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

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 4.2 m 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 15:00 LT. 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.

Dry deposition fluxes and deposition velocities of seven trace metal species at five sites in central Taiwan – a summary of surrogate surface measurements and a comparison with model estimations

Daily air concentrations and dry deposition fluxes of seven metal species were monitored at five sites in central Taiwan for five or six days every month from September 2009 to August 2010. Annual average concentrations at the five sites were in the range of 2.8 to 3.6 ng m−3 for As, 25 to 82 ng m−3 for Mn, 1900 to 2800 ng m−3 for Fe, 69 to 109 ng m−3 for Zn, 18 to 33 ng m−3 for Cr, 60 to 110 ng m−3 for Cu, and 25 to 40 ng m−3 for Pb. Annual average dry deposition fluxes were on the order of 3, 20, 400, 50, 25, 50, and 50 μg m−2 day−1 for As, Mn, Fe, Zn, Cr, Cu, and Pb, respectively. Annual average dry deposition velocities (Vd) for the seven metal species ranged from 0.18 to 2.22 cm s−1 at these locations. Small seasonal and geographical variations, e.g. from a few percent to a factor of 2 for different species and/or at different locations, were found in the measured concentrations, fluxes, and Vds. The measured fluxes and air concentrations had moderate to good correlations for several of the species at several of the sites (e.g. Fe, Zn, and Mn at most of the sites), but had either weak or no correlations for the other species or at the other sites (e.g. As at Sites I and III, Zn and Cr at Site IV, and Cu at most of the sites). The latter cases were believed to have large uncertainties in the flux measurements using surrogate surfaces. Sensitivity tests were conducted for particle Vds using a size-segregated particle dry deposition model, assuming various combinations of three lognormal size distributions representing fine particles (PM2.5), coarse particles (PM2.5–10), and super-sized particles (PM10+), respectively. It was found that the measured dry deposition fluxes can be reproduced reasonably well using the size-segregated particle dry deposition model if the mass fractions of the metal species in PM2.5, PM2.5–10 and PM10+ were known. Significant correlations between the modeled and the measured daily fluxes were found for those cases that were believed to have small uncertainties in the flux measurements.

Dry deposition fluxes and deposition velocities of seven trace metal species at five sites in Central Taiwan - a summary of surrogate surface measurements and a comparison with model estimation

Daily air concentrations and dry deposition fluxes of seven metal species were monitored at five sites in Central Taiwan for five or six days each month from September 2009 to August 2010. Annual average concentrations at the five sites were in the range of 2.8 to 3.6 ng m−3 for As, 25 to 82 ng m−3 for Mn, 1900 to 2800 ng m−3 for Fe, 69 to 109 ng m−3 for Zn, 18 to 33 ng m−3 for Cr, 60 to 110 ng m−3 for Cu, and 25 to 40 ng m−3 for Pb. Annual average dry deposition fluxes were on the order of 3, 20, 400, 50, 25, 50 and 50 μg m−2 day−1 for As, Mn, Fe, Zn, Cr, Cu and Pb, respectively. Annual average dry deposition velocities (Vd) for the seven metal species ranged from 0.18 to 2.22 cm s−1 at these locations. Small seasonal and geographical variations, e.g., from a few percent to a factor of 2 for different species and/or at different locations, were found for measured concentrations, fluxes and Vd. Measured fluxes and air concentrations had moderate to good correlations for several species at several sites, but had weak or no correlations for other species or at other sites, the latter cases were believed to have large uncertainties in flux measurements using surrogate surfaces. Sensitivity tests were conducted for particle Vd using a size-segregated particle dry deposition model, assuming various combinations of three lognormal size distributions representing fine particles (PM2.5), coarse particles (PM2.5-10) and super size particles (PM10+), respectively. It was found that measured dry deposition fluxes can be reproduced reasonably well using the size-segregated particle dry deposition model if the mass fractions of metal species in PM2.5, PM2.5-10 and PM10+ were known. Significant correlations between modeled and measured daily fluxes were found for those cases that were believed to have small uncertainties in flux measurements.