Determining anions and cations in atmospheric aerosols provides key data regarding transport and atmospheric transformation processes in addition to emission sources. Hitherto filters have been used to perform these determinations by collecting the aerosol particles over a longer time period, typically 24 hours. The removal and dissolution of the particles from the filters to water has to be done before performing ion chromatographic (IC) analysis.
Nevertheless, this batch process is only capable of determining averages over a time period of 24 hours or more. Furthermore, the process is laborious with poor temporal resolution, making semi-continuous on-line measurements impossible. Also, there is a possibility for the results to be falsified due to chemical transformations and desorption processes, thereby leaving the reliability of the results somewhat in question.
Particle samplers need to facilitate semi-continuous sampling of aerosols downstream from two different denuder systems and enable sensitive monitoring of changes in the ionic composition of the aerosols.
The sampling systems can be connected to an 850 Professional IC for anions and cations that provides quasi-continuous measurements. The temporal resolution presented is 5-15 minutes, thereby correlating rapid changes with meteorological and other data.
Instrumentation for Air Pollution Analysis
The analysis of harmful effects of air pollution requires semi-continuous measurements of inorganic species in aerosols and their gas phase components in ambient air, rapidly and accurately. Particle-Into-Liquid-Sampler (PILS) paired with wet-chemical analyzers, such as an IC, and the Monitoring instrument for AeRosols and GAses (MARGA) with two integrated ICs are the most promising devices, often referred to as steam collecting devices.
Both devices consist of a condensation particle growth sampler, gas denuders, and control instruments. While PILS is composed of a downstream growth chamber and two consecutive fixed denuders, the MARGA features a Steam-Jet Aerosol Collector (SJAC) and a Wet Rotating Denuder (WRD).
Although, different assemblies are used by the aerosol samplers of PILS and MARGA, the aerosol particles are grown into droplets in a supersaturated water vapor environment in both the samplers. The collected droplets are mixed with carrier water beforehand and then fed continuously into sample loops or pre-concentration columns for on-line IC analysis.
PILS is designed for sampling of aerosols only, while MARGA also performs determination of water-soluble gases. Classical denuders perform gas removal from the air sample upstream of the growth chamber, while MARGA performs gaseous species collection in a WRD for on-line analysis. Aerosols neither dissolve in the WRD nor in the PILS denuders due to their low diffusion speeds of aerosols compared to gases.
Selecting appropriate ion chromatographic conditions enables PILS-IC to provide a precise determination of seven key inorganic species (Na+, K+, Ca2+, Mg2+, Cl-, NO3- and SO42-) in fine aerosol particles, within 4-5 minutes. MARGA can also determine HCl, NH3, SO2, HNO2, and HNO3 simultaneously.
Working Principle of PILS
The PILS is equipped with a vacuum air pump (Figure 1) to draw the ambient air sample at a rate of 16.7L/min through the whole system. A cyclone controls the size of the entering particulate matter (PM), while the two consecutive static denuders remove interfering gas-phase nitrogen and sulfur species. The remaining aerosols then enter the growth chamber, where they are then mixed with injected supersaturated steam.
Figure 1. The PILS.
After being grown into droplets, the aerosol particles are continuously collected on the impactor plate surface. After the removal of the air bubbles in the debubbler unit, the liquid sample consisting of the aerosols is transferred to the loop or the pre-concentration column of an IC (Figure 2). To determine both anions and cations, the sample stream needs to be split into two.
Figure 2. The PILS Set-up.
Working Principle of MARGA
While PILS is designed exclusively for determining aerosols, MARGA quantifies components in aerosols and the gas phase. The key variation lies in the construction of the annular denuders. PILS eliminates interfering gases using two consecutive fixed denuders, while MARGA quantitatively absorbs water-soluble gases in a thin water film on a rotating wall using the WRD (Figure 3).
Figure 3. MARGA.
The dissolution of the aerosols does not take place in the WRD because of their low diffusion rate. Hence, the aerosols reach the Steam-Jet Aerosol Collector (SJAC), where their growth takes place using supersaturated steam condensation.
Therefore, MARGA offers two sample streams, of which one consists of the dissolved gases and the other comprises the dissolved particles. The results from an IC analysis of components in aerosols and the gas phase are shown in the following table:
|Detection limit [μg/m3]
This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.
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