Molecular composition and the impact of fuel moisture content on fresh primary organic aerosol emissions during laboratory combustion of ponderosa pine needles

Abstract

Environmental context Wildland fire smoke and its impacts on air quality and human health are increasing globally. However, uncertainties in organic emissions from these fires hinder our understanding of downwind atmospheric photochemical processes driving the formation of hazardous air pollutants. In this study, we investigated the impact of fuel moisture content on organic species emission during the combustion of ponderosa pine needles, an important fuel source in the western United States. Rationale Pine needles represent an important fuel source in coniferous forest systems in the western United States. During forest fires, they can be easily ignited and help sustain flame on the ground. Methodology In this study, a comprehensive chemical analysis was conducted to examine oxygenated organic compounds (OOCs) present in PM2.5 (particles ≤ 2.5 µm in aerodynamic diameter) formed from burning dry and moist ponderosa pine (Pinus ponderosa) needles (PPN) in the presence and absence of fine woody debris (FWD). The effect of fuel moisture content (FMC), a key parameter that influences smoke formation, has not received much attention. Therefore, we also investigated the effect of FMC on PM2.5 formation and its composition. Thirty three experiments were conducted at the US Forest Service Fire Science Laboratory. PM2.5 was collected onto 47-mm Teflon filters, and silylated extracts were analysed by gas chromatography–mass spectrometry. Results More than 50 OOCs were identified, including levoglucosan and mannosan; n-dodecanoic acid and n-hexadecanoic acid; dihydroabietic acid, and dehydroabietic acid; and a series of intermediate volatile and semivolatile organic compounds. Mass spectra of a wide variety of compounds in electron and chemical ionisation mode are provided. Most of these OOCs were identified in this study for the first time in PPN aerosol, although some were previously reported in pine wood and other biomass burning aerosol. Our results show significant changes in the composition and abundance of particles depending on the amount and type of PPN burned. When compared with dry PPN, moist PPN showed decreased emissions of PM2.5 and OOCs, due likely to the presence of water in the system that partially suppressed the production of OOCs. Discussion Incorporating pine needles in atmospheric models as a contributor to smoke particles generated during forest fires is an essential step towards reducing the current uncertainties regarding the influence of these aerosols on chemical/air mass characteristics, regional meteorology, and the climate.