Foliage surface ozone deposition: a role for surface moisture?

Abstract

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 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 during the subsequent twelve hours from such events, circumstances that were frequent at this boreal site. From the measured fluxes we estimated the ozone conductance using it as the expression of the strength of ozone removal surface sink or total deposition. Further, we estimated the stomatal contribution and the remaining deposition was interpreted and analysed as the non-stomatal sink.The combined time series of measurements showed that both shoot and canopy-scale ozone total deposition were enhanced when moist conditions occurred. On average, the estimated stomatal deposition accounted for half of the measured removal 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 connection was found with ambient relative humidity. The relationship disappeared under 70% relative 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 viaCorrespondence to: N. Altimir (nuria.altimir@helsinki.fi) the theoretical expression of adsorption. Whereas this procedure could predict 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 in the removal of ozone needs to be clarified, in particular: a significant nocturnal stomatal aperture neglected in the estimations, and a potentially large chemical sink offered by reactive biogenic organic volatile compounds.

Ozone deposition into a boreal forest over a decade of observations: evaluating deposition partitioning and driving variables

“…Similar results/reports on long-term measurement of ozone fluxes at the ecosystem level are rather limited (Zhang et al, 2002;Turnispeed et al, 2009;Fares et al, 2010). Nevertheless, extensive ozone deposition studies over different ecosystems have been performed in order to understand deposition mechanisms and possible harmful effect on ecosystems (e.g., Fowler et al, 2001;Lamaud et al, 2002;Goldstein et al, 2004;Holzinger et al, 2005Holzinger et al, , 2006Altimir et al, 2006;Mészáros et al, 2009;Lamaud et al, 2009;Coyle et al, 2009).…”

“…Several mechanisms have been proposed to play a role in creating the non-stomatal deposition. Surface wetness modulates the deposition, usually enhancing it, as it has been showed in many works (summarised in Massman, 2004;Altimir et al, 2006). Fowler et al (2001) propose thermal decomposition as another mechanism, which in Coyle (2005) was suggested to be the main sink on dry surfaces.…”

“…Altimir et al (2004Altimir et al ( , 2006 studied the ozone removal at the site by using the shoot-level chamber as well as the canopy level eddy covariance measurements. The findings indicated that ozone deposition was strongly affected by the humidity conditions of surfaces and non-stomatal deposition contributed from 25 to 50 % of total deposition in dry conditions and from 50 to 60 % in moist conditions.…”

Predicting and partitioning ozone fluxes to maize crops from sowing to harvest: the Surfatm-O<sub>3</sub> model

“…soil and cuticular) pathways. The processes governing each deposition pathways are well identified: stomatal deposition is linked with environmental parameters governing stomatal opening (Emberson et al, 2000a), cuticular deposition increases with relative humidity (Zhang et al, 2002;Altimir et al, 2004Altimir et al, , 2006Lamaud et al, 2009) and soil deposition decreases with relative humidity . Moreover, the partitioning between stomatal, cuticular and soil depositions depends on canopy structure: the ozone transfer from the atmosphere towards the ground is reduced when the canopy height and the leaf area index (LAI) increase (van Pul and Jacobs, 1994;Zhang et al, 2002), while stomatal and cuticular depositions increase with canopy LAI (Zhang et al, 2002;Massman, 2004;Tuovinen et al, 2004).…”

“…In order to quantify the terrestrial ecosystem sink for ozone and to predict the potential effect of ozone on plants, it is necessary (i) on the one hand to predict total ozone deposition to ecosystems and (ii) on the other hand to discriminate the different deposition pathways. To this aim, several studies were performed over forests (Lamaud et al, 2002;Altimir et al, 2004Altimir et al, , 2006, crops Coyle et al, 2009;Lamaud et al, 2009) and grasslands (Zhang et al, 2002(Zhang et al, , 2006. It is currently assumed that ozone deposition follows stomatal and non-stomatal (i.e.…”