Chapter 1 Conceptual Overview of Hemispheric or Intercontinental Transport
1.2. Intercontinental or hemispheric transport of ozone and PM
On the intercontinental or hemispheric scale, the quantity of pollution emitted in one location and the fraction that ultimately reaches a particular downwind location is dependent on three factors:
1) the quantity of the pollutant emitted or produced at the source, 2) the meteorological conditions that transport the pollution from one continent to another, and 3) the physical and chemical transformation processes that modify the quantity and composition of the pollution during transport that lasts from days to weeks. These topics are introduced below.
1.2.1. Major ozone and PM sources
Near the Earth‟s surface (the troposphere) the trace gas ozone has a variety of sources including downward transport from the stratosphere, photochemical production from natural ozone precursor sources such as lightning or wild fires, and photochemical production from ozone
precursors emitted by human activities such as fossil fuel combustion or crop burning. Air pollutants (e.g. CO, NOx) that are directly emitted from a source into the atmosphere are referred to as primary pollutants. In contrast, ozone is a secondary pollutant, meaning that it is produced from chemical reactions involving precursor gases, accounting for approximately 90% of the ozone in the troposphere (10% is directly transported from the stratosphere) [Stevenson et al., 2006]. PM is a complex mixture of extremely small particles and liquid droplets with a broad compositional range, and may have primary and/or secondary sources.
The magnitude and impact of hemispheric and intercontinental scale transport of ozone and PM is initially determined by the global distribution of emissions, and their spatial relation to the major
meteorological transport pathways. Emissions that affect ozone and PM concentrations in the
atmosphere are sulphur dioxide (SO2), nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOC), ammonia (NH3), methane (CH4), organic carbon (OC), black carbon (BC) and carbon monoxide (CO); these emissions are discussed in detail in Chapter 3. Modelling studies indicate that man-made, or anthropogenic, emissions of pollutants have a major influence on the quantity of trace gases and PM found in the atmosphere. These studies estimate that in the troposphere (the lowest layer of the atmosphere) the burden of PM in the form of particulate sulphate and black carbon particles has increased by factors of 3 and 6, respectively, since preindustrial times. Meanwhile, the tropospheric burden of ozone has increased by 30-50% [Horowitz, 2006; Lamarque et al., 2005].
Figure 1.2 provides an example of present day anthropogenic emissions with NOx (= NO + NO2), an important precursor of ozone, as well as some PM (e.g. nitrate particles). Global emissions of anthropogenic NOx from fossil fuel combustion (excluding shipping and aviation) for the year 2005 are shown, along with biomass burning emissions [van der Werf et al., 2006]. Biomass burning emissions can have a large inter-annual variability [Duncan et al., 2003; van der Werf et al., 2006], while anthropogenic NOx emissions tend to shift gradually over time, in response to changing economic conditions and pollution control measures. For example, satellite measurements of column NO2 indicate that NOx emissions increased during 1996-2005 in China (up to 29% per year) and other locations in Asia, while emissions decreased in Europe and the U.S. [van der A et al., 2008]. The most recent bottom-up inventories indicate South and East Asia NOx emissions increased 44% during 2001-2006, with an increase of 55% within China [Zhang et al., 2009a], while NOx emissions decreased by 30% across Europe (1990-2005) [Royal Society, 2008], and by 37% across the U.S.
(1985-2008) [U.S. EPA, 2009].
Figure 1.2. Distribution of global anthropogenic-NOx emissions (top), and biomass burning NOx from natural and anthropogenic fires (bottom) during the year 2005. Data are in units of kilo-tons NOx per 1x1 degree grid cell per year. Anthropogenic NOx emissions are from the latest EDGAR-HTAP inventory (as described in Section 3.2.1), while biomass burning emissions are from the Global Fire Emissions Database version 2 (GFEDv2) [van der Werf et al., 2006]. White boxes encompass the four main TF HTAP source-receptor regions, North America (NA), Europe (EU), South Asia (SA) and East Asia (EA).
This report focuses on four broad emissions regions in the northern hemisphere, North America (NA), Europe (EU), South Asia (SA) and East Asia (EA), all outlined in Figure 1.2. The
largest emissions of anthropogenic NOx in the Northern Hemisphere are located in the eastern United States and southeastern Canada (within NA), western and central Europe (EU), and South Asia (SA) and East Asia (EA). Global emissions of anthropogenic NOx are roughly seven times those of biomass burning NOx emissions [Schultz and Rast, 2007]. By far, the majority of global biomass burning occurs in the tropics and is mainly caused by human activity, with natural wildfires being more prevalent in temperate and boreal forests [Crutzen and Andreae, 1990]. Estimates of natural sources of NOx from lightning and soil vary widely but together are approximately one third of anthropogenic emissions [Schultz and Rast, 2007; Schumann and Huntrieser, 2007].
The anthropogenic NOx emissions shown in Figure 1.2 are representative of the spatial distribution of human activity across the globe, but are by no means proportional to the number of people in the various regions as shown by Figure 1.3. While the Northern Hemisphere contains the great majority of people (88%), 90% of anthropogenic NOx emissions, and roughly 50% of biomass burning NOx emissions, per capita emissions vary widely across nations. Broadly speaking, North America has the highest per capita anthropogenic NOx emissions while South Asia has the lowest (Figure 1.3).
Figure 1.3. Global distribution of human population and anthropogenic NOx emissions in the year 2005. Population data from Center for International Earth Science Information Network (http://sedac.ciesin.columbia.edu/gpw/), emissions data from the latest HTAP inventory as described in Section 3.2.1. The regions designated by North America, Europe, South Asia and East Asia are shown in Figure 1.2.
1.2.2. Major transport pathways
This section briefly introduces the major pathways of intercontinental transport, summarized in Figure 1.4 [Stohl and Eckhardt, 2004]. The processes that govern this transport are described in Section 1.4, while observations of pollution within these pathways are reviewed in Chapter 2. The models that simulate intercontinental transport processes are reviewed in Chapter 4.
The mid-latitudes cover most of the major emissions regions within North America, Europe and East Asia. Transport in this region is dominated by the westerly winds that transport emissions from East Asia across the North Pacific Ocean to North America, from North America across the North Atlantic Ocean to Europe, and from Europe into the Arctic and central Asia. Pollutants that are lofted into the mid- and upper troposphere travel further and faster than pollutants that remain in the lower troposphere due to the stronger winds at altitude, especially in the vicinity of the jet stream. The westerly winds are stronger in winter than summer and therefore pollutants are transported at greater speeds and over greater distances during the winter months.
Transport of pollutants emitted in the tropics is dominated by the tropical easterlies or trade winds at the surface. Of the four major emissions regions in the northern hemisphere, only South Asia is dominated by the easterlies in winter, although emissions from Europe can still travel south to northern Africa, and some emissions from North America and East Asia can also reach the tropics. In summertime the easterlies extend their influence northward allowing more emissions
from NA, EU and EA to influence the tropics. Also during this season the North American and Asian summer monsoons develop driving surface emissions northward, while at the same time producing widespread thunderstorms which rapidly loft pollutants from the surface to the upper troposphere. In the upper troposphere, stronger winds can transport the pollutants further than had they remained close to the surface.
The location of the emissions regions with respect to the dominant atmospheric transport pathways has a strong influence on the frequency and strength of intercontinental pollution transport events. For example, the emissions regions along the east coasts of Asia and North America are at the origins of the North Atlantic and North Pacific mid-latitude cyclone storm tracks, which can loft the emissions into the mid- and upper troposphere and transport them to downwind continents in a matter of days. In contrast, Europe is at the end of the North Atlantic storm track and its emissions stay closer to the surface, especially in winter.
Figure 1.4. Pathways of intercontinental pollution transport in the Northern Hemisphere.
Shading indicates the location of the total column of a passive anthropogenic CO tracer released over the Northern Hemisphere continents after 8-10 days of transport, and averaged over 15 years. Shown are transport pathways in summer (June, July, August; upper panel), and winter (December, January, February; lower panel). Gray arrows show transport in the lower troposphere (< 3 km) and black arrows show transport in the mid- and upper
troposphere (> 3 km). [Image reproduced from Chapter 1, Figure 2, page 6, of Stohl, A. and S.
Eckhardt , Intercontinental Transport of Air Pollution: An Introduction, in Intercontinental Transport of Air Pollution, edited by A. Stohl, with kind permission of Springer Science and Business Media B.V.]
1.2.3. Chemistry and transformation processes
Intercontinental pollution transport occurs on timescales of days to weeks, longer than the atmospheric lifetimes of some pollutants (Table 1.1), and ample time for the trace gases and PM emitted or produced at the source to undergo removal or chemical transformation. By the time the polluted air mass arrives at a downwind continent it is likely to have very different chemical properties than it did at the source. The chemical transformation processes that occur during intercontinental transport are numerous and complex. A few of the most important chemical transformation processes experienced by polluted air masses during transport are depicted in Figure 1.5 and described below, using transport from Asia to North America as an example. The descriptions of these processes are supported by studies conducted across many of the world‟s oceans, and are generally applicable to most intercontinental transport routes.
Table 1.1. Approximate lifetimes of trace gases and PM in the atmospheric boundary layer and the free troposphere. Lifetimes are highly variable depending on altitude, time of day, season, and proximity to emissions regions.
Trace gas or PM Approximate lifetime in the atmospheric boundary layer
Approximate lifetime in the free troposphere
NO2 hours days
SO2 days days to weeks
CO weeks to months weeks to months
VOCs hours to months hours to months
CH4 8-9 years 8-9 years
NH3 days days
PM hours to days days to weeks
ozone hours to days weeks to months
Pollution transported from Asia to North America originates in the atmospheric boundary layer (ABL) of eastern Asia where primary air pollutants such as NOx, SO2, CO, NMVOC and PM are directly emitted from combustion and industrial sources. During daytime, sunlight initializes
photochemical reactions that allow NOx and NMVOC to react and form the secondary pollutant, ozone. Ozone becomes a source of the OH radical, a powerful oxidant that reacts with and transforms many pollutants in the atmosphere. Sometimes referred to as the cleaning agent of the atmosphere, OH is also the dominant species that removes CH4 and CO from the atmosphere, and it plays an important role in the oxidation of SO2 to H2SO4, (sulphuric acid) which is a source of particulate sulphate. Other gases that can react to form PM known as secondary aerosols are NOx, NH3, and NMVOC. While these and many other chemical transformation processes occur within the continental ABL, trace gases and PM are also removed from the atmosphere via wet and dry deposition. Dry deposition occurs when the gases or PM come into contact with Earth‟s surface and are taken up or adsorbed onto surfaces. Dry deposition rates depend on the surface type (e.g. vegetation, water, ice) and the rate at which constituents are transported to the ground. Wet deposition occurs when precipitation transports water soluble trace gases (such as sulphuric or nitric acid) and PM to the Earth‟s surface.
Figure 1.5. General intercontinental transport processes. Blue text on the left applies to continental boundary layer processes, red text apples to low level transport and black/white text applies to high altitude transport.
When weather systems export polluted air masses from the Asian continental ABL across the North Pacific Ocean towards North America, the subsequent removal and chemical transformation depends on whether the air mass is exported to the lower troposphere, generally less than 3 km in altitude, or to the mid- and upper troposphere. Polluted air masses exported to the mid- and upper troposphere experience strong uplift via deep convection or warm conveyor belts (WCBs) (these meteorological processes are described in Section 1.4), rising 3 to 15 km above the Earth‟s surface (Figure 1.5). As these air masses rise and cool, water vapour from the ABL condenses into clouds.
The ensuing precipitation removes most of the water soluble trace gases and PM via wet deposition.
By the time the polluted air mass reaches the mid- or upper troposphere, most of the NOx has been converted to other species such as peroxyacetile nitrate (PAN) or nitric acid and there is relatively little left for continued ozone production [Li et al., 2004; Miyazaki et al., 2005; Stohl et al., 2002b].
Much of the reactive nitrogen is in the form of PAN, an important reservoir of NOx [Singh et al., 1992; Zhang et al., 2008]. The clear, cold and dry conditions of the upper troposphere also lead to significantly different chemistry than at the surface, with, for example, a much longer NOx lifetime.
Eventually, these aged, polluted air masses descend towards the surface. The air warms as it descends, allowing thermal decomposition of PAN that releases NOx. This new supply of NOx causes the resumption of ozone production [Hudman et al., 2004; Zhang et al., 2008], however the
descending air mass can encounter higher concentrations of water vapour in the lower troposphere which destroys some of the newly formed ozone [Real et al., 2007]. Increased ozone concentrations can also lead to the oxidation of any remaining SO2 in the air mass, with the resulting sulphuric acid nucleating new particles that condense to form particulate mass [Brock et al., 2004; Dunlea et al., 2009]. A common location for polluted air masses to descend is the eastern North Pacific Ocean [Cooper et al., 2004; Hudman et al., 2004; Zhang et al., 2008]. Portions of these warming and
photochemically active air masses can eventually descend into the North America ABL and contribute to the local pollution burden. However the dilution that occurs during transport is so extensive that the Asian pollution is difficult to detect in the North American ABL, especially where local sources of pollution are strong [Zhang et al., 2009a].
Different chemical transformation processes occur in air masses that remain in the lower troposphere. The lower troposphere contains more water vapour than the mid- and upper troposphere, which leads to net ozone destruction when NOx concentrations are low. However, if NOx
concentrations are maintained above a critical threshold, ozone production will occur [Logan et al., 1981; Reeves et al., 2002]. Over the oceans, the lowest few hundred meters of the lower troposphere is occupied by the marine boundary layer, which is usually stable, moist and isolated from the air above. Ozone production in the marine boundary layer typically occurs within the first few days of pollution export from the continent, at least during the warm season. During winter net ozone
destruction appears to be more typical [Parrish et al., 1998]. Transport of air masses in the lower troposphere, above the marine boundary layer, can result in NOx being released as PAN leading to enhanced ozone concentrations such as over the Azores in the middle of the North Atlantic Ocean at 3 km [Owen et al., 2006; Val Martin et al., 2008]. However, in the lower troposphere, nitric acid is formed preferentially to PAN and, in the presence of clouds will dissolve into cloud droplets leading to reductions in available NOx. In this case, although substantial ozone may have been produced over the continental source region, ozone destruction, due to high water vapour concentrations will prevail downwind. If ozone and water vapour concentrations remain high, the resulting OH can destroy pollutants such as CO [Real et al., 2008].
Regarding PM, lower tropospheric transport tends to experience less precipitation than air masses that are lofted to the mid- and upper troposphere. As a result, wet deposition is less and PM concentrations are much greater in the lower troposphere than in the mid- and upper troposphere [Heald et al., 2006; van Donkelaar et al., 2008]. Furthermore, greater precipitation in winter appears to explain why lower concentrations of PM are transported to the surface of western North America in winter compared to summer [Jaffe et al., 2005].
FINDING: The quantity of ozone or PM transported from one continent to another is
determined by the initial quantity of these pollutants either emitted or produced in the source continent, the altitude to which the pollutants are exported, the transport speed, and the deposition and chemical transformation that occurs during transport.
1.3. Key concepts for describing intercontinental transport processes