Ground Level Air Pollution Dudley E Shallcross Biogeochemistry Research Centre, School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK.

In this article a short review of the history of ground level air pollution is presented and then the sources and impacts of the main primary pollutants are briefly discussed, together with an introduction to various air pollution data archives and other excellent resources. These archives are a tremendous data resource for schools and colleges and can be used in myriad ways such as providing data for project and course work. Finally, a brief description of how secondary pollutants are formed is given, together with recent evidence of the role of plants and trees in exacerbating the generation of these pollutants. A brief history of ground level air pollution

Air pollution, in the form of smoke, has been in existence throughout antiquity. The onset of the industrial revolution in the 17th century heralded in not only the establishment of large urban conurbations but also a more lethal form of air pollution as anthropogenic (man induced) emissions increased. For example, in London episodes of stagnant smoke combined with dense fog became more frequent causing severe health problems, including pulmonary disease and heart failure. The British physician Harold Des Voeux called these episodes the London smog, an amalgamation of the words smoke and fog. Primary pollutants, i.e. species emitted directly from the combustion source, including sulphur dioxide (SO2) and soot particles, were the reason that these so called primary or combustion smogs were so lethal. Reduction in the emissions of primary pollutants such as sulphur from coal reduced the occurrence of these combustion smog events, however, in London for example, severe combustion smog events continued to occur, peaking in the winter of 1952-1953 (see Box 1 ). The increasing emissions of the oxides of sulphur and nitrogen produced by industrial and domestic combustion, throughout the last century, inevitably led to the acidification of rain. Acid rain (see Box 2 ), a term introduced by the French scientist Ducros in 1845, has been a persistent problem ever since and has only recently been brought under control in Europe via legislation in the 1980s.

After world war two the use of motorised transport became widespread and the number of motorised vehicles in use has been increasing continuously. In Los Angeles in the United States of America, in the late 1940s and early 1950s, a dense haze, later to be termed a photochemical smog, was observed. These photochemical smog events brought with them eye irritation, sore throats, asthma and other respiratory problems. Research showed that the cause of these symptoms was not due to the primary pollutants, as in the case of the combustion smog. It emerged that secondary pollutants formed from the primary pollutants were in fact the cause. These secondary pollutants, such as ozone (O3) and

peroxyacetylnitrate (CH3C(O)O2NO2), better known as PAN arise from the action of sunlight on reactive hydrocarbons and nitrogen oxides, released from motorised transport, e.g. cars. Haagen-Smit and Fox (1956) were the first people to make this link between sunlight and these photochemical smog episodes. These episodes were not confined to Los Angeles, but have been subsequently observed in all major industrial and population centres, irrespective of latitude. The first observations of photochemical smog in Europe were made in the Netherlands in 1965 and ever since then elevated ozone concentrations have been found in every European country.

Primary ground level air pollutants The major primary pollutants in the urban environment have been SO2, NOx (NO and NO2), CO, VOCs (volatile organic compounds such as alkanes, alkenes and ethers) and primary aerosols. These pollutants have a variety of sources and differing health impacts. SO2 All fossil fuels (coal, oil and even gas) contain sulphur, and on combustion will generate SO2 and a tiny fraction of SO3. The rise in global emissions of SO2 since the mid 1800s is marked, increasing from 4 Mt in 1860 to around 200 Mt currently. The amount of sulphur present in fossil fuels varies considerably, coal containing from 0.1% - 4% by mass of sulphur mainly in the form of iron pyrites (FeS2) flakes, with 0.04% sulphur in petrol and up to 0.2% in diesel, while natural gas contains as much as 40% sulphur (in the form of H2S). However, virtually all the sulphur can be removed from natural gas, and considerable efforts are now made to clean up emissions. In coal fired power plants, still a major source of SO2, coal can be separated from iron pyrites via froth flotation prior to burning and any SO2 emerging with the flue gas can be removed by reaction with limestone to form hydrated calcium sulphate or gypsum. Technology exists so that emissions of sulphur from the burning of coal could be lowered to any desired level the main barrier being a financial one, although reduction in efficiency of the power station is also a consideration. The impact of SO2 on human, animal and plant life is well documented. SO2 is relatively soluble and the human respiratory tract is designed so as to present a large moist surface area to incoming air. Consequently, around 95% of the inhaled SO2 has been absorbed before reaching the lungs and only about 1% reaches the alveoli in the lungs themselves. In the winter of 1952-1953 in London, around 4000 deaths were attributed to combustion smogs. Here the combination of a strong temperature inversion, trapping SO2 emissions and the presence of high levels of soot and water vapour combined to make a lethal mixture. SO2 and water could adsorb onto the soot particles and be catalysed by metals on the soot surface to form sulphuric acid. The sub-micron (μm) soot particles could then penetrate deep into the lung where the adsorbed sulphuric acid could damage the cell surface and impair oxygen transfer. People with existing respiratory problems would be particularly affected by the combination of soot and SO2. However, today in countries such as the U.K. domestic fuel is predominantly natural gas which is very low in sulphur, and vehicles themselves produce little SO2, but developing

countries still have a potential SO2 pollution problem. Vegetation is also adversely affected by high doses of SO2, whose incorporation into the plant leads to sulphite and bisulphite ion formation. These ions affect the functioning of guard cells within plants, causing them to open more widely and can lead to loss of water and facilitate the entrance of other gaseous pollutants. SO2 also reacts with many materials and in the urban environment can lead to damage of stonework and steel structures. In the atmosphere, SO2 is removed by dry and wet deposition with a lifetime of a few days typically. NOx NO and NO2, known collectively as NOx are also produced during combustion by the Zeldovitch mechanism O + N2 NO + N (1) N + O2 NO + O (2) Both reaction (1) and (2) are extremely endothermic and therefore will only occur at high temperatures. A second source can also occur from nitrogen in the fuel itself. Primary NOx emissions are dominated by vehicles, typically constituting around 50% of the total burden, although coal is also still an important source. Since temperature of combustion is so critical for NOx production, a great deal of effort has been expended in developing low NOx industrial burners and in the design of engines to operate at a lower peak temperature. The fitting of converters to cars, which catalyse the reduction of NO to N2, has had a marked impact where they have been adopted, in reducing NOx emissions. Despite the efficiency of catalytic converters, the steady increase in vehicle use suggests that NOx global emissions will rise markedly over the next century. In urban areas, during weekdays, NOx concentrations have a typical diurnal cycle following the peak rush hours in the morning and evening. NO2 is less soluble than SO2 and can therefore penetrate deeper into the lung, causing respiratory damage and is more toxic than NO. Urban NOx smogs can occur in both summer (photochemical) and winter (primary build-up). In vegetation NOx can interfere with nitrite and nitrate reductase activity, leading to poor plant function. NO and NO2 are rapidly inter-converted in the atmosphere and NO2 can be removed by wet and dry deposition but is also converted into either nitric acid or organic nitrates, which can also be wet and dry deposited, with a combined lifetime of 1-2 days. CO The combustion of fossil fuels will lead to the production of carbon dioxide, CO2, and one might expect that incomplete combustion would lead inevitably to some CO production. Nevertheless, one single source dominates CO emissions, petrol engine vehicles. Catalytic converters in modern cars can catalyse the oxidation of CO to CO2 with 90% efficiency, although CO2 is non-toxic its build-up in the atmosphere is cause for concern. On a molecule per molecule basis, CO can combine with haemoglobin in the blood (to form carboxyhaemoglobin) some 200 times more efficiently than oxygen combine with haemoglobin. High exposures, 30 ppm for example, can cause headaches and impaired mental agility. CO is

oxidised in the atmosphere, via reaction with the hydroxyl radical (•OH) to form CO2, with a lifetime of approximately a month.

VOCs A myriad of volatile organic compounds exist in the urban environment, including alkanes, alkenes, alkynes, aromatics, halocarbons, carbonyls and alcohols. These VOCs are largely derived from vehicle emissions and solvent use, such as adhesives, aerosol propellants, printing, cleaning of metals and from paints. Industrial emissions of VOCs from the oil industry are another major source that may impact on the urban environment, while natural emissions from plants and trees also make a significant contribution to the urban and regional loading. VOCs play a crucial role in the photochemical formation of ozone (see later) but can themselves be harmful to human health. VOCs such as benzene and 1-3 butadiene are known to be carcinogenic, while many aromatic compounds such as the polyaromatic hydrocarbons (PAH) are mutagens. VOCs are photooxidised (i.e. their oxidation is driven by the action of light) in the atmosphere to form H2O and CO2 ultimately, where the initiator is usually the hydroxyl radical, but the nitrate radical, NO3, and indeed ozone, O3, can also be important oxidants. The lifetimes of VOCs vary enormously, 10 years for methane, months for ethane, weeks for acetylene (ethyne) and a few hours for 1,3 butadiene. Direct photolysis of carbonyl compounds such as formaldehyde (methanal), is an important additional loss process for this class of compounds and can yield in the case of methanal both radicals (60%) and molecular products (40%) HCHO + hν → •H + •HCO (3a) HCHO + hν → H2 + CO (3b) Primary aerosols Anthropogenic activities will inevitably generate primary aerosol; combustion will give rise to soot particle production, whilst actions such as grinding, drilling and crushing will release a range of particles into the atmosphere. Coal burning was the major source of black smoke or soot, but more recently diesel road vehicles have become a major source of black smoke in urban areas. Natural sources of primary aerosol, such as pollen, wind blown dust and soil erosion dominate in the background atmosphere. However, anthropogenic sources far outweigh these natural sources in the urban environment. The impact of aerosol particles on human and animal health is directly related to their size and composition. Large particles (many tens of μm) are removed rapidly from the atmosphere by sedimentation and indeed particles whose diameter is larger than around 10 μm are removed effectively by impaction with the nasopharyngeal region in humans and do not penetrate further into the respiratory system. However, aerosol particulate matter with a diameter of less than 10 μm (PM10) can penetrate beyond the nasal region to the tracheobronchial region. Here, removal by direct impaction is slow and only very small particles, whose diameter is smaller than around 0.1 μm, have Brownian diffusion velocities that are large enough for them to be lost to the walls of the trachea and bronchus. There is then a range of particle diameters, around 0.1-1.0 μm, which can penetrate deep into the lung and are therefore

believed to be linked to respiratory illnesses such as asthma and bronchitis. Since smoke particles are typically 1 μm or less in diameter, they are therefore the dominant toxic aerosol in the urban environment.

In Box 3 table 1 summarises the main sources of these primary pollutants and table 2 summarises their potential health effects. The trends in emissions of these primary pollutants from 1970-2003 are shown in figure 4 . It is clear that emissions of primary pollutants have been declining steadily over the last 30 years as cleaner combustion technology has been adopted in the UK.

Exposure to primary pollutants

It goes without saying that the highest exposure to these primary pollutants will be close to areas of high emissions, such as busy roads. However, there are other factors that must be considered that can at times influence the level of pollutants humans are exposed to and these are building geometry and wind speed and direction. A parameter called the aspect ratio R of a street is defined as the height of the buildings in a street divided by the width of that street. Therefore as R increases so the street becomes narrower, when R is greater than about 0.7 the possibility of skimming flow exists illustrated in Box 4 . If the wind speed exceeds about 1.5 ms-1 and blows perpendicular to the street, i.e. across the top of the buildings from one side of the street to the other, a vortex can be set up that allows pollutants to be recirculated around the street and prevent the pollutants from escaping (see figure 5 ). Box 4 also shows some measurements made at the Marylebone Road in London when the wind was blowing from the south side to the north side of the street at roof level (figure 6 ). A vortex was created that blew pollutants from the north side to the south side of the street at ground level. During the early part of the day the wind was light and no vortex was formed, levels of pollutants were similar on both sides of the street, but as the wind picked up so the pollutant levels at the south side rose dramatically, while that on the north side decreased, so much so that the south side was at times experiencing five times the levels of pollution compared with the north side simply due to the building geometry, wind speed and direction. Wind speed itself is an important factor for exposure, under low wind speed conditions (less than 1 ms-1) pollutants can stagnate at street level and levels can build up, conversely at high wind speeds all major primary pollutants except PM10 are flushed out of the street and exposure tends to be low. At high wind speeds, road dust can be resuspended giving rise to elevated levels of particles, therefore PM10 exposure is a minimum at intermediate wind speeds (around 4 ms-1).

Secondary Pollutants

Once emitted into the atmosphere, primary pollutants can undergo photochemical oxidation, giving rise to the production of secondary pollutants. Of these secondary pollutants, ozone (O3) and some secondary aerosols are believed to be the most detrimental to health. In this section, the production of ozone is briefly discussed and the mechanism for photochemical smog formation is summarised in Box 1 .

O3 Once released into the atmosphere, NOx (•NO and •NO2) is rapidly inter-converted via reaction with ozone already present in the atmosphere, where hν represents a photon of light •NO + O3 → •NO2 + O2 (4) •NO2 + hν → •NO + O(3P) (5) O(3P) + O2 + M → O3 + M (6) Under typical conditions experienced in the urban environment, reactions (4) – (6) are rapid and levels of these pollutants would stabilise quickly if these are the only ones in operation. In order to produce an excess of ozone, i.e. during a photochemical smog episode, alternative reactions converting NO to NO2 other than reaction (4) are required. Since vehicular NOx emissions are predominantly NO, the reaction •NO + •NO + O2 + M → 2•NO2 + M (7) is a mechanism for converting •NO to •NO2, without destroying ozone. However, the reaction is slow and is only important at extremely high •NO levels, i.e. very close to the emission source (e.g. tailpipes of vehicles). The major route for •NO to •NO2 conversion involves the reaction of NO with the hydroperoxy radical (•HO2) and organic peroxy radicals (termed RO2)

NO + •HO2 → NO2 + •OH (8) NO + •RO2 → NO2 + •RO (9) •RO + O2 → •HO2 + R-HO (10)

Reaction (9) produces an alkoxy radical (•RO) that reacts with oxygen in the atmosphere instantly to produce a hydroperoxy radical and a carbonyl compound. Subsequent photolysis of •NO2, via reaction (5) and addition of ••O(3P) free radical to oxygen in reaction (6) leads to net production of ozone. The photooxidation of volatile organic compounds (VOCs) provides the source of peroxy radicals (•RO2) but for simplicity CO is used as the most simple example of photooxidation initiated by the •OH radical: First, •OH is generated by the photolysis of ozone to produce excited O atoms termed O(1D), which can then react with water to yield •OH radicals O3 + hν → O(1D) + O2 (10) O(1D) + H2O → •OH + •OH. (11) Once formed, the OH radical reacts with CO to produce a hydrogen atom and carbon dioxide, the hydrogen atom instantly adds to molecular oxygen, giving rise to a hydroperoxy radical (•HO2). Reaction (8) converts •NO to •NO2 and •HO2 to •OH. The whole sequence of reactions below yields a net production of ozone •OH + CO → •H + CO2 (12) •H + O2 + M → •HO2 + M (13) NO + •HO2 → NO2 + •OH (8)

•NO2 + hν → •NO + ••O(3P) (5) ••O(3P) + O2 + M → O3 + M (6) NET CO + 2O2 → CO2 + O3 Photochemical smog formation is driven then by the presence of NOx, VOCs and sunlight and the spatial extent and severity of the smog will depend on the mix of all three, together with local meteorology. Photochemical smog episodes are therefore restricted to those periods of the year when a given location experiences its most intense periods of sunlight and will be exacerbated by still conditions. In urban areas and their environs, NOx levels are usually very high, and ozone production is said to be NOx – saturated. At these very high levels, NOx can itself retard ozone formation by the reaction of NO2 with the OH radical to form nitric acid •OH + •NO2 + M → HNO3 + M. (14)

Reaction (14) is a radical termination reaction (since two free radicals produce a molecule), since HNO3 is removed by wet and dry deposition and is a sink for •OH. Hence the highest levels of O3 are usually found downwind of cities. In Box 1 figure 2 real data from the NETCEN archive for the Bristol area are to used to illustrate a typical photochemical smog episode and figure 3 summarises the reaction scheme involving reactions (3) – (13) pictorially.

The ratio of VOC/NOx is a critical parameter in determining the rate of ozone production, with a maximum production rate occurring at a ratio of around 8. Reducing photochemical smog formation is not a straightforward issue, one can attempt to reduce initial NOx levels, which will lead to lower total ozone production, but will mean that peak ozone is experienced closer to the city, and therefore greater exposure to humans. Reducing VOC levels will move the ozone peak further away from the source, and typically further away from human exposure, but will not reduce the total ozone produced. Emissions of VOCs from natural sources (e.g. from plants and trees) can be very high in the summer months. If these natural emissions are added to the reactive ‘plume’ leaving a major city they can also prolong photochemical smog episodes and is a factor that is very hard to legislate for. During the recent high ozone episode in August 2003, natural emissions, in concert with NOx from human activity, were largely responsible for extremely high surface ozone observed just north of London. Reduction of surface ozone is extremely important, at 200 ppb. ozone is thought to damage cells at the surface of the bronchia and the alveoli in the lungs themselves, leading to inflammation of the airway in humans. A variety of studies suggest that effects of ozone exposure can be observed above a threshold of around 50 ppb. The debate continues whether ozone induces or exacerbates asthma, particularly in the young and old. In plants, the effect of ozone is more readily observed, where brown spots appear on leaves. On entering the stomata, ozone reacts rapidly, producing hydrogen peroxide and free radicals but does not penetrate far into the plant. Ozone, like SO2 reduces the rate of photosynthesis, and therefore can reduce crop yields. Secondary organic aerosols (SOA)

Aerosol formation in the atmosphere can reduce visibility and affect climate and chemical processes. Secondary organic aerosols are formed via gas-to-particle conversion of either low-volatility or highly soluble organic species. Since the formation of these precursors are often photochemically driven, it is common for secondary aerosol formation to accompany photochemical smog formation. It should be noted that production of SOA can occur naturally, however, during photochemical smog episodes, reduced organic compounds such as alkanes and alkenes are often oxidised to products of the same carbon number, but of a higher molecular weight. These products will include functional groups such as carboxylic acids (-COOH), aldehydes (-CHO), ketones (-C(O)C), alcohols (-OH) and nitrates (-ONO2). All of these functional groups tend to reduce the volatility of the compound relative to its precursor and can increase the atmospheric loading of sub-micron (<10-6 m) particles. Further Reading Several excellent books and web-sites exist and a few representative ones are given here. Air Pollution in general J. Colls, Air Pollution, E & FN Spon, 1997.

PORG Report: This provides a comprehensive discussion of pollution and pollutants in the UK. http://www.edinburgh.ceh.ac.uk/pollution/docs/PORGiv.htm

National Expert Group on Transboundary Air Pollution final report: Further reading on pollution in the UK and Europe.http://www.nbu.ac.uk/negtap/finalreport.htm

Health effects and background to Air Pollution: A great web-site written by Edinburgh medics.http://www.portfolio.mvm.ed.ac.uk/studentwebs/session4/27/introduc.htm Acid Rain: A comprehensive web-site on acid rain. http://schools.ceh.ac.uk/advanced/acidrain/acidrain1.htm Art Exhibition: A fascinating look at smog. http://www.lshtm.ac.uk/art/smog/

Databases available on the web

National Atmospheric Emissions Inventory: Catalogues all estimates of emission sources for each major pollutant in the UK. http://www.naei.org.uk/

NETCEN data: An excellent database of pollutant measurements made

across the UK, some sites date back to the late 1970s. Species listed include, NO, NO2, CO, SO2, PM10 and VOCs. Look at pollution trends over the years, seasonal or even daily variations in your own area. http://www.aeat.co.uk/netcen/airqual/data/autoinfo.html

Dirty Air D.E.Shallcross Education in Chemistry, September 2006