Continuous air quality monitoring for harmful particulate matter and aerosols

Feb 24, 2025

Article

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]. Industrial air pollution monitoring is essential as breathing highly polluted air leads to respiratory problems, heart disease, cancer, and other severe health issues. 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 for continuous ambient air quality monitoring.

What is the difference between particulate matter and aerosols?

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

Particulate matter (PM) is generally defined as small solid particles which are suspended in a gas, while aerosols are finer liquid droplets or solid particles that remain suspended in gases for significant periods of time. Both can negatively affect human health, especially when their diameters are less than 2.5 µm (PM_2.5, Figure 1).

Aerosols and PM can come from natural sources like volcanic eruptions, but also from anthropogenic activities such as industrial operations and transportation. Therefore, industrial air quality monitoring plays a critical role in identifying emission sources, understanding chemical composition, and developing strategies to reduce exposure.

How is PM and aerosol analysis done?

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.

Sample collection 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 [4]. However, this method is only capable of determining averages over periods of time of 24 hours or longer. Furthermore, the method is cumbersome and imprecise. This makes continuous online measurements impossible.

Continuous sampling of aerosol composition is crucial for understanding and addressing air quality challenges. Real-time data provides valuable insights into rapid changes in aerosol composition, allowing quicker responses to pollution events, and supports more accurate scientific investigations of atmospheric processes.

To overcome the limitations of traditional sampling methods, advanced technologies are required for continuous aerosol analysis. Steam collecting devices, such as the Metrohm AeRosol Sampler (MARS) and the 2060 Monitor for AeRosols and Gases in ambient Air (MARGA) (Figure 2), offer real-time, continuous monitoring of aerosol composition. These instruments utilize advanced techniques to collect and analyze aerosols, providing valuable data for air quality assessment and research.

Both instruments (2060 MARGA and MARS) include gas denuders (Wet Rotating Denuder «WRD»; Figure 4, left), a condensation particle growth sampler (Steam-Jet Aerosol Collector «SJAC»; Figure 4, right), as well as pumping and control devices. These instruments apply the method of converting aerosol particles into droplets in a supersaturated water vapor environment. After being mixed with carrier water, the collected droplets are continuously fed into sample loops or preconcentration columns for analysis.

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

While MARS has been designed to sample only aerosols, the 2060 MARGA also 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 gases, aerosols have low diffusion speeds and thus pass through 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 and 24/7 air quality monitoring, the 2060 MARGA M is better suited.

**Table 1.** Differences between the analysis capabilities of the 2060 MARGA and MARS systems.
	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	Suited only for aerosols analysis Aerosols: Cl^-, NO₃^-, SO₄^2-, F^-, NH₄⁺, Na⁺, Ca²⁺, Mg²⁺, K⁺	Analysis of aerosols and gases Aerosols: Cl^-, NO₃^-, SO₄^2-, F^-, NH₄⁺, Na⁺, Ca²⁺, Mg²⁺, K⁺ Gases: HCl, HNO₃, HONO (HNO₂), SO₂, NH₃, HF
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
Single or multiple analysis techniques possible	Single analysis technique
Time resolution	Continuous air monitoring	Continuous air monitoring
Sample collection method	SJAC	WRD and SJAC
Domensions 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

Following is a comparison of results to determine 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 [5], a good correlation would indicate that MARS also measures aerosols with similar accuracy.

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 5). 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 for both systems, but since 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, the concentrations given by MARS and the 2060 MARGA are similar.

Conclusion

Monitoring air pollution is crucial because it allows us to understand the types and levels of pollutants 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 2060 MARGA or MARS from Metrohm Process Analytics to understand the impact of various pollutants 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 2025-02-06).

[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 2025-02-06).

[4] 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

[5] 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