Nitrous acid (HONO) is an important hydroxyl (OH) radical source that is formed on both ground and aerosol surfaces in the wellmixed boundary layer. Recent studies report the release of HONO from nonacidic soils, although it is unclear how soil that is more basic than the pK 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.nitrous acid | nitrite | surface acidity | air pollution | nitrogen cycle N itrous acid (HONO) plays a significant role in regulating the oxidative capacity of the atmosphere as it is readily photolyzed to produce nitric oxide (NO) and hydroxyl radical (OH) (1). The latter product, OH, is an important atmospheric oxidant that leads to ozone and secondary aerosol formation (2, 3). Despite extensive research over three decades (4-12), HONO formation and loss processes are not completely understood, and current atmospheric models do not accurately predict nighttime and daytime levels (13-18). Models and field measurements of vertical HONO gradients indicate that reactions on ground surfaces may explain the inconsistencies (12,13,15,(19)(20)(21) and that the hydrolysis of NO 2 on these surfaces is the most important HONO source at night (22-25) that likely continues through the daytime (19). Recent investigations demonstrate that nitrite derived from microbial activity is a source of atmospheric HONO from nonacidic soil (i.e., soil pH >5) (26, 27). However, the mechanism responsible for HONO release from soil nitrite remains unclear because the pK a of HONO in bulk water is ∼3 (28-30); this suggests that our understanding of bulk aqueous equilibria may not be applicable to understanding the speciation of soil-adsorbed N(III) species [N(III) = NO 2ˉ, HONO, H 2 ONO + ]. Here we report that the surface acidity of boundary layer minerals plays a critical role in the ability of a surface to release HONO or sequester it as nitrite (NO 2ˉ) or the nitroacidium ion (H 2 ONO + ) on the surface. We also demonstrate the disproportionate importance of aluminum and iron (hydr)oxides in controlling HONO uptake and release on soil surfaces.In our pre...

Nitrous acid (HONO) is an important OH radical source that is formed on both ground and aerosol surfaces in the well-mixed boundary layer. Large uncertainties remain in quantifying HONO sinks and determining the mechanism of HONO uptake onto surfaces. We report here the first laboratory determination of HONO uptake coefficients onto actual soil under atmospheric conditions using a coated-wall flow tube coupled to a highly sensitive chemical ionization mass spectrometer (CIMS). Uptake coefficients for HONO decrease with increasing RH from (2.5 ± 0.4) × 10(-4) at 0% RH to (1.1 ± 0.4) × 10(-5) at 80% RH. A kinetics model of competitive adsorption of HONO and water onto the particle surfaces fits the dependence of the HONO uptake coefficients on the initial HONO concentration and relative humidity. However, a multiphase resistor model based on the physical and chemical processes affecting HONO uptake is more flexible as it accounts for the pH dependence of HONO uptake and bulk diffusion in the soil matrix. Fourier transform infrared (FTIR) spectrometry and cavity-enhanced absorption spectroscopy (CEAS) studies indicate that NO and N2O (16% and 13% yield, respectively) rather than NO2 are the predominant gas phase products, while NO2(-) and NO3(-) were detected on the surface post-exposure. Results are compared to uptake coefficients inferred from models and field measurements, and the atmospheric implications are discussed.

Nitrous acid (HONO) is a photochemical source of hydroxyl radical and nitric oxide in the atmosphere that stems from abiotic and biogenic processes, including the activity of ammonia-oxidizing soil microbes. HONO fluxes were measured from agricultural and urban soil in mesocosm studies aimed at characterizing biogenic sources and linking them to indigenous microbial consortia. Fluxes of HONO from agricultural and urban soil were suppressed by addition of a nitrification inhibitor and enhanced by amendment with ammonium (NH4(+)), with peaks at 19 and 8 ng m(-2) s(-1), respectively. In addition, both agricultural and urban soils were observed to convert (15)NH4(+) to HO(15)NO. Genomic surveys of soil samples revealed that 1.5-6% of total expressed 16S rRNA sequences detected belonged to known ammonia oxidizing bacteria and archaea. Peak fluxes of HONO were directly related to the abundance of ammonia-oxidizer sequences, which in turn depended on soil pH. Peak HONO fluxes under fertilized conditions are comparable in magnitude to fluxes reported during field campaigns. The results suggest that biogenic HONO emissions will be important in soil environments that exhibit high nitrification rates (e.g., agricultural soil) although the widespread occurrence of ammonia oxidizers implies that biogenic HONO emissions are also possible in the urban and remote environment.

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