrow] 2 NO + 2 O 2 O + [O.sub.2] + M [right arrow] 2 [O.sub.3] + M / NET: RH + 4 [O.sub.2] [right arrow] R'O + [H.sub.2.O] + 2 [O.sub.3]

and we now have net ozone production. It is interesting that it takes ozone to make ozone.

The other oxidation product, R'O, is generally a carbonyl which can also be photolysed by sunlight. For example, formaldehyde is formed in the oxidation of methane and other hydrocarbons and can contribute to the following sequence:

[CH.sub.2.O] + h[nu] [right arrow] CHO + H CHO + [O.sub.2] + [right arrow] CO + [HO.sub.2] H + [O.sub.2] + M [right arrow] [HO.sub.2] + M 2 [HO.sub.2] + 2 NO [right arrow] 2 [NO.sub.2] + 2 HO 2 [NO.sub.2] + h[nu] [right arrow] 2 NO + 2 O 2 O + 2 [O.sub.2] + M [right arrow] 2 [O.sub.3] + M / NET: [CH.sub.2.O] + 4 [O.sub.2] [right arrow] CO + 2 OH + 2 [O.sub.3]

and we have a chain reaction which produces additional ozone and OH radicals. Note that the chain depends on the presence of NO and [NO.sub.2]. These two nitrogen oxides are interconverted within a matter of two minutes and are collectively referred to as [NO.sub.x].

If the [NO.sub.x] concentration is low, other fates await the radicals:

(eg.) OH + [O.sub.3] [right arrow] [HO.sub.2] + [O.sub.2] [HO.sub.2] + [O.sub.3] [right arrow] OH + 2 [O.sub.2] and [RO.sub.2] + [HO.sub.2] [right arrow] ROOH + [O.sub.2]

which will result in decreased OH and ozone. The oxidation level of the atmosphere therefore depends on the amount of [NO.sub.x] relative to the hydrocarbon concentration.

The ambivalent nature of [NO.sub.x] in ozone production is illustrated in Figure 1, which shows ozone as a function of [NO.sub.x] for varying amounts of a representative mix of butane and propylene (non-methane hydrocarbons: NMHC). (Other hydrocarbons will provide curves with different values but similar shapes depending on their reactivities.) In this example, ozone production is [NO.sub.x] -limited for ratios of hydrocarbons to [NO.sub.x] of 4.8. Additions of [NO.sub.x] will therefore result in ozone production. At NMHC: [NO.sub.x] ratios below 4.8, ozone production is hydrocarbon-limited and additional [No.sub.x] actually leads to ozone destruction by reactions such as (A). Most of Canada is in the [NO.sub.x]-limited region, and the median value of the NMHC: [NO.sub.x] ratio in the Windsor-Quebec corridor is above 10:1.

Addressing the Problem

Until four years ago, there was no federal or provincial legislation limiting the emission of [NO.sub.x], including that from the major source, motor vehicles. Not uncharacteristically, Canada simply adopted the US Environmental Protection Agency's (EPA) position that ozone abatement could best be achieved by limiting hydrocarbon emissions, since EPA believed that ozone production was hydrocarbon-limited in the regions of concern. This strategy resulted in tens of billions of dollars having been spent since 1970 on hydrocarbon reduction, with no dramatic improvements in a number of US urban areas. EPA's strategy was based on the output of a computer model which is now believed to be flawed. Among other points, the model ignored the emissions of natural hydrocarbons. Trees, for example, emit isoprene, which is many times more reactive than alkanes and most other alkenes in ozone production. (Apparently Ronald Readan was a prophet before his time when he said that trees pollute!)

Except in downtown New York and Los Angeles, reduction in hydrocarbon emissions alone failed to produce the expected ozone reduction. No ozone improvement was achieved in Atlanta, largely because of the large input of natural hydrocarbons. And in all rural areas downwind of major cities, ozone production is [NO.sub.x] are more rapid than those that processes that remove [NO.sub.x] are more rapid than those that remove hydrocarbons.

More advanced models now predict that the best strategy for ozone reduction requires the simultaneous reduction in both hydrocarbon and [NO.sub.x] emissions, the relative effectiveness of each depending on hydrocarbon: [NO.sub.x] ratios. The CCME Management Plan suggests that the 82 ppbv ozone target would require a 40 -- 50% reduction in [NO.sub.x]. The Windsor end of the Windsor-Quebec corridor is dominated by transport of ozone and its precursors from the USA, especially Detroit and Cleveland. Canadian controls alone will therefore do little to solve the ozone problem. In the rest of the corridor, [NO.sub.x] control is likely to be more effective in urban areas. Existing emission in the Atlantic provinces are not yet well enough characterized to allow the effects of control strategies to be predicted; however, much of the ozone and its precursors originate in the major coastal areas in the US.

Most attention has been focussed on ozone largely, I suspect, because it is relatively easy to measure. But there are many other secondary pollutants formed in the photochemical oxidation of primary pollutants which are even more harmful than ozone. We have already seen that carbonyls, such as formaldehyde, are formed in the same reactions that make ozone. In addition, the free radicals formed in these reactions can react with [NO.sub.x] to form organo-nitrogen compounds such as nitrosamines, peroxyacynitrates, etc., many of which are known carcinogens. We obviously cannot go into their complex chemistry here. However, any controls designed to reduce ozone levels will have the commensurate benefit of reducing the production of these toxic substances as well.

The CCME Management Plan calls for three phases in achieving its objectives. Phase I, to start immediately, calls for emission targets for [NO.sub.x] and VOC to be negotiated between federal and regional jurisdictions in the designated target areas. Some 31 initiatives have been defined to achieve these targets. These include 27 initiatives to retrofit emission sources and to improve transportation management. Phase II is to begin in 1994. The plan states that "information available on emissions, cause and effect relationships, effectiveness of US control programmes and the means available to further reduce emissions beyond the levels identified in Phase I is inadequate for setting final caps at this time. Consequently, only interim reduction targets will be negotiated as part of Phase I". Phase II therefore aims at establishing emission caps to meet the 82 ppbv objective by 2005. Other non-attainment areas besides the three designated areas (ie., the lower Fraser Valley, the Windsor-Quebec corridor, and Saint John -- western Nova Scotia) will also be investigated, such as airsheds surrounding other large urban areas.

Phase III, to come into effect in 1997, will make final adjustments to emission caps and to all non-attainment areas. The cost of all the measures is estimated to be in the order of $855 million per year.

This is a very ambitious programme. It remains to be seen how much of the plan will be implemented before the Government declares this problem solved and moves to identifying a new "most serious air quality problem."
COPYRIGHT 1991 Chemical Institute of Canada
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Schiff, Harold
Publication:Canadian Chemical News
Date:Aug 1, 1991
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