Heterogenous reactions on atmospheric carbonaceous particles
Abstract
Elevated particle concentrations in the atmosphere have received
significant attention due to their multiple effects from urban to global scales. The
sources and formation mechanisms of these air pollutants, however, are poorly
understood, especially atmospheric reactions involving oxidation and
condensation of gas phase hydrocarbons, catalyzed by atmospheric particles. This
work examines these particle catalyzed, gas to particle transformation processes,
through detailed photochemical modeling of air quality episodes in Texas. Two
heterogeneous reaction pathways were incorporated into a widely used
photochemical model. The pathways that were examined were (1) Heterogeneous
formation of sulfuric acid on carbonaceous particles and (2) Acid catalyzed
condensation reactions of low molecular weight aldehydes. The impact of these
pathways on air quality in Texas was examined for a period during which
wildfires generated significant amounts of carbonaceous atmospheric aerosol.
This episode was chosen for analysis because the rates of the heterogeneous
reactions were expected to be significant during this period, leading to observable
signals in ambient data.
Simulations, together with ambient data, indicated that wood smoke
mediated sulfate formation reactions, not accounted for in most current
photochemical models, may have led to 5-10 µg/m3
of sulfate formation. In
photochemical simulations, wood-smoke mediated sulfate formation was modeled
by calculating the rate of impingement of SO2 molecules on the wood smoke
particles, and then assuming that a fraction of the impingements resulted in
reaction. For reaction probabilities on the order of 0.01, the model predicted
magnitudes, spatial distributions and temporal distributions of sulfate
concentrations consistent with observations.
Simulations, together with ambient data, indicated that acid aerosol
mediated organic aerosol formation reactions, not accounted for in most current
photochemical models, may have led to 1-5 µg/m3
of organic aerosol. In
photochemical simulations, acid mediated organic aerosol formation was modeled
by calculating the rate of impingement of aldehyde molecules on acidic particles,
and then assuming that a fraction of the impingements resulted in reaction. For
reaction probabilities on the order of 0.0005, the model predicted magnitudes,
spatial distributions and temporal distributions of organic aerosol consistent with
observations.
Ambient data and model results in this work demonstrated that the
heterogeneous reactions on carbonaceous particles are significant. The
methodology developed in this work, most importantly, can be applied to other
heterogeneous reactions to be incorporated into a photochemical model.
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