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Soil Nitrite as a Source of Atmospheric HONO and OH Radicals

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Science  16 Sep 2011:
Vol. 333, Issue 6049, pp. 1616-1618
DOI: 10.1126/science.1207687

Abstract

Hydroxyl radicals (OH) are a key species in atmospheric photochemistry. In the lower atmosphere, up to ~30% of the primary OH radical production is attributed to the photolysis of nitrous acid (HONO), and field observations suggest a large missing source of HONO. We show that soil nitrite can release HONO and explain the reported strength and diurnal variation of the missing source. Fertilized soils with low pH appear to be particularly strong sources of HONO and OH. Thus, agricultural activities and land-use changes may strongly influence the oxidizing capacity of the atmosphere. Because of the widespread occurrence of nitrite-producing microbes, the release of HONO from soil may also be important in natural environments, including forests and boreal regions.

Heterogeneous reactions of NO2 have been suggested to be a missing source of nitrous acid (HONO) to the atmosphere (14), and photoenhanced reactions on the surface of soot and other aerosol particles have been proposed to explain the observed enhancement of HONO production rates during the daytime [up to 5 parts per billion (ppb) hour–1] (59). Because of low uptake coefficients for NO2, however, atmospheric aerosols have too little surface area to account for the measured HONO production rates (7, 8). Thus, recent studies suggested that the reduction of N(IV) in NO2 to N(III) in HONO may proceed on ground surfaces (7, 8), and field data indicate that HONO is indeed released from the ground (10, 11).

The dominant sources of N(III) in soil, however, are biological nitrification and denitrification processes (12), which produce nitrite ions (NO2) from ammonium (by nitrifying microbes) as well as from nitrate (by denitrifying microbes), as illustrated in Fig. 1. Nitrites are highly water-soluble, and they can undergo the following reversible acid-base reaction and partitioning between air and the aqueous phase of humid soil

Fig. 1

Coupling of atmospheric HONO with soil nitrite. Red arrows represent the multiphase processes linking gaseous HONO and soil nitrite (acid-base reaction and phase partitioning), green arrows represent biological processes, orange arrows represent heterogeneous chemical reactions converting NO2 and HNO3 into HONO, and blue arrows represent other related physicochemical processes in the N cycle [supporting online material (SOM) text].

NO2(aq)+H+(aq)HNO2(aq)HONO(g)(1)

By convention, HNO2 represents molecular nitrous acid in the aqueous phase (aq), which can reversibly dissociate into H+ and NO2 ions or partition to the gas phase (g), where nitrous acid is designated as HONO. The equilibrium gas-phase concentration over an aqueous solution of nitrous acid, [HONO]*, is determined by the acidity (pH value) and nitrite concentration of the solution (13). [HONO]* is a key parameter controlling the exchange of HONO between the gas and aqueous phase. When [HONO]* is higher than the actual gas-phase concentration, [HONO], nitrous acid will be released from the aqueous phase; otherwise, gaseous HONO will be deposited.

The total nitrite concentration in soil, CN(III), including both NO2 ions and HNO2 molecules, is usually reported in units of micrograms of N per gram of oven-dry soil (μg g–1). The available literature data on CN(III) in different types of soils vary widely from ~0.01 μg g–1 to ~100 μg g–1, and the pH values are in the range of 3 to 8 (figs. S1 and S2). The variability of CN(III) and pH is linked to differences in soil composition, microbiological processes, fertilization, land use, aquatic transport, and atmospheric deposition or release as discussed below (14, 15).

Figure 2A shows the range of [HONO]* calculated for the observed range of CN(III) and pH in soil. The isolines illustrate the functional dependence of [HONO]* on CN(III) and pH at an average θg (gravimetric soil water content) of 0.2 kg kg–1 (16) by assuming ideal solution behavior (13). The data points represent experimental studies that reported both CN(III) and pH for investigated soil samples, suggesting that neutral or alkaline conditions (high pH) favor the accumulation of nitrite, whereas low soil nitrite concentrations tend to coincide with acidic conditions (low pH). This is consistent with a loss of nitrite through enhanced formation of HNO2 and release of HONO under acidic conditions. At low pH, even small amounts of nitrite may still lead to high values of [HONO]*.

Fig. 2

[HONO]* and Fmax. (A) Equilibrium gas-phase HONO concentration, [HONO]*; and (B) maximum HONO release flux, Fmax (in terms of N mass). The lines are logarithmic isopleths. The points represent data pairs of soil pH and total nitrite concentration (CN(III), in terms of N mass) reported from field measurements (clay loam soil, red circles; semiarid pine forest, red triangles; acidic fen soil, red diamonds); from incubation experiments with ambient soil samples (clay loam soil, blue crosses; grazed grassland soil, blue plus signs); and from the chamber experiment in this study (green asterisk); see SOM for details.

The atmospheric HONO concentrations observed in the planetary boundary layer vary from tens of parts per trillion in polar and forest regions (10, 17, 18) to several parts per billion in rural and urban areas (4, 19). These field measurement results are within the range of [HONO]* values calculated for the available data pairs of soil pH and CN(III): [HONO]* ≈ 0.1 to 600 ppb (Fig. 2A). Thus, the release of HONO from soil nitrite may indeed account for the missing source of HONO. To our knowledge, this process has not been considered in previous investigations of atmospheric HONO, but our results are consistent with earlier soil-air exchange studies suggesting that soil nitrite may strongly influence the release of gaseous N compounds from soil (20, 21).

To test the validity of the proposed HONO release mechanism, we conducted a chamber experiment with real soil. A sample of agricultural soil (loam) with CN(III) = 2.0 μg g–1, pH = 6.5, and θg = 0.4 kg kg–1 was placed in a Teflon chamber continuously flushed with dry synthetic air (practically free of HONO) (13). At the chamber exit, the gas-phase concentration of HONO released by the soil was measured with a long path absorption photometer (13). Throughout the 65-hour experiment, we observed strongly elevated [HONO] in the chamber outflow, which implies that the soil indeed released gaseous HONO over a period of more than 2 days. As shown in Fig. 3, [HONO] exhibited a strong increase from ~1 to ~22 ppb as θg decreased because of water evaporation, which is consistent with increasing CN(III) concentration in the soil water and increasing [HONO]* in the soil pore space. After reaching a maximum at θg ≈ 0.05 kg kg–1, [HONO] decreased slightly as θg further decreased. The investigated range of θg = 0.4 to 0.03 kg kg–1 covers characteristic wet and dry ambient soil conditions (16), and the observed dependence of [HONO] on θg closely resembles the previously reported humidity dependence of nitric oxide (NO) emissions from soil (21). This confirms the proposed relation between atmospheric HONO and soil nitrite, because the soil emissions of NO are known to be linked to soil nitrite (2022) (Fig. 1). Throughout the chamber experiment, the deviations between the observed gas-phase concentration [HONO] and the theoretical equilibrium concentration [HONO]* (13) were less than a factor of 3 (Fig. 3), which is small compared with the large variability of ambient soil nitrite concentrations and pH ranging over multiple orders of magnitude (Fig. 2). The deviations can be attributed to the kinetic limitations of mass transfer and to non-ideal solution behavior [adsorption, kelvin, and solute interaction effects on gas-liquid partitioning (23)].

Fig. 3

Chamber measurements of HONO emissions from soil. [HONO], gas phase HONO concentration in the chamber exit (red solid line); [HONO]*, equilibrium HONO concentration (green dashed line); and θg, gravimetric soil water content (black dotted line).

Overall, the results of the chamber experiment clearly confirm that soil nitrite can serve as a strong source of atmospheric HONO. We suggest that HONO produced by heterogeneous reactions of atmospheric NO2 on soil surfaces should also be buffered by the nitrite equilibrium outlined in Eq. 1. With regard to the biogeochemical cycling of N, HONO emissions provide an additional pathway for the release of soil nitrite to the atmosphere (Fig. 1).

To quantify the flux rate of HONO emissions from soil nitrite, we applied a standard formalism describing the atmosphere-soil exchange of trace gases as a function of the difference between the atmospheric concentration and the equilibrium concentration at the soil solution surface (13)F=vt([HONO][HONO]*)(2)Accordingly, the emission flux F at a given value of [HONO]* increases as the atmospheric [HONO] decreases and reaches a maximum (Fmax) when [HONO] approaches zero. The transfer velocity, vt, depends primarily on meteorological and soil conditions, and over continents it is typically on the order of ~1 cm s–1 (13, 24).

Figure 2B shows the range of Fmax corresponding to the [HONO]* values in Fig. 2A with a vt of 1 cm s–1. In the observed range of soil pH and CN(III), Fmax varies from ~1 ng m–2 s–1 up to ~3000 ng m–2 s–1 (N mass flux). This range fully covers the range of surface fluxes corresponding to the strength of the missing HONO source reported from field measurements: ~0.1 to 5 ppb hour–1 (1, 3, 18), equivalent to ~1 to 1000 ng m–2 s–1 for mixed-layer heights of ~100 to 1000 m (13). Even if non-ideal solution behavior and other effects may reduce the actual release of HONO (as suggested by the chamber experiment, which showed a difference between [HONO] and [HONO]* of up to a factor of 3), Fmax would still suffice to explain the missing source.

Besides the magnitude of the missing source, the HONO release from soil nitrite can also explain the characteristic diurnal course observed in field measurements. Figure 4 shows characteristic diurnal courses as observed for the atmospheric concentration and unknown source of HONO at the Xinken site (22.6148°N, 113.5912°E) in an agriculturally active rural region near the megacity cluster of Guangzhou, China (measured from 23 to 30 October 2004) (3). The atmospheric HONO concentration exhibits a maximum in the early morning and a minimum in the afternoon, resulting from the interplay of production, loss, and turbulent mixing within the boundary layer, as observed also at other continental locations (19, 2527). The missing source of HONO (Pmissing), which is required to maintain the daytime concentration level, follows the diurnal course of the photolytic loss rate and exhibits a pronounced noontime maximum (~5 ppb hour–1) (3). By considering the temperature dependence of the equilibria in Eq. 1 (13), the diurnal course of the observed missing source could be matched by soil HONO emission (Psoil) at a daily average [HONO]*¯=15ppb, a mixed-layer height of 300 m, and a modeled diurnal course of vt and [HONO]* as detailed and discussed in (13) and fig. S3.

Fig. 4

Diurnal variation of atmospheric [HONO], Pmissing, and Psoil at the Xinken site. Pmissing is the missing source of HONO required to maintain the daytime concentration level. Psoil is the modeled HONO emission from soil nitrite assuming Embedded Imageand a mixed-layer height of 300 m (13).

Because of enhanced fertilizer use and soil acidification in developing countries (28), the release of HONO from soil nitrite might strongly increase in the course of global change, resulting in elevated OH concentrations and amplified oxidizing capacity of the lower troposphere. Besides fertilization and intensified agricultural use of soils in populated environments, nitrite production and HONO release may also be important in natural environments, including forests and boreal soils, because of increasing N deposition (29), acid deposition, and the ubiquity of (de)nitrifying microbes. For example, the soils in boreal and tropical forests are typically highly acidic (pH ≈ 4 to 5, fig. S2). Thus, even very low soil nitrite concentrations (~0.001 to 0.01 μg g–1) could lead to a substantial release of HONO in such environments. In view of the potentially large impact on atmospheric chemistry and global environmental change, we recommend further studies of HONO release from soil nitrite and related processes in the biogeochemical cycling of N in both agricultural and natural environments.

Supporting Online Material

www.sciencemag.org/cgi/content/full/science.1207687/DC1

Materials and Methods

SOM Text

Figs. S1 to S5

Tables S1 to S3

References

References and Notes

  1. Materials and methods are available as supporting material on Science Online.
  2. Acknowledgments: This work was supported by the Max Planck Society and the European Commission under the projects EUCAARI (grant no. 036833-2) and PEGASOS (grant no. 265148). We gratefully acknowledge X. G. Chi, S. S. Gunthe, M. Shiraiwa, and N. Knothe for comments and discussions.
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