Abstract. Organic aerosols (OA) derived from small-scale wood combustion emissions are not well represented by current emissions inventories and models, although they contribute substantially to the atmospheric particulate matter (PM) levels. In this work, a 29 m3 smog chamber in the ILMARI facility of the University of Eastern Finland was utilized to investigate the formation of secondary organic aerosol (SOA) from a small-scale modern masonry heater commonly used in northern Europe. Emissions were oxidatively aged in the smog chamber for a variety of dark (i.e., O3 and NO3) and UV (i.e., OH) conditions, with OH concentration levels of (0.5–5)  ×  106 molecules cm−3, achieving equivalent atmospheric aging of up to 18 h. An aerosol mass spectrometer characterized the direct OA emissions and the SOA formed from the combustion of three wood species (birch, beech and spruce) using two ignition processes (fast ignition with a VOC-to-NOx ratio of 3 and slow ignition with a ratio of 5).Dark and UV aging increased the SOA mass fraction with average SOA productions 2.0 times the initial OA mass loadings. SOA enhancement was found to be higher for the slow ignition compared with fast ignition conditions. Positive matrix factorization (PMF) was used to separate SOA, primary organic aerosol (POA) and their subgroups from the total OA mass spectra. PMF analysis identified two POA and three SOA factors that correlated with the three major oxidizers: ozone, the nitrate radical and the OH radical. Organonitrates (ONs) were observed to be emitted directly from the wood combustion and additionally formed during oxidation via NO3 radicals (dark aging), suggesting small-scale wood combustion may be a significant ON source. POA was oxidized after the ozone addition, forming aged POA, and after 7 h of aging more than 75 % of the original POA was transformed. This process may involve evaporation and homogeneous gas-phase oxidation as well as heterogeneous oxidation of particulate organic matter. The results generally prove that logwood burning emissions are the subject of intensive chemical processing in the atmosphere, and the timescale for these transformations is relatively short, i.e., hours.

Residential wood combustion (RWC) emits high amounts of volatile organic compounds (VOCs) into ambient air, leading to formation of secondary organic aerosol (SOA), and various health and climate effects. In this study, the emission factors of VOCs from a logwood-fired modern masonry heater were measured using a Proton-Transfer-Reactor Time-of-Flight Mass Spectrometer. Next, the VOCs were aged in a 29 m Teflon chamber equipped with UV black lights, where dark and photochemical atmospheric conditions were simulated. The main constituents of the VOC emissions were carbonyls and aromatic compounds, which accounted for 50%-52% and 30%-46% of the detected VOC emission, respectively. Emissions were highly susceptible to different combustion conditions, which caused a 2.4-fold variation in emission factors. The overall VOC concentrations declined considerably during both dark and photochemical aging, with simultaneous increase in particulate organic aerosol mass. Especially furanoic and phenolic compounds decreased, and they are suggested to be the major precursors of RWC-originated SOA in all aging conditions. On the other hand, dark aging produced relatively high amounts of nitrogen-containing organic compounds in both gas and particulate phase, while photochemical aging increased especially the concentrations of certain gaseous carbonyls, particularly acid anhydrides.

<p><strong>Abstract.</strong> Secondary organic aerosol (SOA) derived from small-scale wood combustion emissions are not well represented by current emissions inventories and models, although they contribute substantially to the global atmospheric particulate matter (PM) levels. This type of SOA is formed via the oxidation of combustion-emitted volatile organic compounds, inducing large changes in the properties of combustion-derived PM. In this work, a 29 m<sup>3</sup> smog chamber in the ILMARI facility of the University of Eastern Finland was deployed to investigate the formation of SOA from small-scale wood combustion. Emissions were oxidatively aged in the smog chamber for a variety of dark (i.e., O<sub>3</sub> and NO<sub>3</sub>) and UV (i.e., OH) conditions, with OH concentration levels of (0.5–5) × 10<sup>6</sup> molecules cm<sup>−3</sup>, achieving equivalent atmospheric aging of up to 18 hours. A soot particle aerosol mass spectrometer (SP-ToF-AMS) was applied to measure real-time concentrations of the major chemical mass fractions of submicron particles in chamber conditions simulating ambient wood combustion plumes. Substantial SOA formation was observed in every experiment performed with three wood species (birch, beech and spruce) and two ignition process (fast ignition with a VOC-to-NO<sub><i>x</i></sub> ratio of 3 and slow ignition with a ratio of 5). Both dark and UV aging increased the SOA mass, and OA enhancement resulted in an average SOA production of 2.0 times the initial OA mass loadings. The enhancement was found to be higher for the slow ignition compared with fast ignition. UV aging revealed faster SOA formation rates than dark aging.<br><br> OA elemental analysis indicated that the oxidatively aged wood combustion emissions exhibited Van Krevelen slopes of approximately −0.7, with slightly steeper slopes for the fast ignition experiments than for the slow ignition cases. These results are, in Van Krevelen space, in the same region as measured ambient OA, and the OA aging was found to follow a similar slope with ambient observations, suggesting that our chamber experiments are well suited to simulate polluted boundary-layer conditions. To investigate SOA composition in detail, positive matrix factorization (PMF) was used for separating SOA, POA, and their subgroups from the total OA spectra. This resulted in two POA factors (POA1 and POA2) and three SOA factors (SOA1, SOA2, and SOA3) representing the three major oxidizers: ozone, the nitrate radical and the OH radical. The SOA of the first factor (SOA1) was likely formed via ozonolysis of unsaturated compounds, whereas SOA2 contained secondary organonitrates from nitrate radical oxidation. The third SOA factor (SOA3) was indicative of SOA formation dominated by the OH radical.<br><br> The PMF results showed that most POA was oxidized after the ozone addition, forming aged POA, and after 7 h of aging (dark + UV aging), more than 75 % of the original POA was transformed. This process may involve evaporation and homogeneous gas-phase oxidation as well as heterogeneous oxidation of particulate organic matter. The results generally prove that logwood burning emissions are subject of intensive chemical processing in the atmosphere, and the time scale for these transformations is relatively short, i.e., hours.</p>

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