Soil surface acidity plays a determining role in the atmospheric-terrestrial exchange of nitrous acid

Significance Nitrous acid (HONO) emitted from soil plays an important role in regulating the oxidizing capacity of the atmosphere. Unexpectedly high emissions of HONO are observed from soils with close to neutral pH, where aqueous acid–base and Henry’s law equilibria would predict that nitrite is the dominant nitrogen species. We show that surface acidity of soil minerals rather than aqueous acid–base equilibria controls nitrite speciation in soil, implying that up to 70% of global soils are capable of emitting HONO due to their acidic or close to neutral pH; this suggests that soil nitrite, whether microbially derived or otherwise deposited, likely plays a more widespread role in terrestrial–atmospheric cycling of nitrogen affecting air pollution and climate than previously thought.

Nitrous acid (HONO) is an important hydroxyl (OH) radical source that is formed on both ground and aerosol surfaces in the well-mixed boundary layer. Recent studies report the release of HONO from nonacidic soils, although it is unclear how soil that is more basic than the p K ₐ of HONO (∼3) is capable of protonating soil nitrite to serve as an atmospheric HONO source. Here, we used a coated-wall flow tube and chemical ionization mass spectrometry (CIMS) to study the pH dependence of HONO uptake onto agricultural soil and model substrates under atmospherically relevant conditions (1 atm and 30% relative humidity). Experiments measuring the evolution of HONO from pH-adjusted surfaces treated with nitrite and potentiometric titrations of the substrates show, to our knowledge for the first time, that surface acidity rather than bulk aqueous pH determines HONO uptake and desorption efficiency on soil, in a process controlled by amphoteric aluminum and iron (hydr)oxides present. The results have important implications for predicting when soil nitrite, whether microbially derived or atmospherically deposited, will act as a net source or sink of atmospheric HONO. This process represents an unrecognized mechanism of HONO release from soil that will contribute to HONO emissions throughout the day.