by Jachym Brzezina, Czech Hydrometeorological Institute
Global climate change and air pollution are two of the most significant environmental threats today. The World Health Organization (WHO) states that air pollution prematurely kills an estimated 7 million people worldwide every year. Data shows that 9 out of 10 people breathe air that exceeds WHO guideline limits. Many of these pollutants are closely linked to climate change.
The battle against air pollution is one of the major focus areas at The Czech Hydrometeorological Institute (CHMI). Headquartered in Prague, CHMI is the Czech national organization responsible for meteorology and climatology, hydrology and air quality. CHMI’s Division of Air Quality monitors ambient air conditions, operates the station network, identifies sources of pollution (source apportionment) and gathers and processes emission and greenhouse gases data.
It is extremely important to monitor pollutant concentrations, their current values as well as long-term trends and try to implement measures that will lead to improved air quality. For these reasons, it is crucial to also have information about pollution sources in order to understand the main causes of poor air quality.
The Use of SEM/EDX to Analyze Air Pollutants
The CHMI Air Quality Department in Brno is using a TESCAN scanning electron microscope (SEM) with energy-dispersive X-ray analysis (EDX) for their research. While SEM/EDX is a common technique used in materials research, it is not as commonly used for air quality analysis. SEM/EDX enables the scientists to look at the individual contamination particles and analyze their size in two dimensions, represent their shape numerically, as well as understand the particle’s elemental composition in order to determine the potential sources of pollution.
Other techniques used more commonly to assess air quality include mass spectrometry, UV-fluorescence, chromatography, and radiometry, all of which determine exact quantitative concentrations of particular pollutants in the air. SEM is only semi-quantitative (it does not provide an absolute concentration in the air) however, it provides other essential quantitative information about the air particles.
Particle Analysis Provides Critical Insight to the Sources of Pollution
Being able to tell whether the particle is regular-shaped or irregular, larger or smaller, in addition to the combination of chemical elements it contains, enables CHMI to describe the air particles much more comprehensively and helps to identify the potential source. For example, particles from combustion processes tend to be regular-shaped, while particles from natural sources are usually irregular and larger.
The combination of elements and their weight ratios are also crucial for source apportionment. For example, some elements such as iron or many others are present in both natural and anthropogenic (human-related) sources and so a single value representing an absolute concentration is not enough to differentiate the two. Knowing the overall combination of elements with iron in a particular particle and its size and shape provides the potential to identify the possible source.
Knowing the particle size and shape is also important in order to determine how deep into the respiratory system the particle can travel. Extremely small particles can even penetrate directly into a person’s bloodstream. Large particles are less likely to enter the respiratory system since they can be trapped by mucous membranes. The chemical composition then determines what effects it might have on a human body.
Common Elements in Air Pollution
Some of the most common elements seen in the samples that CHMI has processed are silicon, calcium, magnesium and aluminium because they are present in soils, which are practically everywhere, and such particles tend to be rather large and irregular. Some elements cannot be analyzed, for example, hydrogen. Two other elements that cannot be quantified are carbon and oxygen because the polycarbonate filter that CHMI uses is made up of these two elements, therefore it is impossible to differentiate particle content from background content.
- Fly ash (Figure 1) is created during coal burning processes. Apart from organic matter, coal also contains mineral components (clay minerals, quarts, calcite, gypsum, pyrite etc.), which melt during coal burning and when they subsequently cool down, they produce spherical particles, so-called cenospheres. As a result of gas generation (eg. SO2, CO) and their expansion, these spherical particles are hollow.
Figure 1: Fly ash particles
- Iron rich particles (Figure 2) are a common pollution source thanks to automobile traffic, which produces smaller and more spherical particles, often containing elements such as iron. Iron-rich particles can originate in the friction of brake pads or of the actual tram rails. One can also see pollen particles in Figure 2.
Figure 2: Iron-rich particles
- Carbonaceous particles (Figure 3) are another interesting example. They are the effect of fireworks on air quality, which involves elements typically used in pyrotechnic products like potassium, strontium, barium, copper or sulphur. Carbonaceous particles are formed during incomplete combustion of organic material. Particles form tightly bound aggregates in the form of straight or branched chains, which further create agglomerates of a size up to several micrometers (µm).
Figure 3: Carbonaceous particles
A Standardized Methodology for Air Quality Monitoring
One of CHMI’s main objectives is to improve air quality. The results from the TESCAN SEM/EDX system can help achieve this because, in order to reduce emissions, it is critical to know the source. It is also possible that, in some cases, the particle is of natural origin. In this case, unfortunately, there is nothing that can be done―even a totally uninhabited planet would have some concentrations of pollutants from natural sources, such as soil erosion, resuspension from animals, volcano eruptions, and wildfires.
Ultimately, CHMI’s wants to develop a standardized methodology for air quality monitoring that can be easily used by scientists worldwide for identification of particles using automated and manual SEM/EDX particle analysis. The center plans to develop a database where all the information from SEM along with meta data (location of sampling, date of sampling, sampler type, etc.) will be stored and create software and statistical tools in order to analyze the data in a consistent way.
About the author: Jachym Brzezina is Head of the Air Quality Department at the Czech Hydrometeorological Institute.