Continuous air quality monitoring for harmful particulate matter and aerosols

Air pollution is defined by the World Health Organization (WHO) as «contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere» [1]. When air pollution levels are high, this can lead to respiratory problems, heart disease, and other illnesses (e.g., cancer). It can also cause acid rain, damage crops, reduce plant growth and productivity, and harm wildlife. Since 99% of the global population breathes air that exceeds WHO quality guideline limits [1,2], this is a widespread issue. Among the various air pollutants, particulate matter and aerosols are of particular concern. This blog article discusses these contributors to air pollution and highlights two instruments dedicated to continuously monitoring air quality parameters.

Brief introduction to particulate matter and aerosols

Particulate matter (PM) is generally defined as small solid particles suspended in a gas, while aerosols are finer liquid droplets or solid particles that remain suspended in gases for significant periods of time. Both of these can negatively affect human health, especially when their diameters are less than 2.5 µm (PM_2.5, Figure 1). Aerosols and PM can be created by natural occurrences such as volcanic eruptions, as well as anthropogenic activities such as industrial operations and transportation. Therefore, it is important to analyze the chemical composition of these pollutants not only to determine the long-term effects after exposure, but also to identify their sources in order to take steps to reduce emissions.

Comparison of particulate matter to the width of as human hair.

Figure 1. Particulate matter size comparison. Adapted from [3].

Once in the air, these miniscule particles can be transported across long distances, causing complications far from their source. The smaller the particles’ size, the deeper they can infiltrate the respiratory system. Several studies have linked PM to health problems (e.g., respiratory issues) and environmental issues (e.g., visibility impairment) [4–6]. While coarser dust particles (PM₁₀) are mostly retained by nasal hair, fine particles (PM_2.5) can penetrate deep into the lungs and cause irritation. Aerosols, on the other hand, are even smaller than PM particles and can therefore stay aloft in the atmosphere for extended periods.

To gain a better insight into the effects of air pollution on our health and the environment, there is a need for accurate measurements that determine the quantity and chemical composition of suspended particles at a high temporal resolution. However, the collection of representative samples and the associated analysis are the most challenging parts of air monitoring.

Air monitoring – then and now

Traditionally, PM and aerosol analysis consists of two steps: sample collection and analysis. To collect representative samples, it is important to use appropriate sampling equipment and techniques.

The sample collection step commonly employs a filtration process. Particles are collected on substrates with filters which are removed after a certain period of time for extraction with deionized water for subsequent analysis [7]. However, this method is only capable of determining averages over a time of 24 hours or more. Furthermore, the method is cumbersome and has poor precision, making continuous on-line measurements impossible.

Continuous sampling is of utmost importance since it will enable sensitive monitoring of changes in the ionic composition of the aerosols. But how can this be done?

Metrohm Process Analytics is a well-known provider of analytical solutions for air and aerosol analysis with a wealth of experience and expertise in the field. We offer a comprehensive portfolio of advanced instruments, software, and accessories that enable accurate and reliable measurement of airborne particles.

The most promising instruments for aerosol collection, often referred to as steam collecting devices, are shown in Figure 2: the Metrohm AeRosol Sampler (MARS) and the 2060 Monitor for AeRosols and Gases in ambient Air (MARGA).

Figure 2. Inline sample preparation for gaseous samples is made simpler using the 2060 MARGA (left) or MARS (right) systems from Metrohm Process Analytics.

Figure 3. A close-up view of the Metrohm AeRosol Sampler (MARS).

Regarding the chemical analysis, the MARS device (Figure 3) is coupled to wet chemical analyzers such as a cation and/or anion chromatograph (IC) or a voltammetric system, while the 2060 MARGA has integrated anion and cation ICs (see video below).

Both instruments include gas denuders (Wet Rotating Denuder «WRD», Figure 4), a condensation particle growth sampler (Steam-Jet Aerosol Collector «SJAC», Figure 5), as well as pumping and control devices. These instruments apply the method of growing aerosol particles into droplets in a supersaturated water vapor environment. Previously mixed with carrier water, the collected droplets are continuously fed into sample loops or preconcentration columns for analysis.

Figure 4. Wet rotating denuder (WRD) for easy gas removal.

Figure 5. Steam-Jet Aerosol Collector (SJAC) featured in the MARS and 2060 MARGA.

MARS vs. 2060 MARGA – which is the right choice?

While MARS has been designed to sample only aerosols, the 2060 MARGA additionally determines water-soluble gases. Compared to the classical denuders which remove gases from the air sample upstream of the aerosol collector (growth chamber), the 2060 MARGA collects the gaseous species in a WRD for online analysis. In contrast to the gases, aerosols have low diffusion speeds and thus pass the WRD without interference.

The 2060 MARGA comes in two configurations: R (research) and M (monitoring). The 2060 MARGA R version is meant for research campaigns such as the study of seasonal air quality variability. When not in use, the ion chromatograph can be uncoupled and repurposed for other laboratory research. For a more permanent solution, the 2060 MARGA M is used for 24/7 air quality monitoring.

Figure 7. MARS enables comprehensive analysis of air samples with IC, VA, or both.

In comparison, the MARS can be used as a preconditioning unit for several analytical techniques (Figure 7) in ambient or industrial environments such as an IC, a voltammetric (VA) instrument, mass spectrometer (MS), or total organic carbon (TOC) analyzer. Alternatively, samples for offline determination can be collected using an autosampler. To immediately evaluate the results, the MARS can also be linked remotely to any analysis system. On the other hand, the 2060 MARGA has two integrated ICs, so no other analytical technique can be coupled.

Table 1. Differences between the 2060 MARGA and MARS. The 2060 MARGA R is for research purposes with a detachable ion chromatograph while the 2060 MARGA M is intended for dedicated air quality monitoring with its two integrated ICs.

	MARS	2060 MARGA
Sample size	Large air samples: 0.5–1.0 m³/h	Large air samples: 0.5–1.0 m³/h
Type of pollutants	Suitable for only aerosols analysis Aerosols: Cl^-, NO₃^-, SO₄^2-, F^-, NH₄⁺, Na⁺, Ca²⁺, Mg²⁺, K⁺	Aerosols and gases analysis Aerosols: Cl^-, NO₃^-, SO₄^2-, F^-, NH₄⁺, Na⁺, Ca²⁺, Mg²⁺, K⁺ Gases: HCl, HNO₃, HONO (HNO₂), SO₂, NH₃, HF
Type of pollutants	MARS can measure various pollutants, such as sulfate, nitrate, and ammonium ions.	MARGA can measure various pollutants, such as sulfate, nitrate, and ammonium ions, as well as trace gases, including sulfur dioxide and ammonia.
Analysis method	Can be paired with different analysis techniques (e.g., IC, VA, etc.)	Two integrated ICs
Analysis method	Single or multiple analysis techniques	Single analysis technique
Time resolution	Continuous air monitoring	Continuous air monitoring
Sample collection method	SJAC	WRD and SJAC
Dimensions in mm (W/H/D)	660/605/605	2060 MARGA R: 660/930/605 2060 MARGA M: 660/1810/605
Intended use	Research	2060 MARGA R – Research campaigns 2060 MARGA M – Dedicated continuous monitoring

The following section compares results to see if there is any correlation between the 2060 MARGA and MARS aerosol sampling and measurement. Since the aerosol results from the 2060 MARGA are known to be accurate [8], a good correlation would indicate that the MARS also measures aerosols accurately.

The graphs below show the aerosol results of the ambient air in Schiedam, the Netherlands, measured between June 6–9, 2022 with both the 2060 MARGA and MARS systems using ion chromatography (Figure 6). The 2060 MARGA has a cycle time of 60 minutes (normal cycle time), whereas the MARS has a 30-minute cycle time. The data show a similar trend between both systems but because the MARS generates twice the data, its aerosol concentration data is higher compared to that of the 2060 MARGA. If the data is corrected to 60 minutes by using a moving average, then the concentrations given by the MARS and 2060 MARGA are similar.

Figure 6. Comparison test showing results for various air quality parameters as measured by the 2060 MARGA and MARS.

Conclusion

Monitoring air pollution is crucial because it enables us to understand the types and levels of pollutants present in the air we breathe. Exposure to air pollution can cause numerous health issues, including respiratory illnesses, cardiovascular disease, and even cancer. It can also harm the environment by causing acid rain, ozone depletion, and contributing to climate change. It is important to measure the air quality using tools such as the MARS or 2060 MARGA from Metrohm Process Analytics to understand their impact and develop effective strategies to reduce exposure. By doing so, we can work towards creating a healthier and more sustainable environment for all.

References

[1] World Health Organization. Air pollution - Overview. https://www.who.int/health-topics/air-pollution (accessed 2023-06-22).

[2] WHO Global Air Quality Guidelines: Particulate Matter (‎PM_2.5 and PM₁₀)‎, Ozone, Nitrogen Dioxide, Sulfur Dioxide and Carbon Monoxide; World Health Organization: Geneva, 2021. https://www.who.int/publications/i/item/9789240034228

[3] US EPA. Particulate Matter (PM) Basics. https://www.epa.gov/pm-pollution/particulate-matter-pm-basics (accessed 2023-06-22).

[4] Venners, S. A.; Wang, B.; Xu, Z.; et al. Particulate Matter, Sulfur Dioxide, and Daily Mortality in Chongqing, China. Environ. Health Perspect. 2003, 111 (4), 562–567. DOI:10.1289/ehp.5664

[5] Zhang, J.; Song, H.; Tong, S.; et al. Ambient Sulfate Concentration and Chronic Disease Mortality in Beijing. Sci. Total Environ. 2000, 262 (1–2), 63–71. DOI:10.1016/s0048-9697(00)00573-8

[6] US EPA. Health and Environmental Effects of Particulate Matter (PM). https://www.epa.gov/pm-pollution/health-and-environmental-effects-particulate-matter-pm (accessed 2023-03-27).

[7] Wang, D.; Jiang, J.; Deng, J.; et al. A Sampler for Collecting Fine Particles into Liquid Suspensions. Aerosol Air Qual. Res. 2020, 20 (3), 654–662. DOI:10.4209/aaqr.2019.12.0616

[8] Läubli, M. Air Monitoring by Ion Chromatography – a Literature Reference Review, 2018. https://www.metrohm.com/en/products/a/ir_m/air_monitoring_icv2.html