Simulating ozone dry deposition at a boreal forest with a multi-layer canopy deposition model

A multi-layer ozone (O-3) dry deposition model has been implemented into SOSAA (a model to Simulate the concentrations of Organic vapours, Sulphuric Acid and Aerosols) to improve the representation of O-3 concentration and flux within and above the forest canopy in the planetary boundary layer. We aim to predict the O-3 uptake by a boreal forest canopy under varying environmental conditions and analyse the influence of different factors on total O-3 uptake by the canopy as well as the vertical distribution of deposition sinks inside the canopy. The newly implemented dry deposition model was validated by an extensive comparison of simulated and observed O-3 turbulent fluxes and concentration profiles within and above the boreal forest canopy at SMEAR II (Station to Measure Ecosystem-Atmosphere Relations II) in Hyytiala, Finland, in August 2010. In this model, the fraction of wet surface on vegetation leaves was parametrised according to the ambient relative humidity (RH). Model results showed that when RH was larger than 70% the O-3 uptake onto wet skin contributed similar to 51% to the total deposition during nighttime and similar to 19% during daytime. The overall contribution of soil uptake was estimated about 36 %. The contribution of sub-canopy deposition below 4.2m was modelled to be similar to 38% of the total O-3 deposition during daytime, which was similar to the contribution reported in previous studies. The chemical contribution to O-3 removal was evaluated directly in the model simulations. According to the simulated averaged diurnal cycle the net chemical production of O-3 compensated up to similar to 4% of dry deposition loss from about 06:00 to 15:00 LT. During nighttime, the net chemical loss of O-3 further enhanced removal by dry deposition by a maximum similar to 9 %. Thus the results indicated an overall relatively small contribution of airborne chemical processes to O-3 removal at this site.