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

摘要

Biomass burning organic aerosol (BBOA) can be emitted from natural forestfires and human activities such as agricultural burning and domestic energygeneration. BBOA is strongly associated with atmospheric brown carbon (BrC)that absorbs near-ultraviolet and visible light, resulting in significantimpacts on regional visibility degradation and radiative forcing. The mixingstate of BBOA can play a critical role in the prediction of aerosol opticalproperties. In this work, single-particle measurements from a Soot-ParticleAerosol 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 massinfluenced by wildfire emissions transported from northern Québec toToronto, representing aged biomass burning plumes. Cluster analysis ofsingle-particle measurements identified five BBOA-related particle types.rBC accounted for 3–14 wt % of these particle types on average. Only oneparticle type exhibited a strong ion signal for K, with mass spectracharacterized by low molecular weight organic species. The remaining fourparticle types were classified based on the apparent molecular weight of theBBOA constituents. Two particle types were associated with low potassiumcontent and significant amounts of high molecular weight (HMW) organiccompounds. Our observations indicate non-uniform mixing of particles withina biomass burning plume in terms of molecular weight and illustrate that HMWBBOA can be a key contributor to low-volatility BrC observed in BBOAparticles. The average mass absorption efficiency of low-volatility BBOA isabout 0.8–1.1 m g based on a theoretical closure calculation. Ourestimates indicate that low-volatility BBOA contributes ∼ 33–44 % of thermo-processed particle absorption at 405 nm; and almost allof the BBOA absorption was associated with low-volatility organics.