Abstract
Abstract. The field application of an aerosol concentrator in conjunction with an atmospheric 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 technique allowing online measurements of organic acids in the gas and particle phase. The detection limit for the acid species in the particle phase was improved by a factor of 7.5 to 11 (e.g. ∼40 ng m3 for pinonic acid) by using the miniature versatile aerosol concentration enrichment system (mVACES) upstream of the mass spectrometer. The APCI-IT-MS was calibrated in the negative ion mode with two biogenic organic acid standards – pinic acid and pinonic acid. Pinic acid was used as a surrogate for the quantification of the total amount of organic acids in the ambient aerosol based on the total signal intensities in the negative ion mode. The results were compared with the total organic signal of a C-ToF-AMS during the HUMPPA-COPEC 2010 field campaign. The campaign average contribution of organic acids measured by APCI-IT-MS to the total submicron organic aerosol mass was estimated to be about 60%, based on the response of pinic acid. Very good correlation between APCI-IT-MS and C-ToF-AMS (Pearson's R = 0.94) demonstrates soft-ionization mass spectrometry as a complimentary technique to AMS with electron impact ionization. MS2 studies of specific m/z ratios recorded during the HUMPPA-COPEC 2010 field campaign were compared to MS2 studies of selected monoterpene oxidation products formed in simulation chamber experiments. The comparison of the resulting fragments shows that oxidation products of the main VOCs emitted at Hyytiälä (α-pinene and Δ3-carene) cannot account for all of the measured fragments. Possible explanations for those unaccounted fragments are the presence of unidentified or underestimated biogenic SOA precursors, or that different products are formed by a different oxidant mixture of the ambient air compared to the chamber ozonolysis.
Highlights
In this paper we describe for the first time the coupling of the miniature versatile aerosol concentration enrichment system with the soft-ionization atmospheric pressure chemical ionization (APCI)-ion trap mass spectrometer (IT-MS), allowing online measurements of the composition of laboratory generated OA and ambient OA during the HUMPPA-COPEC2010 field campaign at Hyytiala, Finland (Williams et al, 2011)
We have demonstrated for the first time the application of an online mass spectrometric technique with the ability to perform MS2 experiments from biogenic organic acids in the field
The APCI-IT-MS data were compared to C-ToF-aerosol mass spectrometer (AMS) data, revealing that the two techniques are complementary mass spectrometric techniques which can lead to an improved understanding of organic aerosol formation and composition
Summary
Online analysis of organic acids in the bulk aerosol was realized by using a Finnigan LCQ ion trap mass spectrometer (IT-MS) (Finnigan MAT, San Jose, USA). The instrument was equipped with a modified atmospheric pressure chemical ionization (APCI) ion source (Kuckelmann et al, 2000). It was operated in the negative mode to form [M−H]− ions. Due to a lower gas-phase acidity of O−2 compared to acetic acid (Bartmess, 2011), most atmospherically relevant organic acids are detected by the O−2 -ionization method. The background of the instrument was measured at the beginning, during and at the end of the campaign by connecting zero air (synthetic air, hydrocarbon free, Westfalen, Germany) to the inlet of the saturator. Gas-phase measurements, which were recorded in 3-h time intervals, were interpolated linearly and subtracted from the total signal resulting in the particle phase signal
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