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The chemical composition of submicron aerosol during the comprehensive fieldcampaign HUMPPA-COPEC 2010 at Hyytiälä, Finland, is presented. Thefocus lies on online measurements of organic acids, which were achieved byusing atmospheric pressure chemical ionization (APCI) ion trap massspectrometry (IT-MS). These measurements were accompanied by aerosol massspectrometry (AMS) measurements and Fourier transform infrared spectroscopy(FTIR) of filter samples, all showing a high degree of correlation. The softionization mass spectrometer alternated between gas-phase measurementssolely and measuring the sum of gas and particle phase. The AMS measurements of C, H and O elemental composition show that theaerosol during the campaign was highly oxidized, which appears reasonabledue to high and prolonged radiation during the boreal summer measurementperiod as well as the long transport times of some of the aerosol. In orderto contrast ambient and laboratory aerosol, an average organic acid pattern,measured by APCI-IT-MS during the campaign, was compared to terpeneozonolysis products in a laboratory reaction chamber. Identification ofsingle organic acid species remains a major challenge due to the complexityof the boreal forest aerosol. Unambiguous online species identification wasattempted by the combinatorial approach of identifying unique fragments inthe MS2 mode of standards, and then comparing these results withMS2 field spectra. During the campaign, unique fragments oflimonene-derived organic acids (limonic acid and ketolimononic acid) and of thebiomass burning tracer vanillic acid were detected. Other specific fragments(neutral loss of 28 Da) in the MS2 suggest the occurrence ofsemialdehydes. Furthermore, an approach to determine the average molecular weight of theaerosol is presented. The campaign average organic molecular weight wasdetermined to be 300 g mol−1. However, a plume of aged biomass burningaerosol, arriving at Hyytiälä from Russia, contained organiccompounds up to 800 Da (MWom≈450 g mol−1), showing that the average molecular weight can varysignificantly. The high measurement frequency of both AMS and APCI-IT-MSenabled the partitioning of selected organic acids between gas andparticle phase as a function of the total particulate mass to be quantified.Surprisingly high fractions of the higher molecular weight organic acids wereobserved to reside in the gas phase. These observations might be aconsequence of large equilibration timescales for semi-solid boreal forestaerosol, as has been recently hypothesized by Shiraiwa and Seinfeld(2012)

atmospheric chemistry and physics

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