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Atmospheric organic aerosols aregenerally classified as primary and secondary (POA and SOA) according totheir formation processes. An actual separation, however, is challenging whenthe timescales of emission and gas-to-particle formation overlap. Thepresence of SOA formation in biomass burning plumes leads to scientificquestions about whether the oxidized fraction of biomass burning aerosol israther of secondary or primary origin, as some studies would suggest, andabout the chemical compositions of oxidized biomass burning POA and SOA. Inthis study, we apply nuclear magnetic resonance (NMR) spectroscopy toinvestigate the functional group composition of fresh and aged biomassburning aerosols during an intensive field campaign in the Po Valley, Italy.The campaign was part of the EUCAARI project and was held at the ruralstation of San Pietro Capofiume in spring 2008. Factor analysis applied tothe set of NMR spectra was used to apportion the wood burning contributionand other organic carbon (OC) source contributions, including aliphaticamines. Our NMR results, referred to the polar, water-soluble fraction of OC,show that fresh wood burning particles are composed of polyols and aromaticcompounds, with a sharp resemblance to wood burning POA produced in woodstoves, while aged samples are clearly depleted of alcohols and are enrichedin aliphatic acids with a smaller contribution of aromatic compounds. Thecomparison with biomass burning organic aerosols (BBOA) determined byhigh-resolution aerosol mass spectrometry (HR-TOF-AMS) at the site shows onlya partial overlap between NMR BB-POA and AMS BBOA, which can be explained byeither the inability of BBOA to capture all BB-POA composition, especiallythe alcohol fraction, or the fact that BBOA account for insoluble organiccompounds unmeasured by the NMR. Therefore, an unambiguous composition forbiomass burning POA could not be derived from this study, with NMR analysisindicating a higher O / C ratio compared to that measured for AMS BBOA. Thecomparison between the two techniques substantially improves when addingfactors tracing possible contributions from biomass burning SOA, showing thatthe operational definitions of biomass burning organic aerosols are moreconsistent between techniques when including more factors tracing chemicalclasses over a range of oxidation levels. Overall, the non-fossil totalcarbon fraction was 50–57%, depending on the assumptions about the<sup>14</sup>C content of non-fossil carbon, and the fraction of organic carbonestimated to be oxidized organic aerosol (OOA) from HR-TOF-AMS measurementswas 73–100% modern

atmospheric chemistry and physics

Primary and secondary biomass burning aerosols determined by proton nuclear magnetic resonance (&amp;lt;sup&amp;gt;1&amp;lt;/sup&amp;gt;H-NMR) spectroscopy during the 2008 EUCAARI campaign in the Po Valley (Italy)