Informal Document No.10 

Convention on Long-range Transboundary Air Pollution 

Executive Body 28th Session 

Hemispheric Transport of Air Pollution 2010 

Executive Summary 

ECE/EB.AIR/2010/10 Corrected 

Prepared by the Co-Chairs of the 

Task Force on Hemispheric Transport of Air Pollution 

Contents 

Paragraphs Page

 I. Introduction.................................................................................................... 1 1 

 II. General findings............................................................................................. 2 2 

 III. Main findings on ozone.................................................................................. 3–22 2 

 A. Observational evidence.......................................................................... 3 2 

 B. Global and regional modelling .............................................................. 4–13 2 

 C. Impacts of intercontinental transport ..................................................... 14–18 4 

 D. Future scenarios for emissions and climate ........................................... 19–22 5 

 IV. Main findings for particulate matter............................................................... 23–38 5 

 A. Observational evidence.......................................................................... 23 5 

 B. Global and regional modelling .............................................................. 24–31 6 

 C. Impacts of intercontinental transport ..................................................... 32–36 7 

 D. Future scenarios for emissions and climate ........................................... 37–38 7 

 V. Main findings for mercury ............................................................................. 39–48 8 

 A. Observational evidence.......................................................................... 41 8 

 B. Global and regional modelling .............................................................. 42–45 8 

 C. Impacts of intercontinental transport ..................................................... 46 9 

 D. Future scenarios for emissions and climate ........................................... 47–48 9 

 VI. Main findings for persistent organic pollutants.............................................. 49–61 9 

 A. Observational evidence.......................................................................... 50 10 

 B. Global and regional modelling .............................................................. 51–57 10 

 C. Impacts of intercontinental transport ..................................................... 58 11 

 D. Future scenarios for emissions and climate ........................................... 59–61 11 

 VII. Main recommendations.................................................................................. 62–68 12 

 Annex 

 Relative Annual Intercontinental Responses.................................................. 14 

I. Introduction 

1. The Task Force on Hemispheric Transport of Air Pollution was created by the Convention on Longrange Transboundary Air Pollution in December 2004 to improve the understanding of the 

intercontinental transport of air pollutants across the Northern Hemisphere. Under the leadership of 

the European Union and the United States of America, the Task Force has organized a series of 

projects and collaborative experiments designed to advance scientific understanding of the 

intercontinental transport of air pollution. The Task Force has convened a series of 15 meetings or 

workshops in a variety of locations in North America, Europe, and Asia, which have been attended by 

more than 700 individual experts from 38 countries. The Task Force intensively cooperated with the 

Convention’s centres and task forces and other relevant international and regional organizations. This

summary, produced by the Task Force Chairs, presents the findings and recommendations of a 

multivolume assessment produced by the Task Force reviewing the state of the science with respect to 

the intercontinental transport of ozone (O3), particulate matter (PM), mercury (Hg) and persistent 

organic pollutants (POPs).1 

 

1

 To be printed in the United Nations Economic Commission for Europe (UNECE) Air Pollution Series. 
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II. General findings 

2. O3, PM, Hg, and POPs are significant environmental problems in many regions of the world. For each 

of these pollutants, observed concentrations or deposition at any given location can be thought of as 

composed of several different fractions, one of which is related to the intercontinental transport of 

anthropogenic emissions. Other fractions may be associated with natural emission sources or local 

and regional anthropogenic sources. Each fraction differs in terms of the emission sources that 

contribute to it, the temporal and spatial variability of the contribution, and the potential for and 

sensitivity to emission controls. In most cases, mitigating local or regional emission sources is the 

most effective approach to mitigating local and regional impacts of these pollutants. However, 

without further international cooperation to mitigate intercontinental flows of air pollution, many 

nations are not able currently to meet their own goals and objectives for protecting public health and

environmental quality. With changing global future emissions, it is likely that over the next 20 to 40

years it will become even more difficult for nations to meet their own environmental policy objectives

without international cooperation to address transboundary and intercontinental flows of air pollution. 

Furthermore, cooperation to decrease emissions that contribute to intercontinental transport of air 

pollution has significant benefits for both source and receptor countries. 

III. Main findings on ozone 

A. Observational evidence 

3. An increasing trend in O3 concentrations has been measured consistently at a number of remote sites 

across the Northern Hemisphere, suggesting an increase in the hemispheric baseline O3 concentration 

of a factor of two during the latter half of the twentieth century. It is likely that much of this change is 

due to increases in anthropogenic emissions of O3 precursors. More recently, a more rapid increase 

has been observed taking place downwind of Eastern Asia in the free troposphere, whereas the 

increases within the boundary layer of Central Europe and North America have slowed down. 

Measurements at some locations on the western coasts of Europe and North America clearly show 

that trans-oceanic air flows can carry O3 concentrations that approach or exceed some air quality 

standards or objectives. 

B. Global and regional modelling 

4. The hemispheric transport of air pollution (HTAP) multi-model experiments have provided the first 

set of comparable estimates of intercontinental source-receptor relationships from multiple models by 

examining the impacts of 20% emission reductions of relevant anthropogenic pollutants in four 

regions, approximately covering the major populated areas of North America, Europe, South Asia, 

and East Asia. These names are used hereinafter to refer to these rectangular regions, which 

encompass more than 75% of the anthropogenic emission sources in the Northern Hemisphere and, in 

some cases, include significant areas of ocean. Specific analyses were also made to quantify the 

impact in the Arctic of emission changes in these four source regions. 

5. The annual average ground-level O3 mixing ratio averaged across the four regions and the ensemble 

of participating models is about 37 parts per billion by volume (ppbv) (± 4 ppbv standard deviation). 

This annual average, region-wide O3 concentration masks large seasonal and geographic variability, 

and large differences across models. However, it provides a useful point of reference for considering 

the magnitude of intercontinental transport. Based on estimates in the literature, about 20–25% of this 

annual average ground-level concentration originates in the stratosphere and a similar fraction is 

formed from natural emissions of precursors. The remainder is due to anthropogenic sources of 

precursors from within the region itself and transported from outside the region. The relative 

contribution of anthropogenic and natural, regional and extraregional sources varies by location, 

season, and year. 

6. Models indicate that for Europe, East Asia, and North America the absolute contribution of 

intercontinental transport to ground-level O3 typically peaks in spring and fall, and is smallest during 

 

Advanced drafts are available at www.htap.org. 
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summer when O3 concentrations reach their highest maxima due to the peak in production from local 

and regional emissions. In these regions in winter, intercontinental transport of O3 may exceed local 

and regional production. For South Asia, which is largely dominated by the summer and winter 

monsoon regimes, the absolute contribution of both intercontinental transport and local and regional 

production of O3 are largest during the winter monsoon period and the annual contribution of 

intercontinental transport does not exceed that of local and regional production. Intercontinental 

contributions to ground-level O3 in the Arctic are larger than production from local and regional 

emissions and peak in April through June, with a secondary maximum in October and November. 

Taken together, these results indicate that decreasing local or regional emissions is more effective at 

decreasing the highest O3 levels, but that there is a significant fraction of tropospheric O3 that is not 

within the control of local and regional political jurisdictions. 

7. To quantify the relative importance of emissions changes outside each of these regions as compared to 

emissions changes inside each of these regions, we defined the Relative Annual Intercontinental 

Response (RAIR) metric, which is the sum of the changes in the annual average, regionally averaged 

concentration within a region due to a 20% decrease in emissions in the three other regions divided by

the sum of the changes in concentration within a region due to a 20% decrease in emissions in all four

regions. The value of the metric ranges from 0%, indicating no intercontinental influence, up to 100%,

indicating that air quality in a region is completely dominated by intercontinental sources. Thus, the

RAIR is a measure of how much benefit a region may receive from emission reductions in other 

regions when emission reductions are coordinated on an intercontinental scale. The RAIR values 

calculated in the HTAP multi-model experiments for some of the different pollutants or parameters 

considered are tabulated in the annex to this document. 

8. For O3, the impact of 20% changes in anthropogenic emissions of nitrogen oxides (NOx), VOCs, CO, 

SO2, and primary PM in one region on ground-level O3 in the other regions varies from 0.07 to 0.37 

ppbv O3 on an annual average, region-wide basis, as estimated by the mean of the model ensemble. 

These values are significant in comparison to the response of ground-level O3 to 20% decreases of 

emissions within the region itself, which vary from 0.82 to 1.26 ppbv. The RAIRs (shown in the 

annex) indicate that, in all four of the source/receptor regions, at least 30% of the total concentration 

changes within each region is related to emission changes in the other three regions. The RAIRs are 

larger for column O3 than for ground-level O3, and the column RAIRs exceed 50% in Europe and East 

Asia. 

9. On an annual average, region-wide basis, the largest source/receptor relationship is the impact of 

North American emissions on European ground-level O3. This is followed by the impact of European 

emissions on South Asian and East Asian ground-level O3. The annual average impact of East Asian 

emissions on North American ground-level O3 is similar to the impact of North American emissions 

on East Asian ground-level O3, but with peaks in different seasons. European emissions have the 

largest influence on Arctic ground-level O3 followed by North American emissions. 

10. Within a region, annual average, region-wide O3 concentrations are most strongly influenced by 

changes in emissions of NOx, VOCs, methane (CH4), and CO, in decreasing order of importance. The 

intercontinental contributions to annual O3 concentrations, however, are most strongly influenced by 

changes in CH4, followed by NOx, VOCs, and CO. Decreasing anthropogenic CH4 emissions in a 

given region was shown to produce roughly the same decrease in intercontinental transport of O3 to 

other regions as a similar percentage decrease in NOx, VOCs, and CO emissions combined. The O3

response to changes in CH4 emissions, however, requires several decades to be fully realized, given 

the relatively long atmospheric lifetime of CH4. 

11. The HTAP multi-model ensemble of O3 simulations was compared to O3 concentrations observed by 

selected ground-based networks and sondes (balloons) in North America, Europe, and Asia. At most 

ground-level sites and for lower and middle troposphere sonde measurements, the model ensemble 

mean generally captures the observed seasonal O3 cycle and is close to the observed regional mean 

ground-level O3 concentration. Furthermore, the model ensemble mean agrees well with regionally 

averaged observed values in all regions in spring and late autumn when intercontinental transport 
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tends to be strongest. This ability gives us some confidence that we can quantitatively represent the 

key processes controlling the formation, transport and removal of O3 and its precursors. 

12. However, there are notable biases in the model ensemble mean: underestimating ground-level O3 at 

high altitude sites, overestimating summertime ground-level O3 concentrations over Japan and the 

eastern United States, and high biases for sonde measurements in the upper troposphere and at polar 

sites. 

13. In the HTAP multi-model experiments, the difference between models in terms of the response of O3

concentrations due to 20% changes in precursor emissions is appreciable in both the source region and 

at distant receptors; the standard deviation is 20–50% of the ensemble mean response. The differences 

between models and between models and observations are due to uncertainties in emissions estimates, 

limitations imposed by the spatial and temporal resolution of the models, and uncertainties in the 

representation of transport, chemistry, and removal processes. 

C. Impacts of intercontinental transport 

14. In terms of exceedances of ambient air quality standards, the highest concentrations of O3 are 

typically associated with stagnant conditions, when the contribution from transport is low and the 

contribution of local and regional sources are most important. However, intercontinental transport has

increased baseline O3 concentrations to the point where they exceed thresholds for protection of 

vegetation in many locations and exceed thresholds for the protection of human health occasionally in 

some locations. As public health-based air quality standards continue to be tightened based on new 

health effects research, the contribution of intercontinental transport to exceedances of such standards 

will continue to increase. 

15. There is considerable evidence from experimental human and animal studies and epidemiological 

studies that exposure to ambient O3 concentrations causes adverse health effects which range from 

minor sensory irritation to premature death. Relatively few studies have tried to quantify the human 

health impacts of intercontinental transport of O3 specifically. Those studies have focused on the 

relationship between annual average concentrations and premature mortality and suggest that 

intercontinental transport can contribute significantly to health impacts of air pollution within a given 

receptor region. For O3, one study based on the HTAP multi-model experiments estimated that 

intercontinental transport of O3 contributes 20% to more than 50% of O3-related premature adult 

mortalities in a given receptor region, subject to large uncertainty. 

16. The sum of the health impacts of transported pollution in downwind foreign regions can be larger than 

the health impacts of emissions in the source region itself. Although the impact on ambient 

concentrations in downwind foreign regions may be much less than in the source region itself, the 

total population exposed in those downwind regions is much greater. The HTAP multi-model 

experiments suggest that emission reductions in North America and Europe will avoid more O3-

related mortality outside these source regions than within the regions themselves. 

17. O3 causes damage to crops, forests, and grasslands, which has important implications for productivity, 

biodiversity, and food security and may be an important contributing factor to the agricultural yield 

gap that currently exists across much of Asia. Global yield losses of four staple crops due to exposure 

to O3 are estimated to be between 3%–16%, depending on the crop, and are valued at $14 billion to 

$26 billion per year. Based on the HTAP multi-model experiments, intercontinental transport may be 

responsible for 5% to 35% of the estimated crop yield losses depending on the location, crop and 

response function used, subject to large uncertainties. 

18. O3 contributes significantly to climate forcing, directly as a greenhouse gas and indirectly by 

damaging plants and inhibiting their natural uptake of CO2. Among O3 precursors, widespread 

decreases in emissions of CH4, CO, and VOCs better reduce net climate forcing over time than 

decreasing NOX, which increases the lifetime of CH4 and, thus, increases climate forcing over decadal 

time scales. Decreasing emissions of CH4, which is a greenhouse gas itself, will result in decreases in 

the direct forcing from CH4 and the direct and indirect forcing of O3, affecting the rate of climate 

change in the coming decades. 
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D. Future scenarios for emissions and climate 

19. The significance of intercontinental transport may change in the future due to changes in the 

magnitude and spatial distribution of anthropogenic emissions. The implications of changes in 

anthropogenic emissions were explored in the HTAP multi-model experiments by considering a set of 

global emission scenarios developed to inform the Intergovernmental Panel on Climate Change’s fifth 

assessment report, known as the Representative Concentration Pathways (RCPs). Three of the four 

scenarios assume some climate change mitigation policy will be adopted, and all four assume that the 

implementation of air pollution control policies will increase as development and income increase. As 

a result, all four of the scenarios suggest that between now and 2050, global emissions of NOx, VOCs, 

and CO will eventually decline. However, the regional distribution of emissions in the Northern 

Hemisphere is expected to shift, with steeper and earlier declines in Europe and North America and 

shallower declines or actual increases in South and East Asia. 

20. The impacts of this redistribution of future emissions and expected changes in future global CH4

concentrations specified by the RCPs were explored using linear approximations of the 

intercontinental source-receptor sensitivities determined under current conditions in the HTAP multimodel experiments. To illustrate the range of future levels of air pollution, we assess the RAIRs 

(provided in the annex) for 2050 under the lowest emissions scenario and for 2030, when global 

emissions peak, under the highest emissions scenario. For North America ground-level O3

concentrations, the RAIR is estimated to increase to around 50% under both the high and low 

emissions scenarios, suggesting that, in the future, changes in emissions of O3 precursors outside the 

region may be as important as changes within the region. For Europe, the RAIR for ground-level O3

increases relatively little under the highest and lowest scenarios, due to simultaneously declining air 

pollution emissions in North America. For East Asia, the RAIR decreases under a high emission 

scenario, under which emissions within the region increase, and increases under a low emission 

scenario, under which emissions in the region decline. For South Asia, the RAIR for ground-level O3

declines under both the low and high scenarios. Note that the sensitivity to changes in geographical 

distribution of emissions within a particular region has not been adequately assessed yet. 

21. Using the linear approximations of the HTAP multi-model experiments, the influence of changing 

CH4 concentrations can be separated from the influence of changing intercontinental transport and 

local and regional emissions over the historical emission trends and future emission scenarios. Under 

future scenarios, expected increases in CH4 concentrations have a large influence on ground-level O3

changes, in some cases offsetting significant decreases in O3 formation associated with local and 

regional emissions. 

22. The HTAP multi-model experiments also examined the potential impacts on ground-level O3 of 

changes in meteorology and transport patterns expected as a result of climate change. Future changes 

in climate are expected to increase the effect of precursor emissions over source regions and reduce 

the effect over downwind receptor regions. However, the impact of these changes on long-range 

transport is relatively small, and is driven by changes in atmospheric chemistry and not by changes in

transport patterns. The effect of natural emission changes and wider climate-related feedbacks have 

not been evaluated fully yet. 

IV. Main findings for particulate matter 

A. Observational evidence 

23. The most tangible evidence for intercontinental transport comes from satellite images of PM crossing 

oceans and continents in discrete plumes, often associated with biomass burning or windblown dust 

events. Within the past few years, satellites have begun to provide quantitative information on 

intercontinental transport of PM, including estimates of the amount of pollution transported, the 

altitude of transport and, in some cases, aerosol properties. Ground-based remote-sensing networks 

and mountain-top measurement sites in Europe, North America and Asia provide large continuous 

data sets that characterize the frequency of occurrence of PM transport events, the meteorological 

conditions responsible for them, and important information on aerosol properties. Evidence of 

intercontinental transport is provided also in the form of long-term trends in ground-based 
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observations from remote islands, which in some cases compare well with the trends in emissions in 

upwind areas. Significant air quality and environmental impacts of the long-range transport of PM 

have been observed, especially in the outflow of the Asian and African continents and in the Arctic. 

B. Global and regional modelling 

24. PM is comprised of different chemical components, some of which are directly emitted as particles by 

natural or anthropogenic sources (primary PM) and others have their origin in gases that are 

transformed through chemical and physical reactions and adsorbed on particles (secondary PM). Both 

primary and secondary PM play a role in intercontinental transport. Conceptually, the components of 

PM can be separated into those of natural origins (including volcanic eruptions, oceans, wind-blown 

soil dust), open biomass burning (which can be both natural and anthropogenic), anthropogenic 

emissions that have been transported on intercontinental scales, and local and regional anthropogenic 

emission sources. 

25. The HTAP multi-model experiments suggest that, in the four regions studied, ground-level 

concentrations of wind-blown soil dust from deserts in Africa, Asia, and the Middle East can be a 

factor of 1.5 to almost 20 higher than that from anthropogenic and open biomass burning sources on a 

region-wide, annual average basis. Of the ground-level PM concentration originating from 

anthropogenic and open biomass burning sources, intercontinental transport of anthropogenic 

emissions accounts for between 5% and 35% on a region-wide, annual average basis. 

26. As is the case with O3, annual average, region-wide statistics mask significant variability between 

seasons, within a given region and across models. The seasonal cycles of PM concentration and 

intercontinental transport vary by chemical component and by region. However, there are substantial 

differences between the seasonal cycles in ground-level concentration predicted by models 

participating in the HTAP multi-model experiments. The model differences are larger for wind-blown 

soil dust than they are for sulphate, black carbon and particulate organic matter, and are larger for the 

Arctic than for the mid-latitude regions. The differences between models reflect large uncertainties in 

emissions and atmospheric processes represented in the models. Despite the substantial differences in 

estimated absolute concentrations, there is much closer agreement between the participating models in 

terms of source attributions on an annual average, region-wide basis. 

27. As shown in the annex, the RAIRs estimated in the HTAP multi-model experiments indicate that 

decreases in intercontinental transport would contribute 5% to 20% of the decrease in region-wide, 

annual average, ground level PM concentrations that results from decreasing anthropogenic emissions 

by 20% in each of the four regions studied. The influence of intercontinental transport is larger for 

aerosol optical depth (AOD) (17–25% for non-soil dust components) and total column loading (24–

37% for sulphate), reflecting the importance of transport above the boundary layer and implying more 

significant contributions to visibility and radiative forcing impacts. 

28. Ground-level PM concentrations or deposition in the Arctic are most sensitive to emission changes in 

Europe. However, total column loadings of PM over the Arctic are equally sensitive to changes in 

emissions from Europe or Asia, due to the fact that Asian emissions have a stronger tendency to be 

lifted and transported aloft than do European emissions. 

29. Ground-level PM concentrations generally respond linearly to changing emissions from both local 

and upwind source regions. However, sulphate and some fraction of particulate organic matter are not 

directly emitted, but are formed in the atmosphere through oxidation, and can be affected by nonlinear systems of chemical reactions. Thus, changes in the intercontinental transport of O3 and its 

precursors can affect downwind PM concentrations, underscoring the multifaceted consequences of a 

rising background of pollution levels in the Northern Hemisphere on local pollution levels. 

30. Many of the same limitations and sources of uncertainty faced in estimating O3 concentrations apply 

to current models for PM, although there are additional challenges as well. In the HTAP multi-model 

experiments, the range of predicted concentrations is about a factor of two for sulphate ground-level 

concentrations in the mid-latitude continental regions, whereas the model differences approach a 

factor of four for black carbon and particulate organic matter ground-level concentrations, and a factor 

of seven for soil dust ground-level concentrations. For the Arctic region, where PM levels are very 
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low, the relative spread of modelled concentration estimates is much greater. A detailed comparison 

between the HTAP-participating models reveals a factor of four difference in the atmospheric lifetime 

of sulphate calculated by the different models. 

31. A comprehensive comparison of HTAP multi-model estimates with PM observations has not been 

conducted. However, available observational data to support such an evaluation are being compiled 

and some comparisons have been conducted for sulphate wet deposition. 

C. Impacts of intercontinental transport 

32. Intercontinental transport of wind-blown soil dust can cause exceedances of ambient air quality 

standards for PM and, in western North America, is likely to interfere with the attainment of visibility 

goals. Intercontinental transport of PM components other than wind-blown dust is not usually 

sufficient to exceed health-based ambient standards. However, there is no threshold for adverse health

effects from exposure to PM. 

33. Based on the HTAP multi-model experiments, the intercontinental transport of PM has influences on 

human mortality that are comparable to O3. While O3 is transported between regions more efficiently, 

the relationship between PM and mortality is stronger. Consequently, the estimated mortalities 

attributable to PM within each source region are much higher, and the contributions of the three 

foreign regions to the mortality in a given home region range from 3% to 5%. Of the total mortalities 

associated with emissions from North America and Europe, 15% and 12%, respectively, are estimated 

to be realized outside of these source regions. 

34. PM deposition also damages a variety of different ecosystems, including forests and grasslands, 

through acidification. Similarly, eutrophication is damaging to biodiversity in sensitive ecosystems of 

low nutrient status. PM can benefit ecosystems by increasing the diffuse solar radiation; however, the

contribution of long-range transport to these effects is very uncertain and may be relatively small. 

35. PM is a significant contributor to climate forcing; intercontinental transport influences the 

distributions of PM and, therefore, the extent and magnitude of its forcing. PM is a mixture containing 

components that mainly cool, including sulphate and organic aerosols, and black carbon that warms. 

Anthropogenic emissions of black carbon, CH4, CO, and VOCs are estimated to have caused a climate 

forcing since 1750 roughly as large as that from anthropogenic CO2. Reductions in PM would 

improve air quality, but for cooling aerosols, including sulphate, nitrate and particulate organic matter, 

this would generally increase warming. Reductions in black carbon would typically benefit both air 

quality and climate. 

36. The Arctic is experiencing rapid climate change. Arctic climate is affected by O3 and PM that are 

transported into the Arctic from other regions, as well as by the climate forcing of O3 and PM outside 

of the Arctic. Deposition of black carbon on snow is understood to be an important positive forcing 

(warming) in the Arctic. 

D. Future scenarios for emissions and climate 

37. As for O3, the impact of future emission changes on the intercontinental transport of PM was 

examined by considering the RCP scenarios up to 2050. All of the scenarios assume some 

implementation of air pollution control policies as development and income increase. By 2050, global 

anthropogenic emissions of SO2 are expected to have declined between 30% and 70%. European and 

North American emissions are expected to continue to decline from their current levels. East Asian 

emissions are expected to continue rising for a while before declining dramatically by 2050. For black

carbon emissions, global emissions are expected to decline between 15% and 50% by 2050, with 

dramatic declines in East Asia, which currently has the largest share of black carbon emissions. Thus,

both the magnitude and composition of the intercontinental transport of PM is likely to change in the 

future. 

38. Climate change is expected to impact PM concentrations through changes in temperature, humidity, 

precipitation patterns, and atmospheric chemistry and dynamics, as well as changes in emissions from 

wind-blown dust, natural forest fires, and biogenic hydrocarbons. The overall impact of these changes 

on the intercontinental transport of PM has not been fully evaluated. 
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V. Main findings for mercury 

39. Hg differs from other major atmospheric pollutants (e.g., O3 and PM) primarily because its 

environmental and health impacts are not directly related to atmospheric mercury burden. While the 

major redistribution pathway of Hg is atmospheric transport, its primary environmental and health 

impacts are in aquatic systems and for aquatic organisms and their consumers. Atmospheric Hg that is 

transferred to aquatic systems through dry and wet deposition mechanisms is converted from an 

inorganic form to methylmercury (MeHg) by microbes in the water and sediments of wetlands, lakes, 

reservoirs, rivers, estuaries and oceans. Unlike other forms of Hg, MeHg biomagnifies in aquatic food 

webs. Consumption of fish or other aquatic organisms with elevated MeHg concentrations is the 

primary route of exposure for humans and other freshwater and marine fish-eating wildlife. 

40. Hg is emitted into the atmosphere as elemental Hg0

 in gaseous form or/and as oxidized, ionic Hg2+ in 

gaseous form or/and bound to suspended particulate matter. Hg0, which has an average atmospheric 

lifetime of six months to a year, dominates atmospheric loading and is typically the form in which Hg 

is transported long distances. Ionic Hg is removed from the atmosphere very quickly through dry 

deposition and wet removal processes. For purposes of apportioning deposition and subsequent 

impacts back to emission sources, Hg deposition can be divided into Hg that was released originally 

from natural sources; legacy Hg that was released from anthropogenic sources originally, was 

deposited, and subsequently re-emitted; newly-released Hg that was released from anthropogenic 

sources primarily as Hg0

 and transported on intercontinental scales; and newly-released Hg that was 

released from local and regional anthropogenic sources primarily as ionic Hg and quickly deposited 

locally and regionally. 

A. Observational evidence 

41. The long atmospheric lifetime of Hg0 results in baseline concentrations of 1.5 to 1.7 ng m-3 Hg0 in the 

Northern Hemisphere and 1.1 to 1.3 ng m-3 Hg0 in the Southern Hemisphere (at sea level). This interhemispheric gradient has been observed consistently in ship-borne observations and is reproduced by 

current global models. Intercontinental transport of Hg has been observed in episodic events of 

elevated Hg0

 concentrations recorded at remote mountain-top sites and during aircraft measurement 

campaigns. Long-term changes in the atmospheric Hg burden have been derived from chemical 

analysis of lake sediments, ice cores, and peat deposits, and observed in firn air samples, and suggest 

that Hg deposition has increased about threefold since pre-industrial times. Decreasing deposition 

trends observed in Europe and North America are consistent with documented regional emission 

reductions; however, global trends in concentrations and deposition are ambiguous and may indicate 

offsetting effects between emission trends in Asia and the other parts of the world, and significant 

recycling of Hg between environmental components. 

B. Global and regional modelling 

42. Based on the HTAP multi-model experiments, natural and re-emitted Hg account for about 35% to 

70% of total Hg deposition on region-wide, annual average depending on the region, whereas 

intercontinental transport of newly-released anthropogenic Hg emissions accounts for about 10% to 

30% of total Hg deposition, on an annual globally-averaged basis. East Asia, which accounted for 

almost 40% of total global newly-released mercury in 2000, is the most dominant among the four 

HTAP source regions accounting for 10% to 14% of the annual Hg deposition found in other regions, 

followed by contributions from Europe, South Asia, and North America. However, where deposition 

is highest, local and regional anthropogenic emission sources are the dominant sources of Hg 

deposition. 

43. The Arctic has no anthropogenic emission sources within the region, so anthropogenic Hg that is 

deposited there comes from intercontinental transport. A large fraction of transported Hg is deposited

in springtime during polar sunrise in atmospheric mercury depletion events, during which Hg0

 is 

rapidly oxidized by photochemical reactants that have built up over the dark winter. 

44. The global Hg models participating in the HTAP multi-model experiments provided relatively 

consistent estimates of the impact of one source region on another despite significant differences in 

emissions and chemistry in each model. The RAIR (see annex) for East Asia is low, suggesting that 
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emission controls within East Asia are much more important for deposition levels in that region than 

are changes in intercontinental transport into the region. For Europe and South Asia, the RAIRs are 

35% and 43%, respectively, suggesting that intercontinental transport makes a significant contribution

to deposition within these regions. For North America, the RAIR is greater than 50%, suggesting 

emission reductions in East Asia, Europe and South Asia decrease region-wide deposition more than 

emissions reductions within North America itself. In the Arctic, Hg deposition could be the most 

effectively controlled by emission reduction in East Asia and Europe due to their proximity to the 

Arctic, prevailing atmospheric circulation patterns and significant contribution of these regions to 

global anthropogenic emissions. 

45. Current global atmospheric Hg models reproduce the observed ground-level Hg0 concentrations to 

within 20% of the sparse observations that are available. The agreement between models and 

observations for Hg wet deposition is weaker, with differences between observed and modelled values 

up to 100%, mainly due to uncertainties in Hg emission rates, Hg oxidation chemistry and estimated 

precipitation rates. 

C. Impacts of intercontinental transport 

46. To better evaluate the main impact that international emissions controls will have on MeHg exposure 

over intercontinental scales, it is necessary to understand the overall chain of human exposure, 

including the linkages between atmospheric and oceanic Hg transport, Hg methylation in marine and 

freshwater ecosystems, exposure to and biomagnification of MeHg in freshwater and marine fish, the 

international trade of fish and fish consumption patterns. These linkages are poorly quantified at 

present. 

D. Future scenarios for emissions and climate 

47. Several recent studies have developed global emission projections for anthropogenic Hg emissions in 

the years 2020 and 2050. Both studies conclude that significant increases in global Hg emissions, up 

to 25% in 2020 and 100% in 2050 as compared to 2005, can be expected if no major changes in 

emission controls are introduced. However, both studies also conclude that the implementation of 

known emissions control technology could maintain or decrease global Hg emissions. The 

intercontinental source-receptor relationships under these future scenarios are not significantly 

different from the source-receptor relationships estimated for current emissions. The large 

contribution of Hg released from aquatic and terrestrial ecosystems (primary natural sources plus reemissions of previously-deposited mercury) to the overall atmospheric deposition dampens the 

relative response of Hg deposition to changes in new anthropogenic emissions, reinforcing the longterm benefit of decreasing the amount of Hg recirculating in the environment by decreasing Hg 

emissions globally. 

48. The impacts of climate change on intercontinental transport of Hg were not explicitly addressed in the 

HTAP multi-model experiments, and there is large uncertainty about how climate change will effect 

natural and recycled emissions of Hg, as well as the atmospheric chemistry and transport of Hg. 

Climate change effects on temperatures, frequency of forest fires, plant growth and decomposition 

will significantly impact the terrestrial-atmospheric exchange of Hg. Likewise, ocean-atmosphere 

exchange of Hg will be affected by changes in temperature, wind speeds and storm frequency, as well 

as changes in atmospheric oxidant and aerosol concentrations. The net effect of these changes, 

however, has not been adequately studied yet. 

VI. Main findings for persistent organic pollutants

49. By definition, POPs have long lifetimes in the environment, often cycling between different 

environmental compartments (i.e., air, water, soil, vegetation, snow and ice). Thus, through direct 

emission and transport or repeated cycles of emission, transport, deposition and re-emission, POPs 

can end up in the environment far from their emission source. The overall potential and dominant 

mechanisms for intercontinental atmospheric transport vary between individual POPs, since these 

have widely different chemical characteristics. 
Page 10 

A. Observational evidence 

50. Evidence for intercontinental transport is provided from observations in remote locations far from

emission sources and in elevated levels in plumes observed at high altitude sites in mountains and 

during an aircraft campaign. Concentrations of POPs are often correlated with other anthropogenic 

pollutants. Existing atmospheric monitoring programmes provide adequate spatial coverage of 

atmospheric concentration information for most POPs in the United Nations Economic Commission 

for Europe (UNECE) region. However, only a few monitoring programmes also analyse POPs in 

precipitation from which total deposition can be estimated. Some long-term air monitoring 

programmes have observed decreasing concentration trends for some POPs subject to international 

emissions controls (e.g., technical Hexachlorocyclohexane or HCH). For other POPs (e.g., p,p’-

Dichlorodiphenyltrichloroethane or p,p’-DDT), at the Arctic stations of Alert and Pallas, observed 

trends show slow or no significant decline in air concentrations in response to international emissions 

controls. 

B. Global and regional modelling 

51. Most POPs do not have natural sources, so observed POPs concentrations or deposition can be 

apportioned into re-emitted POPs that were originally released from anthropogenic sources, newlyreleased POPs that were released from anthropogenic sources and have been transported on 

intercontinental scales and newly-released POPs that were released from local and regional 

anthropogenic sources. The relative importance of each of these fractions varies depending on the 

individual compound of interest and the location of the receptor relative to sources. Apportioning reemitted POPs back to their original sources requires estimating historical emissions and simulating the 

behaviour of POPs in other environmental media over long time periods. As primary emissions are 

decreased or ceased as a result of emission control measures, re-emission becomes increasingly 

important. 

52. As part of the HTAP multi-model experiments, the transport of several POPs (Polychlorinated 

biphenyl-28 or PCB-28, PCB-153, PCB-180, and α-HCH) covering a range of different physical and 

chemical properties was simulated by three models using different approaches to describe pollutant 

transport through environment compartments and different spatial resolutions. Despite their 

differences, the models provide estimates of annual average atmospheric concentrations, the major 

transport pathways of selected POPs and the response to primary emission changes that are within a 

factor of two to three of the ensemble mean. 

53. Differences in the estimated RAIR for POPs presented in the annex are primarily a function of the 

uneven spatial distribution of emissions between regions and the transport characteristics of the 

pollutant. The largest effect on intercontinental transport is seen for a 20% reduction of new α-HCH 

emissions in South Asia, which decreases concentrations in other regions between 2% to 6%. 

Decreases in PCB emissions in Europe produce similar levels of response across the other regions. 

The lowest response in other regions is produced by the reduction of α-HCH in East Asia and North 

America, and for PCBs in East Asia, due to their relatively low contributions to the global emissions 

of these pollutants. 

54. Arctic pollution is mostly sensitive to the changes of emissions in Europe (for all simulated POPs) 

followed by North America (for PCBs) and South Asia (for α-HCH). 

55. The regional differences in the effectiveness of atmospheric POPs transport alone was explored by 

simulating the transport of selected POPs assuming an equal mass of emissions in each source region. 

In contrast to the results with regionally different emissions, the uniform emissions produced the same 

level of response to emission changes for all source-receptor region pairs. 

56. Current POPs models vary widely in the level of detail represented. Model simulations for a subset of 

POPs have been conducted and are typically able to reproduce observed annual concentrations to 

within a factor of three or four, enabling identification of major transport pathways. The POPs that 

have been successfully modelled and evaluated include PCB-28, PCB-153, PCB-180, and α-HCH. In 

some cases, however, the differences between model estimates and observed values can be much 
Page 11 

greater indicating fundamental uncertainties both in emission inventories and in modelling 

approaches. 

57. As with Hg, POPs models must not only simulate the behaviour of pollutants in the atmosphere, but 

they must also simulate the exchange between the atmosphere and other environmental media (such 

as water, soil, snow, ice and vegetation) and the transport and transformations that occur in those 

other media. Observational data from inter-media exchange and transport and transformation studies 

are limited, making it difficult to evaluate models and characterize uncertainties. 

C. Impacts of intercontinental transport 

58. Similar to Hg, POPs are widely distributed through atmospheric transport, but their primary 

environmental impacts are realized through the contamination of food webs. Through processes of 

bioconcentration, bioaccumulation, and biomagnifications, humans and other animals may be exposed 

to much greater concentrations of POPs than observed in the air, water or soil. There is little 

information about long-term trends of POPs in food or human media outside of Western Europe, 

North America and Japan, making it difficult to characterize the global impacts of POPs. The 

traditional diet of indigenous Arctic people, which is high in locally caught fish and game, combined 

with the atmospheric and oceanic transport of POPs into the Arctic and accumulation in the Arctic 

food web results in elevated POPs exposures in native Arctic populations. Studies have demonstrated 

elevated exposures to PCBs in native Arctic populations and have estimated that exposures to 

toxaphene, chlordane, DDT, HCH, dioxins and other dioxin-like POPs can be above levels of concern 

for adverse health effects. Moreover, there is concern about new POPs, which are currently in 

commerce and now just starting to be addressed by international agreements, including 

Pentabromodiphenyl ethers (PBDEs), Perfluorooctane sulfonate (PFOS), and Perfluorooctanoic acid 

(PFOA), that have been detected in Arctic environments, in some cases in increasing amounts. The 

risks of chronic exposure to these chemicals are not well characterized. 

D. Future scenarios for emissions and climate 

59. Over the next 40 years, emissions and patterns of intercontinental transport of some POPs will 

continue to decrease and shift as a result of national and international regulations. For those POPs 

whose use has been banned or strictly limited, shifts may occur as re-emission of legacy pollution 

results in migration or dispersion of the pollutants. For POPs that are still in use as chemicals or are 

unintentionally released from combustion or other industrial processes, differences in regulations or 

economic activity may lead to shifts in the spatial distribution of emissions. In Europe and North 

America, full implementation of the POPs Protocol to the Convention on Long-range Transboundary 

Air Pollution and the Stockholm Convention on Persistent Organic Pollutants is expected to decrease 

emissions by more than 90% for Hexachlorobenzene (HCB) and PCB, more than 60% for 

Polychlorinated dibenzodioxins and dibenzofurans (PCDD/Fs) and 30% to 50% for Polycyclic 

aromatic hydrocarbons (PAHs). 

60. For POPs that are still in commerce and have yet to be addressed by national and international 

regulations, intercontinental flows are expected to increase as continued emissions contribute to the 

stock of the pollutant circulating in the environment. 

61. Climate change may further alter the magnitude and patterns of emissions and intercontinental 

transport of POPs in a variety of ways. Similar to Hg, climate change may significantly alter the 

exchange of POPs between the atmosphere and water, soil, vegetation, sediments, snow and ice. 

There is evidence that climate change phenomena, e.g., elevated temperatures and sea-ice reduction, 

and extreme climate-change induced events, such as forest fires, flooding and glacial melting, will 

remobilize POPs previously-deposited in sinks, e.g., forest soils and vegetation, ocean and lake 

sediments and glaciers. Climate change may also alter the exposures of individuals and populations 

and their vulnerability to chemical exposures. However, our ability to understand the influence of 

these processes is limited by a lack of measurements and organized monitoring of POPs in media 

other than air. 
Page 12 

VII. Main recommendations 

62. Our current understanding of the magnitude of intercontinental flows of air pollution is sufficient to 

conclude (a) that such flows have a significant impact on environmental quality throughout the 

Northern Hemisphere and (b) that coordinated international actions to mitigate these flows would 

yield significant environmental and public health benefits. However, our current ability to quantify 

accurately and precisely the contribution of intercontinental flows on air pollution concentrations or

deposition at any given location or the effect of international emissions controls on pollution levels

and their environmental or public health impacts is limited. 

63. To quantify better the impacts of intercontinental flows of air pollution and the effects of international 

controls, additional efforts are needed to improve further the coverage and resolution of our 

observational systems, the accuracy and resolution of our emissions inventories and projections, the 

fidelity and performance of our models of chemistry and transport and the scope and detail of our 

impact assessments. 

64. Moreover, we need to shift the goal of the science activity from simply developing knowledge to a 

goal of informing action. A programme of monitoring, research, and analysis activities is needed that 

is: 

(a) Intentional, making source attribution on global to intercontinental scales an explicit objective of 

ongoing and future efforts, including efforts to: 

(i) Maintain and expand long-term observational systems for monitoring trends in 

intercontinental transport, air pollution concentrations and characteristics above the boundary 

layer, and contamination in other environmental media; 

(ii) Conduct intensive field studies to improve model descriptions of transport processes, quantify 

the fluxes of pollutants between environmental media (including re-emission to the 

atmosphere) and evaluate emissions inventories; 

(b) Innovative, employing novel techniques and developing new methods where needed, including 

efforts to: 

(i) Quantify transport through the observation of chemical or isotopic fingerprints; 

(ii) Develop new methods for measuring dry deposition, re-emission fluxes and the composition 

of reaction products (e.g., for Hg oxidation); 

(iii) Characterize the relevant properties and environmental chemistry of new chemicals and their 

potential for intercontinental transport; 

(iv) Compare emission estimates to ambient observations using inverse modelling; 

(v) Use adjoint and other advanced modelling techniques for understanding current and future 

source-receptor relationships; 

(c) Integrated, generating new insights by combining and comparing information within disciplines 

and across disciplines, as well as across different pollutants, including efforts to: 

(i) Improve our understanding of environmental transport and fate of air pollutants by bringing 

together information from emissions, observations, and models for multiple pollutants; 

(ii) Co-locate monitoring sites for multiple pollutants, different environmental media and human 

and wildlife exposure; 

(iii) Extend modelling and analysis frameworks to include the fate of air pollutants in other 

environmental media, the exchange between environmental media (including re-emission 

processes), and the exposure to and impacts of transported pollutants; 

(d) Inclusive, engaging a broader community of scientists and air quality management officials in 

developed and developing countries throughout the Northern Hemisphere; 
Page 13 

(e) Supported by institutions and information networks, fostering and facilitating cooperation 

between experts within and across disciplinary and community boundaries. 

65. As we move from developing knowledge to informing collective action, the costs and availability of

controls for emission sources and how they differ across and between continental source regions also 

need to be assessed further. In future work, the implications of sector-based control strategies in 

different regions need to be examined holistically in terms of their multiple impacts on public health, 

ecosystems and climate change at the local, regional and global scales. 

66. The availability of forums for pursuing further international cooperation to mitigate sources of 

intercontinental transport differs depending on the pollutants of interest. The Stockholm Convention 

on POPs and the recently opened negotiations under the United Nations Environment Programme 

(UNEP) on a global instrument on Hg provide forums for furthering global cooperation to mitigate 

sources of POPs and Hg. 

67. For O3 and PM, however, there is no such global agreement, although several regional agreements, in 

varying states of evolution, address at least some of the sources of transported O3 and PM. A range of 

approaches have been suggested for establishing global or hemispheric scale cooperation on 

mitigation of O3 and PM, including a global confederation between existing regional efforts building 

on existing organizational infrastructures and intergovernmental relationships. Such a confederation, 

or interregional partnership, could facilitate information exchange, capacity-building and technology 

transfer between regions, and greatly facilitate connections between the regional air quality 

agreements and other existing global agreements, global institutions and the global scientific 

community. Thus, a global confederation of regional cooperative programmes on air pollution could 

help develop a better and globally-shared understanding of air pollution problems and their solutions 

at the local, regional and global scale while maintaining autonomy and flexibility for regions to 

develop policies and programmes appropriate for their circumstances. Such a confederation may be 

created as a new institution or organized under an existing global institution, such as UNEP, the 

World Meteorological Organization (WMO), or a joint UNEP-WMO venture, as in the case of the 

Intergovernmental Panel on Climate Change (IPCC). 

68. As a subsidiary body of the UNECE Air Convention, the Task Force can continue to play a leadership

role in bringing together different expert communities and connecting different regional and global 

institutions. For the Convention, continuing and expanding such efforts is important to decrease the 

impact of sources outside the Convention and to meet the Convention’s own goals. Irrespective of the 

future role of the Task Force, the potential for a more formal global confederation of regional 

cooperative programmes on transboundary air pollution should be explored. 
Page 14 

Annex 

Relative Annual Intercontinental Responses 

The Relative Annual Intercontinental Response (RAIR) is based on the HTAP multi-model 

experiments in which emissions were decreased by 20% in each of four regions approximating North 

America, Europe, South Asia and East Asia. The RAIR metric is the sum of the changes in the annual 

average, regionally averaged concentration within a region due to a 20% decrease in emissions in the 

three other regions divided by the sum of the changes in concentration within a region due to a 20% 

decrease in emissions in all four regions. Thus, the RAIR is a measure of how much benefit a region 

may receive from emission reductions in other regions when emission reductions are coordinated on 

an intercontinental scale. 

 Receptor Region 

Scenario Pollutant/Parametera North America Europe South Asia East Asia 

2001 O3 Concentration 32% 43% 32% 40% 

2001 O3 Total Column Burden 39% 62% 38% 60% 

2001 Particulate Matter Concentration f 7% 5% 20% 9% 

2001 SO4 Deposition 8% 9% 24% 12% 

2001 Black Carbon Deposition 4% 1% 12% 3% 

2001 Reactive Nitrogen Deposition 3% 4% 13% 7% 

2001 Aerosol Optical Depthb 17% 13% 25% 17% 

2001 SO4 Total Column Burden 25% 25% 37% 24% 

2001 Hg Deposition 61% 35% 43% 10% 

2001 α-HCH Deposition 82% 9% 6% 63% 

2001 PCB-28 Deposition 9% 3% 48% 56% 

2001 PCB-153 Deposition 11% 2% 32% 50% 

2001 PCB-180 Deposition 11% 4% 36% 56% 

2001c

 O3 Concentration 32% 42% 33% 40% 

2030d O3 Concentration 53% 45% 20% 30% 

2050e O3 Concentration 49% 44% 14% 41% 

a

 Concentrations are at ground-level. 

b

 Aerosol Optical Depth calculated for the sum of sulphate, black carbon and particulate organic matter. 

Soil dust was not included. 

c

 Calculated from linear approximations of a subset of models participating in the HTAP multi-model 

experiments. 

d

 Based on Representative Concentration Pathway 8.5, a high emissions scenario. 

e

 Based on Representative Concentration Pathway 2.6, a low emissions scenario. 

f 

 Estimated based on the sum of sulphate and particulate organic matter.