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

Abstract

A multi-layer ozone (O3) 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 O3 concentration and flux within and above the forest canopy in the planetary boundary layer. We aim to predict the O3 uptake by a boreal forest canopy under varying environmental conditions and analyse the influence of different factors on total O3 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 O3 turbulent fluxes and concentration profiles within and above the boreal forest canopy at SMEAR II (Station to Measure Ecosystem-Atmosphere Relations II) in Hyytiälä, 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 O3 uptake onto wet skin contributed ∼ 51% to the total deposition during nighttime and ∼ 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 ∼ 38% of the total O3 deposition during daytime, which was similar to the contribution reported in previous studies. The chemical contribution to O3 removal was evaluated directly in the model simulations. According to the simulated averaged diurnal cycle the net chemical production of O3 compensated up to ∼ 4% of dry deposition loss from about 06:00 to 15:00LT. During nighttime, the net chemical loss of O3 further enhanced removal by dry deposition by a maximum ∼ 9%. Thus the results indicated an overall relatively small contribution of airborne chemical processes to O3 removal at this site.