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 Fe(2+) 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 Fe(2+)(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 Fe(2+) 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.

Ammonia is the most abundant reduced nitrogen species in the atmosphere and an important precursor in the industrial-scale production of nitric acid. A coated-wall flow tube coupled to a chemiluminescence NOx analyzer was used to study the kinetics of NH3 uptake and NOx formation from photochemistry initiated on irradiated (λ > 290 nm) TiO2 surfaces under atmospherically relevant conditions. The speciation of NH3 on TiO2 surfaces in the presence of surface-adsorbed water was determined using diffuse reflection infrared Fourier transform spectroscopy. The uptake kinetics exhibit an inverse dependence on NH3 concentration as expected for reactions proceeding via a Langmuir-Hinshelwood mechanism. The mechanism of NOx formation is shown to be humidity dependent: Water-catalyzed reactions promote NOx formation up to a relative humidity of 50%. Less NOx is formed above 50%, where increasing amounts of adsorbed water may hinder access to reactive sites, promote formation of unreactive NH4(+), and reduce oxidant levels due to higher OH radical recombination rates. A theoretical study of the reaction between the NH2 photoproduct and O2 in the presence of H2O supports the experimental conclusion that NOx formation is catalyzed by water. Calculations at the MP2 and CCSD(T) level on the bare NH2 + O2 reaction and the reaction of NH2 + O2 in small water clusters were carried out. Solvation of NH2OO and NHOOH intermediates likely facilitates isomerization via proton transfer along water wires, such that the steps leading ultimately to NO are exothermic. These results show that photooxidation of low levels of NH3 on TiO2 surfaces represents a source of atmospheric NOx, which is a precursor to ozone. The proposed mechanism may be broadly applicable to dissociative chemisorption of NH3 on other metal oxide surfaces encountered in rural and urban environments and employed in pollution control applications (selective catalytic oxidation/reduction) and during some industrial processes.

Ammonia (NH3) from natural and anthropogenic sources is a globally distributed air pollutant, with emission rates expected to double over the next several decades. Selective catalytic oxidation (SCO) on TiO2 photocatalysts represents a potential strategy for NH3 pollution abatement. We show here that irradiation of TiO2 in the presence of NH3 under atmospheric conditions releases gas-phase nitrous acid (HONO), a potentially harmful precursor to atmospheric ozone, before it can be further reduced to N-2. Chemical ionization mass spectrometry and diffuse reflection infrared spectroscopy measurements indicate that HONO is derived from the reduction of NO2 and NO3-, which are products of the water-catalyzed photooxidation of NH3. This represents the first experimental evidence of HONO formation from NH3 photooxidation and has important implications for understanding SCO processes and devising air pollution control strategies to remove NH3 from ambient air.