New reproducible reference methods for PM₁₀ and PM_2.5

The aim of this work package is to develop new reproducible reference methods for PM₁₀ and PM_2.5, including the design and construction of a demonstration chamber system for the lab-based calibration of PM-measuring instruments using representative ambient aerosols. The target uncertainties lie below 15 %, in line with major gas pollutants, such as SO₂, NO_x and ozone.

WP1 will focus on the challenge of reproducibly generating synthetic aerosol mixtures that i) are stable over prolonged periods of time (hours), ii) capture the main physical and chemical properties of real ambient aerosols such as chemical composition, particle size and concentration and iii) can be tuned with respect to those properties in order to simulate different ambient aerosols. Primary aerosols will be generated in Task 1.1. For the mixing and homogenisation of these primary aerosols and the delivery of the calibration aerosol mixture to the various measuring instruments, a new demonstration aerosol chamber will be designed. The design of the chamber system will be optimised with numerical calculations in Task 1.2 and its operation will be validated by performing dedicated physicochemical analysis of the produced aerosols in Task 1.3. This highly innovative new demonstration aerosol chamber will enable for the first time a lab-based calibration of automatic PM-measuring instruments according to standardised procedures. To this end, a new improved protocol for equivalence testing will be developed involving instrument manufacturers and specialists working at air-quality monitoring networks will also be produced as part of Task 1.4.

Task 1.1: Generation of primary aerosols at various particle sizes and concentrations

The aim of this task is to produce primary aerosols at various particle sizes and mass concentrations (1 μg/m3 – 30 μg/m3) in a stable manner (target reproducibility better than 10 %). Different particle generators, such as Combustion Aerosol STandard (CAST) for combustion aerosols (elemental carbon with some volatile organic fraction), generators for dry dispersion of mineral dust and wet dispersion of organics and semi-volatile materials will be tested in order to define optimal operating points/conditions. The size distribution and number concentration of the produced aerosols will be measured with a Differential Mobility Analysers (DMAS) (or Optical Particle Counter (OPC) and CPCs, respectively, and the mass concentration will be measured gravimetrically. The use of these particle generators will allow controlled variation of the physicochemical composition of the synthetic aerosols mixtures to be generated in Task 1.3. The semi-volatile components and combustion particles will have a variable size distribution with a geometric mean diameter between 50 nm – 300 nm (with Geometric Standard Deviation ideally > 1.6), while the size of the mineral dust particles will extend to the micrometre range.

 

Task 1.2: Development of an Aerosol Chamber for the generation of representative ambient aerosols

The aim of this task is to develop a chamber system for the mixing and homogenisation of the primary aerosols generated in Task 1.1 in order to produce synthetic aerosols that are representative of real ambient aerosols.

These representative aerosols will be tuneable with respect to particle chemistry, size distribution and mass concentration (between 10 μg/m3 and 60 μg/m3). The aim is to achieve an externally mixed homogeneous aerosol comprising the major chemical components found in ambient air (e.g. dust, combustion particles, hygroscopic and volatile substances). The humidity conditions inside the aerosol chamber will be adjustable (i.e. < 10 %, normal conditions, i.e. 40 % – 60 %, and 80 % – 100 %, dry and humid conditions) in order to simulate different environmental conditions. In particular, two humidity regimes, “dry” with 30 %-60 % Relative Humidity (RH) and “humid” with >60 %RH, will be investigated. Regarding the temperature control two possibilities will be evaluated: 1) to simulate different environmental conditions by varying the temperature of the aerosol chamber and the synthetic aerosol in the range 15 ˚C-35 ˚C or 2) to always keep the temperature of the aerosol chamber and the aerosol stable at 20 ˚C and simulate the effects of temperature change by adjusting appropriately the composition of the synthetic aerosol (i.e., higher temperatures would mean lower amount of volatile inorganic salts/organic substances in the form of particulates). The flow rate through the aerosol chamber will be > 5 m3/h to allow at least two PM-measuring instruments to sample simultaneously.

Two groups will perform numerical calculations in order to design an aerosol chamber following two different approaches: i) METAS will design a smaller (<5 m3) ‘Sampling Aerosol Chamber’ (SAC) where the devices under test are placed outside of the chamber and sample aerosol from multiple outlets of the setup and ii) IRSN will design a larger (>5 m3) ‘Volume Aerosol Chamber” (VAC), where the devices under test are placed inside the chamber. The numerical calculations will be experimentally validated (e.g. by characterising the flow and aerosol losses in the chambers). A group of partners having expertise in chamber experiments and in the use of PM-measuring instruments (INRIM, NPL, BAM, LNE, IRSN and METAS) will select the configuration (i.e. design and operating conditions) of the aerosol chamber that best addresses the needs of the stakeholders (A1.2.1). This chamber (either “SAV” or “VAC”) will be further validated within Task 1.3 with respect to the physicochemical composition of the produced aerosols.

 

Task 1.3: Validation of the aerosol chamber

The aim of this task is to validate the aerosol chamber developed in Task 1.2 for the generation of synthetic ambient aerosols by characterising their physicochemical properties under different working conditions (temperature, humidity, varying aerosol composition) and testing the reproducibility of the results (target better than 20 % reproducibility in total mass concentration). Particle mass will be determined using the PM reference method (gravimetric) and an automatic Tapered Element Oscillating Microbalance-Filter Dynamics Measurement Systems (TEOM-FDMS) instrument. Particle size distribution will be measured with aerodynamic particle sizers and optical particle counters, while number concentration will be determined with butanol/water CPCs. Chemical analysis and quantification will be performed for the major chemical components of the aerosol mixtures (made from different sources to reproduce real conditions) using off- and online analytical methods, such as Gas Chromatography/Mass Spectrometry (GC/MS), Ion Chromatography (IC) and OC/EC thermo-optical analysis. The analysis of metals (dust particles) will be carried out in WP2 in A2.3.4.

 

Task 1.4: Development of a protocol for a lab-based calibration of automatic PM instruments

The aim of this task is to establish for the first time a lab-based calibration procedure for automatic PM instruments. This calibration procedure will enable reliable comparison of the performance of automatic PM instruments, which are based on different measuring principles. At the same time, it will render the equivalence testing with the manual reference method more time efficient and accurate, reducing uncertainties down to 15 %. Therefore, a stakeholder-workshop involving instrument manufacturers and air-quality monitoring experts will be organised in order to set up a draft of a new equivalence testing protocol.

The most common automatic PM instruments measure (or calculate) particle mass based on i) an oscillating microbalance (e.g. TEOM, TEOM-FDMS), ii) the absorption of electrons by a loaded filter (beta-absorption instruments) and iii) scattering of light (e.g. Dust Trak, Fidas). At least three automatic PM instruments based on different measuring principles will be tested at METAS according to the new protocol draft using the calibration setup for the generation of synthetic ambient aerosols developed and validated in Task 1.2 and Task 1.3, respectively. The results will be compared to the gravimetric manual reference method. The practical experience gained from this testing campaign will feed back into the protocol and help refine the calibration procedures of automatic PM instruments.