Chemical attribution of enhanced absorption by aged wildfire aerosol in smoke plumes from the Woodbury Fire, Arizona

Fire generated aerosols can impact climate and are detrimental to air quality and human health. The physical, chemical and optical properties of these emissions depend on the burn conditions, e.g. temperature, fuels, oxygen availability. Atmospheric aging modifies these properties, changing the aerosols ability to warm/cool climate. Mixed aerosol compositions of black carbon (BC) with organic material and/or brown carbon (BrC) are believed to enhance the aerosol ability to absorb light but the chemical drivers are complex. Multiple aged-plumes from the Woodbury Fire were observed at Los Alamos National Laboratory from 21-22 June, 2019 providing the opportunity to combine optical (CAPS-SSA 405 nm, PAX 870 nm), size distribution (SMPS), hygroscopicity, and BC mass (SP2) measurements with measurements of chemical composition (SP-AMS). Transport times of the plumes were about 12 hours. Peak submicron number concentration and scattering reached 15,500 per cm³ and 1500 Mm^-1 respectively, many orders of magnitude above the background.

We observe contrasting plume optical properties, with a 21 June plume exhibiting much stronger absorption enhancements of 6 and 2.5 at 450 nm and 870 nm wavelengths relative to fresh BC and larger Ångstrom absorption exponent than the other lower concentration plumes. Enhanced absorption is attributed to organic coatings on BC and BrC. The 21 June plume also exhibited lower modified combustion efficiency (MCE), lower mass fraction of BC (m_BC/m_total), and a higher mass fraction of levoglucosan (f60) and inorganic nitrate on the BC-containing particles as measured by a SP-AMS relative to others. Additional morphological analysis will quantitatively attribute coating enhancements using thickness data from the SP2 and identify unique chemical markers for BC and BrC from the SP-AMS.

Furthermore, laboratory burns will be performed with similar fuel to the Woodbury Fire and aged with a Potential Aerosol Mass (PAM) Oxidation Flow Reactor to link the chemistry and optical properties.

Instruments: Cavity Attenuated Phase Shift & Single Scatter Albedo Monitor (CAPS-SSA); Scanning Mobility Particle Sizer); Photoacoustic Extinctiometer (PAX); Single Particle Soot Photometer (SP2), Soot Particle Aerosol Mass Spectrometer (SP-AMS)