secondary organic aerosol formation in biomass-burning plumes: theoretical analysis of lab studies and ambient plumes
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2017
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Abstract
Secondary organic aerosol (SOA) has been shown to form in biomass-burning
emissions in laboratory and field studies. However, there is significant
variability among studies in mass enhancement, which could be due to
differences in fuels, fire conditions, dilution, and/or limitations of
laboratory experiments and observations. This study focuses on understanding
processes affecting biomass-burning SOA formation in laboratory smog-chamber
experiments and in ambient plumes. Vapor wall losses have been demonstrated
to be an important factor that can suppress SOA formation in laboratory
studies of traditional SOA precursors; however, impacts of vapor wall losses
on biomass-burning SOA have not yet been investigated. We use an aerosol-microphysical model that includes representations of volatility and oxidation
chemistry to estimate the influence of vapor wall loss on SOA formation
observed in the FLAME III smog-chamber studies. Our simulations with
base-case assumptions for chemistry and wall loss predict a mean OA mass
enhancement (the ratio of final to initial OA mass, corrected for
particle-phase wall losses) of 1.8 across all experiments when vapor wall
losses are modeled, roughly matching the mean observed enhancement during
FLAME III. The mean OA enhancement increases to over 3 when vapor wall
losses are turned off, implying that vapor wall losses reduce the apparent
SOA formation. We find that this decrease in the apparent SOA formation due
to vapor wall losses is robust across the ranges of uncertainties in the key
model assumptions for wall-loss and mass-transfer coefficients and chemical
mechanisms.
We then apply similar assumptions regarding SOA formation chemistry and physics to smoke emitted into the atmosphere. In ambient plumes, the plume dilution rate impacts the organic partitioning between the gas and particle phases, which may impact the potential for SOA to form as well as the rate of SOA formation. We add Gaussian dispersion to our aerosol-microphysical model to estimate how SOA formation may vary under different ambient-plume conditions (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, leading to more effective SOA formation. Emissions from large fires, such as intense wildfires, dilute slowly, suppressing OA evaporation and subsequent SOA formation in the near field. We also demonstrate that different approaches to the calculation of OA enhancement in ambient plumes can lead to different conclusions regarding SOA formation. OA mass enhancement ratios of around 1 calculated using an inert tracer, such as black carbon or CO, have traditionally been interpreted as exhibiting little or no SOA formation; however, we show that SOA formation may have greatly contributed to the mass in these plumes.
In comparison of laboratory and plume results, the possible inconsistency of OA enhancement between them could be in part attributed to the effect of chamber walls and plume dilution. Our results highlight that laboratory and field experiments that focus on the fuel and fire conditions also need to consider the effects of plume dilution or vapor losses to walls.
We then apply similar assumptions regarding SOA formation chemistry and physics to smoke emitted into the atmosphere. In ambient plumes, the plume dilution rate impacts the organic partitioning between the gas and particle phases, which may impact the potential for SOA to form as well as the rate of SOA formation. We add Gaussian dispersion to our aerosol-microphysical model to estimate how SOA formation may vary under different ambient-plume conditions (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, leading to more effective SOA formation. Emissions from large fires, such as intense wildfires, dilute slowly, suppressing OA evaporation and subsequent SOA formation in the near field. We also demonstrate that different approaches to the calculation of OA enhancement in ambient plumes can lead to different conclusions regarding SOA formation. OA mass enhancement ratios of around 1 calculated using an inert tracer, such as black carbon or CO, have traditionally been interpreted as exhibiting little or no SOA formation; however, we show that SOA formation may have greatly contributed to the mass in these plumes.
In comparison of laboratory and plume results, the possible inconsistency of OA enhancement between them could be in part attributed to the effect of chamber walls and plume dilution. Our results highlight that laboratory and field experiments that focus on the fuel and fire conditions also need to consider the effects of plume dilution or vapor losses to walls.
| Reference Key |
bian2017atmosphericsecondary
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|---|---|
| Authors | ;Q. Bian;S. H. Jathar;J. K. Kodros;K. C. Barsanti;L. E. Hatch;A. A. May;S. M. Kreidenweis;J. R. Pierce |
| Journal | Journal of agricultural and food chemistry |
| Year | 2017 |
| DOI |
10.5194/acp-17-5459-2017
|
| URL | |
| Keywords |
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