Interference of sea salt in capture vaporizer-ToF-ACSM measurements of biomass burning organic aerosols in coastal locations

Abstract

The capture vaporizer (CV) was developed to reduce uncertainties in non-refractory aerosol composition measurements made using the aerosol mass spectrometer (AMS) and the aerosol chemical speciation monitor (ACSM). Use of the capture vaporizer has achieved this by improving the instruments' collection efficiency to ∼1, but it has also lengthened the aerosol particles' residence times in the instrument, which has changed AMS and ACSM measurements using the standard vaporizer by altering known fragmentation patterns of organic marker species and increasing the likelihood of detecting refractory particles such as sea salt at typical operating temperatures (∼550 °C). This study reports that the changes affected by the capture vaporizer leads to sea salt particles interfering with measurements of biomass burning organic aerosols (BBOA) in environments where both particle sources are present as the ACSM's unit mass resolution is unable to distinguish between different molecules with the same molecular mass. Demonstration of this interference was performed using CV-Time of Flight-ACSM (CV-ToF-ACSM) measurements at two coastal Australian locations: the Kennaook-Cape Grim Baseline Air Pollution Station, Tasmania; and the site of the COALA-2020 (Characterizing Organics and Aerosol Loading over Australia 2020) campaign in New South Wales. Concentrations of BBOA marker ions m/z 60 and m/z 73 were examined at both locations, which showed two distinct branches of points: one where the two marker ions were positively correlated and one that was uncorrelated. This was due to m/z 60 also being a marker for sea salt. A threshold concentration of m/z 73 was established at each location to recognise periods where m/z 60 originated from BBOA. Lower concentrations of m/z 44 and radon when m/z 73 concentration was below the BBOA threshold indicated that m/z 60 concentration during these periods corresponded to inorganic particles of marine origin. Positive Matrix Factorization has also been shown to separate m/z 60 concentration from the two sources. This study suggests that using CV-ToF-ACSMs in coastal locations that are exposed to biomass burning smoke needs to consider sea salt interference when identifying BBOA.