Modeling Biomass Burning Organic Aerosol Atmospheric Evolution and Chemical Aging

Abstract

The changes in the concentration and composition of biomass-burning organic aerosol (OA) downwind of a major wildfire are simulated using the one-dimensional Lagrangian chemical transport model PMCAMx-Trj. A base case scenario is developed based on realistic fire-plume conditions and a series of sensitivity tests are performed to quantify the effects of different conditions and processes. Temperature, oxidant concentration and dilution rate all affect the evolution of biomass burning OA after its emission. The most important process though is the multi-stage oxidation of both the originally emitted organic vapors (volatile and intermediate volatility organic compounds) and those resulting from the evaporation of the OA as it is getting diluted. The emission rates of the intermediate volatility organic compounds (IVOCs) and their chemical fate have a large impact on the formed secondary OA within the plume. The assumption that these IVOCs undergo only functionalization leads to an overestimation of the produced SOA suggesting that fragmentation is also occurring. Assuming a fragmentation probability of 0.2 resulted in predictions that are more consistent with available observations. Dilution leads to OA evaporation and therefore reduction of the OA levels downwind of the fire. However, the evaporated material can return to the particulate phase later on after it gets oxidized and recondenses. The sensitivity of the OA levels and total mass balance on the dilution rate depends on the modeling assumptions. The high variability of OA mass enhancement observed in past field studies downwind of fires may be partially due to the variability of the dilution rates of the plumes.

Phase Behavior and Viscosity in Biomass Burning Organic Aerosol and Climatic Impacts

“…Previous studies have investigated the phase behavior of some types of atmospheric aerosol particles, such as mixtures of secondary organic aerosol with inorganic salts and mixtures of secondary organic aerosol with hydrocarbon-like organic aerosol. − However, the phase behavior of primary BBOA remains understudied and unresolved. Models simulating the microphysics, chemical evolution, concentration, or radiative properties of wildfire smoke typically assume that BBOA comprises only a single phase. ,− On the other hand, Jahn et al identified that some sampled BBOA particles exhibited an organic shell coating an organic core using transmission electron microscopy images, with the caveat that the vacuum conditions required for this type of microscopy may affect the particle morphology. In addition, bulk-phase liquid tar condensates generated from high-temperature wood distillation were reported to have separated into a water-soluble and nonsoluble oily phase, , although it is not known whether phase separation would occur in particles containing this material after equilibration with ambient relative humidity (RH).…”

Biomass burning organic aerosol (BBOA) from wildfires has multiple phases and is more viscous than laboratory generated BBOA

“…After emission the primary BBOA can partially evaporate and secondary BBOA can form on and in the primary BBOA. 48,58 A highly viscous BBOA, as observed here, could limit the rate of evaporation of primary BBOA and the formation rate of secondary BBOA. [103][104][105][106][107][108] A recent modelling study that investigated the evolution of BBOA by evaporation of primary BBOA and the formation of secondary BBOA assumed the BBOA was liquid-like with a diffusion coefficient of 10 -10 m 2 s -1 .…”

“…Current atmospheric models that treat biomass burning organic aerosols typically consider them as single-phased particles. 37,[47][48][49] The results herein show that BBOA, at least in regions where forests primarily consist of pine trees, should be treated as having two separate phases. Depending on the degree of internal/external mixing of organic aerosol in forest fire plumes, these two phases may coexist and be separated within individual particles.…”

“…45,46 Despite the above, models simulating the chemistry, transport, and climatic impacts of wildfire BBOA typically assume that BBOA comprises only a single phase. 37,[47][48][49] Our understanding of the viscosity and phase behaviour of BBOA remains incomplete, in part, because previous studies have focused on samples generated in the laboratory and not collected from the real atmosphere. The composition of BBOA in the real atmosphere may be different than the composition of BBOA in the laboratory for several reasons including atmospheric aging of BBOA, higher degrees of dilution, and burning conditions (e.g.…”

Liquid-liquid phase separation and viscosity in biomass burning organic aerosol and climatic impacts

“…However, the phase behavior of primary BBOA remains understudied and unresolved. Models simulating the microphysics, chemical evolution, concentration or radiative properties of wildfire smoke typically assume that BBOA comprises only a single phase (24,(36)(37)(38). On the other hand, Jahn et al identified that some sampled BBOA particles exhibited an organic shell coating an organic core using transmission electron microscopy images (39), with the caveat that the vacuum conditions required for this type of microscopy may affect the particle morphology.…”