Transformation of logwood combustion emissions in a smog chamber: formation of secondary organic aerosol and changes in the primary organic aerosol upon daytime and nighttime aging
Authors: Tiitta P. , Leskinen A. , Hao L. , Yli-Pirilä P. , Kortelainen M. , Grigonyte J. , Tissari J. , Lamberg H. , Hartikainen A. , Kuuspalo K. , Kortelainen A. , Virtanen A. , Lehtinen K. E. J. , Komppula M. , Pieber S. , Prévôt A. S. H. , Onasch T. B. , Worsnop D. R. , Czech H. , Zimmermann R. , Jokiniemi J. , Sippula O. 
Published: May 2016

Abstract. 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 m3 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., 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 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-NOx 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. 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. 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.