Comparison of three aerosol chemical characterization techniques utilizing PTR-ToF-MS: a study on freshly formed and aged biogenic SOA

Abstract

Abstract. An intercomparison of different aerosol chemical characterization techniques has been performed as part of a chamber study of biogenic secondary organic aerosol (BSOA) formation and aging at the atmosphere simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). Three different aerosol sampling techniques -the aerosol collection module (ACM), the chemical analysis of aerosol online (CHARON) and the collection thermal-desorption unit (TD) were connected to proton transfer reaction time-of-flight mass spectrometers (PTR-ToF-MSs) to provide chemical characterization of the SOA. The techniques were compared among each other and to results from an aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS). The experiments investigated SOA formation from the ozonolysis of β-pinene, limonene, a β-pinene-limonene mix and real plant emissions from Pinus sylvestris L. (Scots pine). The SOA was subsequently aged by photo-oxidation, except for limonene SOA, which was aged by NO 3 oxidation. Despite significant differences in the aerosol collection and desorption methods of the PTR-based techniques, the determined chemical composition, i.e. the same major contributing signals, was found by all instruments for the different chemical systems studied. These signals could be attributed to known products expected from the oxidation of the examined monoterpenes. The sampling and desorption method of ACM and TD provided additional information on the volatility of individual compounds and showed relatively good agreement.Averaged over all experiments, the total aerosol mass recovery compared to an SMPS varied within 80 ± 10, 51 ± 5 and 27 ± 3 % for CHARON, ACM and TD, respectively. Comparison to the oxygen-to-carbon ratios (O : C) obtained by AMS showed that all PTR-based techniques observed lower O : C ratios, indicating a loss of molecular oxygen either during aerosol sampling or detection. The differences in total mass recovery and O : C between the three instruments resulted predominantly from differences in the field strength (E/N) in the drift tube reaction ionization chambers of the PTR-ToF-MS instruments and from dissimilarities in the collection/desorption of aerosols. Laboratory case studies showed that PTR-ToF-MS E/N conditions influenced fragmentation which resulted in water and further neutral fragment losses of the detected molecules. Since ACM and TD were operated in higher E/N than CHARON, this resulted in higher fragmentation, thus affecting primarily the detected oxygen and carbon content and therefore also the mass recovery. Overall, these techniques have been shown to provide

A model framework to retrieve thermodynamic and kinetic properties of organic aerosol from composition-resolved thermal desorption measurements

“…Although based on a small selection of data, these numbers also broadly agree with earlier studies that have used other methods to try to quantify how large of a fraction of monoterpene-derived SOA consists of oligomeric compounds. It has been concluded that these compounds constitute around 50 % of SOA mass in laboratory setups (Baltensperger et al, 2005;Hall and Johnston, 2011;Putman et al, 2012), or even well above 50 % (Gao et al, 2004;Kolesar et al, 2015a), in general agreement with observations made on monoterpene-dominated ambient SOA (Kourtchev et al, 2016).…”

Validity and limitations of simple reaction kinetics to calculate concentrations of organic compounds from ion counts in PTR-MS

“…Note that Eqs. (10) and (11) are analogous to methods presented by Hansel et al (1995) that calculate the volume mixing ratio of volatile organic compounds (VOCs) based on kinetic conditions in the drift tube.…”

High contributions of fossil sources to more volatile organic carbon

“…A number of previous studies have found that at least a part of POC from various sources is semi-volatile, including wood burning, gasoline and diesel vehicle exhausts and cooking (Lipsky and Robinson, 2006;Shrivastava et al, 2006;Robinson et al, 2007;Grieshop et al, 2009bGrieshop et al, , 20 2009cMay et al, 2013aMay et al, , 2013b. Moreover, a significant part of freshly formed SOC is semi-volatile and will contribute to the more volatile OC fraction desorbed at lower temperatures (Holzinger et al, 2010;Salo et al, 2011;Meusinger et al, 2017;Gkatzelis et al, 2018). The relative contributions from fossil and non-fossil sources to OC can be dependent on the volatility.…”

“…It is likely that some of mvOC are of secondary origin, since SOC is formed in the atmosphere and subsequently condenses onto the aerosol particles (Hallquist et al, 2009;Jimenez et al, 2009). It has been shown that SOC formed in chambers 20 initially desorbed at relatively low temperatures (e.g., 200 °C) (Holzinger et al, 2010;Salo et al, 2011;Meusinger et al, 2017;Gkatzelis et al, 2018). However, there is also evidence that the formation of highly oxidized low-volatile compounds can occur in SOA formation (Ehn et al, 2014).…”