Aerosols Without Borders

Photo Credit: Oxy_gen/Shutterstock.com

Stephanie Kappes, Metrohm International Headquarters, Herisau, Switzerland.

Long-term measurements to determine the quantity and chemical composition of suspended particles at high temporal resolution are useful to gain insight into the effects of particulate matter on health and the environment. In the south-east of Scotland, such measurements are being taken as part of the European Monitoring and Evaluation Programme (EMEP), which focuses on the monitoring and evaluation of the long-range, transboundary transmission of air-polluting substances in Europe. The programme serves to regularly equip European governments with the scientific knowledge required to reduce air pollution and mitigate its effects.

The Auchencorth Moss Monitoring Site

Auchencorth Moss is about 20 kilometres south of Edinburgh in Scotland. The landscape is a raised bog - that is, a bog which is exclusively precipitation-fed. In this remote place, the Centre for Ecology & Hydrology (CEH) operates a monitoring site (Figure 1) collecting specialized data on the quality of air and precipitation. Scientists at CEH and other British research institutes analyze this data as part of their research initiatives, including EMEP. Isolated from urban and industrial areas, as well as other local sources of emissions, the area qualifies as a “rural background location” (1). It is therefore ideal for analyzing to what degree crops and natural ecosystems, as well as the rural population, are exposed to airborne pollutants.

A Convention for Clean Air

The UNECE (United Nations Economic Commission for Europe) Convention on Long-Range Transboundary Air Pollution came into force in 1983. The signatory states agreed to take specific action, such as combating air pollution and building a monitoring network across Europe. EMEP operates under this convention and provides it with a scientific foundation. In order to collect the relevant data, the programme follows a monitoring strategy (2). This strategy determines, for example, that long-term, comprehensive monitoring of air pollutants is to take place, whereby temporal resolution is sufficient to investigate not only atmospheric processes, but also individual pollution events.

Hourly Air Pollutant Measurements

The data collected at Auchencorth Moss includes the concentrations of inorganic aerosol components and inorganic reactive gases in the ambient air. These are determined completely automatically on an hourly basis by the Monitor for AeRosols and Gases in ambient Air (MARGA). Marsailidh M. Twigg and her colleagues at the CEH, Metrohm Applikon, and the University of Edinburgh School of Chemistry recently published an evaluation of the results collected between 2006 and 2012 - a period of six and a half years. It was published in the open-access journal Atmospheric Chemistry and Physics Discussions (3).

Measurements with MARGA

MARGA analyzes water-soluble aerosols and trace gases by ion chromatography (IC). The system at Auchencorth Moss takes in ambient air and aerosols contained therein up to a diameter of 10 µm (PM10, where PM stands for particulate matter) through a suitable inlet opening. The air flow is then split into two lines. The first line leads directly into a sampling box. The other directs the air flow initially into a cyclone, which removes particles with a diameter greater than 2.5 µm from the mixture. This ensures that only aerosols in the PM2.5 fraction reach the second sampling box. Each sampling box contains a wet rotating denuder (WRD), which dissolves the water-soluble gases from the passing air flow, and a steam-jet aerosol collector (SJAC), which transfers the water-soluble aerosols into a second solution. The sample solutions are then continuously transferred into the analysis box where they are analyzed by anion and cation chromatography. Further information on the analytical procedure can be found in the original publication which can be downloaded free of charge from http://bit.ly/MARGApub.

Making One-Off Pollution Events Visible

The hourly rhythm of the measurements reveals variations in the composition of aerosols throughout the day, including one‑off pollution events. Given the consistently low concentrations of pollutants, these variations are particularly striking at the rural location of the Auchencorth Moss monitoring site. For example, on the night of 5 November 2012 (called Guy Fawkes Night in the United Kingdom and celebrated with fireworks), the site measured a sharp increase of K+ concentration around midnight; at 2.61 µg/m3 the figure was 40 times the annual average of 0.07 µg/m3.

In addition to variations throughout the day, Twigg and her colleagues were also able to observe more long-term trends, such as seasonal differences in the concentrations of Na+ and Cl–: as the ions at the measuring site primarily originate from the sea, they enter the atmosphere in greater quantities during the winter months as a result of the increase in wind speeds (Figure 2).

Ions Originating From the Sea

Auchencorth Moss is not far from the sea in any wind direction, meaning that the quantity of marine ions is relatively high. Typical ion species of marine origin are Na+, Cl–, SO42–, Mg2+, Ca2+, and K+. The ions are, however, not necessarily exclusively of marine origin. The proportion of ions that are of marine origin can nonetheless be calculated because they occur in the sea in fixed proportions. Twigg and her colleagues took the measured Na+ concentration to be entirely of marine origin and calculated the marine fractions of the other species from this value. In the coarse fraction of aerosols (PM10 minus PM2.5), the marine ions made up 73%, while in the fine fraction (PM2.5), they constituted around 30% of the inorganic ions.

When calculating the marine proportion of Cl– from the Na+ concentration, the scientists found a deficit of Cl– in many of the individual measurements as well as in the average of all measurements: in the coarse fraction, the concentration of marine Cl–calculated exceeded the Cl– concentration that was actually observed. This deficit, in combination with the measured surplus of NO3–, which is not a marine ion, suggests chloride-nitrate exchange. Such an exchange can take place during the long-range transmission of sea salt by reaction with HNO3. This hypothesis is supported by the fact that increased nitrate quantities were generally found to coincide with Cl– deficits in an analysis of the measurements taken in 2012 (Figure 3).

Towards the Origin of Air Masses Using Computer Simulation

There are no sources of emissions in the immediate vicinity of the monitoring site. The long-range transmission of air masses therefore has a considerable impact on the composition of aerosols at Auchencorth Moss. Using backwards trajectories, which are created by computer simulations on the basis of meteorological data, the origin of air masses can be ascertained. In order to retrace the path of air masses, such trajectories were simulated in 3-h intervals for the measuring period between 2007 and 2012 - 17,370 simulations in total. A cluster analysis helped Twigg and her colleagues to group together similar trajectories (Figure 4) and to establish the relationships between aerosol composition and the origin of air masses.

Trajectories 1, 2, and 4, originating from the Atlantic Ocean and the Arctic Circle, were linked with high quantities of Na+ and Cl– (Figure 5). This is even true for the PM2.5 fraction, which is usually determined by secondary (anthropogenic) ions. By contrast, air masses that cross land (trajectories 5 and 6) carry considerably more secondary inorganic aerosols, such as NO3– and NH4+.

Conclusion

The semicontinuous determination of inorganic aerosol components at Auchencorth Moss enables the analysis of both one-off pollution events and long-term trends. When combined with computer simulations, the origin of the air masses and aerosols can also be determined. Twigg and her colleagues were able to establish that a significant proportion of the aerosols, and therefore also air pollution, detected at rural locations like Auchencorth Moss have travelled a great distance. This underlines the importance of international cooperation in the fight to control emissions.

References

1. Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe
2. Unece.org (2015): «EMEP Strategies». Retrieved 27 August 2015 from
3. http://www.unece.org/env/lrtap/emep/strategies.html.
4. M.M. Twigg et al., Atmos. Chem. Phys. 15, 8131–8145 (2015).

Stephanie Kappes has worked as a scientific writer and editor in the marketing department of Metrohm since 2013. She obtained her bachelor degree in nanosciences from the University of Basel in Switzerland. During her masters studies of molecular and cellular life sciences, which she completed at Utrecht University in the Netherlands, and several internships, she focused on the public communication of science.

E-mail:ska@metrohm.com

Website:www.metrohm.com