The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces
2016
Kebede, Mulu A. | Bish, David L. | Losovyj, Yaroslav | Engelhard, Mark H. | Raff, Jonathan D.
Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO₂ on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO₂ reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO₂ by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO₂ hydrolysis mechanism. The magnitude of NO₂-to-HONO conversion depends on the amount of Fe²⁺ present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO₂ and Fe²⁺(aq) present in thin films of water coating the surface, whereas in the range of pH 5–8 HONO stems from reaction of NO₂ with structural iron or surface complexed Fe²⁺ followed by protonation of nitrite via surface Fe–OH₂⁺ groups. Reduction of NO₂ on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO₂] ratios observed during dust storms in urban areas.
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