Mixing state of Carbonaceous Aerosol Emissions from an Ecodesign Woodstove

Abstract. Residential wood burning (RWB) has become an increasingly significant source of carbonaceous aerosols (CAs) in the UK and worldwide. Black Carbon (BC) and Organic Aerosol (OA) fractions of CAs are of particular concern due to their impacts on climate and human health. Measuring the mixing state of CA is important as the mixing state can influence key aerosol properties including light absorption, hygroscopicity, cloud scavenging, atmospheric lifetime, and toxicity. While emissions and mixing states from traditional stoves have been previously reported and characterized, key uncertainties remain in the influence of user behaviour on emissions and the emission characteristics from the modern ‘Ecodesign’ appliances. Recent emissions tests imply that these can emit more pollutants such as polycyclic aromatic hydrocarbons (PAHs) and BC under certain circumstances and here we utilise online instrumentation to probe the mechanisms behind this and whether user behaviour has a role in impacting emissions. Hardwood logs were burned in a controlled test system using a UK Ecodesign-compliant woodstove under five operating protocols: standard, overload, underload, hot-reload and open-door protocols. Instantaneous particle emissions were quantified using a Single Particle Soot Photometer (SP2), a Differential Mobility Sizer (DMS500) and an Aerosols Mass Spectrometer (AMS). Modified combustion efficiency (MCE) was derived from CO and CO₂ concentrations measured by a Fourier-Transform Infrared (FTIR) spectrometer. Three typical combustion phases were observed: a pre-ignition phase, a flaming phase (with rich and non-rich flaming distinguished by an MCE of 0.95), and a smouldering phase. Stove operation can affect emissions by altering particle size, leading to ultrafine particle (UFP) formation (e.g., open-door, underload and overload conditions) and also prolong the rich flaming phase (e.g., hot-reload and overload), increasing PAH emissions. The findings here demonstrate how user behaviour may increase emissions of certain pollutants from modern stoves, partially offsetting the benefits compared to older designs.