The importance of burning conditions on the composition of domestic biomass-burning organic aerosol and the impact of atmospheric ageing

Domestic biomass burning is a significant source of organic aerosol (OA) to the atmosphere; however, the understanding of OA composition under different burning conditions and after oxidation is largely unknown. Compositional analysis of OA is often limited by the lack of analytical standards available for quantification; however, semi-quantitative non-target analysis (NTA) can overcome these limitations by enabling the detection of thousands of compounds and quantification via surrogate standards. A series of controlled-burn experiments were conducted at the Manchester Aerosol Chamber to investigate domestic biomass-burning OA (BBOA) under different burning conditions and the impact of atmospheric ageing. Insights into the chemical composition of fresh and aged OA from flaming-dominated and smouldering-dominated combustion were obtained via a newly developed semi-quantitative NTA approach using ultra-high-performance liquid chromatography high-resolution mass spectrometry. Aerosol from smouldering-dominated burns contained significant organic carbon content, whereas under flaming-dominated conditions it was primarily black carbon. The detectable OA mass from both conditions was dominated by oxygenated compounds (CHO) (approximate to 90 %) with smaller contributions from organonitrogen species. Primary OA (POA) had a high concentration of C8-C17CHO compounds, with both burns exhibiting a peak between C8-C11. However, flaming-dominated POA exhibited a greater contribution of C13-C17 CHO species. More than 50 % of the CHO mass in POA was determined as aromatic by the aromaticity index, largely in the form of functionalised monoaromatic compounds. After ageing, the aromatic contribution to the total CHO mass decreased with a greater loss for smouldering (-53 %) than flaming (-16 %) due to the increased reduction of polyaromatic compounds under smouldering conditions. The O:C ratios of the aged OA from flaming and smouldering were consistent with those from the oxidation of aromatic compounds (0.57-1.00), suggesting that compositional changes upon ageing were driven by the oxidation of aromatic compounds and the loss of aromaticity. However, there was a greater probability of O:C ratios >= 0.8 in aged smouldering OA, indicating the presence of more oxidised species. This study presents the first reported quantitative non-target compositional analysis of domestic BBOA using retention window scaling and demonstrates that compositional changes between burn phase and after ageing may have important consequences for exposure to such emissions in residential settings.