Observations: What is the observational evidence for the intercontinental transport of O3, PM, Hg, and POPs in the Northern Hemisphere?

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.


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].


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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