Abstract
Abstract. A 1-month field campaign of ozone (O3) flux measurements along a five-level vertical profile above, inside and below the canopy was run in a mature broadleaf forest of the Po Valley, northern Italy. The study aimed to characterize O3 flux dynamics and their interactions with nitrogen oxides (NOx) fluxes from the forest soil and the atmosphere above the canopy. Ozone fluxes measured at the levels above the canopy were in good agreement, thus confirming the validity of the constant flux hypothesis, while below-canopy O3 fluxes were lower than above. However, at the upper canopy edge O3 fluxes were surprisingly higher than above during the morning hours. This was attributed to a chemical O3 sink due to a reaction with the nitric oxide (NO) emitted from soil and deposited from the atmosphere, thus converging at the top of the canopy. Moreover, this mechanism was favored by the morning coupling between the forest and the atmosphere, while in the afternoon the fluxes at the upper canopy edge became similar to those of the levels above as a consequence of the in-canopy stratification. Nearly 80 % of the O3 deposited to the forest ecosystem was removed by the canopy by stomatal deposition, dry deposition on physical surfaces and by ambient chemistry reactions (33.3 % by the upper canopy layer and 46.3 % by the lower canopy layer). Only a minor part of O3 was removed by the understorey vegetation and the soil surface (2 %), while the remaining 18.2 % was consumed by chemical reaction with NO emitted from soil. The collected data could be used to improve the O3 risk assessment for forests and to test the predicting capability of O3 deposition models. Moreover, these data could help multilayer canopy models to separate the influence of ambient chemistry vs. O3 dry deposition on the observed fluxes.
Highlights
Ozone (O3) has been widely documented as one of the most dangerous pollutants for plants (Wittig et al, 2009; Matyssek et al, 2012; Gerosa et al, 2015; Marzuoli et al, 2018)
Ozone deposition pathways other than plant stomata are usually grouped as nonstomatal deposition, they include merely physical dry deposition processes and chemical consumption processes due to ambient air chemistry. Among these processes, which still remain to be understood in depth, we find the thermal decomposition on dry surfaces (Cape et al, 2009), the deposition on wet surfaces (Fuentes et al, 1992; Altimir et al, 2004, 2006; Gerosa et al, 2009b), the deposition on soil (Stella et al, 2011), reactions stimulated by light (Coe et al, 1995; Fowler et al, 2001), chemical reactions with nitric oxide (NO) (Dorsey et al, 2004; Rummel et al, 2007; Pilegaard, 2001; Gerosa et al, 2009b) and chemical reactions with biogenic volatile organic compounds (BVOCs) (Fares et al, 2010; Goldstein et al, 2004)
The major aims of this study were (i) to contribute to the understanding of the diel dynamics of O3 fluxes and O3 concentration gradients at five levels above and within a mature broadleaf forest canopy, (ii) to assess the ozone sinks above and within the forest and in particular the amount of O3 deposited on the different forest layers, and (iii) to evaluate the role of the nitrogen oxides (NOx) exchange on the O3 deposition, both at the top of the canopy and at soil level
Summary
Ozone (O3) has been widely documented as one of the most dangerous pollutants for plants (Wittig et al, 2009; Matyssek et al, 2012; Gerosa et al, 2015; Marzuoli et al, 2018). The deposition of O3 on forest ecosystems has been extensively studied over the last 20 years with eddy covariance field campaigns (Padro, 1996; Cieslik, 1998; Lamaud et al, 2002; Mikkelsen et al, 2004; Gerosa et al, 2005, 2009a, b; Launiainen et al, 2013), which were made possible thanks to the development of fast O3 analyzers. Zona et al, 2014; Fares et al, 2010, 2012, 2014; Clifton et al, 2017; Finco et al, 2017) These studies highlighted that an important deposition pathway is represented by the O3 uptake by trees through leaf stomata. The O3 amount entering the stomata strongly depends on the environmental and physiological factors that drive stomata opening (Jarvis, 1976; Emberson et al, 2000), such as, for example, the soil water availability that is positively correlated to the stomatal O3 flux (Gerosa et al, 2009a; Büker et al, 2012)
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