Abstract. To improve the simulation of the heterogeneous oxidation of SO2 and
NOx in the presence of authentic mineral dust particles
under ambient environmental conditions, the explicit kinetic mechanisms were
constructed in the Atmospheric Mineral Aerosol Reaction (AMAR) model. The
formation of sulfate and nitrate was divided into three phases: the gas phase,
the non-dust aqueous phase, and the dust phase. In particular, AMAR established the
mechanistic role of dust chemical characteristics (e.g., photoactivation,
hygroscopicity, and buffering capacity) in heterogeneous chemistry. The
photoactivation kinetic process of different dust particles was built into
the model by measuring the photodegradation rate constant of an impregnated
surrogate (malachite green dye) on a dust filter sample (e.g., Arizona test
dust – ATD – and Gobi Desert dust – GDD) using an online reflective
UV–visible spectrometer. The photoactivation parameters were integrated with
the heterogeneous chemistry to predict the formation of reactive oxygen
species on dust surfaces. A mathematical equation for the hygroscopicity of
dust particles was also included in the AMAR model to process the multiphase
partitioning of trace gases and in-particle chemistry. The buffering capacity
of dust, which is related to the neutralization of dust alkaline carbonates
with inorganic acids, was included in the model to dynamically predict the
hygroscopicity of aged dust. The AMAR model simulated the formation of
sulfate and nitrate using experimental data obtained in the presence of
authentic mineral dust under ambient sunlight using a large outdoor smog
chamber (University of Florida Atmospheric
Photochemical Outdoor Reactor, UF-APHOR). Overall, the influence of GDD on the heterogeneous
chemistry was much greater than that of ATD. Based on the model analysis, GDD
enhanced the sulfate formation mainly via its high photoactivation
capability. In the case of NO2 oxidation, dust-phase nitrate
formation is mainly regulated by the buffering capacity of dust. The measured
buffering capacity of GDD was 2 times greater than that of ATD, and
consequently, the maximum nitrate concentration with GDD was nearly 2 times
higher than that with ATD. The model also highlights that in urban areas with
high NOx concentrations, hygroscopic nitrate salts quickly
form via titration of the carbonates in the dust particles, but in
the presence of SO2, the nitrate salts are gradually depleted by
the formation of sulfate.
Methods and Results of the American Museum Expeditions in the Gobi Desert, 1922–25
