Formation and transformation of biogenic secondary organic aerosols from precursors and real plant emissions

Studies using photochemical “smog”, or aerosol, chambers can provide valuable insights into the complex multiphase processes leading to the formation and transformation of atmospheric particulates. As part of the Aerosol Coupling in the Earth System (ACES) project, a series of novel experiments were carried out at the Manchester Aerosol Chamber in order to investigate the chemistry and microphysics of the formation and transformation of biogenic secondary aerosols under realistic conditions. Both real plant emissions and a selection of compounds covering a wide range of reactivity including isoprene (C5 H8), monoterpenes (isomeric formula C10 H16), sesquiterpenes (isomeric formula C15H24) and oxygenated VOCs have been studied in detail. The chemical composition of the formed SOA was measured on-line using an Aerodyne Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS) and by using a high volume pump and a collapsible chamber, the entire contents of the chamber could be sampled rapidly onto a filter for off-line molecular analyses. Repetitive experiments were carried out and filter samples taken at different experiment times – 2, 4 and 6 hours – allowed the evolution of individual SOA components to be investigated. A hygroscopicity tandem differential mobility analyser (HTDMA) and a cloud condensation nuclei (CCN) counter were used to probe the hygroscopic properties and of the aerosols in the sub- and super-saturated regimes, respectively. A proton transfer mass spectrometer was used to study the evolution of the gas phase oxidation products. For example, the composition of -caryophyllene SOA was studied using liquid chromatography coupled to mass spectrometry. Twelve components were identified based on MS2 fragmentation patterns. The SOA composition was found to be much simpler than seen for monoterpenes and no oligomers were found. Experiments at 50 ppb and 250 ppb, indicate that the distribution of products varied depending on the starting VOC concentration. Low concentrations resulted in a higher proportion of more polar species, indicating the importance of carrying out smog chamber studies as close to ambient concentrations as possible. Results obtained using the ToF-AMS showed that SOA formed using lower initial precursor concentration contained higher fraction of m/z 44 (a typical marker for highly oxygenated organic molecules) compared to SOA produced using high precursor concentration. However, this finding was not consistent for all of the studied precursors as will be discussed in this talk. Experiments have also been conducted whereby a natural ensemble of biogenic precursors was generated and subsequently used as the starting material within the photochemical chamber. The work focused on emissions from three tropical species (found to be mostly isoprene) and contrasted their results with selected boreal plants with monoterpene dominated emissions. The experiments were conducted under controlled, environmentally realistic, conditions of light and temperature. The speciation of the VOC emitted by the plants and flowing into the reaction chamber was determined by GC and PTRMS. Investigation of the aerosol formation and transformation processes was studied using a suite of state-of-the-art instruments. Aerosol formation from all plants was investigated under seeded and un-seeded conditions. This presentation will provide an overview of the results of this work and discuss its main findings.This work was supported by the UK Natural Environment Research Council (NERC) through the Aerosol Properties, PRocesses And InfluenceS on the Earth's climate (APPRAISE) programme. M. Rami Alfarra is supported by NERC’s National Centre for Atmospheric Science (NCAS).