Nitrogen oxides are trace gases that critically affect atmospheric chemistry and aerosol formation1. Vegetation is usually regarded as a sink for these gases, although nitric oxide and nitrogen dioxide have been detected as natural emissions from plants2,3. Here we use in situ measurements to show that solar ultraviolet radiation induces the emission of nitrogen oxide radicals (NOx) from Scots pine (Pinus sylvestris) shoots when ambient concentrations drop below one part per billion. Although this contribution is insignificant on a local scale, our findings suggest that global NOx emissions from boreal coniferous forests may be comparable to those produced by worldwide industrial and traffic sources.
We measured the amount of NOx emitted from P. sylvestris (NOx flux) at the SMEAR II station in southern Finland4 by enclosing individual young shoots inside a specially equipped chamber5,6. The cover of each chamber was made of ultraviolet-transparent quartz glass (which has a transmittance of over 90% for ultraviolet A and B light) so that the plants were exposed to solar ultraviolet radiation. The chambers were closed 2–3 times every hour for 60 s while the gas concentration inside was being monitored.
We used a mass-balance equation to determine the rate of change in NOx concentration inside the chamber during a single chamber closure. To calculate the NOx flux, the solution to the mass-balance equation was fitted to the measured values6.
The NOx concentration increased during closure in an empty chamber, but increased faster when the chamber contained a shoot (Fig. 1a). Ultraviolet radiation also caused a small amount of NOx emission from the chamber walls, which was taken into account in applying the mass-balance equation.
NOx flux from the shoots occurred when they were exposed to solar ultraviolet light: when the shoots were removed from their ultraviolet-transparent chambers, the NOx flux decreased, but was restored when the shoots were replaced (Fig. 1b). We repeated this experiment using two chambers during three sunny days in summer 2001, and found that the measured reduction in NOx emission after removing the shoots, and the increase after inserting the shoots, were statistically significant on each occasion (P = 0.00004).
We confirmed that ultraviolet radiation was inducing NOx emission from the shoots by replacing the quartz covers on the chambers with ultraviolet-opaque methacrylate (plexiglass) covers (negligible transmittance at 290–320 nm). NOx emission decreased from shoots inside chambers with opaque covers, and removing the plants had no effect on NOx flux (Fig. 1c). Swapping the covers had no effect on photosynthesis, which uses visible light.
The uptake of NOx by plants increases as its atmospheric concentration increases, so a compensation point7 is frequently detected2,3,8,9. NOx concentrations at the SMEAR II station are usually below 1 part per billion (p.p.b), but in May 2001 they rose to 6 p.p.b. during an episode when the weather was cloudy and ultraviolet radiation was low (mostly <10 W m−2), causing NOx deposition. This varied linearly with the ambient concentration, and the compensation point was estimated to be around 1 p.p.b.
As ultraviolet radiation induces NOx emission from P. sylvestris shoots, it must also influence the compensation point. Assuming a linear relationship between exposure to ultraviolet light and NOx emission, and between the ambient concentration of NOx and its deposition, the compensation point is above 3 p.p.b. at maximum ultraviolet irradiance. In previous NOx-exchange studies3,8,9, ultraviolet radiation was excluded either by the chamber material or from the light source, causing the compensation-point estimates to be too low.
We do not yet know the origin of the emitted NOx. It may come from plant metabolism2 or be released from pine-needle surfaces as a result of ultraviolet irradiation, in a reaction similar to the one that occurs at the chamber walls. Gas-phase NOx is produced from snow by photolysis of nitrate10 — a reaction that would enable NOx emissions to recycle reactive nitrogen on foliage.
The NOx emission induced by ultraviolet radiation was 1 ng m−2 of pine-needle surface per second under conditions corresponding to a pre-industrial time, when the ambient NOx concentration was less than 0.5 p.p.b. A flux of this magnitude is sufficient to influence the availability of nitrogen to plants and may affect the atmospheric NOx budget11.
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The authors declare no competing financial interests.
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Hari, P., Raivonen, M., Vesala, T. et al. Ultraviolet light and leaf emission of NOx. Nature 422, 134 (2003). https://doi.org/10.1038/422134a
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