Manvendra K. Dubey — Los Alamos National Laboratory
Samantha Bixler — NMTech and LANL
Christian Carrico — Colorado State University
Dian Romonosky — Los Alamos National Laboratory
Allison C Aiken — Los Alamos National Laboratory
Category
Absorbing aerosol
Description
Top: Correlation of measured water uptake (Kext) of BB CAs with fuel inorganic content (blue) and smoke inorganic fraction (red)
Bottom: Evaluation of mixing rules for Kext that use laboratory data (left half) to predict ambient fires (right half) for 4 different fires sampled at LANL as noted
Carbonaceous aerosols (CAs) from biomass burning (BB) have increased substantially with the observed warming and drying of the US. While wildfires are projected to intensify in the future missing knowledge of BB CAs hampers assessments. Observations show that warming effects of BB CAs can dominate over cooling effects due to enhanced light absorption by internal mixing. However, if internal mixing reduces the aerosol lifetime it would lower their atmospheric burden. In order to elucidate mechanisms regulating this tradeoff we report laboratory and field studies of smoke from biomass burning. Fresh emissions from ~32 fuels burned under flaming and smoldering conditions were investigated. We measured aerosol size distribution, absorption, scattering and extinction at multiple wavelengths, water uptake at 85% relative humidity (fRH85%) with a humidity controlled dual nephelometer, and refractory CA mass with a SP2. Trace gases and the ion composition of the fuel and smoke were also measured.
We find that whereas the optical properties of smoke were strongly dictated by the flaming versus smoldering nature of the burn, the observed hygroscopicity was intimately linked to the chemical composition of the fuel. The mean hygroscopicity ranged from nearly hydrophobic (fRH85% = 1) to very hydrophilic (fRH85% = 2.1) values typical of pure deliquescent salts. The κ values varied from 0.004 to 0.18 and correlated well with fuel and smoke inorganic content. Inorganic content was the key driver of hygroscopicity with combustion phase playing a secondary but important role (~20%). Flaming combustion promoted hygroscopicity by generating refractory CA and ions. Smoldering combustion suppressed hygroscopicity by producing hydrogenated organics. We compare our laboratory results with observations of 4 ambient fire episodes that spanned aging times of less than an hour (Los Alamos fires) and 3 days (the 2017 labor day NW wildfires) to evaluate mixing rules that predict κ using our laboratory data and knowledge of the fuel burned. We report wavelength dependent mass light absorption cross-sections to evaluate the nature of absorbing CA species. We plan to report results on aged BBCA with our OH oxidation reactor where we also monitor the size resolved composition of CAs with our SP-AMS. Our goal is to develop a mechanistic framework to predict water uptake and optical properties of biomass burning aerosols as a function of fuel type, fire intensity and age.
