Key concepts for describing intercontinental transport processes

Global, or hemispheric, background and baseline concentrations are terms that are often used interchangeably. To bring order and consistency to the discussion of hemispheric or intercontinental pollution transport, these terms are defined here for the purposes of this assessment report. The term baseline concentration will refer to observations of pollutants, while the term global or hemispheric background concentration will refer to modelled concentrations of pollutants.

A baseline concentration is an observation made at a site when it is not influenced by recent, locally emitted or produced pollution. These baseline sites are typically situated in locations with minimal and infrequent impact from local sources of anthropogenic pollution. Observations may be made continuously and subsequently sorted, or air samples taken only when meteorological

conditions are such that the recorded concentrations are free from the local contamination. Time series of baseline concentrations provide the range and frequency of pollutant concentrations transported to the site from upwind locations. However the requirement that only recently emitted or produced local pollution be excluded means that baseline concentrations may contain traces of local pollutants that were emitted many days earlier and became well-mixed with other air masses. There is no strict definition of “recently” emitted or produced local sources of anthropogenic pollution.

The global or hemispheric background concentration of a pollutant is a model construct that estimates the atmospheric concentration of a pollutant due to natural sources only. It is a

straightforward modelling exercise to quantify the global background concentrations of any pollutant resulting only from natural sources by using a variety of apportionment, labelling and source-receptor techniques. The relatively long-lived air quality pollutants like ozone, CO, and PM, or the greenhouse gases, are pervasive in the northern hemisphere. Because their lifetimes are longer than the time taken to advect around a latitude circle, it is unlikely that a location can be found that only observes naturally occurring concentrations of these pollutants. Therefore, global or hemispheric background concentrations of long-lived pollutants cannot be observed. The global or hemispheric background concentrations of these species can only be determined by models and these

concentrations will always be less than baseline observations which contain natural and anthropogenic contributions. In contrast, for short-lived air quality species like SO2 or NO2 it is possible to find some locations in the northern hemisphere unaffected by anthropogenic emissions allowing the direct observation of global or hemispheric background concentrations. However such locations are

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relatively few and models are still required to estimate the global or hemispheric background concentrations for most places in the northern hemisphere.

In addition to global or hemispheric background concentrations, the terms urban background and rural or regional background are also widely used in the literature. Urban background

concentrations are those observed in urban areas away from the direct influence of heavily-trafficked roads and chimney stacks. Rural or regional background concentrations are those observed at

locations where there is little influence from urban sources of pollution. Note that while urban, rural and regional background concentrations are based on observations, global and hemispheric

background values are model constructs.

In light of the definitions above, care must be taken when considering common expressions such as, „the observed growing hemispheric background ozone concentrations‟. There is little supporting evidence for an increase in natural (not influenced by human activities) sources of ozone and its precursors such as lightning, wild fires and stratosphere-troposphere exchange. In this case, the usage of „background‟ is inappropriate and „observed growing hemispheric ozone levels‟ would be a more appropriate reference to the growing influence of human activities on tropospheric ozone for which there is more supporting evidence.

FINDING: Baseline concentrations refer to observations made at a site when it is not influenced by recent, locally emitted or produced anthropogenic pollution. The term global or hemispheric background concentration is a model construct that estimates the atmospheric concentration of a pollutant due to natural sources only.

1.3.2. Source Attribution and Source-Receptor Relationships

Policy-makers have come to recognise only relatively recently the importance of hemispheric transport of air pollution to the achievement of air quality guidelines, standards and targets. Source attribution and source-receptor relationships are important concepts in this context. They are used to define how much individual source regions contribute to ground-level pollution in absolute and relative terms and how much hemispheric pollution transport contributes to the frequency and severity of pollution episodes.

Source attribution or source apportionment is the process by which a concentration or deposition is split into a number of components (or fractions) that represent a source contribution. In principle, the sum of all components should add up to the observed or modelled concentration (or unity). For the simple case of an inert tracer or for the components of an exactly linear chemical system, different source attribution approaches will give a similar impression of the importance of different sources [Seibert and Frank, 2004]. However, source attribution under one emission scenario may be different from the source attribution after emissions have changed. For example, the source attributions for ozone and PM measured in a highly polluted location are distinctly different for the pre-industrial and present-day atmospheres [Horowitz, 2006; Lelieveld and Dentener, 2000].

Most source attribution studies for ozone have employed the “tagging” or “labelling” methods where pollutants from specific source regions are “tagged” or “labelled” and explicitly tracked in the model using extra model variables. In this way, each molecule or particle, whether “tagged” or not is given the same local, instantaneous removal rate. These tagging techniques have led to some

important findings regarding tropospheric chemistry. Using ozone as an example:

1) An ozone “tagging” analysis on the global scale found that production of ozone in the middle, upper and continental lower troposphere all make significant contributions (10 - 50

%) to ozone concentrations throughout the troposphere [Wang et al., 1998];

2) Transport from polluted continental source regions generally accounts for more than 40%

of the ozone in remote locations [Sudo and Akimoto, 2007];

3) Over Europe, North American and Asian sources of ozone contribute substantially to the annual ozone budget, accounting for approximately 11 and 8 %, respectively, while European

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sources contribute only 9 %. These contributions show marked seasonalities because of the interactions between the chemical and transport processes [Auvray and Bey, 2005].

4) “Labelled” ozone in a global model coupled to a regional model provided a source

attribution of the ozone measured at a rural location in southern England during January 2006 (see Figure 1.6). Averaged over the 12-month study period, two-thirds of the ozone found at this site was advected by large-scale intercontinental processes and one-third from production on the regional scale within Europe. Of the ozone brought by intercontinental transport, the largest contribution came from North America and the western North Atlantic, with smaller contributions from the stratosphere, Asia and the Pacific [Derwent, 2008].

5) A global chemical transport model provided ozone source attributions for 8 locations in North America during March 2001. At most locations intercontinental transport contributes about 10 – 20 ppbv, with significant location-location and day-by-day variability [Fiore et al., 2003].

Whilst these source attributions for ozone go a long way towards answering the policy questions addressing ozone and its hemispheric scale transport, they have some drawbacks and limitations. There are technical issues surrounding the “tagging” and “labelling” schemes that mean that it is difficult to separate the contributions to ozone from natural and anthropogenic sources of VOCs and NOx and from surface and elevated sources such as lightning and aircraft. Also, such source attributions are highly model- and location-specific.

Figure 1.6. Source attribution of the ozone found at a rural location in southern England during 2006 [Derwent, 2008]. Europe-regional refers to the ozone advected directly over the local- and regional-scales to the location. North America to that formed over that continent and over the North Atlantic and east Pacific; Asia to that formed over that continent and over the western Pacific; Europe-intercontinental to that advected around latitude circles and back into Europe; Extra-continental refers to that from interhemispheric transport. [Reprinted from Figure 1 in Derwent, R. G. (2008), New Directions: Prospects for regional ozone in north-west Europe, Atmospheric Environment, 42:1958-1960, with permission from Elsevier.]

The term “source-receptor” relationship describes the sensitivity of concentrations or depositions at a “receptor” location to a change in emissions from a “source” location [Seibert and Frank, 2004]. It is a key concept which is used in this document to assess the impact of emissions from an upwind continent or region on a downwind receptor location [Derwent et al., 2004; Fiore et al., 2002; Wild and Akimoto, 2001]. Source-receptor relationships are different in concept to source

0 10 20 30 40 50 60

01/01/2006 01/02/2006 01/03/2006 01/04/2006 01/05/2006 01/06/2006 01/07/2006 01/08/2006 01/09/2006 01/10/2006 01/11/2006 01/12/2006

O3, ppb

Europe-regional North America Asia

Europe-intercontinental Extra-continental Stratosphere

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attributions [Venkatram and Karamchandani, 1986] and imply a response at a particular receptor location to a change in the emission from a particular source. However, the two terms are linked by the fact that multiplying the source-receptor relationship by the emissions flux yields the source attribution for the simple case of an inert tracer or for the components of an exactly linear system. For ozone, with its non-linear relationship between NOx and VOC precursor emissions, this multiplication does not yield the same perceptions about the importance of intercontinental sources and

intercontinental transport.

Calculations of source-receptor relationships can be classified into source-oriented and receptor-oriented approaches. The source-oriented approach is the most commonly-used approach, where emissions from a particular source region are perturbed and these perturbations are propagated forward in time throughout the model domain. In the receptor-oriented approach, the perturbation in the receptor region from a change in emissions is traced back in time through the model domain.

FINDING: Source attribution and source-receptor relationships are widely applied analytical techniques used to define how much individual source regions contribute to ground-level pollution in absolute and relative terms, and how much hemispheric pollution transport contributes to the frequency and severity of pollution episodes.

1.4. Major types of intercontinental transport processes