Observations of Sesquiterpenes and their Oxidation Products in Organic Aerosols in central Amazonia

Description

Biogenic volatile organic compounds (BVOCs) from the Amazon forest region represent the largest source of organic carbon emissions to the atmosphere globally. These BVOC emissions dominantly consist of volatile and intermediate volatility terpenoid compounds that undergo chemical transformations in the atmosphere to form oxygenated condensable gases and secondary organic aerosol (SOA). We deployed the Semi-Volatile Thermal desorption Aerosol Gas chromatograph (SV-TAG) at a rural site (“T3”) located to the west of the urban center of Manaus, Brazil as part of the Green Ocean Amazon (GoAmazon2014/5) field campaign to measure hourly concentrations of intermediate volatility and semi-volatile BVOCs and their oxidation products in the gas and particle phases during the wet and dry seasons. We speciated 20 sesquiterpenes and 4 diterpenes and measured their concentrations which ranged 0.01-6.04 ng m-3 (1-670 ppqv). Many sesquiterpenes present in ambient air were found to overlap with the sesquiterpene composition in essential oils from the Copaiba tree (Copaifera officinalis Jacq. L.), native to South America and common in the Amazon. However, highly reactive sesquiterpenes prevalent in the oil, in particular β-caryophyllene, were only rarely observed in the ambient air, and only at night when ozone was extremely low, consistent with rapid removal by reaction with ozone. Using synthesized authentic standards, we quantified both gas- and particle-phase products from β-caryophyllene oxidation in the ambient air. We also compared the ambient observations with laboratory-based sesquiterpene oxidation experiments and were able to attribute more than 40 additional compounds within representative filter samples to sesquiterpene oxidation. We estimate the contribution of sesquiterpene oxidation to SOA during the wet season by combining molecular-level measurements from SV-TAG and two-dimensional gas chromatography with those from an additional suite of BVOC and particle measurements deployed in the Amazon. We were able to chemically assign 20% of observed OA as originating from known isoprene and terpene oxidation tracers/chemical signatures, of which two known β-caryophyllene SOA tracers make up 0.03% of total OA during the wet season. We further estimate that sesquiterpenes are likely to contribute at least 3% of total OA based on estimated OA mass attributed to the 40 additional sesquiterpene-derived compounds.