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Intercontinental Transport of Air Pollution

This report was written for the class ATM S 358 to summarize and explain findings from a literature review of research on the intercontinental transport of air pollution.

Table of Contents

Introduction

Intercontinental transport represents long range transport of 1000 km or more and on a timescale of days to weeks. It's not as simple as going straight from point A to B and complexity arises from the transport pathways a plume can take, differing in speed of transport, height in the atmosphere, and chemical processes that can occur. Examples include photolysis from sunlight and reactions with other species leading to the destruction of a pollutant or the formation of a secondary pollutant.

We care about intercontinental transport of pollutants partly for the same reasons we care about air pollution: it perturbs chemical cycling and can lead to pollutant formation, can alter reservoir species through deposition, and poses a risk to the health of humans and the environment. Furthermore, the wide reach of intercontinental transport means all parts of the world are affected and there are implications for global climate change and overall air quality, the effects of which are more significant for pristine regions that otherwise have little local emissions of pollutants.

Global wind patterns play an important role, with westerlies in the midlatitudes pushing plumes of pollutants from east to west and circulations at higher latitudes carrying pollutants towards the poles. Meridional transport at the equator is rarer, so that intercontinental transport is considered on a hemispheric basis mostly. These winds result in some common intercontinental routes that pollutants follow, such as trans-Pacific flow from East Asia to North America, trans-Atlantic flow from North America to Europe. Since the winds vary on a seasonal basis, pollutants also show a seasonal variation in how much is transported and to where. Additionally, verticality must be considered since it depends on the lifetime of a pollutant. Shorter-lived pollutants tend to be transported higher up in the free troposphere from lofting by convection, where transport is quicker, while longer-lived pollutants are carried at both high altitudes and on slower low altitude routes.

Regional Contributions to Ambient Air Pollution

A 2021 study by Li et al. modeled the transport and concentrations of major intercontinental pollutants worldwide and compared the results to ground site measurements and satellite data. They specifically looked at tropospheric ozone, black carbon, and non-sea-salt sulfate. Tropospheric ozone is a relatively long-lived pollutant and so is better able to survive intercontinental transport, although this was found to be highly dependent on precursor emissions. Ozone can be transported both through the free troposphere and across oceans through the surface layer and was found to account for 35% to 60% of non-local surface ozone in North America, Europe, and East and South Asia, which are also the four major regions they identified as being significant sources of anthropogenic pollutants. The model also showed that approximately 37.5% of the tropospheric ozone in the Arctic came from Russia-Belarus-Ukraine, North America, and Europe, most likely due to the proximity of these major polluting regions to the North Pole and the low local emissions of ozone precursors in the Arctic.

Non-sea-salt sulfate is a secondary aerosol primarily formed by the oxidation of SO2 and therefore its concentrations are highly affected by anthropogenic pollutant emissions. The study investigated its transport in the planetary boundary layer; however, its transport there is relatively limited compared to the free troposphere where its lifetime is longer, and so that may result in underestimations of how much intercontinental transport occurs. It was found that emissions from Europe contributed about 45% of the non-sea-salt sulfate concentrations in the Arctic.

This is like black carbon, which is a primary pollutant produced by biomass burning and gas flaring, meaning natural emissions are comparable to anthropogenic emissions. Its transport patterns are similar to those non-sea-salt sulfate, with strengthened transport from South Asia during the summer and winter monsoons. It was found that black carbon travels farther than non-sea-salt sulfate does, although it has a lower relative contribution to pollutant concentrations at receptor regions. Again, the Arctic was heavily impacted by intercontinental transport of black carbon with Europe contributing 17.2% of the total concentrations and therefore being a major contributor to Arctic haze in the winter, when poleward transport and deposition of black carbon is at its maximum due to wind patterns facilitating transport and stable conditions at the Arctic surface slowing down removal of pollutants.

Overall, it was found that impacts on pristine regions such as the Arctic tended to be the most significant, although more heavily polluted regions such as North America, Europe, and East and South Asia could be affected by transport of secondary pollutants and precursors which would react with local pollutants to increase pollution. One limitation of the study was that non-linearities in the concentrations of ozone and black carbon were ignored, leading to possible underestimation of transport.

Anthropogenic Volatile Organic Compounds and Ozone Transport

A 2023 study by Derwent et al. investigated the chemistry of ozone production in intercontinental transport and its impacts at the receptor region, specifically focusing on the role of volatile organic compounds (VOCs) as an ozone precursor. They used a three-dimensional Lagrangian model that simulates the global distributions of ozone and methane to study the transport of tropospheric ozone from North America to Europe resulting from pulses of VOCs in the source region. They analyzed the response over six months at Mace Head, Ireland and Mt Waliguan on the Tibetan plateau, which correspond to ground stations that can give measurements.

The reactions of interest in this study were low-NOx reactions in the NOx/VOC regimes of ozone production. It was found that photochemical oxidation of methane and other organic compounds played an important role in ozone production and they identified a NOx-compensation point, on the order of 10 ppt, where production of ozone by the reaction of HO2 and NO balances the destruction of ozone by reacting with HO2, which determined whether OH reactions would lead to ozone production or destruction. Specifically, below this compensation point, OH reactions would lead to ozone destruction and above it would lead to production.

Transport of ozone plumes produced by pulses of VOC emissions in North America were transported via convection with rapid advection to the middle and upper troposphere, where the plumes were transported by westerlies to Europe. Once the plumes arrived at Europe, they produced a short-term VOC-driven response and a longer-term methane-driven response. In the same month the plumes were transported, it was found ozone destruction occurred in the plumes as a result of the HO2 + NO flux becoming negative due to low NOx concentrations from intercontinental transport. The short-term VOC-driven response occurred 1 to 2 months after the VOC emission pulses and led to net ozone production from the reaction of OH with VOCs above the NOx-compensation point. The longer-term methane-driven response occurred 3 to 4 months after the VOC pulses and was caused by the depletion of OH from reacting with the VOCs, reducing the OH and methane sink, leading to increasing methane concentrations and production of ozone by the CH3O2 + NO and HO2 + NO reactions.

In addition to the production and impacts of intercontinental ozone, Derwent et al. also evaluated the global warmingpotentials of individual VOCs and the two responses. It was found that the longer-term methane-driven response contributed 3.9x as much to global warming as the short-term VOC-driven ozone response. Of the 17 VOCs they selected to analyze in greater detail, isoprene, benzene, ethene, and butane were important to global warming and ozone and methane concentrations. Isoprene contributed the most to ozone production and methane destruction, and despite being emitted in small amounts, had the greatest global warming potential. Ethene also contributed to ozone production and had the 2nd greatest total contribution to global warming, as well as being emitted in significant amounts at 7.2 Tg/yr. Benzene was the only VOC correlated with ozone destruction and NOx loss through HNO3 and organic NOy formation, and had the third greatest total contribution to global warming through methane along with significant emissions of 6.1 Tg/yr. Butane was the second most emitted VOC at 8.2 Tg/yr and contributed significantly to the longer-term methane-driven response. Overall, it was found that the secondary pollutant chemistry of ozone transported intercontinentally resulted in significant implications for climate change as well as pollutant chemistry at receptor regions.

Dust Transport and Deposition

Besides secondary pollutant formation, deposition of pollutants from intercontinental transport is important. Huang et al. modeled dust transport from three major deserts, the North Africa Desert (NFD), Middle East Desert (MED), and East Asia Desert (EAD), in the northern hemisphere. Dust is an important pollutant because it can affect cloud/ice condensation, precipitation patterns, and radiative forcing. It also contributes to haze at the surface, reducing visibility and air quality. Because dust has a long lifetime lasting several weeks, it is able to travel far. It was found that there is significant transport from East Asia to western North America, about 56 Tg/yr, with the contribution from dust comparable to particulate matter originating from other pollutants. Overall, the total dust emission was 4531 Tg/yr (66% from the NFD, 24% from the MED, and 10% from the EAD) as the dominant aerosol composition by mass over land compared to other aerosols such as SO4, organic matter, black carbon, NO3, NH4, and sea salt (although they were unable to model interactions between different aerosols since it is numerically difficult to compute). The highest dust plume concentrations were from 900 to 500 hPa between 40 N and 50 N at 6 μg/m3.

The dust was lifted to higher altitudes by frontal and postfrontal convections, then transported long-range by westerlies and turbulent mixing in the boundary layer. Therefore, there was a correlation with the seasonal variations of extratropical cyclones and midlatitude westerlies, leading to maximum export in spring. The plumes from the NFD and MED split at the Tibetan plateau before recombining with dust from the EAD. Dust from the NFD and MED transported primarily above 600 hPa at 2 μg/m3. Dust from the EAD transported primarily below 600 hPa at 3 μg/m3.

Dry deposition accounted for 5% to 20% of the dust contribution from NFD and MED at 3 g/m2, resulting in the deposition of nutrients such as iron and phosphorus into ecosystems. MED dust transport was significantly impacted by the Indian summer monsoon resulting in maximum wet deposition rates comparable to the average dry deposition rate. Wet deposition overall contributed less dust deposition due to the distance of regions with significant precipitation from the source deserts, so that dry deposition resulted in reduced dust concentrations.

Satellite Observations of Short-Lived Long-Range Transport

Although long-lived pollutants get the spotlight in intercontinental pollutant research, shorter-lived pollutants, such as NO2, can also be transported far. NO2 has a lifetime of a few hours in the planetary boundary layer, which extends to just four days in the free troposphere. Despite this, Zien et al. found through analysis of GOME-2 satellite observations that intercontinental transport events of NO2 were more common than previously thought and that these events may be missed in observations due to the practice of cloud filtering, or removing images with a certain amount of cloud cover since they can make measuring trace gases by satellite more difficult. Zien et al. used an algorithm to detect plumes of NO2 and then verified the sources of these plumes using Lagrangian backtracking.

It was found that NO2 transport was associated with frontal systems which form clouds, specifically cyclones passing over emission regions, that would loft the NO2 into the free troposphere, and therefore transport was most common during autumn and winter. This also highlights the importance of real world observations, as strong convection is not represented well in computational models of atmospheric chemistry. It was also found that NO2 tended to convert to reservoir species, such as PAN, during transport and then converted back to NOx once closer to the surface through interactions with local chemical species at the receptor region. This has implications for chemical cycling at receptor regions, as NOx is a major contributor to tropospheric ozone pollution by perturbing the null cycle and encouraging formation of ozone, which is amplified by the presence of OH and VOCs, leading to photochemical smog and other air quality issues. Furthermore, it may react with OH to form HNO3 or acid rain.

Four major NO2 plume emitting regions were identified: North America, Europe, South Africa, and China. It was found that North America and Europe emit an NO2 plume every 10 days, or every 5 days in winter, and South Africa every 17 days, and China every 7 days. Plume concentrations of NO2 and lifetimes tended to be higher in winter, as well as more significantly more plumes being emitted in each hemisphere's winter. Some limitations of the study were the inability tostudy NO2 plume transport to the Arctic or transport at nighttime due to limitations of the satellite's observation capabilities. Overall, it was found that short-lived pollutants could be intercontinentally transported to impact the chemistry of far-away regions.

Southern Hemisphere Transport and Future Research

Finally, most of the studies either focused on the northern hemisphere exclusively or included the southern hemisphere in addition to it. However, it is difficult to find studies focused on intercontinental transport in the southern hemisphere, specifically due to less population, landmass, and less industrial activity leading to less emissions of pollutants that can be transported intercontinentally, focusing mostly on biomass burning. Sheridan evaluated the intercontinental transport of anthropogenic ozone and particular matter (PM) 2.5 in the southern hemisphere for the year 2008, and projections for 2030, to study the importance of intercontinental transport of pollutants and the role of emissions in the southern hemisphere. Ozone and PM2.5 were chosen because they are good indicators of air quality (and hence the impact at receptor regions) and have relatively long lifetimes compared to their precursors, allowing for greater long-range transport.

For example, because Africa has a strong influence on the pollutant transport to Australia, this results in a significant increase in foreign pollution in Australia, especially the west coast. Africa and mostly Australia contribute to ozone in the southernmost parts of South America, although its distance from emitters means the overall foreign contribution is low. Differences in the RERER values (fraction of pollutant concentrations from foreign sources) from 2008 to 2030, as shown in the table below, can arise from a change in the amount of local pollution (e.g., air quality regulations decreasing emissions) or a change in the amount of foreign pollution transported to an area (e.g., an increase in emissions from an upwind area).

The table by Sheridan showing the calculated RERER values for 2008 and 2030 for Australia (AUS), South America (SAM), Southeast Asia (SEA), and Sub-Saharan Africa (SSA) is reproduced below.

O3 2008 O3 2030 Difference PM2.5 2008 PM2.5 2030 Difference
AUS 0.53 0.73 +0.20 0.20 0.31 +0.11
SAM 0.26 0.34 +0.08 0.18 0.16 -0.02
SEA 0.57 0.54 -0.03 0.61 0.41 -0.20
SSA 0.48 0.45 -0.03 0.31 0.21 -0.10

The lack of studies dedicated to intercontinental transport of pollution in the southern hemisphere is a gap in present research which will continue to widen as industrialization continues. In particular, Africa currently has the most biomass burning of any continent and pollutant emissions are projected to increase from economic growth. Other ongoing questions in intercontinental pollution transport research are better representations in models of convection and non-linearities in secondary pollutant chemistry, interactions between different species and aerosols during transport, and understanding of short-lived long-range transport events.

Citations

Hemispheric Transport of Air Pollution 2010: Part A - Ozone and Particulate Matter. United Nations, 2010. www.un-ilibrary.org, https://doi.org/10.18356/2c908168-en.

Li, Jie, et al. “High-Resolution Modeling of the Distribution of Surface Air Pollutants and Their Intercontinental Transport by a Global Tropospheric Atmospheric Chemistry Source-Receptor Model (GNAQPMS-SM).” Geoscientific Model Development, vol. 14, no. 12, Dec. 2021, pp. 7573-604. Copernicus Online Journals, https://doi.org/10.5194/gmd-14-7573-2021.

Derwent, Richard G., et al. “Investigating the Role of Organic Compounds in Intercontinental Ozone Transport: Reactivity Scales and Global Warming Potentials (GWPs).” Atmospheric Environment, vol. 306, Aug. 2023, p. 119817. ScienceDirect, https://doi.org/10.1016/j.atmosenv.2023.119817.

Huang, Jianping, et al. “Modeling the Contributions of Northern Hemisphere Dust Sources to Dust Outflow from East Asia.” Atmospheric Environment, vol. 202, Apr. 2019, pp. 234-43. ScienceDirect, https://doi.org/10.1016/j.atmosenv.2019.01.022.

Zien, A. W., et al. “Systematic Analysis of Tropospheric NO2 Long-Range Transport Events Detected in GOME-2 Satellite Data.” Atmospheric Chemistry and Physics, vol. 14, no. 14, July 2014, pp. 7367-96. Copernicus Online Journals, https://doi.org/10.5194/acp-14-7367-2014.

Sheridan, Lily. “Intercontinental Air Pollution Transport: A Southern Hemisphere Perspective.” University of Wollongong Thesis Collection 2017+, Jan. 2020, https://ro.uow.edu.au/theses1/1241.