Simulation of heterogeneous photooxidation of SO<sub>2</sub> and NO<sub><i>x</i></sub> in the presence of Gobi Desert dust particles under ambient sunlight

Abstract

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.

Effects of NO<sub>2</sub> and C<sub>3</sub>H<sub>6</sub> on the heterogeneous oxidation of SO<sub>2</sub> on TiO<sub>2</sub> in the presence or absence of UV–Vis irradiation

“…In many developing countries, such as China and India, high concentrations of SO 2 , NO x , and volatile organic compounds (VOCs) coexist in the atmosphere (Zou et al, 2015;Liu et al, 2013;Yang et al, 2009) and result in "complex atmospheric pollution" (Yang et al, 2011) and heavy haze events. Sulfate was found to play important roles in the occurrence of these haze events (Zhang et al, 2011;Z. R. Liu et al, 2017) due to both its high mass concentration in fine particles (PM 2.5 ) and its strong hygroscopicity.…”

A Novel Pathway of Atmospheric Sulfate Formation Through Carbonate Radical

“…The heterogeneous reaction of SO2 on dust particles in the atmosphere is a complicated process, covering a series of reactions taking place through both homogeneous and heterogeneous ways. Many studies pointed out that dust particles in the atmosphere are more likely to possess several water layers (Vlasenko et al, 2006;Pradhan et al, 2010;Yang et al, 2017) through a thermodynamic equilibrium process, and particles are subjected to provide quasi-aqueous medium when RH is over 20 % (Yu and Jang, 2018). Therefore, the adsorption process mediated by formed water layers can be a major route for gas-dust partitioning, and the aqueous radical-initiated reactions are very likely to responsible for the rapid oxidation of SO2 on mineral dust surfaces.…”

On mineral dust aerosol hygroscopicity

“…It can alter the radiative forcing of the earth both directly (Balkanski et al, 2007;Huang et al, 2014;Di Biagio et al, 2017) and indirectly (Cziczo et al, 2013;Karydis et al, 2017). Mineral dust can also change the abundance of reactive trace gases as well as aerosol compositions via heterogeneous reactions (Usher et al, 2003;Dupart et al, 2012;He et al, 2014;Tang et al, 2017;Yu and Jang, 2019). Furthermore, the deposition of mineral dust will bring substantial amounts of nutrients (e.g., Fe and P) into some marine and terrestrial ecosystems, thereby largely affecting biogeochemistry in these regions (Jickells et al, 2005;Okin et al, 2011;Schulz et al, 2012;Li et al, 2017;Tagliabue et al, 2017;Meskhidze et al, 2019).…”