Foliage surface ozone deposition: a role for surface moisture?

Abstract

This paper addresses the potential role of surface wetness in ozone deposition to plant foliage. We studied Scots pine foliage in field conditions at the SMEARII field measurement station in Finland. We used a combination of data from flux measurement at the shoot (enclosure) and canopy scale (eddy covariance), information from foliage 5 surface wetness sensors, and a broad array of ancillary measurements such as radiation, precipitation, temperature, and relative humidity. Environmental conditions were defined as moist during rain or high relative humidity, and the subsequent 12 h from such events, circumstances that were frequent at this boreal site. From the measured fluxes we estimated the ozone conductance as the expression of the strength of the 10 ozone removal surface sink or total deposition. Further, the stomatal contribution was estimated and the remaining deposition was analysed as non-stomatal sink.The combined time series of measurements showed that both shoot and canopyscale ozone total deposition were enhanced when moist conditions occurred. On average, the estimated stomatal deposition accounted for half of the measured removal 15 at the shoot scale and one third at the canopy scale. However, during dry conditions the estimated stomatal uptake predicted the behaviour of the measured deposition, but during moist conditions there was disagreement. The estimated non-stomatal sink was analysed against several environmental factors and the clearest correspondence was found with ambient relative humidity. The relationship disappeared under 70% relative 20 humidity, a threshold that coincides with the value at which surface moisture gathers at the foliage surface according to the leaf surface wetness measurements. This suggests the non-stomatal ozone sink on the foliage to be modulated by the surface films. We attempted to extract such potential modulation with the estimated film formation via the theoretical expression or adsorption isotherm. Whereas this procedure could predict 25 the behaviour of the non-stomatal sink, it implied a chemical sink that was not accountable as simple ozone decomposition. We discuss the existence of other mechanisms whose relevance needs to be clarified, in particular: a significant stomatal aperture

Interactions of forests with secondary air pollutants: Some challenges for future research

“…Perhaps as important are the implications for biological activity on leaf surfaces exposed to ammonium salts. Apart from the ready availability of N as a nutrient source for microorganisms, enhanced periods with liquid water availability could lead to enhanced ion-exchange and leaching across the leaf surface (Tukey, 1970), and may also affect the deposition and reaction of ozone (Altimir et al, 2006). The more favourable conditions for micro-organism growth lead to increased risks of pathogen attack (Huber and Gillespie, 1992).…”

Seasonal variation of ozone deposition to a tropical rain forest in southwest Amazonia

“…This value is not very plausible for a cuticular resistance during daytime, since it is comparable to the daytime bulk stomatal resistance and the latter already explains the observed O 3 deposition to a satisfying degree. However, Zhang et al (2002) and Altimir et al (2006) found for forest and other ecosystems that the nonstomatal canopy resistance (attributed mainly to cuticular deposition) strongly depends on relative humidity and surface wetness. For high values of relative humidity (above 70-80%), often observed during nighttime, they report strongly decreasing canopy resistance values comparable in magnitude to our results.…”

Surface ozone concentrations and ecosystem health: Past trends and a guide to future projections

“…Although this may not be particularly important for O 3 , dry deposition of the gas at the earth's surface is affected by surface wetness (Altimir et al, 2006;Grantz et al, 1995;Lamaud et al, 2002), and by the state of vegetation, in particular its uptake through stomata. Increased CO 2 concentrations may inhibit O 3 uptake directly, by reducing stomatal conductance (Harmens et al, 2007;Sanderson et al, 2007;Tausz et al, 2007).…”