Ozone deposition into a boreal forest over a decade of observations: Evaluating deposition partitioning and driving variables
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 differ-ent time-scales, including daily to inter-annual variation as well as the dependence on environmental variables and con-centration of biogenic volatile organic compounds (BVOC-s). 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 grow-ing 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 deter-mined to vary from 26 to 44 % during day time, increasing from the start of the season until the end of the growing sea-son. By using multi-variate analysis it was determined that day-time total ozone deposition was mainly driven by pho-tosynthetic 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 aver-age level (R 2 = 0.79). The explanatory power of the multi-variate model for ozone non-stomatal deposition was much lower (R 2 = 0.38). The set of common environmental vari-ables and terpene concentrations used in multivariate analy-sis were able to predict the observed average seasonal vari-ation in total and non-stomatal deposition but failed to ex-plain 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 reac-tions 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 sev-eral non-stomatal removal mechanisms. Unknown chemical compounds or processes correlating with monoterpene con-centrations, including potentially reactions at the surfaces, contribute to non-stomatal sink term.