Bulk and molecular-level characterization of laboratory-aged biomass burning organic aerosol from oak leaf and heartwood fuels

Claire F Fortenberry,Dhruv Mitroo,William H Brune,Michael J Walker,Yaping Zhang,Brent J Williams

doi:10.5194/acp-18-2199-2018

Abstract

Abstract. The chemical complexity of biomass burning organic aerosol (BBOA) greatly increases with photochemical aging in the atmosphere, necessitating controlled laboratory studies to inform field observations. In these experiments, BBOA from American white oak (Quercus alba) leaf and heartwood samples was generated in a custom-built emissions and combustion chamber and photochemically aged in a potential aerosol mass (PAM) flow reactor. A thermal desorption aerosol gas chromatograph (TAG) was used in parallel with a high-resolution time-of-flight aerosol mass spectrometer (AMS) to analyze BBOA chemical composition at different levels of photochemical aging. Individual compounds were identified and integrated to obtain relative decay rates for key molecules. A recently developed chromatogram binning positive matrix factorization (PMF) technique was used to obtain mass spectral profiles for factors in TAG BBOA chromatograms, improving analysis efficiency and providing a more complete determination of unresolved complex mixture (UCM) components. Additionally, the recently characterized TAG decomposition window was used to track molecular fragments created by the decomposition of thermally labile BBOA during sample desorption. We demonstrate that although most primary (freshly emitted) BBOA compounds deplete with photochemical aging, certain components eluting within the TAG thermal decomposition window are instead enhanced. Specifically, the increasing trend in the decomposition m∕z 44 signal (CO2+) indicates formation of secondary organic aerosol (SOA) in the PAM reactor. Sources of m∕z 60 (C2H4O2+), typically attributed to freshly emitted BBOA in AMS field measurements, were also investigated. From the TAG chemical speciation and decomposition window data, we observed a decrease in m∕z 60 with photochemical aging due to the decay of anhydrosugars (including levoglucosan) and other compounds, as well as an increase in m∕z 60 due to the formation of thermally labile organic acids within the PAM reactor, which decompose during TAG sample desorption. When aging both types of BBOA (leaf and heartwood), the AMS data exhibit a combination of these two contributing effects, causing limited change to the overall m∕z 60 signal. Our observations demonstrate the importance of chemically speciated data in fully understanding bulk aerosol measurements provided by the AMS in both laboratory and field studies.

Highlights

Atmospheric particulate matter (PM) negatively affects human health (e.g., Kampa and Castanas, 2008), impedes visibility (e.g., Appel et al, 1985), and impacts the global energy balance through direct radiative forcing or by acting as cloud condensation nuclei (e.g., Kanakidou et al, 2005)
According to aerosol mass spectrometer (AMS) mass spectra, the biomass burning organic aerosol (BBOA) measured in these experiments is chemically consistent with BBOA from similar oak fuel sources, though with key differences related to combustion conditions (Cubison et al, 2011; Ortega et al, 2013; Reece et al, 2017; Weimer et al, 2008)
Based on our results, a BBOA plume exhibiting high concentrations of aliphatic leaf wax components may be attributed to canopy or leaf litter devolatilization and combustion, while a plume with high concentrations of levoglucosan and lignin decomposition products could be attributed to heartwood combustion

Summary

Introduction

Atmospheric particulate matter (PM) negatively affects human health (e.g., Kampa and Castanas, 2008), impedes visibility (e.g., Appel et al, 1985), and impacts the global energy balance through direct radiative forcing or by acting as cloud condensation nuclei (e.g., Kanakidou et al, 2005). Organic aerosol (OA) particles comprise 20–90 % of submicron PM. Fortenberry et al.: Bulk and molecular characterization of BBOA from oak fuels (PM1) and may consist of thousands of distinct organic compounds (Goldstein and Galbally, 2007; Ng et al, 2010; Zhang et al, 2007). Given the multitude of organic compounds in the atmosphere and the numerous chemical reactions they can experience during atmospheric processing (e.g., Goldstein and Galbally, 2007; Kroll et al, 2009), laboratory studies are needed to fully understand the chemical composition and oxidative evolution of source-specific primary OA (POA, aerosol emitted directly into the atmosphere) and secondary OA (SOA, formed from gas-phase material that partition into the particle phase following photooxidation)

Methods

Results

Conclusion