Tropospheric Chemistry

Peter Borrell

An essay written for the Encyclopedia of Ecology and Environmental Management, Editor: Peter Calow, Blackwell Science, Oxford, 1998.

Tropospheric chemistry refers to the chemical processes occurring in the troposphere by which trace substances released into the atmosphere from natural and anthropogenic sources are oxidised. The reactions form stable compounds, such as carbon dioxide and water, or more soluble compounds, such as peroxides or acids, which are rained out or deposited from the atmosphere. See also Stratospheric Chemistry.

The troposphere
The lower atmosphere, below about 15 km altitude, is termed the troposphere because of its turbulent air motions. A boundary layer of about 1 km thickness is often envisaged as an interface between the earth's surface and the free troposphere. The troposphere is characterised by a general decrease in temperature with height, from ca 15 ºC at the surface to ca -50 ºC at about 15 km. Above this, in the lower stratosphere, the temperature increases with height, the changeover region being known as the tropopause.

The troposphere contains an enormous range of inorganic and organic trace substances. Many of them, such as methane, nitrous oxide, dimethyl sulphide, are released by natural processes. Many, such as the nitrogen oxides, a variety of volatile organic compounds (VOCs) and sulphur dioxide are released by man's activities. Others, such as ozone, carbon monoxide and nitric acid, are formed in the atmospheric oxidation of the compounds released from both natural and anthropogenic sources.

Initiation of tropospheric oxidation: the hydroxyl radical, OH
Oxidation of organic compounds occurs through a series of cyclic chain reactions. These are usually initiated by the reactive hydroxyl radical, OH, which is the principal agent of attack on trace substances in the atmosphere. Hydroxyl radicals are formed by the photolysis of ozone at short wavelengths (< 310 nm) in the ultraviolet:

O3 + hn ( l < 310 nm) = O2 + O(1D) (1)

An electronically excited state of the oxygen atom, O(1D), is formed that can then react with methane or water to give one or two hydroxyl radicals:

O(1D) + H2O = OH + OH      (2)
O(1D) + CH4 = OH + CH3     (3)

Tropospheric ozone itself may either be formed by photochemical oxidation, as indicated below, or it may be brought down by vertical atmospheric mixing, from the stratosphere into the free troposphere or from the free troposphere into the boundary layer.

Tropospheric oxidation of methane
As an example of an oxidation chain reaction, consider the oxidation of methane, CH4. Methane is a major trace constituent having an average volume ratio of ca. 1500 ppb in the troposphere (ppb = part per billion). The attack of the hydroxyl radical abstracts hydrogen from CH4 to form water:

The methyl radical, CH3, formed at the same time reacts with oxygen forming a methylperoxy radical, CH3O2. In a continental atmosphere during the day, this will probably react with nitric oxide, NO, released by combustion processes, to form a methoxy radical, that in turn reacts with oxygen to form formaldehyde, HCHO: Like methane, the formaldehyde molecule itself may be oxidised by OH in a similar sequence of reactions, the end result being the formation of CO or CO2 and water.

The hydroperoxy radical, HO2, from reaction (7) oxidises NO to NO2 and regenerates an OH radical:

The reaction sequence, (4) to (8), constitutes a chemical chain reaction with OH as the chain carrier: OH is consumed in reaction (4) and regenerated in reaction (8). The OH radical can thus be regarded as a catalyst, only a small amount being needed to oxidise much larger amounts of CH4. In the same reaction chain, two molecules of NO are also oxidised to nitrogen dioxide, reactions (6) and (8).

Volatile organic compounds (VOCs), such as hydrocarbons, are all oxidised by similar mechanisms and so, in sunlight when enough NOx is present, fuel the synthesis of ozone.

Carbon monoxide, another major trace constituent with an average volume ratio of about 190 ppb in the troposphere, is also oxidised in a chain reaction. Following an initial attack by OH; a hydrogen atom is formed which adds to oxygen to form a hydroperoxy radical, HO2:

An OH radical is then regenerated by reaction (8).

Ozone formation
In reactions (5) and (7), the peroxy radicals convert a molecule of NO to NO2. During the day time, the NO2 can be photolysed to form an oxygen atom, that can then add to oxygen to form a molecule of ozone, reactions (11) and (12):

Reactions (11) and (12) can be reversed by the rapid reaction of NO with ozone: The forward and back reactions, (11), (12) and (13), with their dependence on the concentration of NOx ([NOx] = [NO] + [NO2]) and the light intensity, determine the local concentration of ozone. On a sunny summer afternoon the ozone volume ratio may be between 60 and 80 ppb in the boundary layer. Overall, the oxidation of VOCs and CO in the continental boundary layer generates ozone, provided enough NOx is present.

Oxidation and ozone in remote and background atmospheres
In remote locations on earth, largely free from man's emissions and with low concentrations of NOx (< 1 ppb), trace substances are still oxidised. However, little ozone is formed photochemically. Instead the peroxy and other radicals may react together, rather than with NO, and form organic peroxides, hydrogen peroxide and other compounds, that may be rained out or deposited directly into the biosphere.

In the free troposphere in the northern hemisphere, above the boundary layer, the volume ratio of ozone varies between about 30 ppb in winter to 60 ppb in summer. The concentration levels appear to have more than doubled in the last hundred years, with most of the increase being attributed to man's activities. The additional ozone is formed photochemically, as shown above, either in the free troposphere itself or in the boundary layer, from which it is transferred up by vertical mixing. As in remote areas the NOx concentrations in the free troposphere are generally low, so that hydrogen peroxide and other compounds are formed from additional reactions of the peroxy and hydroperoxy radicals.

Photochemical air pollution; summer smog
In the polluted boundary layer, the concentrations of the primary pollutants, VOCs and NOx emitted by traffic and industrial sources, are much higher than in the free troposphere above. In the presence of sunlight, high concentrations of ozone and other photo-oxidants are produced, with ozone volume ratios reaching 400 ppb in bad cases. The resulting condition is known as a photochemical smog. It was first characterised in Los Angeles in the nineteen fifties, and is prevalent in cities that have a warm sunny climate. The pollutants are trapped within the boundary layer under a meteorological temperature inversion (temperature increasing with height). The high pollutant concentrations facilitate rapid photochemistry and the inversion prevents the polluted air being diluted by mixing with cleaner air from above. The growth in population and traffic throughout the world means that most sub-tropical and tropical cities suffer from chronic smog conditions. Summer smog episodes are also regional scale phenomena in the populated temperate parts of the world during sunny periods accompanied by stagnant high pressure conditions.

A frequent constituent of smog is the powerful lachrymator PAN, peroxyacetylnitrate. It is formed from acetaldehyde, CH3CHO, a product of the photo-oxidation of ethane and other hydrocarbons:

As PAN can dissociate again to form NO2, it can act as a transport agent. It is formed in areas of high pollution and can release NO2 in remote areas far from the pollution source.

Controlling air pollution: problems with complex chemistry
While the reduction of VOC and NOx emissions is clearly the way to reduce photochemical smog formation, the complexities of the chemistry make cost-effective measures difficult to devise. For example, in a city in summer, there may be very high VOC and NOx concentrations, which themselves are undesirable or harmful. Oxidation takes place rapidly by the mechanisms shown above, forming PAN and other products. Very little ozone may in fact be measured in the city itself because it is "titrated out" as soon as it is formed by the high concentrations of NO, reaction (13). However, downwind of the city, in the suburbs and surrounding countryside, the measured ozone concentrations are greater: there has been more time during transport from the city for the photochemical reactions to occur, and NOx may be removed faster from the air than the VOCs. Reaction (13) is then less effective and the ozone concentration increases. Analogous effects are likely to be observed when measures to lower NOx emissions within the city are taken. The concentration of ozone may remain the same or even increase with small reductions of NOx, and it may require very large reductions to achieve any decrease in ozone at all.

Much current policy-related work is devoted to determining whether the conditions within given localities are "NOx limited" or "VOC limited" and whether, within an area, policy should be directed towards control of NOx, VOC or both, to achieve a cost effective reduction in pollution.

Acid rain: the formation of sulphates and nitrates
Another example of complex chemical transformations during transport through the air is provided by the oxidation of sulphur dioxide, SO2, emitted from the combustion of fossil fuels. Sulphuric acid, H2SO4, is formed which acidifies rain and snow. The oxidation process involves not only chemical reactions in the gaseous phase, but also reactions within the droplets of cloud water as the air passes through clouds.

The recent reductions in SO2 emissions in Europe have brought about a decrease in acidification but the result is not as great as was perhaps hoped. The increasing NOx emissions from traffic have increased the rain-out of nitric acid, a product from photo-oxidation.

The complexity of tropospheric chemistry: atmospheric modelling
The reactions mentioned are only a few of the very many that occur in the atmosphere. A good chemical model scheme describing the chemistry of the troposphere contains perhaps fifty or so chemical compounds and radicals, and perhaps two hundred reactions. The variation of solar radiation with time and wavelength must also be included. It is still a simplified scheme and it constitutes one of a number of modules in an atmospheric model.

Such an atmospheric model is intended to simulate, computationally, the production, distribution and transport of pollutants within a geographical region under particular meteorological conditions. As well as the chemical module, there must be a meteorological driver; a module to describe the dynamic motions of the atmosphere, such as advection, vertical mixing and deposition; modules to include the physics and chemistry of clouds and aerosols; and also an emission module, containing estimates of anthropogenic and natural emissions for a given region.

To be reliable, a large simulation model requires a good scientific understanding of the many parts of this interdisciplinary field. As these models are needed to support modern policy for air pollution abatement, it should be no surprise that research is being vigorously pursued in all the areas related to tropospheric chemistry.

For an excellent account of tropospheric chemistry see: Wayne, R. P. (1991) Chemistry of Atmospheres, Clarendon Press, Oxford, chapter 5.
 

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