Single-particle characterization of biomass burning organic aerosol (BBOA): evidence for non-uniform mixing of high molecular weight organics and potassium

Alex K Y Lee,Megan D Willis,Robert M Healy,Cheol-Heon Jeong,John C Wenger,Jon M Wang,Greg J Evans,Jonathan P D Abbatt

doi:10.5194/acp-16-5561-2016

Abstract

Abstract. Biomass burning organic aerosol (BBOA) can be emitted from natural forest fires and human activities such as agricultural burning and domestic energy generation. BBOA is strongly associated with atmospheric brown carbon (BrC) that absorbs near-ultraviolet and visible light, resulting in significant impacts on regional visibility degradation and radiative forcing. The mixing state of BBOA can play a critical role in the prediction of aerosol optical properties. In this work, single-particle measurements from a Soot-Particle Aerosol Mass Spectrometer coupled with a light scattering module (LS-SP-AMS) were performed to examine the mixing state of BBOA, refractory black carbon (rBC), and potassium (K, a tracer for biomass burning aerosol) in an air mass influenced by wildfire emissions transported from northern Québec to Toronto, representing aged biomass burning plumes. Cluster analysis of single-particle measurements identified five BBOA-related particle types. rBC accounted for 3–14 wt % of these particle types on average. Only one particle type exhibited a strong ion signal for K+, with mass spectra characterized by low molecular weight organic species. The remaining four particle types were classified based on the apparent molecular weight of the BBOA constituents. Two particle types were associated with low potassium content and significant amounts of high molecular weight (HMW) organic compounds. Our observations indicate non-uniform mixing of particles within a biomass burning plume in terms of molecular weight and illustrate that HMW BBOA can be a key contributor to low-volatility BrC observed in BBOA particles. The average mass absorption efficiency of low-volatility BBOA is about 0.8–1.1 m2 g−1 based on a theoretical closure calculation. Our estimates indicate that low-volatility BBOA contributes ∼ 33–44 % of thermo-processed particle absorption at 405 nm; and almost all of the BBOA absorption was associated with low-volatility organics.

Highlights

Biomass burning is one of the major global sources of primary particulate matter, contributing approximately 42 and 74 % of global black carbon (BC) and primary organic aerosol, respectively (Bond et al, 2004)
Even though K+ has been considered as a tracer of biomass burning aerosol particles, our results demonstrate that K+ is not uniformly mixed in biomass burning organic aerosol (BBOA) and that over 80 % of BBOA particles observed here are characterized by low K+ content (i.e. < 1 wt % by assuming relative ionization efficiency of K+ (RIEK) of 2.9)
High molecular weight organic compounds were clearly observed in some BBOA particles

Summary

Introduction

Biomass burning is one of the major global sources of primary particulate matter, contributing approximately 42 and 74 % of global black carbon (BC) and primary organic aerosol, respectively (Bond et al, 2004). Typical climate forcing calculations only consider BC and dust as light-absorbing aerosol components, there is growing evidence that atmospheric brown carbon (BrC, i.e. light-absorbing organic particulate matter with wavelengthdependent absorption in the ultraviolet–visible spectral region) is strongly associated with biomass burning organic aerosol (BBOA), resulting in significant impacts on regional. Y. Lee et al.: Single-particle characterization of biomass burning organic aerosol visibility and radiative forcing (Kirchstetter et al, 2004; Chen and Bond, 2010; Lack et al, 2012, 2013; Saleh et al, 2014; Lu et al, 2015; Washenfelder et al, 2015)

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