10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Combustion Conditions Leading to Primary Brown Carbon Emissions in Diesel Exhaust and Biomass Combustion
Vilhelm B. Malmborg, Axel C. Eriksson, Sandra Török, Christina Andersen, Louise Gren, Christoffer Boman, Robert Lindgren, Kirsten Kling, Sam Shamun, Martin Tunér, Yilong Zhang, Shawn Kook, Per-Erik Bengtsson, JOAKIM PAGELS, Lund University, Sweden
Abstract Number: 1688 Working Group: Combustion
Abstract The occurrence of light absorbing carbon (LAC) with enhanced absorption in the UV region (Absorption Angstrom Exponent, AAE >1) is well documented from measurements early in the soot formation process in lab-flames and diesel engines (Malmborg et al. 2017). In the atmospheric community, additional UV absorption is most often discussed in relation to biomass combustion emissions, and is then often referred to as Brown Carbon (BrC). Neither, the combustion conditions resulting in the BrC emission, nor the chemical components responsible for the absorption are well known for real world sources.
In this study we investigated BrC emissions from; 1) a mini-CAST flame soot generator using nitrogen dilution to vary flame height and temperatures, 2) a modern heavy duty diesel engine, operated on diesel and biodiesel fuels, where flame temperatures are varied using Exhaust Gas Recirculation (EGR; a common NOx reduction technique), 3) a Nordic wood stove used for domestic heating, and 4) a natural draft gasifier cookstove using pelletized woody biomass.
The optical properties of the aerosol was characterized with either an Aethalometer (model AE33 Magee Sci.) or a custom made 4-λ diode laser in-situ extinction device. The particle chemical composition was investigated using a Soot Particle Aerosol Mass Spectrometer (SP-AMS). PM was collected on quartz filters for thermal-optical analysis of Elemental and Organic Carbon (EC/OC; EUSAAR 2) and on TEM grids for High-Resolution Transmission Electron Microscopy (HR-TEM) analysis. In select cases a thermal denuder (300 °C) or a high temperature furnace (up to 900 °C) was used to investigate volatile and non-volatile fractions, and the effects of thermal processing on particle composition and optical properties.
The mass absorption cross section values of emissions from the flame soot generator decreased with increasing nitrogen dilution, in accordance with the literature. At the same time the AAE increased from just above 1 to 3.5 and HR-TEM analysis showed that the mean carbon lattice fringe-length decreased. Hence, reducing the flame temperature using N2 produces less mature, less graphitized, soot. Thermal processing removed semi-volatile OA such as PAHs. However, the AAE was still far above 1.0. The cases with elevated AAE were associated with significant fractions of the aerosol in the OC3-4 channels and “pyrolytic carbon” in the thermal-optical analysis, as well as large carbon clusters in the mid-carbon and fullerene carbon range detected by the SP-AMS.
Similar trends were obtained using increasing levels of EGR in the heavy duty diesel engine. AAE increased from ~1 to 2.5 with increasing EGR and was only partially reduced upon removal of PAHs with the TD at 300 °C. Again elevated fractions of OC 3-4 and pyrolytic carbon were found in the thermal optical analysis for LAC with high AAE.
Emissions from the conventional well insulated wood heating stove showed in general AAE.
A clear trend between primary BrC emissions and combustion conditions is emerging. It appears that BrC emission cannot be assigned to neither the organic fraction, nor the refractory soot core. Instead, BrC is more likely related to the slow rates at which particle properties matures in flames with reduced temperatures.
Finally, it is interesting to note that all of the cases resulting in elevated BrC emissions also resulted in elevated PAH emissions, which suggests these emissions are of health relevance.
This work has received financial support from the Swedish research councils VR and FORMAS.
Malmborg, V. B., et al. Env. sci. & techn. 51 (2017): 1876-1885.
Martinsson J. et al. Env. sci. & techn. 49 (2015) 14663-14671.