Hyphenation of a EC / OC thermal–optical carbon analyzer to photo-ionization time-of-flight mass spectrometry: an off-line aerosol mass spectrometric approach for characterization of primary and secondary particulate matter

摘要

Source apportionment and characterization of primary and secondary aerosolsremains a challenging research field. In particular, the organic compositionof primary particles and the formation mechanism of secondary organicaerosols (SOAs) warrant further investigations. Progress in this field isstrongly connected to the development of novel analytical techniques. Inthis study an off-line aerosol mass spectrometric technique based on filtersamples, a hyphenated thermal–optical analyzer photo-ionization time-of-flight mass spectrometer (PI-TOFMS) system, was developed. The approachextends the capability of the widely used particulate matter (PM) carbon analysis (forelemental / organic carbon, EC / OC) by enabling the investigation of evolvedgaseous species with soft and selective (resonance enhanced multi-photon ionization, REMPI) and non-selective photo-ionization (single-photonionization, SPI) techniques. SPI was tuned to be medium soft to achievecomparability with results obtained by the electron ionization aerosol mass spectrometer (AMS). Different PM samples including wood combustionemission samples, smog chamber samples from the reaction of ozone withdifferent SOA precursors, and ambient samples taken at Ispra, Italy, inwinter as well as in summer were tested. The EC / OC–PI-TOFMS techniqueincreases the understanding of the processes during thermal–opticalanalysis and identifies marker substances for the source apportionment.Composition of oligomeric or polymeric species present in PM can beinvestigated by the analysis of the thermal breakdown products. In the case ofwood combustion, in addition to the well-known markers at m/z ratios of 60and 73, two new characteristic masses (m/z 70 and 98) have been revealed aspotentially linked to biomass burning. All four masses were also thedominant signals in an ambient sample taken in winter time in Ispra, Italy,confirming the finding that wood burning for residential heating is a majorsource of PM in winter at this location. The summersample from the same location showed no influence of wood burning, but seemsto be dominated by SOAs, which was confirmed from the comparison with chamberexperiment samples. The experiments conducted with terpenes as precursorsshowed characteristic masses at m/z 58 and 82, which were not observable inany other emission samples and could serve as a marker for SOA from terpenes.