In tropical forests organic matter makes up a large fraction of fine particulate mass and the contribution can be up to 90 % in the submicrometer size region, mainly due to the formation of secondary organic aerosols (SOA) from the oxidation of biogenically released VOCs. Despite the abundance of organic material and the important role these particles play in the rainforest boundary layer, the ambient composition of submicrometer organic aerosols remains poorly understood. Several different biogenic VOCs are released in tropical regions but it is believed that especially isoprene is emitted in large amounts from these ecosystems. In fact, global biogenic emissions of isoprene (600 Tg yr−1) are sufficiently large that the formation of SOA in even small yields results in substantial production of atmospheric particulate matter. However, especially the NOx concentrations have been found to be highly influential in SOA production. As described above the Amazon Basin exhibit huge variations of gaseous and particulate pollution with clean conditions in the rainy season and polluted conditions in the dry season, when biomass burning activities result in much higher concentrations of e.g. nitrogen oxides. Field measurements of particle phase BVOC products at the ATTO site at the different seasons can deliver insights into the underlying chemical processes to link biogenic VOCs and SOA formation under clean and polluted conditions. Another highly interesting aspect of biogenic SOA formation in tropical regions is the question about their involvement in new particle formation. Here the challenge lies on the identification of extremely low-volatile products (ELVOCs) from BVOC oxidation in combination with size-segregated aerosol sampling. Consequently, ultra-trace analysis of organic compounds is needed to shed light on the early involvement of organic compounds in particle formation in pristine atmospheres. Therefore, the focus of this WP lies on the chemical characterization of submicrometer organic aerosol by sophisticated mass spectrometric techniques, aiming on the identification and quantification of individual molecular marker compounds to understand the chemical influences on aerosol formation and growth.
Aerosol sampling at ATTO will be performed at various heights and at different seasons. The analysis will be done using an existing high resolution mass spectrometer (Q-Exactive Orbitrap), usually in combination with liquid chromatography. The main goal is the identification of marker compounds, e.g. isoprene low NOx marker (IEPOX-products), isoprene high NOx marker (specific organic nitrates), marker compounds for monoterpene oxidation and ageing (MBTCA) and sesquiterpene oxidation products as well as selected ELVOCs.
The chemical characterization of aerosol particles at various forest measurement stations (tropical, boreal and temperate forests) have been conducted since several years by the Hoffmann group (e.g. van Eijck et al., 2014; Wildt et al., 2014, Andreae et al., 2015). Currently, the exceptional performance of new MS technologies (high resolution mass spectrometry, Orbitrap MS) in combination with soft ionization technologies is explored to enable the examination of marker molecules with high mass resolution, sub-ppm mass accuracy and high sensitivity.