Abstract
Abstract. Source apportionment and characterization of primary and secondary aerosols remains a challenging research field. In particular, the organic composition of primary particles and the formation mechanism of secondary organic aerosols (SOAs) warrant further investigations. Progress in this field is strongly connected to the development of novel analytical techniques. In this study an off-line aerosol mass spectrometric technique based on filter samples, a hyphenated thermal–optical analyzer photo-ionization time-of-flight mass spectrometer (PI-TOFMS) system, was developed. The approach extends the capability of the widely used particulate matter (PM) carbon analysis (for elemental / organic carbon, EC / OC) by enabling the investigation of evolved gaseous species with soft and selective (resonance enhanced multi-photon ionization, REMPI) and non-selective photo-ionization (single-photon ionization, SPI) techniques. SPI was tuned to be medium soft to achieve comparability with results obtained by the electron ionization aerosol mass spectrometer (AMS). Different PM samples including wood combustion emission samples, smog chamber samples from the reaction of ozone with different SOA precursors, and ambient samples taken at Ispra, Italy, in winter as well as in summer were tested. The EC / OC–PI-TOFMS technique increases the understanding of the processes during thermal–optical analysis and identifies marker substances for the source apportionment. Composition of oligomeric or polymeric species present in PM can be investigated by the analysis of the thermal breakdown products. In the case of wood combustion, in addition to the well-known markers at m/z ratios of 60 and 73, two new characteristic masses (m/z 70 and 98) have been revealed as potentially linked to biomass burning. All four masses were also the dominant signals in an ambient sample taken in winter time in Ispra, Italy, confirming the finding that wood burning for residential heating is a major source of PM in winter at this location. The summer sample from the same location showed no influence of wood burning, but seems to be dominated by SOAs, which was confirmed from the comparison with chamber experiment samples. The experiments conducted with terpenes as precursors showed characteristic masses at m/z 58 and 82, which were not observable in any other emission samples and could serve as a marker for SOA from terpenes.
Highlights
Atmospheric aerosols are complex mixtures of small solid and liquid particles in the air
The carbon analyzer enables the stepwise heating of filter samples and the determination of bulk parameters (OC, elemental carbon (EC), total carbon (TC))
The PI-TOFMS system allows for the fast analysis of the evolving gaseous species
Summary
Atmospheric aerosols are complex mixtures of small solid and liquid particles in the air. The organic content of secondary particles (secondary organic aerosol, SOA) can account for up to 88 % of the organic carbon fraction in hours of the highest photochemical activity (Lim and Turpin, 2002) They can derive from nucleation of gaseous precursors, condensation of gases on particles or chemical reactions in the troposphere (Hallquist et al, 2009; de Gouw et al, 2005). Depending on whether the C = C double bond is located inside or outside the ring they are referred to as endocyclic or exocyclic terpenes (Larsen et al, 2001) They have higher PM-forming potential than typical aromatic species, such as xylene (Seinfeld and Pankow, 2003) and play a major role in aerosol formation, especially in rural areas and in times of high photochemical activity. The unknown comparability to real atmospheric processes and products, due to much higher concentrations of precursors and oxidants in laboratory studies, lack of other VOCs, and effects of the chamber walls, still presents a major challenge for smoke chamber studies (Jaoui et al, 2008; Hallquist et al, 2009)
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