Determination of anions and cations in aerosols by ion chromatography

Importance of the topic

Atmospheric aerosols play a critical role in air quality, climate forcing and human health. Determining their ionic composition reveals source emissions, transport pathways and chemical transformation processes. Traditional filter‐based collection offers only daily averages, limiting insight into rapid changes. Semi‐continuous, on‐line methods overcome these limitations by delivering high temporal resolution data that support dynamic process studies and regulatory monitoring.

Objectives and study overview

This work compares two advanced aerosol samplers—Particle-Into-Liquid Sampler (PILS) and Monitor for AeRosols and GAses (MARGA)—both coupled to ion chromatography (IC). The goal is to evaluate their performance in measuring major inorganic ions in fine particles (PM2.5) and, in the case of MARGA, water-soluble gases. Key metrics include sampling efficiency, temporal resolution and detection limits for anions, cations and selected gas species.

Methodology and instrumentation

Both systems draw ambient air at ~16.7 L/min and remove interfering gases before growing particles into droplets via supersaturated steam. In PILS, two static denuders eliminate NH3 and SO2 upstream of a cyclone (PM2.5 cutoff) and a downstream growth chamber. Collected droplets impact a wetted plate and mix with carrier water containing an internal standard. MARGA features a Wet Rotating Denuder (WRD) to absorb soluble gases into a thin water film, followed by a Steam-Jet Aerosol Collector (SJAC) for particle growth. Liquids are debubbled and directed either to sample loops or to preconcentration columns prior to IC analysis.

Used instrumentation

• Particle-Into-Liquid Sampler (PILS) with static denuders, cyclone separator and growth chamber
• Monitor for AeRosols and GAses (MARGA) including Wet Rotating Denuder and Steam-Jet Aerosol Collector
• Ion Chromatograph equipped for simultaneous anion and cation analysis (split flow for separate columns)
• Steam generator, vacuum pump and debubbler for continuous sample delivery

Main results and discussion

PILS achieves 4–5 min cycle times for quantifying Na+, K+, Ca2+, Mg2+, Cl–, NO3– and SO42– in PM2.5. Extending analysis to 10–15 min allows detection of low-molecular-weight organic acids (acetate, formate, oxalate). MARGA provides parallel streams for dissolved gases (HCl, HNO3, HNO2, SO2, NH3) and particles, enabling simultaneous quantification within 10 min. Detection limits (µg/m3) are approximately:

• Gases: HCl 0.05, HNO3 0.05, HNO2 0.08, SO2 0.10, NH3 0.08
• Particles: Cl– 0.05, NO3– 0.05, SO42– 0.08, NH4+ 0.08, Na+ 0.08, K+ 0.10, Ca2+ 0.08, Mg2+ 0.10

The two systems deliver semi‐continuous measurements over a week without manual intervention, capturing rapid concentration changes and minimizing artifacts from filter handling and chemical transformation.

Benefits and practical applications

These on‐line techniques offer:

• High temporal resolution (5–15 min) for process studies
• Low detection limits in the ng/m3 to µg/m3 range
• Reduced labor and faster data turnaround compared to filter methods
• Capability to correlate ionic composition with meteorological parameters and emission events

Future trends and applications

Advances may include miniaturized or portable samplers for field deployments, integration with optical or mass spectrometric detectors for chemical speciation, and automated data processing with machine learning for source apportionment. Coupling with remote sensing and atmospheric models can further elucidate aerosol impacts on climate and health.

Conclusion

PILS and MARGA paired with IC provide robust platforms for semi‐continuous monitoring of inorganic ions in aerosols and, in the case of MARGA, water-soluble gases. They overcome limitations of conventional filter sampling by delivering near real-time data with high sensitivity and temporal resolution, supporting air quality research and regulatory monitoring.

References

• D.A. Orsini, Y. Ma, A. Sullivan, B. Sierau, K. Baumann and R.J. Weber, Refinements to the Particle-Into-Liquid-Sampler for ground and airborne measurements of water soluble aerosol composition, Atmospheric Environment 37, 1243–1259 (2003)
• A. Khlystov, G.P. Wyers and J. Slanina, The Steam-Jet Aerosol Collector, Atmospheric Environment 29(17), 2229–2234 (1995)