Online atmospheric pressure chemical ionization ion trap mass spectrometry (APCI-IT-MSn) for measuring organic acids in concentrated bulk aerosol – a laboratory and field study

[摘要] The field application of an aerosol concentrator in conjunction with anatmospheric pressure chemical ionization ion trap mass spectrometer(APCI-IT-MS) at the boreal forest station SMEAR II at Hyytiälä,Finland, is demonstrated in this study. APCI is a soft-ionization techniqueallowing online measurements of organic acids in the gas and particle phase.The detection limit for the acid species in the particle phase was improvedby a factor of 7.5 to 11 (e.g. ∼40 ng m³ forpinonic acid) by using the miniature versatile aerosol concentrationenrichment system (mVACES) upstream of the mass spectrometer. The APCI-IT-MSwas calibrated in the negative ion mode with two biogenic organic acidstandards – pinic acid and pinonic acid. Pinic acid was used as a surrogatefor the quantification of the total amount of organic acids in the ambientaerosol based on the total signal intensities in the negative ion mode. Theresults were compared with the total organic signal of a C-ToF-AMS duringthe HUMPPA-COPEC 2010 field campaign. The campaign average contribution oforganic acids measured by APCI-IT-MS to the total submicron organicaerosol mass was estimated to be about 60%, based on the response ofpinic acid. Very good correlation between APCI-IT-MS and C-ToF-AMS(Pearson's R = 0.94) demonstrates soft-ionization massspectrometry as a complimentary technique to AMS with electron impactionization. MS² studies of specific m/z ratios recorded duringthe HUMPPA-COPEC 2010 field campaign were compared to MS²studies of selected monoterpene oxidation products formed in simulationchamber experiments. The comparison of the resulting fragments shows thatoxidation products of the main VOCs emitted at Hyytiälä (α-pineneand Δ³-carene) cannot account for all of themeasured fragments. Possible explanations for those unaccounted fragmentsare the presence of unidentified or underestimated biogenic SOA precursors,or that different products are formed by a different oxidant mixture of theambient air compared to the chamber ozonolysis.

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