Ambient Air Pollution from Wood Burning of Domestic Heating

Thesis from Jiexia Zheng
University of Stuttgart, 2020

Residential wood burning is an important component of domestic heating in Europe. With the advantage as renewable energy, wood burning has, however, become a general environmental issue due to wood smoke pollution in terms of particulate matter (PM) and PM-bound substances like polycyclic aromatic hydrocarbons (PAHs), black carbon (BC) and organic matter (OM), which are well-known for their negative effects on both environment and human health. This pollution problem is serious especially under inversion weather conditions.

The purpose of this thesis is to quantify the wood burning contribution to ambient PM10 during winter heating periods in a residential area in Dettenhausen, Germany. Firstly, the air quality measurements in this area were carried out by sampling ambient PM samples and measuring gaseous pollutants (CO and NOx) and black carbon in winter 2013/2014 and winter 2014/2015. PM10 mass concentrations were determined and the filter samples were chemically analyzed in laboratories for PM-bound polycyclic aromatic hydrocarbons and levoglucosan. A certain number of filters was also analyzed for organic carbon (OC) and elemental carbon (EC) content.

Concentrations of these air pollutants give an overview of the pollution situation in this area. The results for PAHs show that the carcinogenic potential of total PAHs is 65 % for both winter periods, with most of daily average benzo[a]pyren (BaP) exceeding the EU annual limit value of 1 ng/m³. Black carbon (BC) was measured for the first time in this area and the daily concentration, respectively diurnal trend, through winter 2014/2015 shows a significant contribution from biomass burning. The PM pollution due to wood smoke could be recognized by the substantial levoglucosan found in all PM samples. Levoglucosan was evaluated through its correlations with PM10 and other pollutants like BaP, BC, and gaseous pollutants (CO and NOx). However, the contribution from residential wood burning could not be quantified directly from these ambient data. According to the application principle and calculation procedure of using levoglucosan as wood burning tracer, the emission ratios of levoglucosan measured directly at emission sources is required.

Therefore, wood combustion experiments at a wood stove were carried out for the investigation of PM-bound levoglucosan. The sampling was carried out with a dilution technique, which was realized through the application of a dilution tunnel built according to EPA method 5G (US Environmental Protection Agency 2000). Various parameters, defining as operation, combustion and sampling conditions, were applied to investigate their potential influence on the levoglucosan emission levels. The experiment design included two sampling methods for two types of wood which are commonly used in a residential area Dettenhausen: beech hardwood and spruce softwood.

Firstly, experiments with continuous simultaneous sampling before dilution (hot flue gas) and after dilution (diluted flue gas) were carried out and the comparison of the results proved that a dilution tunnel was necessary to cool down and dilute the hot flue gas, which enabled the condensation of levoglucosan on PM at low temperature and to be collected on filters. The levoglucosan measured behind dilution should be used for the determination of emission ratios since it represents the realistic situation. Therefore, levoglucosan was further investigated by sampling only after dilution for different combustion phases. The results show that levoglucosan was emitted mainly at the burn-up phase and the main-burning phase, irrelevant of airflow settings at the wood stove. The comparison between different experimental parameters revealed that cold-start resulted in a relative high concentration of levoglucosan due to the low combustion temperature, but this influence was only limited to the burn-up phase. The dilution ratio could be regulated between 5 and 10, but it was shown not to have an obvious effect on the measured levoglucosan level. Based on these results, a suitable sampling procedure for levoglucosan was suggested and different emission ratios of levoglucosan were calculated.

By estimating the local wood consumption situation, the calculation of wood burning PM contribution with the levoglucosan tracer method was realized after the derivation of local conversion factors which can correlate the levoglucosan to wood burning PM in ambient air. The ambient concentration of levoglucosan was multiplied by the chosen local conversion factors. The obtained values suggested an average contribution of wood burning to total ambient PM10 is in the range of 16 to 25 % in winter 2014/2015 in the studied area. The comparison with black carbon data showed that biomass burning black carbon has an average contribution of 30 % to the total black carbon. However, the biomass black carbon fraction cannot be compared directly with the wood burning PM10 fraction calculated from levoglucosan because black carbon and PM10 possess different particle size distributions. A better comparison requires further investigations, which can be one of topics in the future of work.

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