Constraining a hybrid volatility basis set model for aging of wood burning emissions using smog chamber experiments
Geoscientific Model Development Discussions
Semi-volatile and intermediate volatility organic compounds (SVOCs, IVOCs) are not included in the current non-methane volatile organic compounds (NMVOCs) emission inventories but may be important for the formation of secondary organic aerosol (SOA). In this study, novel wood combustion aging experiments performed at different temperatures (263&thinsp;K and 288&thinsp;K) in a ~7&thinsp;m<sup>3</sup> smog chamber were modelled using a hybrid volatility basis set (VBS) box model,
... BS) box model, representing the emission partitioning and their oxidation against OH. We combine aerosol-chemistry box model simulations with unprecedented measurements of nontraditional volatile organic compounds (NTVOCs) from a high-resolution proton transfer reaction mass spectrometer (PTR-MS) and with organic aerosol measurements from an aerosol mass spectrometer (AMS). In so-doing, we are able to observationally-constrain the amounts of different NTVOCs aerosol precursors (in the model) relative to low-volatility and semi-volatile primary organic material (OMsv) which is partitioned based on current published volatility distribution data. By comparing the NTVOCs/OMsv ratios at different temperatures, we determine the enthalpies of vaporization of primary biomass burning organic aerosols. Further, the developed model allows for evaluating the evolution of oxidation products of the semi-volatile and volatile precursors with aging. More than 30,000 box model simulations were performed to retrieve the combination of parameters that fit best the observed organic aerosol mass and O:C ratios. The parameters investigated include the NTVOC reaction rates and yields as well as enthalpies of vaporization and the O:C of secondary organic aerosol surrogates. Our results suggest an average ratio of NTVOCs to the sum of non-volatile and semi-volatile organic compounds of ~4.75. The mass yields of these compounds determined for a wide range of atmospherically relevant temperatures and organic aerosol (OA) concentrations were predicted to vary between 8 and 30&thinsp;% after 5&thinsp;hours of continuous aging. Based on the reaction scheme used, reaction rates of the NTVOC mixture range from 3.0 &times; 10&ndash;11&thinsp;cm<sup>3</sup>&thinsp;molec<sup>&minus;1</sup>&thinsp;s<sup>&minus;1</sup> to 4.0 &times; 10&ndash;11&thinsp;cm<sup>3</sup>&thinsp;molec<sup>&minus;1</sup>&thinsp;s<sup>&minus;1</sup>. The average enthalpy of vaporization of SOA surrogates was determined to be between 55,000&thinsp;J&thinsp;mol<sup>&minus;1</sup> and 35,000&thinsp;J&thinsp;mol<sup>&minus;1</sup> which implies a yield increase of 0.03&ndash;0.06 % K<sup>&minus;1</sup> with decreasing temperature. The improved VBS scheme is suitable for implementation into chemical transport models to predict the burden and oxidation state of primary and secondary biomass burning aerosols.