Secondary organic aerosol formation in biomass-burning plumes: theoretical analysis of lab studies and ambient plumes

摘要

Secondary organic aerosol (SOA) has been shown to form in biomass-burningemissions in laboratory and field studies. However, there is significantvariability among studies in mass enhancement, which could be due todifferences in fuels, fire conditions, dilution, and/or limitations oflaboratory experiments and observations. This study focuses on understandingprocesses affecting biomass-burning SOA formation in laboratory smog-chamberexperiments and in ambient plumes. Vapor wall losses have been demonstratedto be an important factor that can suppress SOA formation in laboratorystudies of traditional SOA precursors; however, impacts of vapor wall losseson biomass-burning SOA have not yet been investigated. We use an aerosol-microphysical model that includes representations of volatility and oxidationchemistry to estimate the influence of vapor wall loss on SOA formationobserved in the FLAME III smog-chamber studies. Our simulations withbase-case assumptions for chemistry and wall loss predict a mean OA massenhancement (the ratio of final to initial OA mass, corrected forparticle-phase wall losses) of 1.8 across all experiments when vapor walllosses are modeled, roughly matching the mean observed enhancement duringFLAME III. The mean OA enhancement increases to over 3 when vapor walllosses are turned off, implying that vapor wall losses reduce the apparentSOA formation. We find that this decrease in the apparent SOA formation dueto vapor wall losses is robust across the ranges of uncertainties in the keymodel assumptions for wall-loss and mass-transfer coefficients and chemicalmechanisms.We then apply similar assumptions regarding SOA formation chemistry andphysics to smoke emitted into the atmosphere. In ambient plumes, the plumedilution rate impacts the organic partitioning between the gas and particlephases, which may impact the potential for SOA to form as well as the rateof SOA formation. We add Gaussian dispersion to our aerosol-microphysicalmodel to estimate how SOA formation may vary under different ambient-plumeconditions (e.g., fire size, emission mass flux, atmospheric stability).Smoke from small fires, such as typical prescribed burns, dilutes rapidly,which drives evaporation of organic vapor from the particle phase, leadingto more effective SOA formation. Emissions from large fires, such as intensewildfires, dilute slowly, suppressing OA evaporation and subsequent SOAformation in the near field. We also demonstrate that different approachesto the calculation of OA enhancement in ambient plumes can lead to differentconclusions regarding SOA formation. OA mass enhancement ratios of around 1calculated using an inert tracer, such as black carbon or CO, havetraditionally been interpreted as exhibiting little or no SOA formation;however, we show that SOA formation may have greatly contributed to the massin these plumes.In comparison of laboratory and plume results, the possible inconsistency ofOA enhancement between them could be in part attributed to the effect ofchamber walls and plume dilution. Our results highlight that laboratory andfield experiments that focus on the fuel and fire conditions also need toconsider the effects of plume dilution or vapor losses to walls.