INTRODUCTION		Return to other reports prepared for DETR
OZONE CHEMISTRY
OZONE CONCENTRATIONS IN THE UK
OTHER PHOTOCHEMICAL OXIDANTS		Access the authors' website here
HYDROCARBONS IN THE UK
OXIDES OF NITROGEN
EFFECTS OF OZONE ON VEGETATION		Published by: The Centre for Ecology and Hydrology - Edinburgh, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK ISBN: 0-870393-30-9
EFFECTS OF OZONE ON MATERIALS
EFFECTS OF OZONE ON HUMAN HEALTH
OZONE IN A EUROPEAN PERSPECTIVE
PHOTOCHEMICAL OXIDANTS AND THE EFFECTS OF EMISSIONS CONTROLS

INTRODUCTION

OZONE CHEMISTRY

		The formation of photochemical ozone results from the sunlight-initiated oxidation of volatile organic compounds (VOCs) in the presence of the nitrogen oxides, NOx (= NO and NO2). The rates and mechanisms of the oxidation of VOCs are generally well characterised and quantified by laboratory study, although some significant uncertainties remain for particular classes of VOC (e.g. aromatic hydrocarbons and terpenes).
		The chemistry interconverting oxidised nitrogen compounds is reasonably well understood, and explains the formation of a range of inorganic and organic species which can act as reservoirs for NOx and hydrogen-containing free radicals. At night, the nitrate radical (NO3) plays a central role in the chemistry of oxidised nitrogen species.
		Certain chemical reactions in the condensed phase, either on surfaces or in liquid droplets, may influence the concentrations of key species involved in the gas phase mechanisms and are as yet poorly understood.

OZONE CONCENTRATIONS IN THE UK

		The UK has a rural network of 17 stations and an urban network of 36 (1/5/97) stations which provide the broad regional spatial patterns in O3 concentrations throughout the country and the concentrations in representative urban areas respectively. The data are freely available to the public and distributed by CEEFAX/TELETEXT, a freephone number (0800 55 66 77) and the INTERNET (/netcen/airqual/).
		The annual mean ozone concentrations in the UK vary between regions and with topography. The largest mean concentrations occur in rural areas, and in all areas mean concentration increases with altitude. The low altitude (< 200 m) concentrations are largest along the south coast of England.
		Annual mean ozone concentrations are generally lower in urban areas, by on average 20% to 40% of the nearby rural concentration, representing a 5 to 10 ppb reduction.
		Ozone concentrations are, on average, larger at the coast by about 20%, the effect being restricted to the area within 5 to 10 km of the coast.
		There are clear annual and diurnal cycles in ozone concentration in the UK (and elsewhere in Europe) with a spring maximum and an autumn minimum and mid afternoon peak and nocturnal minimum respectively.
		Concentrations of ozone exceed thresholds for effects on vegetation and human health throughout the UK. The largest and most frequent exceedances occur in southern England and especially in rural areas of SE England.
		Maps of mean ozone concentrations and the thresholds for effects on vegetation and human health have been constructed for the UK. The maps are subject to considerable uncertainties as a consequence of the limited number of monitoring stations and the complexity of the landscape.
		These maps show exceedance of critical levels for effects on vegetation over large areas of the UK (detailed in the summary of effects in chapter 7.1 and in section 3.4).
		Maps of the average exceedance of the EPAQS ozone standard (8 hour running mean over 50 ppb) from 1990 to 1995 show exceedance of the 97th percentile threshold throughout approximately 98% of the UK.
		Trends in annual mean O3 concentration are significantly positive (at the 5% level) at 9 sites. At sites which are appreciably influenced by local pollutant sources, the trend is negative but small and variable, averaging -0.1% per year.
		The magnitude of peak concentrations has declined, with the average monthly peak concentration during the period 1986 to 1994 being 20 ppb to 30 ppb smaller than during the period 1972 to 1985 and a decline in the monthly 95th percentile showed a decline averaging 0.8 ppb per year. This is a very important improvement in air quality in the UK as the peak concentrations are associated with human health and vegetation effects.
		The exceptionally hot sunny summer of 1995 led to an expectation of peak ozone concentrations similar in magnitude to those recorded during the similarly hot sunny summer of 1976. However, although concentrations were higher than those recorded in recent years, they failed to reach the levels observed in 1976. This was partly due to reductions in emissions of precursor species in some European countries but also to air circulation patterns over Europe and the UK, which frequently brought relatively clean air into the UK from N. Europe.
		The main sink for ozone in the UK is dry deposition to the ground, which over the year represents a total of ca 1500 kt O3 over the country.
		In urban areas, the removal of O3 by reaction with NO represents a further sink for O3. The depletion of O3 in the UK is greater on average than the photochemical production, so that at the downwind coast, the concentrations are generally smaller than those upwind. On a larger scale the photochemical production of O3 and other oxidants in the UK 'plume' downwind of the coast extends over several hundreds of km.
		Peak concentrations of ozone are positively associated with other pollutants such that during periods with concentrations of ozone in excess of 60 ppb, the concentrations of SO2 and NO2 are also substantially larger than their mean values.

OTHER PHOTOCHEMICAL OXIDANTS

		Peroxyacetyl nitrate (PAN), hydrogen peroxide and nitric acid are the most important products of photochemistry besides ozone, but none are thought to reach damaging concentrations in Britain.
		Hydrogen peroxide concentrations measured at Harwell, Oxfordshire, between 1992 and 1994 show no evidence of a long-term trend since 1988. Average concentrations were 0.2-0.3 ppb, with hourly maximum not exceeding 3 ppb.
		Long-term PAN measurements have been made in eastern Scotland and western Ireland. Annual concentrations (geometric mean) were 50-100 ppt in Scotland and 30 ppt in Ireland, smaller than at Harwell (ca 175 ppt). The Harwell data showed no long-term trend between 1987 and 1994, although data capture was low in 1992-94.
		PAN concentrations were only correlated strongly with ozone concentrations during photochemical episodes. Large (>0.5 ppb) winter PAN concentrations were not always associated with large ozone concentrations.
		PAN episodes in eastern Scotland occurred when anticyclonic conditions brought polluted air from mainland Europe north over England.

HYDROCARBONS IN THE UK

		Total emissions of VOCs in the UK have increased from 2266 kt in 1970 to 2337 kt in 1995. The current inventory indicates that total annual emissions have declined since 1989.
		Mobile sources and solvent usage are the two largest source categories and together account for about 70% of UK emissions of volatile organic compounds.
		Speciated inventories are now available for over 100 VOC compounds.
		Emissions of volatile organic compounds of biogenic origin in the UK were given as 50-100 kt yr -1 in the last PORG report. Since then further work has failed to reduce the uncertainty in this estimate, one model predicting less than this and another predicting more.
		The UK has now established a unique national network of hydrocarbon monitoring sites across the UK reporting data hourly on 25 species from 11 urban and 1 rural site.
		Network sites generate data on two carcinogenic compounds, benzene and 1,3 butadiene.
		The UK National Air Quality Strategy has recommended air quality standards for benzene and 1,3 butadiene. Network monitoring data show that current urban concentrations of these compounds are below the standards.
		A complete year of data is available for 1995 and 1996 for most sites and it is apparent that the urban concentrations of the C2-C8 hydrocarbons are heavily influenced by motor vehicle emissions and natural gas leakage.
		Local episodes of elevated concentrations are observed at many sites. These are caused by reduced dispersion of local primary emissions of hydrocarbons.
		Measurements of a series of oxygenated hydrocarbons have been made at Harwell, Oxfordshire as part of the EMEP network. Acetone is the most abundant oxygenated hydrocarbon measured and methyl glyoxal the most abundant bi-functional oxygenated organic compound.
		Consideration of rural hydrocarbon measurement data suggest that concentrations may have declined substantially over the period 1970-1990.
		The measured hydrocarbon concentration data have been compared with emission inventories. The automatic measurements point to the national inventory underestimating the emissions of C2-C8 hydrocarbons by about 40%.
		Direct, on board measurements of exhaust emissions from motor vehicles are in good agreement with kerbside measurements for all VOCs except acetylene.
		Methane is an important VOC in the atmosphere, making a significant contribution to regional ozone formation. Computer modelling studies have shown that methane accounts for 15-20% of ozone formed photochemically over north West Europe.
		The methane concentrations monitored at Mace Head, Ireland continued to rise up until 1991, when the annual increase slowed somewhat. The increase picked up again in 1994 and 1995; background methane concentrations now stand at about 1805 ppb.
		An analysis has been made of the contributions to photochemical ozone formation which can be sustained by the observed concentrations of 87 individual hydrocarbons reported by the monitoring networks and determined from detailed surveys. The top 10 most important hydrocarbons are i-butene > propene > ethene > isoprene > 1,2,4- trimethylbenzene > m+p-xylene > 1,3,5- trimethylbenzene > transbut-2-ene > toluene > transpent-2-ene
		Biogenic hydrocarbons are unlikely to account significantly for the elevated ozone concentrations observed in the United Kingdom. These will contribute to background ozone concentrations and hence may indirectly influence peak concentrations. Two biogenic hydrocarbons, isoprene and alpha-pinene, have been found to account for 5.5% and 0.4% respectively, of the ozone forming potential of the observed distribution of hydrocarbons in the United Kingdom.

OXIDES OF NITROGEN

		Emissions of NOx in the UK peaked in 1990 at 822 kt NOx-N and in 1994 were 675 kt.
		Most UK NOx emissions (83%) are exported from the country, as NO2, by the wind. The deposition of oxidized nitrogen as dry deposited NO2 and wet deposited NO3 - of 142 kt N represents 17% of emissions.
		In some large conurbations NOx concentrations have declined over the last 5 years in line with national emissions (~ -20%). However, NO2 concentrations in such areas have changed little as they are governed by the supply of O3 for oxidation of the primary emissions of NO.
		Close to major conurbations there is evidence that rural NO2 concentrations have increased since the 1980s, and throughout the UK, NO2 concentrations are substantially larger than SO2. However, most rural areas show a small decline in NO2 concentrations over the period 1987 to 1995.
		In urban areas NOx concentrations exceed the European Commission guide values and NO2 concentrations exceed the European Union limit values. These exceedances are expected to decline as urban NOx emissions decline over the next 5 years.
		Epidemiological studies have shown that day to day variations in concentrations of NO2 are associated with adverse health effects. These studies include both time-series studies of daily hospital admissions and geographical studies comparing lung function in different areas. Although these effects many not be as marked as those associated with ozone and particles they are significant and, from the health standpoint, NO2 remains an important air pollutant.
		The toxicity of ambient air pollution mixtures characterised by raised concentrations of NO2 seems greater than that of NO2 itself at similar concentrations. This apparent anomaly could be explained by associations between NO2 and other pollutants.
		Studies of the effects of exposure to NO2 indoors have shown an association between the presence of gas cookers and respiratory symptoms in asthmatic and non-asthmatic subjects, mostly children. More work is needed to identify the causative agent though a number of workers have suggested that NO2 may be an important factor.
		In a study of an episode of pollution in London in 1991, characterised by high levels of NO2 and particles, adverse health effects were detected but it was not possible to distinguish between the possible effects of particles and NO2.
		Because of new knowledge of the important phytotoxic effects of nitric oxide (NO) the present critical level for impacts of nitrogen oxides on vegetation is a combined value for NO plus NO2 (termed NOx). The critical level is 30 mg m -3 (NO2 equiv.) as an annual mean and applies to NOx for all categories of vegetation.
		Estimates of NOx distribution have been derived, using urban and rural conversion factors, from mapped values of NO2. The critical level for effects of NOx on vegetation is exceeded over 29% of the UK land area, particularly in rural central and southern England, and in and around major urban areas. In areas where the critical level is exceeded there may be adverse direct effects of NOx on vegetation although the nature of the effects is uncertain. Semi-natural vegetation is potentially the most sensitive to NOx exposure because it is often limited by the supply of nutrient nitrogen.

EFFECTS OF OZONE ON VEGETATION

		The assessment of environmental risk related to ozone effects on vegetation is currently based around the critical level concept. Critical levels are defined using the AOT40 index - the cumulative exposure over 40ppb during a growing season.
		Since the last report the knowledge of critical levels for ozone effects on vegetation has greatly improved. At an international workshop in Kuopio, Finland in April 1996, revised critical levels of ozone were agreed to prevent damage to the most sensitive crops, forests, and semi-natural vegetation. This has provided the necessary basis to quantify the spatial patterns of exceedance of these critical levels in the UK.
		Using the assessment method and critical level values recommended at this workshop, the critical level for forests, an AOT40 of 10000 ppb h over six months, is only exceeded in southern Britain. The area in exceedance of the critical level represents 15% of UK land area, and 23% of UK forest area. In contrast, the critical level for crops and semi-natural vegetation, an AOT40 of 3000 ppb h over 3 months, is exceeded almost throughout England and Wales, and in many parts of Scotland. The area in exceedance of the critical level represents 81% of UK land area, 91% of UK area of arable crops, and 76% of UK area of semi-natural vegetation. There is limited experimental evidence of impacts on vegetation in certain of the areas where the critical level is exceeded.
		Exceedance of the critical level does not mean that there will be damage to vegetation, but only that the risk of damage exists for sensitive species and conditions. Furthermore, the degree of exceedance of the critical level cannot be used directly to estimate the extent of damage to vegetation, or to assess the economic impacts of ozone on vegetation. This is because exposure to high levels of ozone is correlated with high temperatures and irradiances, and high vapour pressure deficits. For example, there is considerable variation between years in the AOT40 values found in the UK, but the years with high values of AOT40, and wider exceedance of the critical level, are often the hotter and drier years in which accumulated soil moisture deficit is greater, and hence ozone uptake by vegetation is lower.
		AOT40 values are calculated over a fixed three or six month growing period, but this may not reflect actual UK growth periods. Furthermore, the ozone sensitivity of vegetation is not constant, but varies with the stage of plant development. The timing of ozone episodes varies between years and sites in the UK, and this will significantly alter the impact on vegetation.
		Analysis of the seasonal patterns of AOT40 across the UK shows large variations from year to year. Data averaged over several years showed a more consistent pattern:- in April and May, there was relatively little difference in AOT40 values across the UK, but for the rest of the summer months, there was a large gradient from southern sites, where large additional AOT40 exposures occurred, to remote northern or western sites, at which almost no additional AOT40 exposure occurred.
		Recent studies have shown large gradients in AOT40 values between the standard measurement height of the monitoring network, and the height of the vegetation, in the case of crops and semi-natural vegetation. This means that exceedance of the critical level estimated using the national network may significantly overestimate the true area of exceedance in the UK.
		There is an urgent need to estimate more accurately the real impact of ozone on vegetation in areas of the UK where the critical level is exceeded. This will require further experimental and modelling work, so that the effect of factors such as variation in sensitivity between species, the effects of climatic and edaphic factors, and the timing of ozone episodes, can be quantified. A key focus of this work will be the development of methods to map the uptake of ozone by vegetation, and to quantify the relationship between this uptake and observed effects.
		Within the UK vegetation effects assessment it has not been possible to modify the maps of AOT40 for the urban reduction in exceedance.

EFFECTS OF OZONE ON MATERIALS

		Ozone has a damaging effect on certain materials, including polymers, rubbers, surface coatings and textiles. In combination with N- and S- pollutants it has been shown to corrode metals and with S-compounds damage some types of stone.
		Material damage is related to the annual mean ozone concentration, which is lowest in urban areas where the density of at-risk materials is highest .
		A preliminary critical level for material damage was set at 20 ppb as an annual average concentration, at a UNECE workshop in 1993. This level is currently exceeded in all of the UK with the exception of some urban areas.
		Estimates of the current cost of damage are of the order of several £100 million for the UK. However the methods used to produce these estimates are very uncertain and can only give an indication that the costs are substantial.
		As measures to control the emissions of VOCs and NOx come into effect, ozone levels may rise in urban areas with an associated increase in the cost of damage to materials.

EFFECTS OF OZONE ON HUMAN HEALTH

		Ozone is the most irritant of the common air pollutants and exposure to concentrations commonly encountered in the UK has been shown to produce inflammation of the respiratory tract.
		In terms of effects on standard physiological indices of lung function there is no evidence to suggest that asthmatic subjects are more sensitive to ozone than other individuals. However there is evidence based on studies of the inflammatory response to the airways induced by exposure to ozone to suggest that asthmatic subjects are a little more sensitive than others.
		The combination of ozone and acid aerosols may have a more marked effect than exposure to ozone alone.
		Long term exposure to ozone in the USA at average concentrations greater than those encountered in the UK has been shown to be associated with a chronic decline in lung function.
		A recent study has demonstrated that ozone levels in London are associated with changes in daily mortality rates. Cardiovascular and respiratory mortality were affected with the effects being most marked during the summer season.
		Recent time-series studies of the effects of daily variations in concentrations of ozone and indices of ill-health including hospital admissions and mortality have shown that there may be a threshold of effect at about 40-60 ppb ozone, 8 hour average concentration.
		Depending on the assumptions made about thresholds of effect for ozone, a wide range of hospital admissions related to ozone could be derived. These range from 0.35% to 6.1% of total admissions for respiratory disorders. The Department of Health Committee on the Medical Effects of Air Pollutants will investigate this further and report later in 1997.
		Because admission to hospital for respiratory disease is a relatively rare event, the effect of ozone on hospital admissions, already small in relative terms, is likely to have only a small public health impact. However, panel studies indicate that ozone also increases the risk of a worsening of symptoms or increased medication requirement. Although this is a small relative effect, a large number of people are at risk and when the widespread exposure to ozone is also taken into account, the health impact of increased ozone levels is likely to be greater than the effect on hospital admissions suggests.
		New evidence from the UK and from elsewhere confirms the conclusions of the Department of Health Committee on the Medical Effects of Air Pollutants that, at the population level, air pollution has a detectable but relatively small effect on the provocation of existing asthma, and is unlikely to play a role in the incidence of new asthma.

OZONE IN A EUROPEAN PERSPECTIVE

		Ozone is the major gaseous pollutant throughout Europe and ambient concentrations regularly exceed thresholds for effects on vegetation and human health during the summer in most countries.
		The magnitude of exceedance of effect thresholds for both crops and human health increases along a gradient from the UK towards central Europe. In this respect the UK, along with the Scandinavian countries, are the areas least affected by episodes of elevated ozone in Europe.
		The transport distances of pollutants and their precursors and timescales for chemical production of ozone lead to long range transport of ozone and other photochemical oxidants throughout Europe. As a consequence we share the same air and its associated pollutants. Policies to reduce the frequency and magnitude of elevated ozone episodes require control measures throughout Europe.
		Policies to reduce the Northern hemisphere tropospheric background ozone concentration requires control measures throughout the Northern hemisphere.
		The European ozone monitoring data (1980 - 94) show no consistent trends in mean concentrations throughout the period. A small reduction in mean concentrations has been reported for the Netherlands, although the underlying cause is unclear.

PHOTOCHEMICAL OXIDANTS AND THE EFFECTS OF EMISSIONS CONTROLS

		Regional scale ozone formation and its control is an important policy issue in a number of international policy fora, including in particular, the UN ECE and the European Union.
		The work of the UN ECE has concentrated mainly on the impacts of ozone on crops and trees and has introduced the concept of AOT40 to characterise critical exposure levels of ozone. It is likely that reductions in NOx and hydrocarbon emissions approaching the maximum feasible will be required if these critical levels are not to be exceeded in future years across Europe.
		The work of the European Union has focused mainly on human health effects and the role played by motor vehicles and their fuels in the deterioration of urban air quality. Proposals are being drawn up for emission and fuel quality standards for the year 2000 and beyond. The Commission have adopted an air quality objective of 90 ppb, 99-percentile hourly mean ozone concentration, a level which is not normally exceeded in the United Kingdom except in the most photochemically active summers.
		The National Air Quality Strategy has set a challenging health-based provisional air quality objective for the UK of 50 ppb as an 8-hour rolling mean expressed as a 97th percentile. VOC and NOx emission reductions approaching the limits of technical and economic feasibility will be required, going far beyond current policies, if the ozone air quality objective in the National Air Quality Strategy is to be met in the future.
		Further consideration will need to be given to background tropospheric ozone if strict vegetation-based environmental criteria expressed in AOT40 terms are not to be exceeded in the UK and in the rest of Europe. This will require action on ozone precursor emissions at a global scale.

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