Abstract
Abstract. We develop and evaluate a method to estimate O3 deposition and stomatal O3 uptake across networks of eddy covariance flux tower sites where O3 concentrations and O3 fluxes have not been measured. The method combines standard micrometeorological flux measurements, which constrain O3 deposition velocity and stomatal conductance, with a gridded dataset of observed surface O3 concentrations. Measurement errors are propagated through all calculations to quantify O3 flux uncertainties. We evaluate the method at three sites with O3 flux measurements: Harvard Forest, Blodgett Forest, and Hyytiälä Forest. The method reproduces 83 % or more of the variability in daily stomatal uptake at these sites with modest mean bias (21 % or less). At least 95 % of daily average values agree with measurements within a factor of 2 and, according to the error analysis, the residual differences from measured O3 fluxes are consistent with the uncertainty in the underlying measurements. The product, called synthetic O3 flux or SynFlux, includes 43 FLUXNET sites in the United States and 60 sites in Europe, totaling 926 site years of data. This dataset, which is now public, dramatically expands the number and types of sites where O3 fluxes can be used for ecosystem impact studies and evaluation of air quality and climate models. Across these sites, the mean stomatal conductance and O3 deposition velocity is 0.03–1.0 cm s−1. The stomatal O3 flux during the growing season (typically April–September) is 0.5–11.0 nmol O3 m−2 s−1 with a mean of 4.5 nmol O3 m−2 s−1 and the largest fluxes generally occur where stomatal conductance is high, rather than where O3 concentrations are high. The conductance differences across sites can be explained by atmospheric humidity, soil moisture, vegetation type, irrigation, and land management. These stomatal fluxes suggest that ambient O3 degrades biomass production and CO2 sequestration by 20 %–24 % at crop sites, 6 %–29 % at deciduous broadleaf forests, and 4 %–20 % at evergreen needleleaf forests in the United States and Europe.
Highlights
Surface ozone (O3) is toxic to both people and plants
This paper demonstrates a reliable method to estimate O3 fluxes at 103 eddy covariance flux towers spanning over 2 decades to enable O3 impact studies on ecosystem scales
If we subtract the mean seasonal cycle from both synthetic and observation-derived daily Fs,O3, the residual correlation is R2 = 0.5–0.7. This represents the skill of SynFlux at reproducing within-month and interannual variability. These results suggest that synthetic Fss,yOn3 is a reliable estimate of stomatal O3 uptake into plants that can be used at flux tower sites without O3 measurements
Summary
Surface ozone (O3) is toxic to both people and plants. Present-day and recent historical O3 levels reduce carbon sequestration in the biosphere (Reich and Lassoie, 1984; Guidi et al, 2001; Sitch et al, 2007; Ainsworth et al, 2012), perturb the terrestrial water cycle (Lombardozzi et al, 2012, 2015), and cause around $ 25 billion in annual crop losses (Reich and Amundson, 1985; Van Dingenen et al, 2009; Avnery et al, 2011; Tai et al, 2014). Surface deposition is 20 % of the total loss in tropospheric O3, making it an important control on air pollution (Wu et al, 2007; Young et al, 2013; Kavassalis and Murphy, 2017) This O3 deposition flux includes stomatal uptake into leaves, where O3 can cause internal oxidative damage, and less harmful non-stomatal deposition to plant cuticles, stems, bark, soil, and standing water (Fuhrer, 2000; Zhang et al, 2002; Ainsworth et al, 2012). (1)–(3) from surface observations, using some additional information from remote sensing and models This enables the estimation of FO3 and Fs,O3 , as described in Sect.
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