Nitrous acid formation on environmental surfaces containing titanium and iron

Mineral dust represents one of the largest mass fractions of the global aerosols. Iron oxide and titanium dioxide are components of mineral dust that are found in appreciable amounts. Ammonia from livestock, agriculture, industry, and waste treatment sources is an important air pollutant and precursor of light scattering aerosols. Ammonia is relatively unreactive in the gas phase and it was previously thought that it's primary fate in the environment was deposition to surfaces. Our results show that ammonia photooxidation on metal oxides found in aerosols and on boundary layer surfaces such as titanium dioxide represents an unrecognized sink for ammonia and a source of atmospheric NOx, which is precursor to ozone. The photochemical uptake of ammonia and emission of photochemical reaction products such as nitrous acid (HONO), nitric oxide (NO), and nitrogen dioxide (NO2) were studied using a coated-wall flow tube coupled to a chemical ionization mass spectrometer (CIMS) and chemiluminescence NOx analyzer. Our results show uptake kinetics exhibit an inverse dependence on ammonia concentration as expected if the reaction proceeds via a Langmuir-Hinshelwood mechanism. It is demonstrated that the mechanism of NOx formation is humidity dependent. Water catalyzed reactions promote HONO and NOx formation up to a relative humidity of 50%, while formation of unreactive NH4+ appears to limit further NOx formation in the higher humidity range.;We also investigated the kinetics and mechanisms of NO2 adsorption on iron oxide surfaces. Although this is well investigated due to the importance of mineral dust in atmospheric nitrogen cycling, comparatively little is known about the role of iron redox chemistry in controlling NO2-to-HONO conversion on iron oxide and iron bearing clay mineral surfaces. We investigated the uptake of NO2 on iron oxide and clay minerals. Our results show that the amount of HONO formed depends on the surface pH and the availability of surface Fe2+, with the amount of HONO emitted following the trend: Fe 3O4 > alpha-FeOOH > gamma-Fe2O3 > alpha-Fe2O3 in the absence of sunlight. However, the amount of HONO formed on the iron oxide surface does not scale linearly with the Fe2+ content of the substrate and secondary losses of HONO (e.g., retention of nitrate and nitrite) become important at high Fe2+ concentrations and higher surface pH. Additionally, it is also found that the amount of HONO formed photochemically depends on the presence of organic ligands with the HONO flux following the trend: EDTA > citric acid > oxalic acid > formic acid > acetic acid. Furthermore, the flux of HONO from photolysis of mineral dust from Egypt and Arizona show a stronger correlation to the amount of Fe-bearing minerals, rather than TiO2 content. In light of the abundance of redox active iron oxides in mineral dust, soil and clay minerals, these results imply that reduction of NO2 by Fe2+ may be an important source of HONO to the atmospheric boundary layer than previously thought.