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

Abstract

Abstract. This study scrutinizes a decade-long series of ozone deposition measurements in a boreal forest in search for the signature and relevance of the different deposition processes. The canopy-level ozone flux measurements were analysed for deposition characteristics and partitioning into stomatal and non-stomatal fractions, with the main focus on growing season day-time data. Ten years of measurements enabled the analysis of ozone deposition variation at different time-scales, including daily to inter-annual variation as well as the dependence on environmental variables and concentration of biogenic volatile organic compounds (BVOCs). Stomatal deposition was estimated by using multi-layer canopy dispersion and optimal stomatal control modelling from simultaneous carbon dioxide and water vapour flux measurements, non-stomatal was inferred as residual. Also, utilising the big-leaf assumption stomatal conductance was inferred from water vapour fluxes for dry canopy conditions. The total ozone deposition was highest during the peak growing season (4 mm s −1 ) and lowest during winter dormancy (1 mm s −1 ). During the course of the growing season the fraction of the non-stomatal deposition of ozone was determined to vary from 26 to 44 % during day time, increasing from the start of the season until the end of the growing season. By using multi-variate analysis it was determined that day-time total ozone deposition was mainly driven by photosynthetic capacity of the canopy, vapour pressure deficit (VPD), photosynthetically active radiation and monoterpene concentration. The multi-variate linear model explained the high portion of ozone deposition variance on daily average level (R 2 = 0.79). The explanatory power of the multivariate model for ozone non-stomatal deposition was much lower (R 2 = 0.38). The set of common environmental variables and terpene concentrations used in multivariate analysis were able to predict the observed average seasonal variation in total and non-stomatal deposition but failed to explain the inter-annual differences, suggesting that some still unknown mechanisms might be involved in determining the inter-annual variability. Model calculation was performed to evaluate the potential sink strength of the chemical reactions of ozone with sesquiterpenes in the canopy air space, which revealed that sesquiterpenes in typical amounts at the site were unlikely to cause significant ozone loss in canopy air space. The results clearly showed the importance of several non-stomatal removal mechanisms. Unknown chemical compounds or processes correlating with monoterpene concentrations, including potentially reactions at the surfaces, contribute to non-stomatal sink term.

Plant surface reactions: an opportunistic ozone defence mechanism impacting atmospheric chemistry

“…Measured ozone deposition fluxes could not be explained by modelled stomatal and known non-stomatal sinks, such as reactions with measured VOC in the gas phase (Wolfe et al, 2011a, b). The same observation was made by Rannik et al (2012) in a Scots pine dominated field site in Hyytiälä. All these studies assume the presence of yet unmeasured highly reactive semi-or low-volatile compounds, which have a similar temperature-dependent emission pattern as mono-and sesquiterpenes.…”

Direct measurement of NO<sub>3</sub> reactivity in a boreal forest

“…Previous studies of O 3 loss in forests have highlighted the potential role of unidentified, reactive organic compounds 15 (Kurpius and Goldstein, 2003;Goldstein et al, 2004;Holzinger et al, 2006;Rannik et al, 2012). In contrast to monoterpenes, that react only slowly with O 3 (rate constants are ≈ 10 -16 -10 -17 cm 3 molecule -1 s -1 , IUPAC (2017), sesquiterpenes (e.g.…”

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…This partly reflects the range of different processes involved, from adsorption and chemical take-up in plant stomata (Fowler et al, 2009) to iodine-mediated removal at the ocean surface (PradosRoman et al, 2015). Analysis of observational studies of the processes controlling ozone deposition shows that the importance of different deposition pathways differs by location as well as by season and from year to year at each location (Rannik et al, 2012;Clifton et al, 2017). Comparison of dry deposition fluxes from 15 global models involved in the TF HTAP model intercomparison project (Table 1) found that differences in ozone dry deposition are strongly influenced by differences in land cover classification used in models (Hardacre et al, 2015).…”