In situ submicron organic aerosol characterization at a boreal forest research station during HUMPPA-COPEC 2010 using soft and hard ionization mass spectrometry

Abstract

Abstract. The chemical composition of submicron aerosol during the comprehensive field campaign HUMPPA-COPEC 2010 at Hyytiälä, Finland, is presented. The focus lies on online measurements of organic acids, which were achieved by using atmospheric pressure chemical ionization (APCI) ion trap mass spectrometry (IT-MS). These measurements were accompanied by aerosol mass spectrometry (AMS) measurements and Fourier transform infrared spectroscopy (FTIR) of filter samples, all showing a high degree of correlation. The soft ionization mass spectrometer alternated between gas-phase measurements solely and measuring the sum of gas and particle phase. The AMS measurements of C, H and O elemental composition show that the aerosol during the campaign was highly oxidized, which appears reasonable due to high and prolonged radiation during the boreal summer measurement period as well as the long transport times of some of the aerosol. In order to contrast ambient and laboratory aerosol, an average organic acid pattern, measured by APCI-IT-MS during the campaign, was compared to terpene ozonolysis products in a laboratory reaction chamber. Identification of single organic acid species remains a major challenge due to the complexity of the boreal forest aerosol. Unambiguous online species identification was attempted by the combinatorial approach of identifying unique fragments in the MS2 mode of standards, and then comparing these results with MS2 field spectra. During the campaign, unique fragments of limonene-derived organic acids (limonic acid and ketolimononic acid) and of the biomass burning tracer vanillic acid were detected. Other specific fragments (neutral loss of 28 Da) in the MS2 suggest the occurrence of semialdehydes. Furthermore, an approach to determine the average molecular weight of the aerosol is presented. The campaign average organic molecular weight was determined to be 300 g mol−1. However, a plume of aged biomass burning aerosol, arriving at Hyytiälä from Russia, contained organic compounds up to 800 Da (MWom≈450 g mol−1), showing that the average molecular weight can vary significantly. The high measurement frequency of both AMS and APCI-IT-MS enabled the partitioning of selected organic acids between gas and particle phase as a function of the total particulate mass to be quantified. Surprisingly high fractions of the higher molecular weight organic acids were observed to reside in the gas phase. These observations might be a consequence of large equilibration timescales for semi-solid boreal forest aerosol, as has been recently hypothesized by Shiraiwa and Seinfeld (2012).