Chemical Characterization of Particles
The chemical characterisation of aerosol particles is one of the central topics in atmospheric science. Highly relevant topics such as health effects, climate, and cloud formation are strongly linked to the chemical composition of particles. Furthermore, a better understanding of particle sources can only be achieved with detailed knowledge on their composition. Typical anthropogenic sources such as industrial emissions, traffic exhaust, and biomass burning, as well as natural sources such as sea salt and dust resuspension are characterised by very different and often very specific chemical particle constituents. Their determination helps to better elucidate the sources of aerosol particles in the atmosphere. Special focus in atmospheric research is currently laid on the study of so-called secondary formation pathways. Besides direct emission, particles can also be formed within the atmosphere by oxidation of gaseous species to less volatile compounds, which form new particles or new particle mass by nucleation or condensation, respectively. Similarly, semi-volatile compounds can first condense onto particle surfaces and/or dissolve into the aqueous particle phase and then be transformed to non-volatile compounds which permamently form part of the particles. Such formation pathways are highly complex and a large pool of measurement techniques is needed to study such processes in the field. To study the chemical composition of aerosol particles as well as to elucidate important chemical processes in the atmosphere, TROPOS organizes and participates in field campaigns all around the world.
Yearly mean values of mass concentration PM10, content of main water soluble ions (1993–2017) and organic (OC) and elemental carbon (EC) (2003–2017). The OC/EC detection follows the thermographic procedure (VDI 2465/2). The OC/EC concentrations were estimated from the thermo-optical procedure for 2015, 16 and 17. The numbers of measuring days are given below the year. The error bars are the positive standard deviation of daily particle mass concentration means. (Figure: Gerald Spindler / TROPOS)
MARGA
In collaboration with the Umweltbundesamt the MARGA system (Monitor for AeRosols and Gases in ambient Air) is used at the research station of the Leibniz-Institut für Troposphärenforschung (TROPOS) in Melpitz. The instrument was developed by Metrohm Applikon (Netherlands) and uses a combination of a wet-rotating denuder (WRD) and a steam-jet-aerosol-collector (SJAC) to capture the watersoluble ions in particles (Cl-, NO3-, SO42-, NH4+, Na+, K+, Mg2+, Ca2+) and the corresponding trace gases (HCl, SO2, NH3, HNO3, HNO2) in a time resolution of one hour. After passing a teflon coated PM10-inlet the sample air enters the WRD which captures the water soluble gases with an absorbance liquid (pure water). Connected to the WRD is the SJAC where the air is mixed with steam of purified water. The supersaturation in the SJAC causes particles to grow rapidly into droplets. These droplets containing the dissolved inorganic ions are transferred into a sample reservoir. The resulting liquid solutions of the denuder and of the SJAC are analysed online via ion chromatography with conductivity detection.
Aerosol mass spectrometry technics
Two different aerosol mass spectrometers are used to measure the aerosol particle chemical composition (Organics, nitrate, sulfate, ammonium and non-seasalt chloride). The High Resolution Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS, Aerodyne Inc, USA) is deployed during specific intense measurements periods while the Aerosol Chemical Speciation Monitoring (ACSM, Aerodyne Inc, USA) is used for long time monitoring and continuously measured since June 2012. While both instruments are based on a similar concept, there are some major differences therefore they will be described separately.
HR-ToF-AMS
By crossing the aerodynamic lenses, aerosols are sampled and focused into a narrow beam. Particles are transmitted into the high vacuum of the instrument and accelerated to a velocity inversely related to their aerodynamic diameter. Particle beam is them impacted on a vaporizer (a resistivity-heated surface at 600°C) and flashed vaporized. The resulting vapors are ionized by electron impact (EI, 70eV) and detected by a High Resolution Time of Flight mass spectrometer (HR-ToF). Before reaching the vaporizer, the aerosol beam is controlled by a spinning wheel (approx. 150Hz) equipped by two slits. This wheel allows two alternative detection modes: by blocking/unblocking the particle beam, the AMS measures the bulk chemical composition of the particle (MS-mode) and when particle beam is controlled by the slits of the wheel, the AMS provides the particle size resolution (PTOF-mode). Because soot, crustal material and sea salt cannot be detected and the transmission efficiency of the aerodynamic lenses; the AMS is commonly considered to provide non-refractory PM1 aerosol chemical composition. The instrument is typically working at a 5 min time resolution.
ACSM
Although, the ACSM is based on a similar approach than the HR-ToF-AMS, the main differences are the absence of size distribution information and a simple unit mass resolution quadrupole mass spectrometer. In absence of rotating wheel, the chemical composition of particles is obtained by alternatively measuring ambient air and particle-free air. In addition, internal naphthalene standard located inside the chamber is used for monitoring the performance of the instrument over long time period. The ACSM is measuring with a 30 min time resolution.
