Photochemical transformation of residential wood combustionemissions: dependence of organic aerosol composition on OHexposure

Abstract. Residential wood combustion (RWC) emits large amounts of gaseous and particulate organic aerosol (OA). In the atmosphere, the emission is transformed via oxidative reactions, which are under daylight conditions driven mainly by hydroxyl radicals. This continuing oxidative aging produces secondary OA and may change the health- and climate-related properties of the emission. In this work, emissions from two modern residential logwood combustion appliances were photochemically aged in an oxidation flow reactor with various OH exposure levels, reaching up to 6 × 1011 s cm−3 (equivalent to one week in the atmosphere). Gaseous organic compounds were analysed by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS), while particulate OA was analysed online by an aerosol mass spectrometer (AMS) and offline by thermal-optical analysis and thermal desorption-gas chromatography mass spectrometry. Photochemical reactions increased the mass of particulate organic carbon by a factor of 1.3–3.9. The increase in mass took place during the first atmospheric equivalent day of aging, after which the enhancement was independent of the extent of photochemical exposure. However, aging increased the oxidation state of the particulate OA linearly throughout the assessed range, with ∆H:C/∆O:C slopes between −0.17 and −0.49 in van Krevelen space. Aging led to an increase in acidic fragmentation products in both phases. For the volatile organic compounds measured by PTR-ToF-MS, the formation of small carbonylic compounds combined with the rapid degradation of primary volatile organic compounds such as aromatic compounds led to a continuous increase in both the O:C and H:C ratios. Overall, the share of polycyclic aromatic compounds (PACs) in particles degraded rapidly during aging, although some oxygen-substituted PACs, most notably naphthaldehydic acid, increased, in particular during relatively short exposures. Similarly, the concentrations of particulate nitrophenols rose extensively during the first atmospheric equivalent day. During continuous photochemical aging, the dominant reaction mechanisms shifted from the initial gas phase functionalisation/condensation to the transformation of the particulate OA by further oxidation reactions and fragmentation. The observed continuous transformation of OA composition throughout a broad range of OH exposures indicates that the entire atmospheric lifetime of the emission, from fresh to shortly aged to long-term aged emission (representative of long-range transported pollutants), needs to be explored to fully assess the potential climate and health effects of OA emissions.