Determination of the biogenic secondary organic aerosol fraction in the boreal forest by NMR spectroscopy
The study investigates the sources of fine organic aerosol (OA) in the\nboreal forest, based on measurements including both filter sampling\n(PM1) and online methods and carried out during a one-month campaign\nheld in Hyytiala, Finland, in spring 2007. Two aerosol mass\nspectrometers (Q-AMS, ToF-AMS) were employed to measure on-line\nconcentrations of major non-refractory aerosol species, while the water\nextracts of the filter samples were analyzed by nuclear magnetic\nresonance (NMR) spectroscopy for organic functional group\ncharacterization of the polar organic fraction of the aerosol. AMS and\nNMR spectra were processed separately by non-negative factorization\nalgorithms, in order to apportion the main components underlying the\nsubmicrometer organic aerosol composition and depict them in terms of\nboth mass fragmentation patterns and functional group compositions.\nThe NMR results supported the AMS speciation of oxidized organic aerosol\n(OOA) into two main fractions, which could be generally labelled as more\nand less oxidized organics. The more oxidized component was\ncharacterized by a mass spectrum dominated by the m/z 44 peak, and in\nparallel by a NMR spectrum showing aromatic and aliphatic backbones\nhighly substituted with oxygenated functional groups\n(carbonyls/carboxyls and hydroxyls). Such component, contributing on\naverage 50% of the OA mass throughout the observing period, was\nassociated with pollution outbreaks from the Central Europe. The less\noxidized component was enhanced in concomitance with air masses\noriginating from the North-to-West sector, in agreement with previous\ninvestigations conducted at this site. NMR factor analysis was able to\nseparate two distinct components under the less oxidized fraction of OA.\nOne of these NMR-factors was associated with the formation of\nterrestrial biogenic secondary organic aerosol (BSOA), based on the\ncomparison with spectral profiles obtained from laboratory experiments\nof terpenes photo-oxidation. The second NMR factor associated with\nwestern air masses was linked to biogenic marine sources, and was\nenriched in low-molecular weight aliphatic amines. Such findings provide\nevidence of at least two independent sources originating biogenic\norganic aerosols in Hyytiala by oxidation and condensation mechanisms:\nreactive terpenes emitted by the boreal forest and compounds of marine\norigin, with the latter relatively more important when predominantly\npolar air masses reach the site.\nThis study is an example of how spectroscopic techniques, such as proton\nNMR, can add functional group specificity for certain chemical features\n(like aromatics) of OA with respect to AMS. They can therefore be\nprofitably exploited to complement aerosol mass spectrometric\nmeasurements in organic source apportionment studies.