TF HTAP Answers to Policy-Relevant Questions

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TF HTAP Answers to Policy-Relevant Questions

The Task Force on Hemispheric Transport of Air Pollution (TF HTAP) was created by the Convention on Long-range Transboundary Air Pollution (LRTAP Convention) in December 2004 to facilitate research to improve the understanding of the intercontinental transport of air pollutants across the Northern Hemisphere. Findings from TF HTAP’s work have been documented in a series of technical reports [Keating 2007; Dentener 2010; Pirrone 2010; Dutchak 2010; Keating 2010a], and more recently, a special issue of the journal Atmospheric Chemistry and Physics (ACP) [Dentener 2019].

TF HTAP is a scientific expert group, but its work and findings have important policy implications for the LRTAP Convention, as well as for other international efforts and national governments. The TF HTAP chairs have attempted to distill the policy-relevant findings and messages from the TF HTAP’s work at several times in the past. The HTAP 2010 comprehensive assessment was summarized in a question and answer format in Volume D of the report [Keating 2010a], in an executive summary [Keating 2010b], and in an informal note from the TF HTAP chairs to the Executive Body’s 28th session [Keating 2010c]. Findings from the ACP special issue have been summarized in an informal note from the TF HTAP chairs to the EMEP Steering Body [Keating 2020].

Distilling policy-relevant messages from scientific analyses is often quite challenging, especially where there is significant scientific uncertainty. Scientific answers are often carefully crafted to avoid overstating findings, but the nuances and caveats can lead to confusing language. Effective communication often requires more dialogue than can be achieved through a traditional report drafting and review process.

To facilitate more dialogue and effective communication between TF HTAP and policy audiences, particularly the LRTAP Convention’s Working Group on Strategies and Review (WGSR), we have created this online document. The document is organized around policy-relevant questions and the initial answers to the questions draw upon the HTAP 2010 assessment, the ACP 2019 special issue, and other related literature. We invite contributors to comment on and suggest revisions to both questions and answers. Through iterative drafting and dialogue, we hope to be able to refine the answers to be useful to policy makers as the answers also continue to evolve with new research and analysis, which in turn may be guided by the science-policy dialogue around this document.

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    Observations from instruments and collectors on the ground, connected to balloons, in aircraft, or on satellites provide a wealth of evidence that concentrations and deposition of O3, PM, Hg, and POPs are influenced by atmospheric transport between continents and, in some cases, around the globe.

    Ozone

    For O3, evidence of intercontinental transport comes from direct O3 measurements as well as measurements of precursor gases. Plumes of elevated O3 have been observed in the free troposphere and at high elevation sites. Most importantly, an increasing trend in baseline O3 concentrations, i.e., concentrations in air masses without the contribution from local anthropogenic emissions, has been measured consistently at a number of remote sites across the Northern Hemisphere. Measurements suggest that during the latter half of the 20th century, concentrations of O3 at northern mid-latitudes increased by a factor of two or more. It is likely that much of this change is due to increases in anthropogenic emissions of O3 precursors [Dentener 2010].

    O3 observations through 2014 collected for TOAR show peak O3 values strongly decreasing in North America and Europe, and strongly increasing in parts of East Asia. However, the trends are more mixed for summer daytime average O3 concentrations in North America and Western Europe, with some sites showing significant increases [Chang 2017; Schultz 2017]. Methane concentrations have been increasing at a rate of about 6 ppb per year in 2007-2013 and accelerating to 10 ppb per year during 2014-2018 [Dlugokencky 2020].

    Particulate Matter

    Some of the most tangible evidence for intercontinental transport of air pollution comes from satellite images of PM concentrations, often associated with forest or grass fires and windblown soil dust storms, which travel across oceans and continents in visible plumes. Analyses of satellite observations over the period 1992-2012 have shown decreasing trends in North America and Europe and strong increasing trends in South and East Asia [Boys 2014].

    Ground-based lidar networks and mountain top measurement sites in Europe, North America and Asia provide large continuous data sets that characterize the frequency of occurrence of aerosol transport events, the meteorological conditions responsible for them, and important information on aerosol properties. Evidence of intercontinental transport is also provided in the form of long-term trends in surface concentration and wet deposition observations from remote islands and other remote locations, which in some cases are comparable to the emission trends in upwind areas [Dentener 2010].

    Mercury

    The 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. Such events observed in North America have been linked, based on backward trajectories and correlation with other atmospheric pollutant concentrations, such as co-emitted carbon monoxide (CO), to air masses originating over Asia. Analysis of such events suggests that Asian emissions have been underestimated in available emissions inventories. Evidence for intercontinental transport of Hg into the Arctic, which has no primary anthropogenic sources, is also provided by observations of elevated levels of Hg in the tissue of Arctic wildlife [Pirrone 2010].

    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. Such evidence from both hemispheres suggests about a threefold increase of Hg deposition since pre-industrial times, emphasizing the importance of anthropogenic sources to current Hg levels in the environment. Measured deposition trends in Europe and North America are consistent with regional emission controls. However, global trends in concentrations and deposition are ambiguous, which may indicate off-setting effects between emission trends in Asia and the other parts of the world and significant recycling of Hg among environmental components. Our ability to understand these trends and the cycling of Hg is limited by the sparsity of long-term observations for Hg concentrations and deposition [Pirrone 2010].

    Persistent Organic Pollutants

    POPs have long lifetimes in the environment, often cycling among 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 among individual POPs, since these have widely different chemical characteristics. Evidence for intercontinental transport is provided from observations in remote locations far from emission sources and in elevated levels in plumes observed at mountain top sites and during aircraft campaigns. Concentrations of POPs are often correlated with other anthropogenic pollutants [Dutchak 2010].

    Existing atmospheric monitoring programmes provide adequate spatial coverage of atmospheric concentration information for most POPs in the United Nations Economic Commission for Europe region. The introduction of passive samplers that can measure air concentrations has significantly enhanced the spatial coverage of observations in other regions of the world. However, only a few monitoring programmes also analyse POPs in precipitation from which total deposition can be estimated [Dutchak 2010].

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    For O3, a large fraction of the observed concentrations is due to non-anthropogenic emission sources, including precursor emissions from vegetation, fires, lightning, and soils, and intrusion from the stratosphere. The complex and varying background sources of ozone precursor emissions are augmented by and interact with varying anthropogenic sources of precursor emissions within a region and outside a region. The contribution of each of the natural and anthropogenic components varies on different time scales from annual to hourly and by location and elevation. Thus, the contribution of and sensitivity to extra-regional emission sources depends on the metric, or policy objective, of interest.  The highest O3 concentrations are typically associated with local or regional emissions sources, stagnant meteorology, and high temperatures. However, natural and extra-regional anthropogenic sources alone can produce O3 concentrations that exceed health-protective levels.

    The impact of extra-regional anthropogenic emissions is more important for ground-level ozone than for particulate matter or sulfur or nitrogen deposition. 

    • Extra-regional emissions are more important for column ozone burdens and ozone direct radiative forcing than for surface ozone concentrations.
      • For surface ozone, the impact of extra-regional emissions sources is highest in Spring and lowest during Summer.
      • For surface ozone, the impact of extra-regional emission sources is larger for longer-term average concentrations than for peak concentrations that tend to be driven by local emissions.

    Methane, in combination with nitrogen oxides, is an important ozone precursor on a global basis, and projected increases in methane emissions could, over time, offset the ozone decreases associated with decreases of emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs). 

    For Europe, ground-level ozone concentrations are more sensitive to anthropogenic emissions outside of Europe than to anthropogenic emissions inside of Europe. This is the case for all European sub-regions, all seasons, and for a range of ozone metrics including annual and seasonal averages, SOMO35, and POD1.[1] The relative influence of these extra-regional emissions on ozone in Europe varies by location, season, and ozone metric. Extra-regional anthropogenic emissions of NOx and VOCs outside of Europe are estimated to contribute between 2-12 ppb of ozone depending on the season, with an annual average contribution of 4-8 ppb of ozone depending on the model used. The contribution of anthropogenic methane emissions to ozone in Europe is estimated to be about 5-8 ppb (on the basis of 6mDMA1).[2]

    For North America, region-wide annual average ozone concentrations appear equally sensitive to anthropogenic emissions outside of North America and emissions inside of North America. Extra-regional anthropogenic emissions of NOx and VOCs outside of North America and changes in global methane, together, are estimated to contribute between 8-13 ppb of ozone in the western United States, 2-12 ppb of ozone in the central United States, and 2-10 ppb of ozone in the eastern United States, depending on the season.

    Maritime shipping is well understood to have significant impacts on air quality in coastal regions. In the HTAP2 experiments, all maritime shipping was considered to be extra-regional, leading to an increase in the estimated extra-regional contribution to air pollution levels in all regions compared to HTAP1. Further analysis is needed to understand the source-receptor relationships associated with shipping in different coastal regions subject to ECAs (Emission Control Areas) compared to shipping emissions in the open ocean.

    PM2.5 and Deposition

    As with O3, observed PM2.5 concentrations are the result of contributions from non-anthropogenic sources, including emissions from vegetation, natural fires, wind-blown dust, sea-salt, and volcanos. Anthropogenic sources from near and far add to these natural sources. The contribution of each of the natural and anthropogenic components varies on different time scales from annual to hourly and by location and elevation. Both natural and anthropogenic sources can affect PM2.5 concentrations on intercontinental scales.

    The relative impact of extra-regional anthropogenic sources on PM2.5 concentrations is smaller than on O3 concentrations. The sensitivity of the deposition of sulfur (S), oxidized nitrogen (NOy), and reduced nitrogen (NHx) to changes in extra-regional anthropogenic emissions is similar to the sensitivity of PM2.5 concentrations. In North America, 83% of S deposition, 83 % of NOy deposition, and 93% of NH3 deposition are due to sources within North America.  In Europe, the fractions are 64%, 66%, and 88% for S, NOy, and NHX deposition, respectively.  However, in the region defined by Russia, Ukraine, and Belarus, only 39%, 41%, and 45% of S, NOy, and NHX deposition, respectively, are due to emissions within the region.

    Mercury

    Based on the HTAP1 multi-model experiments, natural and re-emitted Hg account for about 35% to 70% of total Hg deposition on a region-wide, annual average basis depending on the region, whereas intercontinental transport of newly-released anthropogenic Hg emissions accounts for about 10% to 30% of total Hg deposition, on a region-wide, annual average basis. East Asia, which accounted for almost 40% of total global newly-released Hg in 2000, is the most dominant among the four HTAP1 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.

    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 Hg depletion events (AMDEs), during which Hg0 is rapidly oxidized by photochemical reactants that have built up over the dark winter. Current models show significant deviation in estimates of Hg deposition to the Arctic due to the uncertainties in the model formulation of the processes related to AMDEs and transport to the polar region.

    The global Hg models participating in the HTAP1 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. In these global models, the change in Hg deposition in one region is linearly related to the change in emissions in another region, provided that the relative proportion of Hg0 and ionic Hg in the anthropogenic emissions stays the same in both regions. Increases in Hg0 emissions lead to enhanced export, whereas increases in ionic Hg result in higher local deposition in the region itself.

    In the Arctic, Hg deposition could be most efficiently controlled by emission reductions in East Asia and Europe due to their proximity to the Arctic, prevailing atmospheric circulation patterns, and the significant contribution of these regions to global anthropogenic emissions.

    Within a receptor region, proximity to the intercontinental source region has a small effect on the spatial distribution of Hg deposition. For example, relative Hg deposition decrease in North America caused by emission reduction in East Asia is somewhat higher in the western part of North America, whereas response to the European emission reduction prevails in eastern North America.

    The response of Hg deposition to changes in emissions varies with season. Intercontinental transport may be a factor of two more important in summer than in winter due to more active mixing of the boundary layer with air aloft. Thus, seasonal variation of the deposition response to emission reduction in other continents can reach 30% of the annual mean. Distinctly different seasonal cycles characterize the transport between adjacent regions such as East Asia and South Asia or Europe and South Asia, which are driven by changes in monsoonal systems. For example, the most significant effect of South Asian emission reduction on Hg deposition in East Asia takes place during the first half of a year, whereas the opposite is true of the effect of East Asian sources on South Asia, which is most significant in the second half. Similarly, South Asian sources have a somewhat larger influence on Europe in winter; whereas European sources more strongly affect Hg deposition to South Asia in summer.

    In the year-long simulations, the large contribution of natural sources and re-emitted legacy Hg to deposition dampens the relative response of Hg deposition to the reduction in new anthropogenic emissions, whether associated with local and regional sources or intercontinental transport. If integrated over much longer time scales, the decrease in deposition will be larger as decreasing new emissions will slowly decrease the amount of legacy Hg that is cycling in the environment.

    Persistent Organic Pollutants

    As part of the HTAP1 multi-model experiments, the transport of several POPs (Polychlorinated biphenyl-28 (PCB-28), PCB-153, PCB-180, and α-HCH), covering a range of different physical and chemical properties and for which some emissions and observational data are available, was simulated by three models. The three models used different modelling approaches to describe pollutant transport through environmental compartments and different spatial resolutions. Despite their differences, the models provide generally consistent estimates of annual average atmospheric concentrations, the major transport pathways of the selected POPs, and responses to primary emission changes. In most of the cases, the differences between the mean response and the model estimates are within a factor of 2 to 3.

    As with the other pollutants, the largest impact on deposition, in most cases, results from changes in local and regional emissions within the region, especially for regions with higher emissions (North America and Europe for PCBs, and South Asia and Europe for a-HCH).

    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).

    The regional differences in the efficiency of atmospheric POPs transport alone were 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, about 1.5%-2%. A slightly more significant effect is obtained for the closest pair of regions, South Asia’s influence on East Asia (2-3%). From the equal emission simulations, transport to the Arctic is most efficient from Europe, producing a 5-6% response. A slightly lower response in the Arctic can be seen for emission changes in North America and East Asia (about 4%) and the Arctic response is weakest for South Asian emissions (about 2%).


    [1] SOMO35 = the annual sum of the positive differences between the daily maximum 8-hour average ozone value and the cutoff value set at 35 ppb; POD1 = phytotoxic ozone dose (accumulated stomatal flux above a threshold of 1 nmol/m2/s) accumulated over a growing season

    [2] 6mDMA1 = the running mean of the 6-month average of the daily maximum 1-hour ozone concentration

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    The current levels of intercontinental transport and hemispheric baseline concentrations of O3 and PM are a result of emissions that, on a global basis, increased rapidly between 1950 and 1990. Since 1990, global emissions leading to O3 and PM concentrations have experienced little change or have begun to decrease. In Europe and North America, which have been the dominant sources of anthropogenic emissions until recent decades, emissions of most precursors are constant or declining, due to the implementation of air pollution control policies. In East Asia and South Asia, emissions of precursors have risen dramatically in recent years, due to economic growth and development in these regions.

    Between 1990 and 2010, anthropogenic emissions of PM increased little globally, but shifted geographically, with a 30% decline in Europe and North America and a 50% increase in Asia [Klimont 2017]. Similarly, between 2000 and 2010, global anthropogenic emissions of ozone precursors (NOX, CO, and NMVOCs) grew modestly. However, emissions in Europe and North America decreased by 10% to 50% while emissions in South Asia and East Asia and other regions of the world increased by 10% to 50% [Turnock 2018]. Globally, anthropogenic CH4 emissions increased by 17% between 1990 and 2012, with decreases in Europe, little change in North America, and strong increases in East Asia, South Asia, and other regions of the world. The changing spatial patterns of emissions have shifted ozone precursors into the tropics where ozone production is more efficient.  Zhang et al.[2016] suggest that this equatorward shift of emissions has increased the total global ozone burden more than the combined effect of the increase in global methane emissions and the increase in the total mass of non-methane precursor emissions.

    Between the HTAP1 simulations based on 2001 and the HTAP2 simulations based on 2008-2010, there were no significant changes in the sensitivities of O3 and PM2.5 concentrations in Europe and North America to 20% perturbations in anthropogenic emissions in other regions. However, changes in emissions patterns have changed the absolute and relative contributions of regional and extra-regional sources.  For example, Jonson et al. [2018] found that for European annual average surface O3 levels, the contribution of European emissions had decreased significantly between 2001 and 2010, while the contribution of North American emissions had declined far less and the contribution of East Asian emissions had increased. Thus, the relative contribution of extra-regional sources increased. Decreases in emissions in North America over the last decade are likely to have contributed to flattening or declining trends observed recently at remote sites in Europe.  Likewise, controls on the electricity and industrial sectors in China have led to decreasing emissions of some pollutants in the last 3-5 years. The implications of these recent emissions decreases and their influence on observed trends at remote sites in North America and Europe should be explored.

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    The intercontinental transport of O3, PM, Hg, and POPs contribute to serious public health problems and damage to natural and agricultural ecosystems in many parts of the world. O3 and PM also contribute significantly to climate change on regional and global scales.

    Ozone

    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.

    The highest concentrations of O3 are typically associated with stagnant conditions, when the contribution from intercontinental transport of air pollution 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 concentrations that exceed such standards will continue to increase.

    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.  One study based on the HTAP1 multi-model experiments estimated that intercontinental transport of O3 contributes from 20% to more than 50% of O3-related premature adult mortalities in a given receptor region, subject  to large uncertainty.

    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 HTAP1 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.

    O3 causes damage to crops, forests, and grasslands, which has important implications for productivity, biodiversity, and food security. Recent experimental studies on field crops, adult forest stands and different grassland ecosystems have found significant impacts associated with ecologically realistic free-air O3 fumigations that mimic the observed increases in baseline O3 concentrations.

    Global crop yield losses of four staple crops due to exposure to O3 are estimated to be between 3% and 16%, depending on the crop, and are valued at $14 billion – $26 billion per year. Based on the HTAP1 multi-model experiments, intercontinental transport may be responsible for about 5% to 35% of the estimated crop yield losses depending on the location, crop, and response function used. However, there is significant uncertainty in these estimates, part of which is due to the limited representativeness of available exposure-response functions based on threshold indices (e.g., accumulated ozone exposure over a threshold of 40 parts per billion (AOT40) and the sum of all hourly ozone concentrations greater than 0.06 parts per million (SUM06)).

    O3 contributes significantly to climate forcing, directly as a greenhouse gas and indirectly by damaging plants and inhibiting their natural uptake of carbon dioxide (CO2). Among O3 precursors, widespread decreases in emissions of CH4, CO, and VOCs will decrease net climate forcing.

    Decreasing NOx may increase climate warming over decadal time scales because less NOx leads to less hydroxyl radical, increasing the lifetime of CH4, which is a greenhouse gas itself. Over time, the increase in radiative forcing from the increased lifetime of CH4 is greater than the decrease in radiative forcing from decreased O3 formation. Decreasing emissions of CH4, however, 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.

    The radiative forcing exerted by O3 is not globally uniform, but extends from the location of precursor emissions over regional and intercontinental scales. This inhomogeneous forcing affects climate change at the global scale and at the regional scale, influencing atmospheric heating and dynamics and ultimately patterns of temperature and precipitation. The largest climatic impacts do not necessarily occur where the radiative forcing occurs and may occur downwind of the source region.

    PM

    For PM, the experimental and epidemiological evidence for effects on mortality is stronger than it is for O3. Although the highest PM concentrations are typically associated with local and regional emission sources, intercontinental transport events associated with forest fires or dust storms do produce concentrations that exceed short-term public health standards. On a longer-term basis, current levels of intercontinental transport of PM interfere with the ability to meet visibility targets for natural surroundings in western North America. Intercontinental transport of PM components other than wind-blown dust or from fires is not usually sufficient to exceed health-based ambient standards.

    Only a few studies have tried to quantify the human health impacts of intercontinental transport of fine particles specifically. Those studies conclude that contributions to PM from emissions within a region are expected to be much more important for human health than emissions from intercontinental transport. However, the impacts of transported PM are still significant. 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 among 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.

    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 POM, this would generally increase warming. Reductions in black carbon would typically benefit both air quality and climate.

    Mercury

    Hg differs from other major atmospheric pollutants (e.g., O3 and PM) in that its environmental and health impacts are not directly related to its atmospheric burden. While the major redistribution of Hg is via the atmosphere, its primary environmental and health impact is in aquatic systems and for aquatic organisms and their consumers. Atmospheric Hg that is deposited directly or indirectly into aquatic systems 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 wildlife.

    Thus, to understand the main impact that emission controls will have on MeHg exposure over intercontinental scales, it is necessary to understand the linkages among atmospheric and oceanic transport, methylation in marine ecosystems, exposure and biomagnification in migratory marine fish, the capture and international trade of seafood, and seafood consumption patterns. These linkages are presently poorly quantified.

    Persistent Organic Pollutants

    By definition, POPs are persistent, bioaccumulative, and toxic. Their adverse effects on human health and wildlife range from various forms of acute toxicity to chronic effects, including carcinogenicity and developmental and reproductive effects. It is the chronic effects from low dose exposures that are most relevant with respect to the impacts of intercontinental transport.

    Similar to Hg, POPs are widely distributed through atmospheric transport, but their primary environmental impacts are realized through the contamination of food webs. 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 on human health. In addition to elevated human exposures, studies have documented elevated concentrations of POPs in wildlife in remote environments.

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    In Europe, changes in emissions outside Europe and global methane concentrations will largely drive future annual average O3 levels. Without additional controls, global methane emissions are expected to grow, increasing O3 mortality in Europe in 2050 by up to 8,000 additional premature deaths compared to 2010 levels. Implementation of mitigation policies, largely outside of Europe, can decrease methane emissions overall and decrease O3 mortality in Europe by up to 2000 premature deaths per year compared to 2010 levels, a difference of 10,000 deaths per year between the highest and lowest global CH4 emissions scenarios.

    In North America, the difference between the highest and lowest global CH4 emissions scenarios corresponds to a difference of up to 5,000 deaths per year in 2050. The sectors with substantial mitigation potential are fossil fuel production, waste and wastewater management, and agriculture, with the largest emissions in China, followed by Latin America, Africa, India, and North America [vanDingenen 2018].

    Shipping makes a significant contribution to both O3 and PM2.5 levels in Europe and North America, particularly in coastal regions. The impact of planned emissions control policies under the International Maritime Organization have been examined on the regional scale. However, the impact of the emission controls on intercontinental transport has not. TF HTAP is organizing analysis to examine the global impact of controls in each of the IMO designated Emission Control Areas.

    Air pollution in South Asia is expected to continue to increase largely because of changes in emissions within the region. Implementation of clean technologies and climate change mitigation policies could substantially decrease pollution levels with large benefits for human health, crops, and ecosystems. The benefits of emissions decreases in South Asia would mostly accrue to the region itself, and then downwind regions in East Asia and the Middle East. However, the global implications of emissions reductions in South Asia should be further explored.

    Since 2012, TF HTAP has organized a series of workshops with TFIAM and CIAM to explore future global scenarios and the potential for mitigation. IIASA’s work under the ECLIPSE project has contributed significantly to these discussions. Sessions have focused on improving our understanding of emissions and mitigation potential for marine shipping; residential heating, cooking, and lighting; agricultural burning; transportation; electricity production; and various sources of methane.

    The HTAP1 multi-model experiments also examined the potential impacts of changes in meteorology and transport patterns expected as a result of climate change on O3 concentrations using a set of three models that simulated climate changes between the periods 2000 and 2100. Future changes in climate are expected to increase the effect of O3 precursor emissions over source regions and reduce the effect over downwind receptor regions. However, the magnitude of these effects is relatively small, and is driven mostly by changes in atmospheric chemistry and not by changes in transport patterns. The effect of natural emission changes and wider climate-related feedbacks are potentially important, but have not been evaluated fully yet.

    Since 2012, TF HTAP has organized a series of workshops with TFIAM and CIAM to explore future global scenarios and the potential for mitigation. IIASA’s work under the ECLIPSE project has contributed significantly to these discussions. Sessions have focused on improving our understanding of emissions and mitigation potential for marine shipping; residential heating, cooking, and lighting; agricultural burning; transportation; electricity production; and various sources of methane.

    Mercury

    Two recent studies have developed global emission projections for anthropogenic Hg emissions in the years 2020 and 2050. One of the studies was based on the IPCC Special Report on Emissions Scenarios (SRES), used in previous IPCC assessments. The other study was developed for the United Nations Environment Programme’s (UNEP’s) Mercury Programme. Both studies conclude that significant increases, up to 25% in 2020 and 100% in 2050 as compared to 2005, in global Hg emissions can be expected if no major changes in emission controls are introduced and economic activity continues to increase. In both studies, the largest increase in emissions is projected from coal combustion for electricity generation in Asia. However, the implementation of available emission control technology could stabilize or decrease these emissions.

    As part of the HTAP multi-model experiments, three of the scenarios for 2020 were simulated by the participating models. The scenarios ranged from a 25% increase to a 65% decrease in global emissions over 2005 levels. The participating models predicted corresponding increases in deposition of 2-25% and decreases in deposition of 25-35% in four source regions, respectively, in annual simulations. In remote regions, such as the Arctic, the changes were predicted to be smaller – ranging from 1.5-5% increases to 15-20% decreases.

    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 natural sources and re-emitted legacy Hg to deposition dampens the relative response of Hg deposition to changes in new anthropogenic emissions, reinforcing the long term benefit of decreasing the amount of Hg re-circulating in the environment by decreasing Hg emissions globally now.

    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, 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.

    Persistent Organic Pollutants

    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, geographic shifts in transport 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 LRTAP POPs Protocol, the Stockholm Convention, and other national legislation is expected to decrease emissions by more than 90% for hexachlorobenzene (HCB) and PCB, more than 60% for pentachlorodibenzodioxins and furans (PCDD/Fs), and 30-50% for PAHs.

    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. Climate change may further alter the magnitude and patterns of emissions and intercontinental transport of POPs. Similar to Hg, climate change may significantly alter the exchange of POPs among 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.

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    The intercontinental transport of air pollution is driven by several processes including atmospheric circulation, the global distribution of emissions, chemical and physical transformations, and interactions with/transformation in other environmental media.

    Atmospheric Circulation

    In the mid-latitudes of the Northern Hemisphere, the general circulation is dominated by westerly winds that flow from Asia across the North Pacific Ocean to North America, from North America across the North Atlantic Ocean to Europe, and from Europe into Asia. Long-range transport of pollutants can produce distinct plumes in the mid- and upper troposphere, but to be relevant to air quality in a downwind continent, the pollution must descend to the surface. As the plumes descend they are diluted and can be difficult to distinguish from local pollution, especially in receptor regions with relatively high emissions.

    In the tropics, intercontinental transport is generally from east to west, guided by the trade winds throughout the lower and mid-troposphere.

    In the Northern Hemisphere polar region, the cold and stable lower troposphere forms a dome over the Arctic, largely isolating the region from low latitude pollution that is emitted into warm air masses that ascend into the mid- and upper troposphere above the Arctic. Pollutant transport into the Arctic lower troposphere occurs preferentially from Europe when the outer regions of the Arctic dome pass over northern Europe, take up fresh emissions and then retreat back into the Arctic.

    Emissions

    The magnitude and impact of hemispheric and intercontinental scale transport of air pollutants is initially determined by the global distribution of emissions, and their spatial relation to the major meteorological transport pathways described above.

    For example, the intense emission regions along the east coasts of North America and Asia are at the origins of the North Atlantic and North Pacific mid-latitude cyclone storm tracks, which can loft the emissions and transport them to the free troposphere above downwind continents in a matter of days. With Western Europe located at the end of the North Atlantic storm track, its emissions are not lofted to the same extent as those on the east coasts of North America and Asia. Instead, European emissions are exported at relatively low altitudes and have a strong impact on the Arctic.

    Chemical and Physical Transformations

    Intercontinental pollution transport occurs on timescales of days to weeks, longer than the atmospheric lifetimes of some pollutants, and ample time for the trace gases and PM emitted or produced at the source to undergo removal or chemical transformation. By the time a polluted air mass arrives at a downwind continent, it is likely to have very different chemical properties than it did at the source.

    Interactions With and Transformations in Other Environmental Media

    Because of the cycling of Hg and POPs among the atmosphere and land, ocean, and vegetative surfaces, models of the atmospheric transport of Hg and POPs must account for the emission or re-emission of Hg and POPs from surfaces and the chemical and physical processes that occur in surface media that permanently remove the substances from circulation in the environment. Air-sea exchange of semi-volatile POPs and Hg allows for multiple cycles through the atmosphere and ocean. Ocean transport is expected to be most important for persistent substances that are highly water-soluble and have low vapour pressure, such as perfluorinated acids, which are a relatively new class of POPs.

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    Our ability to represent the processes that drive intercontinental or global flows of air pollutants in quantitative, predictive models varies across the different pollutants of interest. In general, our confidence in the predictions of the models decreases from O3 to PM to Hg to POPs, as the complexity of the processes that must be represented increases and as the available observational data base decreases.

    We can evaluate our modelling capabilities by comparing model estimates of concentrations and deposition to the observed magnitudes, patterns, and trends for each pollutant. We can also compare the estimates of different models, which give us some sense of the lower bound of the uncertainty in our modelled estimates. Estimating the current level of uncertainty or confidence in modelled estimates of intercontinental source-receptor relationships and identifying major areas of uncertainty have been main objectives of the HTAP multi-model experiments.

    Ozone and PM

    Our confidence in model estimates of the sensitivity of O3 in Europe and North America to anthropogenic emissions in other regions has not changed since HTAP1. Models are able to reproduce broad spatial and seasonal patterns of observations, but biases can be as large as the estimates of extra-regional contributions. Thus, we have some confidence in the ability of models to qualitatively describe the role of regional and extra-regional sources and processes.  However, uncertainty in quantitative estimates of extra-regional contributions generally are the same order of magnitude as the estimates themselves.

    The range of estimates of surface O3 concentrations in the HTAP2 ensemble is large and of similar magnitude to that in HTAP1, despite having used the same emissions in all HTAP2 models. Regional models, which have higher spatial resolution, are less biased than global models for surface O3 when compared to observations, which is to be expected. The larger errors in global models are particularly important for threshold-based metrics, such as AOT40. Nevertheless, the best performing global models have less error than the worst performing regional models. The largest sources of error are found in temporal processes acting on longer time scales (weeks or months), including emissions, their interaction with chemistry, and long-range transport processes.

    PM

    The range of estimates for PM2.5 narrowed between HTAP1 and HTAP2. Global and regional models perform similarly for PM2.5, with a tendency to underestimate PM2.5 concentrations in Europe and North America.  Removing this low bias may significantly increase estimates of PM2.5 exposure and health impacts.

    Our estimates of emissions sources globally have improved as previously unaccounted sources have been included (e.g. sources of black carbon). However, uncertainties in emissions inventories in some parts of the world remain high. For example, HTAP2 emissions for India were greater by a factor of 1.5 to 2 for NOx, NMVOC, and SO2 as compared to an inventory created at the national scale (Venkataraman 2018). Satellite observations have helped improve estimates of emissions trends in many parts of the world.

    Mercury

    Current global atmospheric Hg models reproduce the observed ground-level Hg0 concentrations to within 20% of the sparse observations that are available and reproduce the pronounced inter-hemispheric gradient in baseline Hg0 concentrations that has been observed. 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. There are larger differences across model estimates of wet deposition in areas where there are little observational data. Significant differences exist among models for dry deposition, which is believed to contribute as much as wet deposition to total Hg deposition and for which there is little observational data.

    Using identical inputs for new anthropogenic emissions, the four models participating in the HTAP1 multi-model experiments predict comparable Hg deposition levels in Europe, North America and South Asia, but the estimated Hg deposition levels are a factor of 1.5 higher in East Asia. The level of agreement is striking, given the significant differences among models in the assumptions used concerning emission rates for natural or re-emitted Hg and the oxidation pathways for Hg0. For deposition in the Arctic, the differences across models are much higher, with a spread greater than a factor of four, due to differences primarily in the representation of atmospheric Hg depletion events.

    The relative contributions of the major source regions to region-wide average Hg deposition in different regions are very similar among the models. The most significant deviations in the modelling results are seen in areas with large anthropogenic and natural and secondary emissions.

    This is due to large uncertainties in the natural and secondary emission estimates and to the differences in spatial resolution of the participating models, which varied by a factor of four.

    Although there are significant uncertainties in the anthropogenic emissions from some source categories and regions of the world, the magnitude of new anthropogenic emissions is thought to be much less than the emissions of re-emitted Hg that was previously deposited. However, the rates of re-emission from terrestrial and aquatic systems, especially from the ocean which may emit twice as much Hg as anthropogenic sources, are not well characterized.

    Persistent Organic Pollutants

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

    Current POPs models vary widely in the level of detail represented. Model simulations for a subset 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 selected PCBs, HCHs, and PAHs. In some cases, however, the differences between model estimates and observed values can be much greater indicating fundamental uncertainties both in emission inventories and in modelling approaches.

    For some POPs, there are significant uncertainties associated with the pollutant’s physical- chemical properties, such as Henry’s Law constants, vapour pressures, and octanol-air partition coefficients. These physical-chemical parameters are used in the models to predict how the pollutant will move among media. However, the total environmental lifetime and long-range transport potential of a POP are not intrinsic substance properties. The fate of pollutants also depends on the characteristics of the environment in which it is found. Little is known about how the abundances and biodegradation of POPs vary in heterogeneous media such as soil, snow, oceans, and lakes.

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    Our current understanding of the magnitude of intercontinental flows of air pollution is sufficient to conclude that such flows have a significant impact on environmental quality throughout the Northern Hemisphere and that coordinated international actions to mitigate these flows would yield significant environmental and public health benefits. However, our current ability to accurately and precisely quantify the contribution of intercontinental flows on air pollution concentrations or deposition at any given location or the effect of international emission controls on pollution levels and their environmental or public health impacts is limited. To be able to better quantify the impacts of intercontinental flows of air pollution and the effects of international controls, additional efforts are needed to improve 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. Moreover, we need to shift the goal of the science activity from simply developing knowledge to a goal of informing action.

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    O3, PM, Hg, and POPs are significant environmental problems in many regions of the world. Mitigation of intercontinental transport is not a substitute for emission reductions at the local and regional scale. In most cases, concentrations within a source region are more sensitive to emission changes within that region. However, without further international cooperation to mitigate intercontinental flows of air pollution, it is likely that many nations will not be able to meet their own goals and objectives for protecting public health and environmental quality over the next 20 to 40 years.

    Efforts to mitigate the sources of intercontinental flows of air pollutants result in significant benefits for both the source and receptor countries in terms of decreased impacts on public health, decreased damage to ecosystems, and depending on the mix of pollutants, decreased contribution to climate change. In fact, countries outside of a source region may benefit collectively more from emission decreases in a source region than the source region itself. One country’s actions can also lessen the costs of emissions control needed in other countries. Thus, there are significant benefits to both source and receptor countries to cooperate in decreasing emissions that contribute to intercontinental transport of air pollution.

    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 Persistent Organic Pollutants and the Minamata Convention on Mercury Pollution provide forums for furthering global cooperation to mitigate sources of POPs and Hg.

    A range of approaches has been suggested for establishing global or hemispheric scale cooperation on mitigation of O3 and PM, including negotiating a new international agreement; incorporating O3 and PM into an existing global agreement, such as the United Nations Framework Convention on Climate Change (UNFCCC) or the Vienna Convention on the Protection of the Ozone Layer; expanding the geographic scope of the LRTAP Convention; or developing a global framework for cooperation within existing regional agreements.

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