Quantifying Black Carbon Emissions from Residential Wood Burning by a Combination of Bottom-Up and Top-Down Approaches

Abstract

Black carbon (BC) is an air pollutant of emerging concern as more is understood regarding its multiple adverse impacts onto human health, global climate change and local visibility. While residential wood burning (RWB) for home heating is a significant contributor to global BC emissions, research into its impacts is limited by a lack of accurate emissions data. Typically, the estimation of regional emissions of air pollutants is carried out by measuring emissions from individual pollution sources. However, because of the highly-variable nature of BC emissions from RWB, this traditional ‘bottom-up’ methodology results in poor emission estimates on a regional scale. An alternative quantification method is the ‘top-down’ estimation of source contributions to measured ambient pollutant concentrations by source apportionment modelling. This study combines the results of a bottom-up emission inventory with top-down source apportionment data compiled by Davy et al. (2016) to better quantify BC emissions from RWB. The study focusses on BC emissions and the resulting concentrations observed within Auckland, and includes the first emission inventory constructed for BC within a New Zealand setting. Results of this study suggest that RWB is a significant source of BC emissions in Auckland, the second highest next to on-road diesel vehicles. From a combination of bottom-up and top-down BC quantification methodologies, it is determined that 145 tonnes of BC were emitted into the air within the Auckland Region in 2013 from approximately 100,000 residential wood burners, 30% of the region’s total estimated BC emissions. This equates to a weighted-average emission factor of 0.97 g of BC emitted per kg of wood burnt for residential wood burners in use across Auckland. Concentrations of BC vary widely across Auckland and are generally found to be decreasing as diesel vehicle emissions controls improve. However, BC concentrations associated with RWB remained static between 2006 and 2013, despite decreased RWB activity. This lack of improvement, coupled with increased concentrations of BC from RWB in early winter, suggests that the inefficient operation of wood burners may be a key driver of BC concentrations. Given the significant effects on both human health and climate, it is vital to advance programmes to help to reduce BC emissions from RWB.